Journal of Petrology Advance Access originally published online on July 1, 2004
Journal of Petrology 2004 45(8):1539-1564; doi:10.1093/petrology/egh015
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Journal of Petrology 45(8) © Oxford University Press 2004; all rights reserved
Abundance and Partitioning of OH in a High-pressure Magmatic System: Megacrysts from the Monastery Kimberlite, South Africa
1 DIVISION OF GEOLOGICAL AND PLANETARY SCIENCES, CALIFORNIA INSTITUTE OF TECHNOLOGY 170-25, PASADENA, CA 91125-2500, USA
2 MINERAL SERVICES CANADA INC., 205–930 HARBOURSIDE DRIVE, NORTH VANCOUVER, B.C. V7P 3S7, CANADA
RECEIVED JULY 14, 2001; ACCEPTED DECEMBER 15, 2003
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ABSTRACT |
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TOP
ABSTRACT
INTRODUCTIONConcentrations of OH, and major and trace elements were determined in a suite of mantle-derived megacrysts that represent the crystallization products of a kimberlite-like magma at
5 GPa and 1400–1100°C. OH concentrations, determined by single-crystal Fourier transform infrared spectroscopy, display the following ranges (ppmw H2O): olivine 54–262, orthopyroxene 215–263, garnet 15–74, clinopyroxene 195–620, and zircon 28–34. High OH concentrations in olivine imply mantle conditions of origin, with limited H loss during ascent. OH is consistently correlated with megacryst composition, exhibiting trends with Mg-number that are similar to those of other minor and trace elements and indicating a record of high-pressure magmatic evolution. H substitution is not coupled to minor elements in olivine, but may be in ortho- and clinopyroxene. The OH–Mg-number trends of garnet and clinopyroxene show inflections related to co-precipitation of ilmenite, suggesting minor element (Ti) influence on OH partitioning. During differentiation, relative OH enrichment in clinopyroxene and olivine is consistent with proportional dependence on water activity, whereas that in garnet suggests a higher power-law dependence and/or influence of temperature. Inter-mineral distribution coefficients for OH between cpx, opx, olivine and zircon are thus constant, whereas partitioning between these minerals and garnet shows a factor 4–10 variation, correlated regularly with composition (and temperature). Calculation of solid–melt partition coefficients for H at 5 GPa over a range of magmatic evolution from 1380 to 1250°C yields: ol 0·0053–0·0046, opx 0·0093–0·0059, cpx 0·016–0·013, gt 0·0014–0·0003, bulk (garnet lherzolite–melt) 0·0063–0·0051. These are consistent with experimental studies and similar to values inferred from mid-ocean ridge basalt geochemistry, confirming the moderate incompatibility of H in mantle melting. KEY WORDS: hydrogen; megacryst; mantle; trace element; water
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INTRODUCTION |
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There is now a substantial body of information indicating that OH is incorporated in trace but measurable amounts in the structure of most common minerals whose chemical formulae suggest that they are nominally anhydrous. OH occurring in this manner may constitute the dominant reservoir of H in the Earth's interior (Bell & Rossman, 1992a
; Meade et al., 1994; Smyth, 1994; Ingrin & Skogby, 2000), and is believed to play an important role in the physical properties of the mantle (Mackwell et al., 1985; Karato, 1990; Hirth & Kohlstedt, 1996). This form of H may also influence the evolution of the hydrosphere through its behavior in mantle melting (Dixon et al., 1988) and through the isotopic fractionation that may attend reactions involving this species (Bell & Ihinger, 2000). The basis for understanding these phenomena lies in quantitative knowledge of the partitioning behavior of H (and D) between various phases (minerals, melt, fluid) as a function of the various conditions relevant to the Earth's mantle.
Most studies to date on natural minerals have dealt with the infrared spectroscopic characteristics and abundance of OH in individual minerals, whereas relatively few (e.g. Rossman & Smyth, 1990; Beran et al., 1993; Peslier et al., 2002) have examined assemblages of coexisting minerals. Furthermore, there have been few (e.g. Arredondo et al., 2001) attempts to use nominally anhydrous minerals in tracing the behavior of hydrous volatiles in igneous or metamorphic systems, despite the suggestion some two decades ago by Martin & Donnay (1972) that this could probably be done.
The experimental work necessary for extracting quantitative information about conditions and processes from the natural observations has received attention over the past decade (Geiger et al., 1991; Bai & Kohlstedt, 1992, 1993; Kohlstedt et al., 1996; Lu & Keppler, 1997; Withers et al., 1998; Matveev et al., 2001; Rauch & Keppler, 2002). These studies establish the relationship between OH in nominally anhydrous minerals and the fugacity of hydrous species with which the minerals have equilibrated at high pressures and temperatures. However, the complexities of natural geological systems require that these fundamental experimental investigations in simple chemical systems or under a limited range of conditions be complemented by studies on natural assemblages in order to be more useful in geology.
In this study, we examine in detail the relationship between OH content and other compositional features in a suite of co-magmatic megacryst minerals (garnet, olivine, orthopyroxene, clinopyroxene and zircon) from the Monastery kimberlite, South Africa. The compositional dependence of partitioning behavior of OH among high-pressure minerals is described and the relationship of the OH concentrations to water fugacity and concentration in the source is evaluated quantitatively with reference to the problems of hydrogen mobility under igneous conditions. These results complement high-pressure experimental studies in contributing to a foundation for understanding the behavior of hydrogen in the Earth's mantle.
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THE MONASTERY MEGACRYST SUITE |
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Overview and significance of megacrysts
‘Cr-poor megacrysts’ (Eggler et al., 1979
) or ‘discrete nodules’ (Nixon & Boyd, 1973) are a common variety of mantle-derived xenolith sampled by kimberlite, characterized by large grain size and a distinctive range of chemical compositions. The wide range of compositions and temperatures recorded are consistent with megacrysts being differentiation products of a mantle-derived magma (Boyd & Nixon, 1973; Gurney et al., 1979; Schulze, 1984, 1987). The composition of this magma, its physical setting and exact relationship to kimberlite, as erupted at the surface, are still under debate (Harte, 1983; Mitchell, 1986; Davies et al., 2000). The ultra-coarse grain size and incompatible element enrichment evokes an analogy with crustal pegmatites, and an origin of the megacrysts in coarse veins and magmatic apophyses marginal to a partially molten diapir or magmatic intrusion has been proposed (Harte & Gurney, 1981). Analogous megacrysts in alnöites and alkali basalts are commonly considered to be broadly cognate with the host magmatic event (Irving, 1974; Schulze, 1987; Neal & Davidson, 1989) and a similar relationship of kimberlite megacrysts to their host is often proposed (e.g. Jones, 1987; Smith et al., 1995).
Of relevance to this study are the observations (1) that these minerals record some of the highest pressures and temperatures encountered in mantle xenoliths, that temperatures span a wide and continuous range, but that pressure of origin is relatively restricted (Gurney et al., 1979), and (2) that local chemical and isotopic equilibrium is a strong feature, unlike many peridotite xenoliths of both the low-temperature, granular, and high-temperature, sheared varieties.
