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Journal of Petrology | Volume 43 | Number 7 | Pages 1389-1413 | 2002
© Oxford University Press 2002
Mineral Chemistry of Mildly Alkalic Basalts from the 25 Ma Mont Crozier Section, Kerguelen Archipelago: Constraints on Phenocryst Crystallization Environments
1DEPARTMENT OF EARTH AND ENVIRONMENTAL SCIENCES, UNIVERSITÉ LIBRE DE BRUXELLES CP 160/02, AVENUE F. D. ROOSEVELT 50, B-1050, BRUSSELS, BELGIUM
2DEPARTMENT OF EARTH, ATMOSPHERIC AND PLANETARY SCIENCES, MASSACHUSETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGE, MA 02139, USA
3LABORATOIRE DE GÉOLOGIE–PÉTROLOGIE, UNIVERSITÉ JEAN MONNET, CNRS–UMR 6524, SAINT-ÉTIENNE, FRANCE
Received July 18, 2002; Revised typescript accepted January 11, 2002
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ABSTRACT |
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Phenocryst compositions and mineral–melt equilibria in the mildly alkalic basalts from the 25 Ma Mont Crozier section on the Kerguelen Archipelago are used to estimate the depths at which magmas stalled and crystallized and to constrain the role of crustal structure in the evolution of magmas produced by the Kerguelen mantle plume. The Crozier section, of nearly 1000 m height, consists of variably porphyritic flows (up to 21 vol. % phenocrysts), dominated by plagioclase ± clinopyroxene ± olivine ± Fe–Ti oxides. Feldspars show an extreme range of compositions from high-Ca plagioclase (An88) to sanidine and variable textures that are related to extensive fractionation, degassing, and mixing in relatively low-pressure (sub-volcanic) magma chambers. Although clinopyroxene is a minor phenocryst type (0–3 vol. %), its non-quadrilateral components, principally Al (1·9–8·6 wt % Al2O3), vary widely. The results of clinopyroxene–liquid thermobarometry and clinopyroxene structural barometry indicate that the Crozier magmas crystallized at pressures ranging from
1 kbar to 11–12 kbar with high-Al clinopyroxene recording the highest pressures of crystallization. High-Al clinopyroxene-rich cumulates may represent an important component of the seismic crust-to-mantle transition zone at 14–16 km depth. High-pressure, high-Al clinopyroxene crystallization became important in the mildly alkalic basaltic magmas from the Kerguelen Archipelago as ascending magmas stalled and fractionated at or near the crust–mantle interface, which became deeper as a result of progressive crustal thickening as the archipelago moved from a ridge-centered setting at 40 Ma to an intraplate position by 25 Ma. KEY WORDS: Kerguelen Archipelago; Mont Crozier; mildly alkalic basalts; phenocrysts; high-Al clinopyroxene
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INTRODUCTION |
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Phenocryst compositions and mineral–melt equilibria are sensitive to variations in the critical intensive parameters of magmatic systems (temperature, pressure, water activity, oxygen fugacity, etc.) and these variations can be used to constrain the crystallization environments of phenocrysts in basaltic lavas. Basaltic lavas from the Kerguelen Archipelago in the SE Indian Ocean, which is the third largest oceanic island after Iceland and Hawaii and the emergent part of the giant Kerguelen oceanic plateau, are mostly low-MgO basalts to trachybasalts with 4–6 wt % MgO (Weis et al., 1998
; Yang et al., 1998; Frey et al., 2000, 2002). These low-MgO basalts are extensively fractionated magmas that underwent significant crustal processing in magma reservoirs or chambers during ascent. Most of these lavas were erupted between 29 and 24 Ma (Nicolaysen et al., 2000) and there is a temporal change from older tholeiitic–transitional basalts to younger mildly alkalic basalts (Gautier et al., 1990; Frey et al., 2000). The Kerguelen Archipelago initially formed in a ridge-centered setting along the Southeast Indian Ridge at 40 Ma and then migrated to an intraplate setting as the ridge moved to the north relative to the stationary hotspot beneath the archipelago (Mutter & Cande, 1983). Combined ridge-centered and intraplate magmatism related to the Kerguelen mantle plume has produced anomalously thick (15–20 km) basaltic–gabbroic crust beneath the Kerguelen Archipelago (Recq et al., 1994; Charvis et al., 1995) that the younger magmas traversed. Thus, there may be important information preserved in the phenocrysts of the younger, mildly alkaline archipelago-forming lavas on the influence of crustal structure on the evolution of magmas produced by the Kerguelen mantle plume.
