Journal of Petrology Advance Access originally published online on May 23, 2007
Journal of Petrology 2007 48(7):1369-1386; doi:10.1093/petrology/egm022
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Crystallization and Degassing in the Basement Sill, McMurdo Dry Valleys, Antarctica
1Division of Earth and Ocean Sciences, Nicholas School of the Environment and Earth Sciences, Duke University, Durham, NC 27708, USA
2Department of Geoscience, University of Nevada, Las Vegas, NV 89154-4010, USA
RECEIVED AUGUST 8, 2006; ACCEPTED MARCH 30, 2007
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ABSTRACT |
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The Basement Sill is part of the Ferrar Large Igneous Province exposed in the McMurdo Dry Valleys, Antarctica. The sill is
330 m thick in the Bull Pass area and 450 + m thick in the Dais area, 12 km to the west, and is characterized by phenocryst-free lower and upper margins and an orthopyroxene-rich central ‘tongue’ (opx 1–5 mm in size). Halogen variations in apatite from a suite of samples collected along vertical transects through the sill were examined to evaluate the process of crystallization-induced degassing (i.e. second boiling) and its effects on magma chemistry. Apatite grains from any given sample are generally unzoned with respect to Cl and F concentrations, but may vary by 20–30 mol% in the halogen site between grains. Overall average Cl/F mass ratios increase with height from the lower margin to the center of the sill, and then decrease to near zero towards the top margin where the rocks are relatively oxide-rich. The Cl/F trend parallels those of bulk MgO and grain size. The upper margin contains abundant mafic pegmatoids and the apatite in these segregations has lower Cl/F ratios compared with that in the host-rocks, although REE show no measurable difference. Numerical modeling illustrates that a cooling and crystallizing sill initially develops two separate vapor-saturated zones at the lower and upper margins owing to the irreversible heat loss to the cooler country rock. Vapor separating from the lower zone migrates upward into hotter silicate liquid, where it is resorbed, thus increasing the Cl/F mass ratio of the liquid. This process leads to saturation and precipitation of apatite from the liquid with a higher Cl/F ratio than would otherwise occur. Volatile enrichment can also aid compaction and grain growth in the central part of the sill. In contrast, the relatively Fe-rich, Cl-poor nature of the upper zone rocks suggests that these rocks may have crystallized from more evolved, degassed silicate liquid, possibly compacted out of the underlying crystal mush. In addition, as vapor sourced from the lower and central parts of the sill ascends into the cooler upper zone of the sill, the vapor may be localized (along with late interstitial silicate liquid) to form pegmatoids at temperatures at which Cl is less favored in apatite and can be leached from existing apatite by the ascending vapor, the latter causing the observed decrease in the Cl/F mass ratio of apatite in the (evolved) pegmatoids. KEY WORDS: Ferrar Igneous Province; halogens; fluid; apatite
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGENERAL GEOLOGY AND PREVIOUS...METHODSRESULTSDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAAPPENDIXREFERENCES |
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Magmatic volatile phases can play a number of important roles in the generation, crystallization, and eruption of mafic magmas (Roggensack et al., 1997
; Huppert & Woods, 2002; Yang & Scott, 2002; Cervantes & Wallace, 2003; Wallace, 2003; Cashman, 2004; Gonnerman & Manga, 2005; Hammer, 2006). However, in plutonic rocks the evidence of degassing is difficult to detect given the fugitive nature of volatile fluids (Candela & Blevin, 1995). Halogen variations in apatite are one mechanism that has been found useful in deciphering the complex degassing history of layered intrusions and associated mafic rocks (e.g. Brown & Peckett, 1977; Boudreau & McCallum, 1989; Willmore et al., 2000).
This study details the halogen composition of apatite in dolerite from the Basement Sill from the west Bull Pass and Dais areas (where the sill is 330–350 and 450+ m thick, respectively; Marsh, 2004) of the Ferrar Igneous Province in the McMurdo Dry Valleys, Antarctica. The crystallized magma bodies provide an excellent field laboratory for the study of vapor migration in a solidifying crystal pile as both were emplaced as an orthopyroxene-phyric mush (Marsh, 2004) with a relatively simple emplacement and crystallization history that can otherwise complicate the understanding of the compositional evolution of vapor from a basic igneous intrusion.
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The Ferrar Large Igneous Province is located along strike of the Trans-Antarctic Mountains, which run north–south and divide Antarctica into the Eastern and Western ice sheets (Gunn & Warren, 1962
; McKelvey & Webb, 1962; Gunn, 1966; Borg et al., 1990; Elliot & Fleming, 2004). The province was produced during the Jurassic-age (Heimann et al., 1994; Knight & Renne, 2005) fragmentation of Gondwanaland. In the McMurdo Dry Valleys in Southern Victoria Land the Ferrar consists of a series of four vertically interconnected dolerite sills (Fig. 1), interconnecting dykes and extrusive rocks (i.e. the 500 m thick Kirkpatrick flood basalt province). The high-aspect ratio dolerite sills vary in thickness from 100 to 500 m and crop out over an area of the order of 10 000 km2. In order from bottom to top they are called the Basement Sill, Peneplain Sill, Asgard Sill and Mt. Fleming Sill (Gunn & Warren, 1962; Hamilton, 1965). The sills are typically separated from one another by several hundred meters, except in the area of Bull Pass, where the Basement Sill and Peneplain Sill are in contact with one another and were interpreted by Marsh (2004), on the basis of field evidence, to have formed from the top down.