Trace element concentrations in the minerals, and the kimberlite association inferred from isotopic studies (Jones, 1987; Nowell et al., 1999), suggest that the megacryst parent magma was enriched in volatiles and incompatible trace elements. The Monastery megacrysts thus provide the opportunity to study in detail the behavior of H in mantle minerals under high P–T, equilibrium conditions in a hydrous mineral–melt system covering a range of chemical bulk compositions.
Chemical evolution of the Monastery megacryst suite
The large size, mineral diversity and great abundance of the megacryst suite at the Monastery kimberlite in the southeastern Free State, South Africa, has led to a number of studies on these samples, with the result that they represent one of the best documented kimberlite-hosted megacryst assemblages. The Monastery megacryst suite comprises olivine, garnet, clinopyroxene, orthopyroxene, ilmenite, phlogopite and zircon. Because these megacrysts commonly occur as isolated crystals, dispersed within the host kimberlite matrix, the petrological relationships of the minerals to one another are not immediately obvious and must be established by examination of (much rarer) samples where one or more minerals are intergrown. Detailed analysis of such relationships combined with extensive chemical analysis have revealed several associations of megacrysts, summarized in Table 1 (Gurney et al., 1979, 1998; Moore et al., 1992; Hatton, 1998). The relationships between these groups remain under investigation.
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In the most easily understood group, referred to as the ‘main silicate trend’ (MST; Moore et al., 1992
; Gurney et al., 1998), the studies of Jakob (1977), Gurney et al. (1979), Moore (1986) and Moore et al. (1992) show that crystallization begins with multiple saturation at the liquidus (1400°C) with magnesian olivine (Fo 88), orthopyroxene, garnet and highly subcalcic clinopyroxene [Ca/(Ca + Mg) 0·3]. Pressure, determined on a rare four-phase peridotite with mineral compositions the same as the most primitive megacrysts, ranges from 4·5 to 6 GPa depending on choice of thermobarometer combination (Moore, 1986; D. R. Bell, unpublished data, 2003). At lower temperature (1250°C), ilmenite (commonly in the form of lamellar pyroxene–ilmenite intergrowths) replaces olivine in the sequence. Detailed study of coexisting mineral occurrences and geochemistry (Moore, 1986; Moore et al., 1992; Gurney et al., 1998; Hatton, 1998) has shown that no hydrous mineral (i.e. phlogopite) crystallizes together with these minerals. This group, or petrological association, forms the major focus of this study. The compositional relationships of the major silicates are shown in Fig. 1.
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The other groups of megacrysts (groups 2 and 3; Moore et al., 1992
) include the minerals ilmenite, calcic clinopyroxene, Fe-rich olivine, phlogopite and zircon. Their relationship to the MST megacrysts was discussed by Moore et al. (1992), but does not affect the present study in a major way. In terms of the OH evolution of group 2 and 3 megacrysts, it is noteworthy that phlogopite appears to be a significant part of the assemblage over much of their evolution. The bulk of this study focuses on the MST megacrysts. In addition, a suite of olivines belonging to the high-Fe (group 2) association has been analysed, together with zircon from this group. A single calcic clinopyroxene (group 3) was analysed.
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SAMPLES |
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Samples selected for this study were a subset of large collections previously studied by Moore (1986)
. Samples derive both from mine tailings dumps and from loose kimberlite boulders in the open pit and surrounding areas. Most come from a single phase of the intrusion (the Quarry kimberlite). The minerals for study were chosen on the basis of previously determined electron microprobe analyses (Moore, 1986 and unpublished data) to span the complete compositional range of each mineral observed for the large sample suites. This is illustrated in Fig. 1. For olivine, orthopyroxene and garnet the selection criterion was 100Mg/(Mg + Fe) (Mg-number), whereas clinopyroxene samples were chosen on the basis of 100Ca/(Ca + Mg) (Ca-number), a temperature index that is also well correlated with Mg-number in this mineral. Zircon samples were all from bi- or polymineralic intergrowths and were chosen on the basis of the Mg-number of the coexisting olivine or ilmenite. Original grain sizes for the monomineralic megacrysts, excluding olivine, ranged from 2 to 15 cm, whereas intergrowths occur more typically on the scale of millimeters to 1 cm. OH contents of coexisting minerals were determined on one pair each of garnet–clinopyroxene, garnet–olivine and olivine–zircon. In addition, the OH contents of three clinopyroxene samples coexisting with ilmenite in the form of lamellar intergrowths were determined.
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METHODS |
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Infrared spectroscopy
Our procedure for quantitative IR spectroscopic analysis of minerals, described by Bell & Rossman (1992a
, 1992b) and Bell et al. (1995b), uses doubly polished plates prepared in the appropriate crystallographic orientation from large, clear fragments handpicked from coarsely crushed pieces of megacryst. For the optically biaxial minerals, more than one grain was often required to generate spectra polarized in all three principal vibration directions of the optical indicatrix. For the isotropic garnets, analyses were based on an unpolarized measurement performed in random crystallographic orientation. Degree of homogeneity in OH content of the Monastery olivines was investigated by the analysis of multiple spots within individual grains and by analysis of multiple and petrographically different subgrains from individual ‘megacrysts’.
All OH concentrations were derived from integrated intensities of the OH absorptions from 4000 to 2800 cm–1, using the molar absorption coefficients given in Table 2. We estimate that the uncertainty (95% confidence level) in the integrated absorbances and, therefore, in the relative OH concentrations, is approximately 10% for garnet, pyroxenes and zircon and 15% for olivine. The higher uncertainty in olivine derives from variable contribution of molecular water OH (see below), which cannot be unambiguously resolved, and also the generally lower signal-to-noise ratio of olivine spectra caused by internal absorption and scattering of light owing to microscopic imperfections. Absolute accuracy of the OH concentrations varies according to precision and accuracy of the calibration method for the individual minerals and depends on consistent choice of baseline. For each mineral, it may also depend on the precise features of the OH absorption spectra, which are generally not constant and depend on mineral composition. These quantitative issues have been discussed further by Bell et al. (1995b, 2003). In the present case, these uncertainties in the molar absorptivity are expected to be minor and, in the absence of more precise information, have been ignored in calculation of OH concentrations.
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Bell et al. (2003)
presented a calibration of OH in olivine that differed substantially from the widely used previous calibration of Paterson (1982). The olivine OH concentrations determined here are based upon the results and methods of Bell et al. (2003), but were calculated from the summed integrated absorbances in the ß and 
polarizations only, because the low absorption in the 
polarization was considered to introduce unwarranted uncertainty. In all tables and figures, the OH concentrations are expressed as parts per million H2O by weight. This does not carry the implication of water molecules within the mineral structure, but follows the common convention of expressing mineral analyses in terms of weight fraction of an oxide component appropriate to the assumed cationic charge (H = +1). In the text, we refer to the hydrous component as OH. Strictly speaking, this could include water molecules (which have hydroxyl groups) as well as hydroxide in the mineral structure. However, unless indicated to the contrary, the term OH generally refers here to hydroxide (hydroxyl) bound in the mineral structure. The symbol H denotes the element hydrogen, irrespective of chemical species.