In this paper, we present the first detailed study of phenocryst compositional variations in a stratigraphic section from the Kerguelen Archipelago. The 25 Ma Mont Crozier section on the Courbet Peninsula exposes a sequence, of 1000 m height, of mildly alkalic, low-MgO basaltic to trachyandesitic lava flows that, together with contemporaneous lavas from the Southeast Province (Weis et al., 1993; Frey et al., 2000), mark the transition from flood basalts to more shield-like volcanism on the Kerguelen Archipelago. The isotopic compositions of basalts from the Mont Crozier section have been interpreted as reflecting those of the Kerguelen mantle plume during formation of the main part of the archipelago (Weis et al., 2002). Our study, based on nearly 600 analyses of plagioclase, clinopyroxene and olivine phenocrysts, demonstrates that although the majority of the observed phenocryst assemblages were produced during low-pressure (subvolcanic) crystallization, there is an important role for relatively high-pressure (5–12 kbar) fractionation of high-Al clinopyroxene in the evolution of mildly alkalic basalts from the Crozier section that is not evident in the older tholeiitic–transitional basalts. The high-pressure fractionation stage requires the presence of deep-seated magma reservoirs, perhaps near the crust–mantle interface beneath the Kerguelen Archipelago. This suggests that changes in alkalinity and decreases in the supply rate of magmas related to the Kerguelen mantle plume were linked to the progressive thickening of the lithosphere beneath the Northern Kerguelen Plateau.
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GEOLOGIC SETTING OF THE KERGUELEN ARCHIPELAGO |
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The Kerguelen Archipelago is located in the southern Indian Ocean on the northern part of the large Kerguelen oceanic plateau (Fig. 1a). Geophysical surveys reveal that the crust of the archipelago and the underlying Northern Kerguelen Plateau (NPK) is in the range of 15–20 km thick (Recq et al., 1994
; Charvis et al., 1995), much thicker than typical oceanic crust. The NKP appears to be made of magmatic products derived from both the Kerguelen mantle plume and oceanic crust produced along the Southeast Indian Ridge at 40 Ma. Based on the above geophysical studies and on extensive geochemical studies of basalts from the archipelago (Gautier et al., 1990; Weis et al., 1993, 1998; Yang et al., 1998; Frey et al., 2000, 2002; Doucet et al., 2002) and from Ocean Drilling Program (ODP) Site 1140 on the northern part of the NKP (Weis & Frey, 2002), there is no evidence that continental material was involved in the formation of the NKP and the archipelago. The existence of such a thick crustal structure, devoid of continental material, is a critical factor in the evolution of the archipelago lavas as it favors the existence of multiple magma reservoirs and polybaric fractionation and may also represent a potential contaminant for magmas as they ascend towards the surface.
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The 6500 km2 Kerguelen Archipelago consists of a central island of 100 km width surrounded by numerous smaller islands (Fig. 1b). Intense Quaternary glacial erosion produced a regional topography with deeply incised valleys and fjords, allowing examination and sampling of thick stratigraphic lava sections. Based on Ar–Ar geochronology (Nicolaysen et al., 2000; Doucet et al., 2002), the volumetrically important flood basalts on the archipelago can be divided into older tholeiitic–transitional basalts (29–26 Ma) and slightly younger (25–24 Ma) mildly alkalic basalts (Fig. 1b). The younger alkalic basalts are characterized by steadily increasing Al2O3 with decreasing MgO, whereas the tholeiitic–transitional basalts show progressively decreasing Al2O3 with decreasing MgO (Gautier et al., 1990; Yang et al., 1998; Frey et al., 2000). These differences indicate that fractionation of plagioclase played a relatively minor role in the formation of the mildly alkalic basalts on the Kerguelen Archipelago (Damasceno et al., 1999).