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The compositions of the Ferrar sills are dominantly quartz-normative dolerite. The lowermost exposed sill, the Basement Sill, is composed of a lower phenocryst-free gabbronorite chilled marginal zone, an orthopyroxene phenocryst-bearing central zone, identified as the ‘Opx tongue’ by Marsh (2004
), and a phenocryst-free gabbronorite upper margin (Gunn & Warren, 1962; Hamilton, 1965; Marsh, 2004). In the West Bull Pass area, the lower chilled margin is dark, aphanitic to microcrystalline, contains plagioclase, orthopyroxene, augite and inverted pigeonite, and has an abrupt contact with the overlying opx tongue. Moving up from the lower margin, the grain size and opx content of the sill both increase toward the center of the Opx tongue, and then both decrease toward the top margin. The increase in orthopyroxene modal abundance in the center of the sill manifests itself in a strong change in bulk-rock MgO concentration from 7 wt % in both the upper and lower margins to 20 wt % in the tongue (Marsh, 2004). The dark-colored upper margin superficially mimics the lower chilled margin and is comprised dominantly of plagioclase, orthopyroxene, augite and inverted pigeonite; however, the upper margin has a more Fe-rich bulk composition and, for the most part, comparatively larger grain sizes (fine- to medium grained) relative to the aphanitic lower chilled margin. The difference in grain size between the lower and upper chilled margins probably reflects higher country rock temperatures above the sill at the time of emplacement owing to the high magma flux feeding the Kirkpatrick flood basalts and the formation of earlier sills (i.e. Peneplain Sill) above the Basement Sill (Hersum et al., submitted). Oxides (magnetite and ilmenite) are particularly abundant at the very top of the upper margin. Another difference between the lower and upper margins is the common presence of mafic pegmatoids and granophyric segregations beginning in the upper part of the opx tongue and continuing to the top of the sill (Zavala & Marsh, 2001, 2002). The pegmatoid segregations are composed dominantly of plagioclase and pyroxene (the latter to several centimeters in length) with minor modal amounts of quartz, alkali feldspar, amphibole, apatite, biotite and oxides. The pegmatoids are enriched in SiO2, FeO and incompatible elements (e.g. K, Ba, Rb, Nb and Ti) and have higher Ab:An (25%) ratios in plagioclase relative to plagioclase in the host-rock. The liquids that crystallized to yield the pegmatoids are inferred to represent the end products of the crystallization of late-state interstitial liquids that were probably expelled (i.e. filter pressed) from the surrounding gabbronorite during compaction of the magma (Geist et al., 2005).
The abrupt contact between the lower and upper margins and the Opx tongue manifests itself in a distinct break in slope noted in a crystal size distribution study (Zieg & Forsha, 2005) as well as a dramatic rise in temperature (200°C) inferred from two-pyroxene thermometry (Simon & Marsh, 2005). These results reflect directly the presence of the extensive Opx tongue and suggest that the pyroxene phenocrysts in the tongue were entrained in the ascending magma after having been texturally equilibrated at a much deeper level of the magmatic plumbing system than the present level of sill emplacement. Notably higher concentrations of Cr and Al in the Opx phenocrysts in the tongue relative to the upper and lower margins lend additional support to this hypothesis (Simon, in preparation). The magma that formed most of the Basement Sill probably intruded as a crystal-rich mush (i.e. silicate liquid + phenocrysts; Marsh, 2004; Charrier & Marsh, 2005; Petford et al., 2005) and the spatial position of the tongue reflects the inability of the large orthopyroxene phenocrysts to settle through the viscous lower margin.
Finally, the Dais Intrusion makes up the Dais, a topographic feature about 12 km west of the Bull pass area, and is an extension of the Basement Sill. It differs from the sill in the Bull Pass area in being thicker (the exposed portion is 450 m thick; the lower contact is not exposed) and in having more pronounced modal layering, particularly in the upper part of the Opx tongue (Marsh, 2004). The layering is defined by sharp changes in the amounts of orthopyroxene and plagioclase and ranges in thickness from centimeter to several meter scale.
Petrology of late-crystallizing minerals
All halogen-bearing minerals are present in only minor modal abundance and none appear to have been early liquidus (i.e.cumulus) phases. Apatite is the principal mineral of interest as it is the most commonly occurring (and probably the first precipitating) halogen-bearing mineral. Neither mica nor amphibole shows any textural evidence of crystallizing before apatite. This is consistent with evidence from layered intrusions crystallized in a low-pressure environment (such as the Skaergaard Intrusion) where apatite is the first halogen-bearing ‘cumulus’ mineral observed after 90% crystallization (e.g. Wager & Brown, 1967). The crystallization sequence of a representative chilled margin composition was modeled using MELTS (Ghiorso & Sack, 1995), which predicts that apatite is the sole halogen-bearing mineral during cooling. Also, unlike the micas and amphiboles, which have a strong crystal-chemical control on halogen substitution, the apatite halogen solid solution (i.e. Cl 
F) is ideal at high temperatures (e.g. Volfinger et al., 1985; Tacker & Stormer, 1989).