Electron microprobe
Initial wavelength-dispersive electron microprobe analyses for sample selection were performed on a Cameca Camebax instrument at the University of Cape Town (UCT), operating at 15 kV. Data processing followed the PAP method (Pouchou & Pichoir, 1991) used by Cameca. Garnets and pyroxenes were reanalyzed at Caltech on a JEOL 733 instrument, operated at 15 and 20 kV accelerating voltage and beam currents from 30 to 500 nA (measured on a Faraday Cup) depending on element abundances. Olivine trace element analyses were performed on a JEOL-JXA-8900 microprobe at the Geophysical Laboratory, Carnegie Institution of Washington (GL), operating at 20 kV and 300 nA. In addition to the high beam currents, extended counting times up to 4 min on both peak and background were used for low abundance elements in all minerals. Detection limits (approximated as 3
B) are in the range 10–30 ppm. Corresponding counting statistic errors for individual trace element analyses with >100 ppm concentration levels are typically less than 10% relative. Data reduction for both JEOL instruments was by the CITZAF method of Armstrong (1988, 1991). Analyses were performed on multiple grains picked from crushed samples. In general, good agreement in major element concentration was found between the laboratories, although a difference in Mg-number of 0·7 was noted between the UCT and GL olivine analyses. This is a confirmation of both analytical accuracy, to the extent that it is required for establishing the compositional trends, as well as mineral homogeneity, because the analyses were carried out on different grains from the same sample.
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RESULTS |
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Petrography and microstructure of olivine
Olivine megacrysts occur as large, fractured and partially serpentinized masses, up to
10 cm in size, embedded in kimberlite matrix, from which they are commonly liberated by mining and processing procedures. Some ‘megacryst’ olivines show evidence of recrystallization, to the extent that they may more accurately be termed porphyroclastic dunites. Such samples comprise variable proportions of large, often strained porphyroclasts, small (sub-millimeter) neoblasts and euhedral recrystallized tablets. They are visually heterogeneous from one grain of the polycrystalline aggregate to another, as well as within individual grains, where a variety of dislocations, tilt walls, microfaults, healed fractures and inclusions may be present in individual aggregates. Many of the features, both in terms of mineral habit and fabric, and internal mineral microstructure are identical to those seen in olivines from high-temperature deformed lherzolites (Boullier & Nicolas, 1973, 1975; Gueguen, 1977, 1979; Drury & Van Roermund, 1988, 1989) and in Fe-rich dunites and peridotites from the Kimberley pipes (Dawson et al., 1981). Noting that any of these structures might be accompanied by OH groups or H2O molecules, the petrographic features visible with the optical microscope to a magnification of 400x of each sample analysed have been recorded (Table 3).
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A consistent difference in appearance was noted between the two chemical groups of olivines. At 400x in transmitted light, the low-Fe olivines had a pervasive and relatively uniform fine ‘granular’ appearance, indicating the presence of structural heterogeneities at the limit of optical resolution. In contrast, the high-Fe group were optically homogeneous under these conditions of examination.
Characteristics of OH spectra
Representative mineral spectra are illustrated in Fig. 2a–e. In general, these are similar to previously published spectra of OH in mantle minerals.
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Garnets
The garnet spectra (Fig. 2a) are typical of OH in kimberlite megacrysts (Bell & Rossman, 1992b
; Matsyuk et al., 1998). The spectra show a regular composition-dependent change in shape, which is discussed further, with reference to compositional dependence of OH partitioning involving garnet, in a later section.
Pyroxenes
Clinopyroxene and orthopyroxene spectra (Fig. 2b and Fig. 2c, respectively) are generally similar to those described by Skogby et al. (1990), and show some composition-dependent variability. Most obvious is a change in the relative proportions of the 3600 cm–1 and 3540 cm–1 bands in the clinopyroxene spectrum, which increase and decrease respectively with increasing compositional evolution (higher Ca-number).
Olivine
Olivine IR OH spectra are similar to those previously reported by Miller et al. (1987), but whereas that study described a large number of different types of spectra, the spectra of samples studied here are relatively uniform. However, the two compositional groupings of olivines have OH spectra that are consistently different. As noted by Miller et al. (1987) for most olivines, these samples exhibit by far the strongest absorption for IR radiation polarized with E || [100] (i.e. ), with absorption intensity for most bands decreasing in the order [100] () > [001] (ß) > [010] (). This emphasizes the need for consistent orientation of olivine samples for comparative studies. Typical spectra from each of the two compositional groups of olivines are shown in Fig. 2d.
In the spectra of both compositional groups of olivine and in both the [100] and [001] polarization directions, the most intense band occurs at 3572 ± 2 cm–1. Its frequency is independent of composition, as appears to be the case for most bands from the data at hand. The intensity of the 3572 cm–1 band in the [100] polarization direction is well correlated with total integrated OH absorbance and could therefore also be used as a crude indication of the total OH concentration in these samples.
Notable differences between the spectra of the two compositional groups include the enhanced intensity of bands on the high-frequency side of this main band in the more Fe-rich olivines ([100] polarization) and reduced intensity of the 3526 cm–1 band, relative to the main 3572 cm–1 band, in the Fe-rich olivines ([001] polarization). Both groups of spectra indicate the possibility of some molecular water in the samples (manifested by a broad band centered near 3420 cm–1), but this is difficult to quantify unambiguously because of overlap with the mineral OH bands. In calculation of the integrated absorbances and resultant H2O contents of Table 3, it has been attempted to screen out this component by estimating its abundance from the [100] spectra. This possible molecular water may occupy the micro-cracks that pervade many of the samples, or may be associated with the minute inclusions described above.
Zircon
The zircon spectra (Fig. 2e) are very similar to that of sample 1229 from Kimberley (Woodhead et al., 1991), consisting of two sharp pleochroic bands superimposed on a small amount of broad, featureless absorption. Upon heating to 400°C, much of this broader component is incorporated into the sharper bands, suggesting that it may have originally been bound in the zircon structure. This behavior has previously been reported by Woodhead et al. (1991).
OH concentrations
Tables 3–5 list the OH concentrations, expressed as ppm H2O by weight (the usage throughout this paper) for the Monastery megacryst minerals. The relative abundances of OH are similar to those observed in these minerals from other mantle environments and concentrations fall within the ranges previously defined for these minerals from the mantle (Bell & Rossman, 1992a, 1992b; Kurosawa et al., 1993; Snyder et al., 1995; Matsyuk et al., 1998; Peslier et al., 2002), decreasing in the order clinopyroxene (291–620 ppm) > orthopyroxene (215–263 ppm) > olivine (43–245 ppm) > garnet (15–74 ppm) > zircon (28–34 ppm). Multiple analysis of the megacrysts used in calibration studies has revealed homogeneity (Bell et al., 1995b). Here, general intra-crystalline homogeneity of megacrysts is supported by our occasional multiple analyses. This is generally, but not rigorously, true of other elemental concentrations in megacrysts (except for edge effects related to reaction with host kimberlite liquids). A notable exception is the variability in OH observed between different textural domains in a deformed and annealed olivine sample ROM250-OL42, discussed below.
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For zircon, the range in concentration determined on two grains from the same sample is greater than that observed between all three samples and is approximately at the limit of analytical precision, i.e. the total variation observed could be caused by analytical uncertainties.