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THE MONT CROZIER BASALTIC SECTION |
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Mont Crozier is the culminating summit of the Courbet Peninsula with a height of 978 m (Fig. 2). The present study concerns a section sampled in 1993 (samples ‘OB93-’), which represents the thickest section sampled on the archipelago. It is also one of the best-exposed, thickest stratigraphic sections exposed or drilled on any oceanic island (e.g. the Pilot Hole of the Hawaii Scientific Drilling Project penetrated to a depth of 918 m; Stolper et al., 1996
). Nicolaysen et al. (2000) determined whole-rock 40Ar/39Ar ages from two samples from the Crozier section (Fig. 2): a basalt flow from the top of the section (OB93-111) gave an age 24·53 ± 0·67 Ma and a basalt flow from the bottom of the section (OB93-204) gave an age of 24·82 ± 0·19 Ma. Sampling of the Crozier section was focused on the massive lava flows, although 20–40% of the section consists of oxidized scoria and highly vesicular reddish levels that represent both tops to the massive flows and blocky aa-type flows. Of the 95 samples collected in 1993, 44 were selected for petrographic observation and geochemical analysis, according to the key criteria of minimal alteration and stratigraphic distribution. Modal abundances of phenocrysts, microphenocrysts and groundmass were determined on thin sections from all 44 samples by point-counting with 1500 points per sample (Table 1).
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Using morphological, petrological and geochemical criteria, we subdivided the Mont Crozier section into four stratigraphic units, from base to summit: A, B, C and D (Fig. 2). The basal unit A (from 0 to 200 m) is composed of the thickest lava flows and is in stratigraphic continuity with an important near-horizontal plateau on the Courbet Peninsula called the Plateau du Tussok. Unit B (200–400 m) corresponds to the top of this plateau. Units C (400–600 m) and D (800–978 m) are the upper units of the section and are characterized by thinner lava flows and more abundant scoriaceous, oxidized layers. Most samples are porphyritic (up to 21 vol. % phenocrysts) with abundant plagioclase phenocrysts, minor clinopyroxene, olivine, and Fe–Ti oxide phenocrysts, and rare amphibole and apatite phenocrysts. One sample from the Crozier section was recognized as a dike in the field (OB93-140, Fig. 2). On the profile of Fig. 2, we have reported four dikes mentioned in the ‘77-’ section (Giret, unpublished field notes, 1977), which was located along the same profile as the ‘OB93’ section.
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PETROGRAPHY AND MINERAL CHEMISTRY |
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On the basis of the variation in phenocryst assemblages and textures, we selected 15 samples for detailed electronprobe microanalysis of plagioclase, clinopyroxene, and olivine phenocrysts and groundmass grains. Analyses were performed on a JEOL JXA-733 Superprobe at the Massachusetts Institute of Technology (accelerating voltage 15 keV; beam current 10 nA) using the Bence & Albee (1968)
correction procedure with the modifications of Albee & Ray (1970). Calibration was made with natural and synthetic standards. Additional plagioclase phenocryst transects were obtained by electronprobe microanalysis on a CAMECA SX100 electron microprobe at the Université Blaise Pascal, Clermont-Ferrand, France (accelerating voltage 15 keV; beam current 10 nA) using a ZAF correction procedure, and natural and synthetic standards. For the MIT analyses, compositions were systematically determined on cores and rims of multiple grains from individual thin sections. All analyses are consistent with mineral stoichiometry. Representative compositions of plagioclase are given in Table 2, of clinopyroxene in Table 3, and of olivine in Table 4.
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Plagioclase
In contrast to the tholeiitic–transitional basalts from the northern part of the Kerguelen Archipelago, where plagioclase is a relatively minor phenocryst type, plagioclase is abundant in the Crozier section (up to 17 vol. %). The increased abundance of plagioclase in these mildly alkalic lavas is at odds with their whole-rock geochemistry, which shows continuously increasing Al2O3 contents for decreasing MgO contents for all lava types (aphyric to porphyritic) (Damasceno et al., 1999), suggesting that most of the plagioclase crystallized at relatively low pressures in sub-volcanic magma chambers just before eruption. We analyzed 136 phenocrysts, microphenocrysts and microlites for a total number of 351 points. Plagioclase phenocryst and groundmass microlite compositions range continuously from An88Ab11Or01 to An05Ab40Or55 (Fig. 3). Stratigraphic, and thus temporal, compositional variation of plagioclase phenocrysts and microlites is notable (Fig. 4). In the lower part of unit A (0–200 m), phenocryst–microlite compositions decrease systematically from An80–42 to An48–10. In the 200–800 m interval (units B and C), compositions remain in a relatively uniform band of An80–45. In the upper unit D, the microlite compositions decrease to the most albitic compositions observed in the entire section (An8). The most evolved plagioclase compositions are found in units A and D, whereas the highest An contents are observed in unit B (OB93-186, An88).