The apatite grains analyzed in this study are interstitial to early formed minerals (plagioclase and pyroxene), and are typically associated with other late-crystallized phases such as quartz, Fe–Ti oxides and, relatively rarely, biotite. Apatite modal abundance is qualitatively correlated with (unpublished) whole-rock P2O5 concentrations, suggesting that apatite is the dominant phosphorus-bearing phase in the rocks. In most instances apatite displays a characteristic acicular habit, forming thin rods usually no more than a few tens of microns in width but locally up to about 300 µm in length. In the central part of the sill, however, grains can be considerably thicker and have a tabular habit (Fig. 2).
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Finally, apatite from a few samples of the exposed part of the Dais was analyzed to see if lateral compositional variation is present in the Basement Sill between the Bull Pass and Dais Intrusion.
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METHODS |
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Apatite grains in polished thin sections were analyzed by wavelength-dispersive spectrometry (WDS) using the Cameca Camebax electron probe microanalyzer (EPMA) at Duke University, Durham, North Carolina, USA. Typical analytical conditions were: 15 keV acceleration voltage, 15 nA beam current, and a focused beam (a requirement given the typically small size of individual grains). Peak and background counting times for Cl and F were in the ranged of 20–30 s and 10–15 s, respectively. Other elements were counted on the peak for 30 s and background for 15 s. To minimize volatilization, F and Cl were counted first. Standards included natural chlorapatite (RM-1, Morton & Catanzaro, 1964) and fluorapatite (from Wilberforce, Ontario) for Ca, P and the halogens, and allanite (102522, #42; Frondel, 1964
) for La and Ce for which the analyte lines are relatively free from interferences (Roeder, 1985). Standard intensities for F and Cl using the apatite standards were also checked against fluorite, topaz and halite. Fluorine was analyzed using a synthetic Si–W layered diffracting crystal. The OH– ion and H2O were calculated by difference based on ideal OH– site occupancy. Previous work with these analytical conditions (see Willmore et al., 2000) has shown no significant apatite orientation effects on the analyses as reported by Stormer et al. (1993). However, to minimize any potential orientation effects, preference was given to grains cut parallel to the c-axis. |
RESULTS |
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Apatite composition
Representative apatite compositions are listed in Table 1 (a complete set of analyses is available as an Electronic Appendix at http://www.petrology.oxfordjournals.org/) and those from west Bull Pass are shown as a function of height in the sill in Fig. 3. The mole fraction of F ranges from 0·4 to end-member fluorapatite (and a few analyses for which the mole fraction F exceeds 1·0), whereas Cl ranges from nears its limit of detection (LOD
0·01 wt %) to a maximum of about 0·5 mole fraction. As has now been observed in a number of studies (e.g. Boudreau et al., 1986, 1992, 1995; Willmore et al., 2000), individual apatite grains from any given sample tend to be homogeneous and not significantly zoned in Cl and F (e.g. Table 1, analysis 5 and 6), whereas different grains in the same sample can vary by as much as 30+ mol% in the hydroxyl/halogen site. Thus, detailed stratigraphic changes can be masked by this within-sample variation. However, overall there remains a broad increase in Cl and decrease in F from the lower margin towards the center of the sill, and Cl then decreases to below LOD in the upper margin. These stratigraphic changes in Cl broadly parallel trends in the bulk-rock MgO and grain size (Fig. 3).
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The compositions of Ferrar apatite along with compositional fields from a number of other mafic intrusions are plotted in an F–Cl–OH ternary diagram in Fig. 4. The few analyses from the Dais overlap those from the Bull Pass area, suggesting that there are no significant regional variations in apatite chemistry. The Cl-poor nature of most Ferrar apatite is typical of intrusions such as the Skaergaard (Greenland), Great Dyke (Zimbabwe), Kiglapait (Canada), and Munni Munni (Australia), where the mole fraction of the chlorapatite component is generally less than 20 mol% (see Boudreau, 1995
). Except for the few Cl-rich samples from the central part of the Opx tongue, none are as Cl-rich as is seen in the lower portions of the Stillwater and Bushveld intrusions below the economically important platinum-group element zones of these two intrusions. Apatite in the Stillwater and Bushveld intrusions is strongly enriched in the end-member chlorapatite component throughout rather sizeable stratigraphic thicknesses.
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) and the Dais Intrusion (+) with those from the Stillwater and Bushveld complexes (only the range below the platinum reefs is shown), as well as the Munni Munni and Great Dyke. Inset shows the range of apatite from associated sill, dike and marginal rocks associated with these other intrusions. After Boudreau (1995Variations in apatite compositions across a pegmatoid–host-rock contact
Apatite was analyzed in three samples collected across a pegmatoid–host-rock contact from the Dais layered intrusion: one from within the pegmatoid segregation, one from the pegmatoid–gabbronorite host-rock contact and one 3 m into the host-rock. The samples were collected from the upper part of the Dais intrusion stratigraphically above the Opx tongue. Data from these apatite analyses are shown in Fig. 5 and example analyses are provided in Table 1; analysis 10 is apatite from within the pegmatoid and 11 is apatite from the host gabbro. Apatite within the pegmatoid is Cl poor (all <0·2 wt % Cl) and most grains are nearly end-member fluorapatite. Furthermore, apatite in the pegmatoid contains even less Cl than apatite in the surrounding upper zone host-rock. Bulk analysis of the pegmatoid demonstrates that the concentrations of incompatible trace elements are elevated between two and three times their concentrations in the host-rock (Geist et al., 2005
); however, the concentrations of rare earth elements (REE) in apatite are essentially unchanged from host gabbronorite to the pegmatoid.