OH variability in olivine and petrographic correlations
OH concentrations in these megacrysts constitute the highest values measured in mantle olivines, consistent with the hypothesis that they crystallized from a magma or fluid enriched in volatiles and incompatible elements at high pressure. High OH in olivine is also favoured (Wright & Catlow, 1994) by the high Fe3+ environment of these samples (Bell & Mattioli, 1994). With the exception of a rare boron-bearing olivine from Tayozhnoye, Siberia (Sykes et al., 1994), similar concentrations have been previously found only in samples derived from kimberlites.
Analysis of different subgrains from the same ‘megacryst’ reveals both homogeneity and heterogeneity. Four spots in a single grain (13.1) from sample ROM250-OL13 are identical in OH content within uncertainty of measurement and are also indistinguishable from a second grain (13.2) with different internal structure from the same ‘megacryst’. Although these samples are petrographically different, all contain features indicating deformation. In another sample (ROM250-OL54), slight heterogeneity (15%) was seen among four oriented cubes. This is slightly greater than the heterogeneity that might be expected from crystallographic misalignment, but could not be correlated with any visible (or other chemical) feature.
In contrast, petrographically different grains from ROM250-OL42 have significantly different OH contents. A highly strained porphyroclast traversed by shear fractures (OL42.1) has OH content similar to other megacrysts whereas two euhedral tablets (OL42.2, OL42.3) that appear to have recrystallized and annealed, although still showing some traces of relic deformation features, have much lower OH contents (Fig. 3). Annealed grains in other samples do not show this dramatic reduction in OH content, and it appears that, in general, there is little relationship of the OH content to grain microstructure or inclusion content. There is little variation in other chemical components between porphyroclasts and recrystallized neoblasts or tablets in these samples, making chemical origins for the OH differences unlikely. For this reason we suggest that the lower OH content of the OL42 tablets may be due to their annealing during ascent or emplacement of the kimberlite (Mercier, 1979) when water pressures are likely to be reduced from those in their mantle source.
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Water fugacities and the preservation of mantle-derived OH in olivine
Because of the high diffusion rates measured for hydrous components in olivines (Mackwell & Kohlstedt, 1990
; Kohlstedt & Mackwell, 1998, 1999), pyroxenes (Ingrin et al., 1995; Hercule & Ingrin, 1999; Carpenter et al., 2000) and garnets (Wang et al., 1996), it has commonly been asserted that the measured levels of OH in mantle-derived minerals bear little relation to those in the source region. Although such inference appears to be a logical conclusion drawn from sound observations, the evidence from natural samples presented below suggests otherwise.
Bai & Kohlstedt (1992) and Kohlstedt et al. (1996) determined the OH content of olivine saturated with a pure water fluid phase at pressures from 0·05 to 13 GPa. This saturation curve, representing the ‘solubility’ of H in olivine, is shown in Fig. 4, along with the measured concentrations from this study reported in Tables 3 and 5. This figure demonstrates that, apart from tablets believed to be annealed during ascent (see above), the samples here reflect OH contents that could only have been attained at pressures in excess of 3 GPa. These are minimum pressures because the OH contents are those calculated for aH2O = 1, i.e. water pressure equal to total pressure. Petrological studies of Monastery megacrysts are not compatible with their precipitation from pure water and suggest in contrast an incompatible-element-enriched silicate melt, with aH2O more probably in the range of 10–1 to 10–2 (see also discussion below). H2O activities calculated from the olivine OH concentrations using the relationship established by Kohlstedt et al. (1996) are in this range (Table 3), but are somewhat higher than expected. At this stage the a–X relations of hydrous olivines at PH2O < Ptotal remain to be established. In summary, however, the OH levels of these olivines were attained at substantial depths in the mantle, not necessarily different from their depths of origin. In particular, they are not compatible with resetting to crustal environments.
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Correlations of OH with mineral composition
Concentrations of OH and other elements as a function of a differentiation index [Ca/(Ca + Mg) in clinopyroxenes, Mg-number in other phases] are plotted in Figs 5–8. All minerals show systematic variation in OH and other minor or trace element content with these indices, although the variation for OH in olivine is poorly defined as a result of a degree of sample heterogeneity. Chemical trends for many elements in these figures are characterized by two sections of different slope, corresponding to crystallization in the presence and absence of ilmenite, respectively (Gurney et al., 1979
). In the following section we discuss the trends for each mineral in turn.
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Garnet
OH in garnet megacrysts for Monastery is positively correlated with Fe-enrichment as previously noted by Bell & Rossman (1992b)
and illustrated in Fig. 5a. There is a change in slope in this correlation at Mg-number 74, the H2O enrichment trend flattening at more Fe-rich compositions. The change in slope of the H2O–Mg-number trend corresponds to the point where ilmenite begins to crystallize from the parent magma (Gurney et al., 1979), and appears in several other element trends with Mg-number (Fig. 5b–k). For some elements the trend flattens or reverses (Na2O, TiO2, V) whereas for others it steepens (Sc, Zr). The trends of the divalent transition metals (MnO, Ni) are not affected.
Clinopyroxene
OH in clinopyroxene is similar to garnet in displaying an initial enrichment, followed by change in slope associated with ilmenite crystallization (Fig. 6a). One anomalous OH-rich clinopyroxene is present (MON26, a gt–cpx pair) in the initial ilmenite-free section of the trend, the remaining samples showing a slight positive correlation between OH and Ca-number. In contrast to garnet, the slope of the trend actually reverses and OH decreases in concentration in the ilmenite-present section of the trend. A similar trend is seen for K (Fig. 6e) and TiO2 (Fig. 6b) in clinopyroxene. In detail, it can be seen that OH occupies a position intermediate between these two elements. As in garnet, the divalent transition metals Mn (not shown) and Ni do not show a break in slope. The most calcic clinopyroxene, belonging to the low Zr/Nb group 3 association, has a very low OH content like some of the olivines of the low Zr/Nb group 2 suite.
Orthopyroxene
The three orthopyroxene samples show an apparent trend of decreasing OH with Fe-enrichment (Fig. 7a), although this trend is, strictly speaking, not outside the estimated experimental uncertainty. From the respective trends of minor element oxides and Ca-number with Mg-number (Fig. 7b, c and e), it is apparent that the orthopyroxene OH contents are not correlated with components affected by ilmenite crystallization, such as TiO2, but are correlated with Al2O3, Ca-number and Na (not shown).
Olivine
For olivines of the low-Fe (MST) association, the scatter in OH data allows no clearly defined correlation with Fe-enrichment to be established (Figs. 3 and 8a), but is consistent with either no variation or a shallow positive correlation. Other trace elements show well-defined correlations with Mg-number in this group (Fig. 8b–e), supporting the consistent chemical evolution seen in clinopyroxene and garnet precipitated over the same temperature interval. The Fe-rich olivines contrast with this behavior and fall into two groups (Fig. 3). One group, which includes the three zircon–olivine intergrowths, shows a negative correlation between OH and Fe-enrichment, whereas the other group is represented by two samples with OH contents well above this trend. All the Fe-rich olivines are characterized by essentially constant trace element levels that are independent of Mg-number (Fig. 8b–e). With the exception of Ti, the trace element concentrations in these samples resemble those in low-temperature peridotites, whereas those in the MST olivines are similar to those in high-temperature peridotites (Hervig et al., 1986).