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DISCUSSION |
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To summarize, apatite in the Basement Sill shows significant Cl/F variations both within individual samples and as a function of stratigraphic position, with the highest Cl/F ratios occurring in the middle of the sill. Assuming that the apparent rapid quench typical of the lower chilled margin preserves apatite halogen compositions in equilibrium with the initial (unfractionated) liquid, the initial apatite is characterized by X(Cl)
0·05 and X(F) 0·7.
Despite the complexity of the magma dynamics indicated by the characteristics of the Opx tongue, a notable feature of the sill is that, once emplaced, it cooled uniformly from the top and bottom such that the silicate liquid would have become vapor-saturated nearly simultaneously at both margins. The sill took some 1000 years to solidify and evidence for post-solidification low-temperature hydrothermal alteration is absent (Simon & Marsh, 2005). As mentioned above, the finer grain size in the lower margin relative to the upper margin suggests that the upper margin may have cooled slightly more slowly; the proximity of the Peneplain Sill above the Basement Sill may have played a role in this (see Hersum et al., submitted). Thus, the rapid crystallization time and preservation of magmatic halogen ratios in trace phases such as apatite provide the opportunity to study the loss of vapor evolved from the upper margin to the overlying country rock and the migration of vapor evolved from the lower margin as this vapor ascended through the sill.
Prior to the interstitial liquid becoming saturated in a halogen-bearing phase (e.g. apatite, biotite, amphibole or vapor), one would not expect there to be fractionation of halogens by crystallization of the observed anhydrous minerals. Ignoring vapor for the moment, and although it appears that apatite is the first halogen-bearing mineral phase to crystallize, the crystallization of apatite, biotite or amphibole should all incorporate F in preference to Cl (e.g. Speer, 1984; Candela, 1986), particularly in rocks with high Mg/Fe ratio (e.g. Volfinger et al., 1985). Considering only apatite, Beswick & Carmichael (1978) noted that fluorapatite is generally more stable (i.e. crystallizes earlier) than chlorapatite and hydroxyapatite at magmatic temperatures and the phase equilibrium interpretations of Tacker & Stormer (1993) are consistent with their observations. This suggests that apatite behaves similarly to other halogen- and OH-bearing minerals in that F-bearing end-members have significantly higher thermal stabilities (Patiño Douce & Roden, 2006). Hence, Cl/F variations in apatite are analogous to Mg/Fe variations in olivine or orthopyroxene (Cawthorn, 1994), such that an increase in the Cl/F ratio of apatite implies both an increase in the Cl/F ratio of the interstitial silicate liquid and that Cl-rich apatite formed from the primary Basement Sill silicate liquid should be stable only at lower temperatures. Thus, the Cl/F mass ratio in the silicate liquid should increase after apatite and other Cl- and F-sequestering mineral phases begin to crystallize from the interstitial liquid. An exsolving vapor will effectively decrease the Cl/F mass ratio of the silicate liquid as degassing progresses (as discussed more fully below). As pockets of interstitial silicate liquid that are saturated in apatite or other halogen-bearing phases become isolated during the final crystallization of interstitial liquid, apatite of different compositions can be produced. This is considered to be the main cause of the observed compositional variation in apatite within individual samples.
The overall stratigraphic changes in the Cl/F and OH/F ratio of apatite can be caused by several crystallization mechanisms, some of which have been discussed previously (Boudreau et al., 1986; Boudreau & McCallum, 1989; Boudreau & Kruger, 1990; Mathez & Webster, 2005). The effect of temperature and of variations in the activities of F and Cl on apatite compositions have been described by Boudreau et al. (1992), who considered the equilibrium between apatite and an aqueous fluid, for which thermodynamic data are available, that are both in equilibrium with a silicate liquid. Although the activities of the halogens in a silicate liquid are not expressed explicitly by such a treatment, an increase in the activity or chemical potential of either of the halogens in any phase must occur in all other coexisting halogen-bearing phases as well.
Let us consider the exchange reaction between apatite (ap) and an aqueous fluid (af) in which HCl and HF are present as neutral species:
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denotes the mole fraction of component i in phase j and 
denotes the activity of i in phase j. Activity coefficients for these species are not known and, thus, are not included in the apparent exchange constant. The relationship between fluid composition and apatite composition is given by
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), Boudreau et al. (1992) fitted expressions for log K to a simple temperature-dependent function. Shown in Fig. 6 are calculated apatite compositions that result from changing either the Cl/F or OH/F ratio of the silicate liquid at a fixed temperature of 1100°C (dashed lines) or from changing the temperature at fixed HCl/HF and H2O/HF activity ratios of 3·0 and 10·0, respectively (continuous lines). The model trends in Fig. 6 suggest that the observed variations in apatite compositions may be caused by changing either the HCl/HF and H2O/HF ratios by approximately an order of magnitude, or by varying the temperature at which apatite equilibrates by up to 500°C.