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ORIGIN OF OH CORRELATIONS |
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The above discussion serves to illustrate that, with the exception of certain olivine samples, the OH component in these megacrysts displays a coherent variation that is similar in its consistency and its inter-mineral variability to other minor and trace components. This suggests that, despite the high diffusivities of hydrous components in olivine, pyroxenes and garnet, these minerals preserve a signature that was established in their mantle source. The mantle origin of this OH is further confirmed by the high (subcrustal) pressures of water required by OH concentrations in olivine. In this section we explore in some detail the origin of these correlations.
Garnet
There are no correlations to suggest that H incorporation in this suite of garnets is governed by the stoichiometric charge balance requirements of other minor and trace element substitutions. The H abundances on an atomic basis are an order of magnitude lower than possible indices of charge imbalance, such as Ti + Zr + P – Na, which means that much more sensitive probes of garnet crystal chemistry (or more hydrous garnets) are required to evaluate conclusively the substitution mechanisms as far as minor elements are concerned.
The positive correlation of OH and Fe content for the garnet megacrysts is qualitatively consistent with progressive enrichment of incompatible elements in an evolving magma and its crystallization products (Bell & Rossman, 1992b). However, the initial OH enrichment for the ilmenite-free section of the garnet trend is from 15 to 57 ppm, or about a factor of four. This greatly exceeds the enrichment in Ti, for example, over the same interval. Petrological modeling (Bell et al., 1995a) indicates that the degree of crystallization over this interval is in the range of 20–40%, implying an increase in melt H (or Ti) concentration by a factor of 1·25–1·55. If the petrological understanding of these samples is correct, then a threefold increase in the partitioning of OH between garnet and melt with Fe-enrichment is implied. At the very least there is a change in the distribution coefficient (D) for OH relative to those for other incompatible components over this interval of composition.
The flattening of the OH trend from the point of ilmenite entry is the opposite to what is expected for constant (or increasing) partitioning of OH between garnet and melt, because the co-crystallization of ilmenite will retard Fe-enrichment for a given degree of crystallization (Schulze, 1984). This should result in steepening of the trend, as observed for incompatible elements such as Zr (Fig. 5e) and Sc (Fig. 5i). The suppression of OH enrichment may result from changing (decreasing) D, or from the buffering effect of a hydrous phase entering the assemblage at this point. As discussed above, there is no hydrous mineral that crystallizes at this point (or at any stage in the evolution of the MST suite). However, the possibility of vapor phase saturation occurring at this point is worth some discussion and is examined below. In this section, the discussion is confined to possible causes of changing D, which, as implied above, involves first a progressive increase in Dgt–liq, followed by a decrease.
A partial explanation for the above behavior may be found in the apparent association between Ti and a certain component of the garnet OH, namely, that with OH absorption at 3512 cm–1 (Bell & Rossman, 1992b). Presumably, all OH sites in garnet increase proportional to an increase in water activity in the system, but the number of ‘3512 cm–1’ sites available progressively increases and then decreases as TiO2 in the garnet changes (Fig. 5c). This results in accentuated build-up in OH over the initial interval where both Ti in garnet and melt water activity increase in response to differentiation, followed by a subdued increase at Mg-number <74, where the number of available OH sites associated with Ti decreases in response to the sudden reversal in Ti content of the garnet. This effect is nevertheless not sufficient to account for the apparent over-enrichment of OH during the initial stages of crystallization, because the proportion of 3512 cm–1 OH in the spectrum does not exceed about 50% (Fig. 2a).
The fact that H is enriched above the levels expected for simple proportional dependence on melt H content suggests a power-law dependence on the activity of melt H-species, e.g. cOH = k(aH2O)n, where n > 1, and implies a reaction stoichiometry for OH incorporation with multiple H atoms per garnet formula unit. From the above discussion on Ti-associated OH sites it is apparent that the incorporation reactions are more complex than the simple hydrogarnet substitution (Flint et al., 1941) and appear to involve melt activities of other components (such as Ti and Al). This, combined with the unknown dependence of aH2O on bulk H concentration in the melt, implies that quantitative extraction of a physically meaningful value of n is not possible at this stage (see also Poirier & Duba, 1997).
Clinopyroxene
The evolution of OH in clinopyroxene before ilmenite crystallization is consistent with constant or slightly decreasing partitioning of OH between clinopyroxene and its parent magma. The downturn in H2O and K associated with the ilmenite-influenced downturn in TiO2 at Ca-number >36 (Fig. 6) suggests an influence of Ti on the incorporation of these components into clinopyroxene. Gasparik (1985) has argued that Ti incorporation into synthetic kosmochlor (NaCrSi2O6 clinopyroxene) is accompanied by a vacancy, and Smyth et al. (1991) have shown a correlation between vacancy and OH concentrations in omphacites. The approximately constant 1:1 ratio of Ti to H (Fig. 9a) in Monastery clinopyroxenes suggests a crystal-chemical association between the two. This association was, however, not found in other studies of natural clinopyroxenes (Skogby et al., 1990; Smyth et al., 1991) and may be coincidental in the present study. The fact that a similar behavior to OH is displayed by K, but at a factor of 10 lower atomic concentrations (Fig. 9b) indicates that this kind of trend can be produced without a specific 1:1 substitutional relationship.
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It is not clear if the lower OH content of the calcic clinopyroxene reflects phlogopite co-crystallization, a different parent magma composition, different P–T conditions of origin, or aspects of the pyroxene crystal chemistry. All these factors may play a role in the OH content. Al and Fe3+ were identified by Skogby et al. (1990)
as probable factors in OH incorporation into clinopyroxene. The progressive decrease in proportional abundance of the 3540 cm–1 band in the Monastery megacrysts suggests a change in proportion of OH associated with Fe (Skogby, 1994).
Sample MON26, which falls above the general OH–Ca-number trend for clinopyroxene is also enriched in K, Ni and Cr compared with the average trends for these elements. This sample may be the product of local interaction of the magma with phlogopite lherzolite wallrock (the common mantle peridotite in the Monastery kimberlite) or a partial melt thereof. Such interaction in megacryst suites is suggested by occasional Sr or Nd isotopic heterogeneity in megacryst samples from any one suite (Jones, 1987; Hops et al., 1992).
Orthopyroxene
The apparently continuous decline in orthopyroxene OH content with megacryst suite differentiation is opposite to that observed in garnet, and differs from what trends may be discerned from the clinopyroxene and olivine data in its monotonic quality. However, the positive correlation between Al and OH is consistent with experimental observations of Rauch & Keppler (2002) on the strong enhancing effect of Al upon OH incorporation in orthopyroxene. The megacryst orthopyroxenes also contain strong OH absorptions at high wavenumber of the type identified by Rauch & Keppler (2002) to result from OH associated with Al. The abundance variations and the spectral characteristics therefore both suggest a strong, and possibly dominant crystal chemical influence on OH incorporation in orthopyroxene.
Olivine
The data in Table 5 and Figure 8a–e indicate that there is no major or trace element determined in this study with which OH is consistently positively correlated in abundance for the entire sample suite. A large number of plots attempting to relate H to minor element substitutions or charge balance criteria were examined with no correlations observed (e.g. Fig. 10a and b). What was observed, however, were relatively strong correlations among the minor elements, excluding H; for example, the approximately 1:1 relationship between Na and Al + P (Fig. 10c) suggests a charge-coupled substitution. Similarly well-defined correlations (but not at 1:1) exist between other minor element combinations (e.g. Fig. 10d). It was generally observed that the addition of H to such plots disrupts the well-defined systematics. This suggests a negligible role of coupled trace element substitutions in controlling OH abundance for olivine. This result conflicts with the study of Kurosawa et al. (1997), who reported a correlation between total monovalent (including H+) cations and trivalent cations (Al + Cr) in olivines from garnet peridotites. Our result is, however, in agreement with a number of experimental and theoretical studies of OH in olivine that predict substitution mechanisms for H involving the major olivine ions Fe, Mg, Si and O (Bai & Kohlstedt, 1992, 1993; Wright & Catlow, 1994; Kohlstedt et al., 1996).
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Because of the scatter in the OH data, the precise behavior of H during chemical evolution of the MST olivines cannot be determined. However, the overall lack of variation in comparison with the Fe-rich olivines, and the fact that a relatively consistent relationship is permitted by the error bars, suggests that abundances are close to those established during the magmatic stages of olivine history. However, the trend is indistinct because of the uncertainties regarding contributions from inclusions (e.g. molecular water), discussed above, as well as the possible effects of limited H loss or gain. The compositional evolution of these olivines in the absence of ilmenite is illustrated by the positive correlation between TiO2 and Fe-enrichment (Fig. 8b). Other elements, such as Ca and Al, whose melt activities are effectively buffered by garnet and clinopyroxene are likely to be controlled in olivine by temperature-dependent partitioning (Agee & Walker, 1990
; Köhler & Brey, 1990). The origin among the high-Fe (group 2) olivines of both high-OH and low-OH olivines is problematic and further obscured by a poor understanding of the P–T–fH2O conditions of origin of this suite. The buffering effect of ilmenite on the TiO2 contents of these olivines can be seen in Fig. 8b, contrasting with the well-defined trend of Ti enrichment for the Fe-poor olivines (that lack coexisting ilmenite). A similar explanation involving buffering of OH by phlogopite may explain the negative correlation of OH with Fe-enrichment in one group of Fe-rich olivines. It is also possible that the low-OH olivines have simply lost H and that the negative OH–Fe correlation in these samples reflects some internally buffered state.
Vapor phase saturation?
Although saturation in phlogopite or other hydrous minerals is believed not to be a factor in the evolution of MST suite megacrysts, it is possible that an H-rich fluid phase separates from the magma at the point where the trends in OH for garnet and clinopyroxene change slope. Evidence from experimental studies (Dalton & Presnall, 1998; Wyllie & Lee, 1999) of kimberlitic liquids in equilibrium with mantle minerals at these pressures suggests that with falling temperature the liquid retains or increases its CO2 content, becoming carbonatitic, and precipitating a carbonate mineral at low temperatures, rather than saturating in CO2. This suggests that a vapor phase that evolves in equilibrium with differentiating megacryst magma should be water rich. However, as the solubilities of water in silicate and carbonate melts in these conditions are of the order of tens of weight percent, the exsolution of a water-rich vapor implies very high concentrations of water in the melt. The water contents estimated for the megacryst magma from trace element systematics (see below) are considerably below expected saturation levels, even after 20–40% crystallization, and exsolution of a water-rich vapor is therefore considered unlikely.
Preservation of mantle signatures: a discussion
The coherent OH trends in garnet and clinopyroxene provide evidence that H has not been lost in a haphazard manner from these samples during eruption. This seems surprising in the light of high measured diffusivities for hydrous components in these minerals (Ingrin et al., 1995; Wang et al., 1996; Hercule & Ingrin, 1999; Carpenter et al., 2000). Some disturbance in the case of olivine remains a possibility, but petrological factors in the source may also play a role. The coherent trends also argue against re-equilibration of the samples in a hydrous crustal environment, and it has been demonstrated that mantle water pressures are required for the OH levels in most olivines. One possible interpretation is that the H is completely decoupled (owing to grossly different diffusive timescales) from the silicate framework of the mineral and responds rapidly to external crustal conditions, whereas the silicate structural framework is essentially frozen, preserving a structure acquired in the mantle. In this case, the recorded H is some product of hydrogen fugacity in the kimberlite magma, or its immediate post-emplacement conditions, and the internally buffered metastable environment of the mineral structure, quenched from upper-mantle conditions. This type of dependence of OH incorporation on prior formation conditions has been demonstrated experimentally for olivine by Bai & Kohlstedt (1993) and for pyroxenes by Skogby (1994) and Guilhaumou et al. (1999).
Compositional trends of Monastery MST megacrysts for major and trace elements are remarkably regular and it may be expected that the environment for OH incorporation would be similarly coherent. However, it seems unlikely that an environment of kimberlite emplacement, cooling and hydrothermal alteration in a number of regimes could be so uniform that the imposed crustal H-species activity resulted in no disturbance of these mantle trends. It seems far more likely that the OH trends observed derive from mantle processes, and that the activity gradients required for H disturbance during ascent are not attained in kimberlite emplacement. Similar conclusions were drawn by Bell & Rossman (1992b) and Matsyuk et al. (1998) from detailed studies of garnets from diverse mantle environments.
Summary of OH correlations
For clinopyroxene and garnet, the relationship of OH to Mg-number (or Ca-number in cpx) is characterized by two sections of different slope, the change occurring where ilmenite becomes a co-crystallizing phase. For clinopyroxene and garnet this occurs during the magmatic evolution of a single petrological association, whereas for olivine, which also exhibits a more scattered relationship owing to later disturbance, it appears that two different petrological associations are involved. The initial positive correlation of OH with Fe-enrichment that occurs for the more Mg-rich samples is qualitatively what is expected for the evolution of a hydrous magma by crystallization of nominally anhydrous phases. However, the quantitative evaluation of these initial trends and the change (or reversal) in slope that occurs for the petrologically more evolved samples demonstrate that the controls on OH incorporation are not straightforward. The limited orthopyroxene data appear to confirm this inference, suggesting possible influence of Al on OH incorporation.
Two possible origins for the change in trend at the ilmenite-saturation point are considered. One is that minor element substitutions effected by ilmenite buffering change the partitioning behavior of OH. The other is that a hydrous phase begins to play a role in the magmatic evolution of the system at this point. There is at present no firm evidence for the latter, but the former possibility is supported by similar behavior to H for other trace elements, such as K and Na. We therefore interpret the regularly changing H2O and minor element concentrations, illustrated in Figs 5–8, to reflect changing elemental concentrations in the evolving melt phase and the variation in response to temperature, mineral chemistry and melt chemistry of mineral–melt distribution coefficients.