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As noted above, there do not appear to be any halogen-sequestering minerals crystallizing in the sill prior to apatite saturation; hence, fractional crystallization of the observed halogen-free minerals alone cannot lead to variations in halogen ratios as a function of stratigraphic position. That is, because P behaved as an incompatible trace element during the crystallization of the Basement Sill, and assuming as a first approximation that apatite crystallization was controlled only by the P concentration in the silicate liquid, then all interstitial silicate liquid should have reached apatite saturation after the same amount of crystallization. The system would have followed a single liquid line of descent with respect to apatite crystallization and apatite would have begun to crystallize at about the same temperature throughout the intrusion. Because this is inconsistent with a large temperature range in apatite equilibration, temperature effects alone cannot be the major cause of the stratigraphic variation in apatite composition. One would also expect the same for biotite and amphibole crystallization.
However, the separation of an aqueous fluid phase from a crystallizing silicate liquid can fractionate the halogens owing to the strong affinity of Cl relative to F for an exsolved aqueous fluid phase relative to silicate liquid (Sourirajan & Kennedy, 1962; Helgeson, 1964; Burnham, 1967; Roedder, 1984, 1992; Bodnar et al., 1985; Candela, 1986; Chou, 1987; Shinohara et al., 1989; Chou et al., 1992; Metrich & Rutherford, 1992; Sterner et al., 1992; Anderko & Pitzer, 1993; Cline & Bodnar, 1994; Lowenstern, 1994; Piccoli & Candela, 1994; Webster et al., 1999, 2004; Webster, 2004; Webster & DeVivo, 2004; Mathez & Webster, 2005). Furthermore, as discussed below, migration of exsolved vapor through a crystallizing magma can readily redistribute halogens from one region to another, leading to variations in halogen ratios in both the silicate liquid and apatite.
We suggest that vapor evolved from the lower half of the sill and subsequently migrated upward through the crystal mush where it encountered hotter, fluid-undersaturated silicate liquid (i.e. silicate liquid that had not reached crystallization-induced fluid saturation). The free vapor phase would have had a high Cl/F mass ratio owing to the strong mass transfer of Cl from silicate liquid to vapor relative to F (Dingwell & Scarfe, 1983; Webster & Holloway, 1990; Candela & Piccoli, 1995). As the vapor ascended into hotter silicate liquid, the vapor would have been resorbed by the silicate liquid; hence, resulting in elevated volatile concentrations and higher Cl/F ratios in the interstitial silicate liquid in the central part of the sill. This process would ultimately yield higher Cl/F ratios in apatite crystallized from this interstitial silicate liquid. In contrast, early formed apatite crystallized from the upper margin of the Basement Sill and its associated pegmatoidal and granophyric segregations grew from more evolved silicate liquid that had already degassed and preferentially lost an unquantified amount of Cl to the exsolved volatile phase. In addition, aqueous fluids separating from the lower and eventually from the middle of the intrusion migrated into cooler rocks of the upper margin. For aqueous fluids with a fixed HCl/HF ratio, the fluids would be in equilibrium with lower Cl/F apatite and could leach Cl from pre-existing apatite.
Evidence from pegmatoids
The strong enrichments in SiO2, FeO and incompatible trace elements in the pegmatoid (Geist et al., 2005) are consistent with these segregations having crystallized from late-stage, evolved interstitial silicate liquid. The lack of variation in rare earth element (REE) apatite noted in this study suggests that the higher bulk-rock REE concentrations in the pegmatoids simply reflect a higher amount of this evolved liquid than is seen in the host-rock, but that the evolved liquid in both the pegmatoid and the interstitial liquid of the host otherwise have similar REE concentrations. However, the lower Cl/F ratio of apatite in the pegmatoid as compared with host apatite implies that the apatite equilibrated with a liquid or vapor with lower Cl/F ratio or that this ratio was fixed but equilibrated at a lower temperature, or both.
It is suggested that the pegmatoidal texture developed as vapor, evolved from the lower parts of the sill, used these regions of higher amounts of silicate liquid (i.e. pegmatoids) as preferential conduits through which to ascend. Further, these aqueous fluids were relatively Cl-poor and were not effective at moving significant quantities of REE. Data from saline aqueous inclusions in quartz hosted by granophyre of the Capitan Pluton (New Mexico) suggest that the light REE (LREE) may be fractionated from the heavy REE (HREE) during volatile phase exsolution, with the LREE having been scavenged by the saline aqueous fluid (Banks et al., 1994). In another study, whole-rock analyses of potassically altered and unaltered granodiorite also showed enrichment in LREE in the aqueous-fluid-altered rocks (Taylor & Fryer, 1980). The ability of aqueous fluids to scavenge preferentially the REE from crystallizing silicate liquid has also been shown experimentally (Flynn & Burnham, 1978; Sakagawa, 1989; Reed, 1995; Reed et al., 2000). Although the mass transfer of individual REE from silicate liquid to vapor is a complex function of the total salinity of the vapor phase (Candela, 1990) and the concentrations of all other chloride-bound elements in the vapor (Reed, 1995; Reed et al., 2000), in general, the exsolution of a Cl-bearing aqueous fluid from a silicate liquid will lead to a decrease in the LREE/HREE mass ratio of the silicate liquid. The partition coefficients describing the simple mass transfer of La and Lu between silicate liquid and vapor increase from approximately 0·01 and 0·001 when the Cl concentration of the vapor is 0·01 molal to 2 and 0·7 when the Cl concentration of the vapor is 2 molal. Thus, if ascending vapor is resorbed into stratigraphically higher, hotter silicate liquids (as discussed below), this should increase the LREE concentration of that interstitial silicate liquid and, even upon subsequent vapor saturation of that parcel of silicate liquid, the LREE/HREE mass ratio might be expected to be higher than that of silicate liquid stratigraphically lower in the magma chamber. The fact that REE concentrations in the pegmatoid and host-rock apatite are equivalent suggests that the late-state interstitial silicate liquid that crystallized to form the pegmatoids did not resorb a significant quantity of ascending vapor and, further, that the vapor exsolving from this segregated silicate liquid was of a low enough salinity or temperature that it did not effectively fractionate the REE.
Modeling degassing in a cooling and crystallizing intrusion
Vapor separation in the cooling and crystallizing Basement Sill was modeled using a modified version of the IRIDIUM program (Boudreau, 2003). This program uses a simplified version of the MELTS free energy minimization program (Ghiorso et al., 1994; Ghiorso & Sack, 1995) coupled with one-dimensional heat and mass transfer to model a number of igneous processes such as compaction and silicate liquid percolation through a porous assemblage. The program has been modified to include a more complete C–O–H–S fluid (see the Appendix). It also has been modified to allow for an upper and lower thickness of material to act as a thermal sink; originally the program simply assumed a fixed heat loss from the top and bottom of the magma column. This modification allows more realistic modeling of the thermal profile of a sill that is losing heat by thermal conduction into cooler country rock.
The initial conditions of the model assume a 350 m thick, pyroxene-phyric sill that is allowed to cool and crystallize as heat is lost to the over- and underlying country rocks. Because we are interested specifically in the effects that volatile saturation and volatile migration may have on halogen ratios, the model initially assumes a uniform mush column. (The phenocryst-free marginal rocks are ignored as they probably represent an initial phenocryst-free leading edge of intruding magma that was essentially chilled prior to the emplacement of the Opx tongue.) Paleodepth estimates suggest country rock temperatures as low as 150°C (Simon & Marsh, 2005; Hersum et al., submitted), but as the rocks are likely to have been preheated by intrusion of the overlying Peneplain sill prior to emplacement of the Basement Sill, we used a uniform initial country rock temperature of 300°C in the model. Pressure at the top of the sill is 100 MPa, based on an intrusion depth of 1 km and assuming a lithostatic pressure gradient of 100 MPa/km.
The bulk composition of the magma (i.e. silicate liquid + crystals) is composed of 60% chilled marginal composition taken from Gunn (1966) at the temperature at which this liquid is saturated with orthopyroxene (1130°C) plus 40% orthopyroxene of mg-number 85. To this is added 0·6 wt % H2O, 0·01 wt % CO2 and 0·03 wt % S. The initial volatile contents of the Ferrar magmas are not known, and we have used a value that falls within the high end of water contents observed in mafic magmas [see summary by Boudreau et al. (1997)]. This value was used to prevent free energy minimization problems at low liquid fractions. Also, the model calculation does not go to 100% crystallization so that the actual quantity of vapor generated is relatively small. After 90% crystallization the liquid has lost most of its CO2 and is degassing nearly pure H2O (as discussed below); this composition vapor would simply continue with additional crystallization. Finally, because the chilled margin composition of Gunn is not in equilibrium with the added orthopyroxene and volatiles, the initial calculated solid assemblage is actually a mix of olivine (10%) and orthopyroxene (25%). Over the course of the run, olivine is replaced with orthopyroxene by peritectic reactions with the silicate liquid.
Chlorine and F are treated as trace elements in the calculation, and are assumed to have vapor–silicate liquid partition coefficients of 100 and 0·1, respectively. These partition coefficient values are within the range observed in both natural and experimental systems across a wide range of silicate liquid compositions (Carroll & Webster, 1994; Saal et al., 2002). Chlorine and F are otherwise assumed to not partition into any solid phases. Minerals that crystallize over the course of the reaction include orthopyroxene, olivine, plagioclase, magnetite, alkali feldspar and an immiscible Fe–S liquid. In addition to crystallization, the mush column is allowed to compact following the model of Shirley (1986). Any vapor that is generated is assumed to move upward at a fixed velocity equal to 1·56 cm/day; this vapor ascent velocity is one-half the characteristic compaction velocity at the start of the simulation. Because each time step is calculated from the compaction velocity profile (which slows down over time as the sill solidifies), this value was selected so that the vapor moves about one space step for each time step over the course of crystallization. This number is otherwise arbitrary, as there are no models that express how bubbles move through a crystallizing and compacting mush column. Nor does vapor degassing or migration affect compaction rates in the model, except for the effect of dissolved H2O in the silicate liquid on viscosity of the latter.
None of the results presented below are qualitatively affected by modest changes in any of the above initial model parameters, as long as the mush column is not initially vapor-saturated. For example, changing the volatile concentrations of the initial silicate liquid or vapor velocity can change the point at which the silicate liquid becomes vapor saturated or the time scale at which vapor fronts move, but the relative change in CO2/H2O exsolved over time will be similar to that discussed below. In this respect, the main conclusions are robust.