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INTER-MINERAL PARTITIONING OF OH |
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Ideally, the study of inter-mineral partitioning should proceed by analysis of spatially adjacent mineral grains, a procedure that is difficult in megacryst suites because coexisting intergrown megacrysts are relatively uncommon. However, the systematic relationships between composition (both major and trace elements) and degree of chemical differentiation for each mineral species are such that equilibrium partitioning relationships between mineral species are readily established by interpolation between analyses of occasional coexisting pairs (e.g. Gurney et al., 1979
). We use this principle to calculate inter-mineral partition coefficients for OH. The validity of this procedure is confirmed by measurements of OH partition coefficients in the few samples where physically coexisting pairs are available for analysis and, in general, for other elements where more data on coexisting pairs are available (Moore, 1986; Bell et al., 1995c). Coexisting compositional relationships described in the literature, supplemented by data reported here, were parameterized by a series of linear or second-order polynomial equations (Table A1, Appendix). For any given megacryst mineral, these equations allow calculation of the major element compositions (either Mg-number or Ca-number) of the other minerals with which it is in equilibrium. The H2O contents of these minerals can then be calculated from the H2O–Mg-number trends established by individual mineral analysis using regression equations derived from the data in Figs 5a, 6a, 7a and 8a (Table A1, Appendix). The inter-mineral distribution coefficients were thus calculated from the measured OH content of a given mineral sample and the calculated OH content of the phases with which it is predicted to be in equilibrium.
Partitioning relationships involving garnet
The calculated distributions of OH between garnet and clinopyroxene, orthopyroxene and olivine as a function of garnet Mg-number are shown in Fig. 11a–c, respectively, together with the measurements of OH partitioning in coexisting garnet–clinopyroxene (MON26) and garnet–olivine (MON24) pairs. The agreement between the D values calculated from the garnet data and those from the clinopyroxene data is a reflection of the well-defined trends that H2O follows with Mg-number and Ca-number in these minerals, respectively. Despite the fact that the trend of OH incorporation into both garnet and clinopyroxene changes slope at the point of ilmenite entry (Figs 5a and 6a), the distribution of OH between these phases displays a monotonic variation, with no change in the trend of inter-mineral partitioning. The calculations for olivine are not as precise as for the other phases because olivine is substantially more heterogeneous with respect to OH than the other minerals. In the case of olivine we have assumed, based on observations discussed above, that the most OH-rich measurement represents the closest approach to the equilibrium mantle value.
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It is immediately obvious from Fig. 11 that partitioning of OH between garnet and other phases is not constant. However, all three minerals display a similar sense of variation in their partitioning of OH with garnet. The first-crystallized clinopyroxene incorporates about 50 times more OH than its coexisting garnet, diminishing to a factor of about 5x for the clinopyroxenes coexisting with the most Fe-rich garnets.
varies from 18 at high Mg-number to 4 at low Mg-number, where the minerals coexist with ilmenite. Olivine and garnet coexist over a more restricted range of the differentiation sequence than garnet, clinopyroxene and orthopyroxene, but the overall variation in 
(from 14 to 3) is similar to that of , about half the relative change in 
.
Much of the change in inter-mineral DH2O involving garnet occurs over the first 50–80°C of the megacryst evolution and is related to very low OH content of garnet under these conditions. Possible crystal chemical reasons for rapidly changing OH in garnet have been discussed above, but temperature is an additional factor that may influence OH incorporation. For example, Lu & Keppler (1997) observed that OH content of Dora Maira pyrope decreased with rising temperature under hydrothermal upper-mantle conditions.
Olivine–orthopyroxene–clinopyroxene partitioning
In contrast to partitioning relationships involving garnet, the distribution of OH between olivine, clinopyroxene and orthopyroxene is relatively uniform. This is shown in Fig. 12, where the calculated DH2O values for cpx/ol, cpx/opx and opx/ol are plotted as a function of temperature. The temperatures are monotonically related to Ca-number in clinopyroxene and are calculated from synthetic coexisting mineral compositions interpolated from the natural dataset of coexisting phases. As a result of the uncertainties in accuracy of various thermometers, and the fertile compositions of the megacrysts, the average of two modern calibrations (Brey & Köhler, 1990; Taylor, 1998) is used in these figures. In all three cases, the DH2O values are more or less consistent with a single distribution coefficient for each mineral pair, independent of mineral composition and temperature over the range investigated. This is a noteworthy point, considering the variation in the OH abundances of the individual minerals and the changes in slope exhibited by their OH trends. The partitioning of OH between ortho- and clinopyroxene in megacrysts [
] is coincident with the range of values of Peslier et al. (2002) [
] and those derived from the data of Bell & Rossman (1992b) for peridotites and a garnet websterite [
]. These latter values are, however, somewhat less accurately determined because of the analytical method employed in that study (unpolarized spectra in some cases). In contrast, partitioning of OH between olivine and the pyroxenes is grossly different. 
is far higher in the study by Bell & Rossman (1992b) because of the very low OH concentrations in olivines from basalt-hosted spinel lherzolite xenoliths. That study also employed a different calibration for the OH content of olivine, which resulted in concentration estimates 0·7 times lower than would be derived from the calibration used here.
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Olivine–zircon partitioning
The three olivine–zircon intergrowths yield a mean
, assuming that the heterogeneities seen in both olivine and zircon in ROM181 are correlated. These samples represent a more chemically evolved system than the other samples, and this partition coefficient is likely to represent temperatures, and possibly pressures, lower than those of the other megacrysts. |
PARTITIONING OF OH BETWEEN MINERALS AND MELT |
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Absolute values of
are difficult to infer because of uncertainties in parent magma H2O content. It is often proposed that megacryst magmas are the precursor magmas to kimberlites. However, direct analysis of the H2O content of kimberlite rocks is inappropriate, because they have exchanged with and been altered by hydrothermal fluids (Sheppard & Dawson 1975), in addition to being subject to degassing processes. An alternative approach is provided by the observation that water in undegassed mantle-derived magmas exhibits systematic concentration variations in relation to other trace elements (Dixon et al., 1988; Michael, 1988, 1995; Danyushevsky et al., 2000). The megacryst parent magmas derive from mildly depleted source regions that are geochemically very similar to ocean island basalt (OIB) sources and, unlike the erupted kimberlite, show minimal effects of interaction with enriched continental lithosphere (Jones, 1987; Nowell et al., 1999).
We have used the correlations of H2O with Ce, K2O and P2O5 in oceanic basalts to provide three independent estimates of the H2O content of the melt parental to the Monastery megacrysts. The water content was derived by extrapolating the trends in Ce/H2O vs Ce, K2O/H2O vs K2O and P2O5/H2O vs P2O5 observed for a large literature database of mid-ocean ridge basalt (MORB) and OIB to the abundances of Ce, K2O and P2O5 in the megacryst magma. The abundances of Ce, K2O and P2O5 in the megacryst magma derive from analyses of the Monastery kimberlite, corrected for the petrographically observed assimilation of lithospheric mantle. These methods give estimates of 
, 
, and 
respectively, with a mean of 2·8 ± 1 wt % H2O. The error estimates represent approximately 95% confidence levels. Sources of error include scatter within the basalt data arrays and uncertainties in the extrapolations from the basalt data to the higher incompatible element concentrations observed in kimberlite-like melts. Given the higher pressure of megacryst and kimberlite genesis, the K and P derived estimates may be more reliable because both these elements, like H, show evidence for enhanced incorporation into mantle minerals at high pressures (Bishop et al., 1978; Edgar & Vukadinovic, 1993; Kohlstedt et al., 1996; Harlow, 1997). In contrast, Ce shows some evidence for a decrease (Sims & De Paolo, 1997).