Model results for times of 0, 31, 107, 210 and 432 years are shown in Fig. 7. Heat loss and crystallization lead to crystallization-induced vapor saturation (i.e. second boiling) initially occurring in the upper and lower margins as the solidification fronts propagate in toward the center of the sill. The extents of these zones at the labeled times are shown in the ‘Mass vapor’ plot in Fig. 7. Although the initial silicate liquid is relatively H2O-rich, the initial vapor separating from both the upper and lower zones is CO2-rich (i.e. high initial CO2/H2O ratio) owing to the much lower solubility of CO2 (Eggler et al., 1974; Mattey, 1991; Lowenstern, 2000). As degassing progresses the vapor separating at any stratigraphic level becomes more H2O-rich and the CO2/H2O ratio continually decreases. The concentration of H2S in the vapor remains low at all times. Spatially, this is expressed by the vapor in the upper and lower margins being more H2O-rich than the more interior gases at any given time, at least during the early stages of degassing. At some time between 61 and 107 years the upper and lower vapor saturation zones merge and the entire sill becomes vapor-saturated. Over time, CO2 is effectively flushed from the sill such that by 432 years the fluid is essentially all H2O with a few mol% H2S, except in the center of the sill where the concentration of CO2 remains high.
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In the upper zone, progressive devolatilization and the separation and loss of vapor from the top of the intrusion to the overlying country rock results in the preferential loss of Cl from the interstitial silicate liquid. Also, although Cl is continually being advected through the sill in vapor evolved and ascending from silicate melt in the lower margin, the Cl content of interstitial liquid in the upper zone remains approximately constant. In contrast, the concentration of F in the interstitial liquid increases as crystallization proceeds. The net result is a decrease over time in the Cl/F mass ratio in the upper part of the sill.
The generation and migration of vapor is more complicated in the lower zone. Vapor exsolved near the lower margin ascends into hotter interstitial liquids that are not yet vapor-saturated. This results in the resorption of vapor in the hotter, interstitial silicate liquid. This process has four effects. First, as Cl is preferentially partitioned into the aqueous fluid relative to F, the resorption of vapor with a high Cl/F mass ratio into the interstitial silicate liquid leads to the formation of a peak in the Cl concentration in the interstitial silicate liquid that migrates upward through the magma with time. Second, the enrichment of volatile components in stratigraphically higher interstitial silicate liquid owing to resorption causes this interstitial silicate liquid to achieve volatile saturation and exsolve a vapor phase earlier than would have occurred based on the original volatile concentrations in this silicate liquid fraction. This results in the lower vapor-saturated zone moving into the center of the sill faster than the upper zone. Third, this upward advancing ‘wet front’ produces a peak in the mass of (H2O-rich) vapor evolved that also moves upward a short distance behind the peak in Cl concentration in interstitial liquid. Fourth, progressive degassing from the lower margin produces more extreme depletions in CO2 in the fluid than occurs in the upper zone. This difference occurs because, unlike the upper margin, the lower margin does not have a source of CO2 from degassing interstitial liquids below it.
The migration of both the ‘Mass vapor’ and Cl peaks continues once the upper and lower vapor saturation zones merge. However, the peak Cl/F ratio of the interstitial silicate liquid declines over time. Although not shown, once the Cl peak passes above the center of the sill it encounters progressively colder magma and, as pointed out above, this will not favor increasing the Cl concentration of apatite. Furthermore, this cooler magma contains more evolved interstitial silicate liquid, which contains a higher concentration of F, and hence again Cl/F ratios are not strongly elevated.
Additional compaction effects
Shown in Fig. 8 are snapshots at 210 years of the phase mode profile (wt%), bulk MgO, mass quantity of vapor, solid and liquid velocities and the per cent deformation of the solid matrix. The profiles in most of these plots show the effects of compaction. At 210 years, the model solid and liquid velocities in the margins are near zero. Thus, active compaction of the mush column at this time is slowing considerably and is occurring mainly in the central part of the sill. The bulk MgO concentration has evolved from a straight profile to an ‘s’-shaped profile owing to compaction. It should be noted that, in the model presented here, the sill was assumed to be uniform in its initial phenocryst content. Although this is not entirely consistent with the conclusion of Marsh (2004), we highlight that the new model results would be superimposed on any more complicated initial distribution of phenocrysts. In the lower part of the intrusion, compaction leads to an increase in MgO as solids displace interstitial liquid. In contrast, for reasons discussed in some detail by Boudreau & Philpotts (2002), in the upper part of the sill the solid assemblage is undergoing extensional deformation. Briefly, this occurs because the increasing solidification of interstitial silicate liquid towards the upper contact decreases the permeability and hence the solid velocity decreases upward in the upper half of the sill. In effect, solids in the center of the sill can move down faster than those near the upper margin. The effect is a zone of extension that is maximized at about two-thirds up in the sill. Although it is beyond the scope of this study, we note that in the Dais intrusion the tensional region is characterized by the appearance of pronounced mafic-felsic layering.
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Finally, it is noted that the migration of exsolved vapor affects the compaction-driven silicate liquid velocity. In general, the silicate liquid would have a positive velocity value as the liquid moves upward in response to solids moving downward (i.e. compacting). However, vapor ascends faster than silicate liquid, hence there is a local negative liquid velocity as a ‘backflow’ of silicate liquid displaces ‘lost’ vapor. This is most evident where a negative liquid velocity peak is associated with the vapor mass peak in the lower part of the sill, but it is also present in the upper part of the sill.
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CONCLUSIONS |
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Apatite compositions and numerical modeling both suggest that degassing in a mafic sill can be relatively complicated. Like the magma itself, the vapor evolved from a cooling magma can undergo a protracted period of compositional evolution. This affects the estimation of volatile abundances of elements such as S, Cl and F. Degassing from the lower margin of the sill can influence strongly the volatile-element budget of the overlying magma column as crystallization proceeds. Variation in halogen ratios of apatite through the vertical extent of a sill provide a convenient, if difficult, method for elucidating its volatilization history. The model results agree broadly with the fluid inclusion record preserved in the Stillwater Complex (Hanley et al., 2005
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Supplementary data for this paper are available at Journal of Petrology online.
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APPENDIX |
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TOPABSTRACTINTRODUCTIONGENERAL GEOLOGY AND PREVIOUS...METHODSRESULTSDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAAPPENDIXREFERENCES |
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The modeling described in this paper uses an updated version of the IRIDIUM program (Boudreau, 2003
). This program uses a simplified version of the MELTS program (Ghiorso et al., 1994; Ghiorso & Sack, 1995) for phase equilibrium determinations that are coupled with mass and heat transfer models. The main simplification from MELTS is the use of simpler solid solution models for some of the minerals (Boudreau, 1999). The updates described here include the addition of CO2 and FeS as system components, which allows the model to exsolve both a C–O–H–S fluid and to precipitate sulfides and C-bearing phases from the liquid.
CO2 solubility in IRIDIUM essentially incorporates the model of Papale (1999). It has been modified as follows: the CO2–oxide liquid interaction parameters have been recalibrated to the liquid components and interaction parameters used in MELTS (the Papale model used those of the program SILMIN, an earlier version of MELTS). The thermodynamic properties of pure CO2 and all other gas species are calculated by the general equation of state for supercritical gases of Duan et al. (1996). Gases are otherwise assumed to mix ideally. Possible gas species include H2O, CO2, CO, CH4, H2S, and SO2; however, for the modeling done here only H2O, CO2 and H2S are considered. Graphite (C), calcite and dolomite are possible precipitating solids. Finally, because of its low solubility at low to moderate pressures, the model treats the CO2 liquid component as not having any effect on the other liquid species other than acting as a dilutant to the mole fraction, X.
The program uses liquid FeS (liquid troilite) for the silicate liquid component for sulfur. Sulfur fugacity is calculated by the method of Wallace & Carmichael (1992). Immiscible sulfide liquid is assumed to be an ideal solution of two components, FeS and FeO. Other possible sulfide phases that can precipitate are pyrrhotite, pyrite and troilite.
FeS–oxide liquid interaction parameters were calibrated (with some arbitrary adjustment because of uncertain sulfide and silicate liquid compositions in the experimental data) from the data of Haughton et al. (1974), Wendlandt (1982), Mavrogenes & O’Neill (1999) and O’Neill & Mavrogenes (2002). Because these studies invariably tabulated total iron only, FeO and Fe2O3 are calculated by the method of Kress & Carmichael (1991) based on experimental f(O2), temperature and bulk composition. These experimental studies are mostly of basalt–intermediate composition, but the lower solubility calculated for more silicic compositions appears reasonable. Above 1 GPa, sulfide saturation values are significantly lower than expected. As for CO2, the interaction parameters do not affect the calculation of the chemical potential of other liquid components other than a minor effect on mole fractions.
Examples of program output are shown in Figs A1–A4. Figure A1 shows an example of gas speciation as a function of f(O2) for a gas-saturated mid-ocean ridge basalt (MORB) magma. It is necessary to note an important caveat regarding oxygen and sulfur fugacity as calculated by MELTS/IRIDIUM; that is, the liquid interaction parameters used by MELTS were not explicity calibrated against oxygen fugacity. Because of this, when f(O2) is calculated from reactions between liquid components used by MELTS, it is not the same as that calculated by the empirical Kress & Carmichael (1991) model incorporated into MELTS. Nonetheless, gas speciation for S-bearing species is broadly consistent with the model results of Shi (1992), for example.
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Figure A2 shows the changes in both gas and magma composition as a vapor-saturated MORB magma undergoes polybaric decompressional fractional degassing from 2001 to 1 bar. This can be compared with similar plots of Papale (1999
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Figure A3 shows the S concentration at sulfide saturation (SCSS) as a function of total iron concentration in the liquid for a selection of liquids taken from Haughton et al. (1974), and Fig. A4 shows the SCSS as a function of sulfur and oxygen fugacity, again for a sample from Haughton et al. (1974).
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ACKNOWLEDGEMENTS |
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The senior author would like to thank Bruce Marsh for his efforts to make the Field Laboratory Workshop in the McMurdo Dry Valleys, Antarctica, a great success and for the chance to participate in the study of these very interesting rocks. This paper was significantly improved by comments from W. Bohrson, D. Geist, J. Hanley and an anonymous reviewer. The work was supported by National Science Foundation Grant EAR 04-07928.
*Corresponding author. Telephone: 919-684-5646. Fax: 919-684-5833. E-mail: Boudreau{at}duke.edu
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REFERENCES |
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Anderko A, Pitzer KS. Equation-of-state representation of phase equilibria and volumetric properties of the system NaCl–H2O above 573 K. Geochimica et Cosmochimica Acta (1993) 57:165