The calculated range of D values is compared with other estimates in Table 6. Values for Dopx–melt are comparable with, but slightly lower than those deduced by Dobson et al. (1995) for orthopyroxenes in a boninitic glass. Dol–melt values are approximately an order of magnitude lower than those estimated by Sweeney et al. (1997), who used elastic recoil detection analysis (ERDA) to analyze H in their experimental run products. That study, however, is not consistent with previous experimental work, in that inferred water contents of the olivines in the water-undersaturated melts exceed by up to a factor of 10 the saturation values measured for olivine in equilibrium with pure water (Kohlstedt et al., 1996). Closer agreement is evident between our study and olivine–melt DH2O of 0·002–0·005, calculated by combining the results of Kohlstedt et al. (1996) with broad estimates of H2O solubility in silicate melt at 5 GPa (30–40 wt %).
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The bulk DH2O for peridotite–melt, calculated by combining the individual mineral D values with their modal proportions, is
0·005–0·006. This is similar to the value of 0·01 inferred for basalt generation (Dixon et al., 1988; Michael, 1988; Danyeshevsky et al., 2000). In considering the melting of heterogeneous mantle, the presence of garnet pyroxenite bodies within peridotite is predicted to have a relatively minor effect on bulk DH2O because calculated DH2O for a 50–50 garnet–clinopyroxene mixture under the conditions examined here is 0·008. |
IMPLICATIONS FOR PLANETARY EVOLUTION |
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This study confirms previous results that pyroxenes and olivine are significant hosts for water in the upper mantle. Relative mineral abundances combined with the observed partitioning suggest that olivine is the dominant reservoir of H (
40%) in garnet peridotite at 5 GPa, with enstatite and diopside each containing about 30% of the total H. Garnet hosts less than 1%. OH is strongly partitioned into the liquid phase during high P–T igneous processes in planetary interiors. However, from the results presented here and previous studies of basalts genesis, a consistent picture is emerging that this solid-incompatibility is only moderate. Water may therefore not be removed as readily from the Earth's interior as some other volatiles (e.g. Cl and some noble gases). The very high ratio of surface to interior hydrogen in the Earth, which is similar to the distribution of elements with much higher incompatibility, such as Cl, Ba or U, indicates that partial melting of the mantle, as observed today, is not responsible for maintaining this distribution of H. More extreme partitioning is required, such as that expected in fluid-mediated processes of H transfer in subduction zones.
The increasing solubility of H in olivine as a function of pressure (Kohlstedt et al., 1996) and its higher measured concentrations in deep natural samples, such as these megacrysts, suggests that H compatibility increases with depth. The reported decline in OH content of garnet at high pressure (Withers et al., 1998) does not affect this tendency because of the low relative contribution of garnet to the overall OH budget of the mantle. Together with the lower extents of adiabatic partial melting at depth, the increasing H compatibility in mantle assemblages implies that the efficiency of H extraction by melting decreases with depth in the upper mantle. Pressure-dependent partitioning of H therefore acts to restrict degassing of H2O to the shallowest part of the mantle. In addition to retarding H transfer to the exosphere, this will also reduce the tendency of high-pressure, low fraction melts to strengthen the mantle through H removal, as may occur at shallower levels (Karato, 1986; Hirth & Kohlstedt, 1996).
Moderate compatibility of H2O during melting also implies that geophysically significant concentrations (100–200 ppm, approximately those of the MORB source) may be retained in mantle residues during hydrous melting of the mantle above subduction zones. The predicted weakening effect of this H suggests easier remixing of such mantle compared with dehydrated oceanic lithosphere, and therefore a possible means to disperse modest quantities of recycled water into the upper mantle.
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SUMMARY AND CONCLUSIONS |
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Cogenetic megacryst minerals from the Monastery kimberlite display correlated variations in OH, major and minor element composition which reflect bulk chemical evolution as a result of igneous differentiation as well as the effects of temperature and composition on inter-phase partition coefficients. During this evolution, water concentration in the parent magma is expected to increase monotonically, there being no independent evidence for vapor saturation. In the initial stages, dominated by silicate crystallization, OH contents of garnet and clinopyroxene appear to reflect this increase. H in garnet increases more rapidly (by a factor of 3–4) than the inferred melt H build-up. In later stages, where ilmenite crystallization is significant, the partitioning of H between melt and minerals is changed, apparently by a crystal chemical effect of Ti on the ability of garnet and clinopyroxene to incorporate H, although inter-mineral H partitioning changes smoothly through this transition. In clinopyroxene, a very similar pattern of compositional evolution to that of OH is seen for K.
From experimental evidence, the OH contents of megacryst garnets and olivines are consistent with their petrogenetic history in the mantle as crystallization products of a water-bearing melt at 50 kbar pressure, although some olivines with recrystallized textures appear to have equilibrated with lower water fugacities as they annealed during or after transport from the mantle. The regular behavior and consistent partitioning systematics observed throughout the suite for OH and all other elements suggests that H contents of garnet and pyroxenes are not generally disturbed from original mantle values. More scattered trends in olivine may result from hydrous inclusions and/or partial H loss in some grains or portions of grains.
Calculated and measured partitioning of OH between coexisting minerals is either invariant (cpx/ol = 3·0 ± 0·5; cpx/opx = 2·0 ± 0·3; opx/ol = 1·6 ± 0·2) or a smooth function of composition and temperature (1380–1150°C) for partitioning involving garnet. The inter-mineral partitioning relationships indicate that although higher OH concentrations may be found in modally less important clinopyroxene, just under half the water in nominally anhydrous mantle peridotite is contained in olivine, which may therefore represent the dominant mineral reservoir of water in the Earth's upper mantle and in other rocky planetary interiors.
The calculated distribution coefficients for water between these minerals and their parent magma (
; 
; 
; 
; 
, all values ±40% uncertain) are broadly consistent with bulk partitioning relationships established for water from petrogenetic studies on MORB, and from laboratory measurements of H solubility in minerals and melts. A consistent picture for the partitioning behavior of H in the principal minerals of the upper mantle is emerging.
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APPENDIX |
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Table A1: Equations used in calculation of coexisting megacryst compositions
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ACKNOWLEDGEMENTS |
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This paper has benefited from discussions with J. E. Dixon, P. D. Ihinger and E. M. Stolper, helpful reviews by J. Dixon and H. Keppler, and the perceptive comments of an anonymous reviewer. We thank Mr S. Gasson of Winburg, South Africa, for permission to visit and sample the Monastery kimberlite, and J. J. Gurney and A. P. le Roex (University of Cape Town) for supporting supplementary studies on the Monastery megacrysts. We acknowledge the assistance and expertise of J. T. Armstrong (Caltech) and C. Hadidiacos (Geophysical Laboratory) in electron probe trace microanalysis. This research was supported by NSF grants EAR 88-16006, EAR 91-040459 and EAR 92-18980 to G.R.R. D.R.B. acknowledges support from the DOE Office of Basic Energy Sciences (DE-FG02-93ER14400) and the Geophysical Laboratory, Carnegie Institution of Washington.
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FOOTNOTES |
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* Corresponding author. Present address: Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287-1604, USA. Telephone: (480) 965-0163. Fax: (480) 965-2747. E-mail: David.R.Bell{at}asu.edu
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