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The most commonly observed texture in plagioclase phenocrysts is zoning (Fig. 5a and b), although sieve-textured and honeycomb-textured plagioclase phenocrysts are present in select samples (Fig. 5c and d). Phenocryst core compositions range from An50 to An88 (Figs 3 and 4); the only exceptions are sieve-textured phenocrysts where core compositions as low as An15–20 are found (e.g. OB93-173; Fig. 4). There is typically a shift of 
An = 15–20 between phenocryst and groundmass microlite compositions. Unzoned and reversely zoned phenocrysts are rarely observed, but do occur in samples where sieve-textured plagioclase is present. An extreme case of phenocryst–microlite disequilibrium is illustrated by sample OB93-111 with a An of 50 (Fig. 4). This sample is conspicuously banded in thin section and consists of thin bands of dark basaltic material, which contain relatively calcic plagioclase phenocrysts, and bands of clear, possibly trachytic, material, which contain ternary feldspar microlites. Throughout the Crozier section, plagioclase phenocryst compositions are distributed in three main compositional domains (Fig. 6a): high An (67–88), intermediate An (57–67), and low An (50–55). This general distribution can be found within individual phenocrysts as shown by the profile from sample OB93-136 (Fig. 6b—the same profile as shown in Fig. 5b). This profile reveals the presence of a small calcic core (An73–80), followed by an abrupt decrease in An (An = 12), a relatively thick zone of oscillatory zonation (An60–65), a small compositional increase near the margin (An = 7), and an abrupt drop in the rim (An = 21) to the microlite composition (<An55).
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Clinopyroxene
Although clinopyroxene is a minor phenocryst phase in the mildly alkalic basalts from the Crozier section (0–3 vol. %; Table 1), it was clearly the most important fractionating phase at depth and played a major role in controlling the geochemistry of the Crozier lavas (Damasceno et al., 1999, 2000). A total of 185 analyses were acquired on 96 clinopyroxene phenocrysts and microlites. Cations were calculated on a six-oxygen basis following the procedure of Lindsley (1983), where ferric iron was calculated from charge-balance considerations. Clinopyroxene phenocrysts occur mostly in unit C. Although the quadrilateral components in clinopyroxene phenocrysts show limited variation (Fig. 3b), typical of clinopyroxene compositions from alkalic basalts, the mg-number in phenocrysts and groundmass grains shows some important stratigraphic variations (Fig. 7). There is a systematic decrease in mg-number in unit A from 0·80 to 0·65, followed by an increasing trend in unit B from 0·65 to 0·85. Unit C shows a constant, but large compositional range (mg-number = 0·65–0·85). The upper unit D contains groundmass clinopyroxene with the lowest mg-number (<0·60). None of the observed clinopyroxene phenocrysts appear to be reversely zoned with respect to mg-number.
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More significant variations in clinopyroxene chemistry are evident when considering the non-quadrilateral components (Fig. 8). The clinopyroxene phenocrysts can be divided into two main groups based on Al contents and both types can be found within individual samples:
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- Al-rich augite with high Al2O3 (5·0–8·6 wt %), high AlVI/AlIV (0·35–0·70), high Na2O (0·40–0·85 wt %), and relatively high enstatite contents (En 52–56);
- Al-poor augite, characterized by relatively low Al2O3 (1·9–4·5 wt %), low AlVI/AlIV (0–0·28), low Na2O (0·40–0·60 wt %), and relatively low enstatite contents (En 40–52).
The stratigraphic variation of AlVI, which is sensitive to the pressure of crystallization, yields additional information and shows that AlVI-rich clinopyroxene phenocrysts occur predominantly in the upper part of the Crozier section (units C and D) (Fig. 7). The textures and zoning patterns of the two clinopyroxene phenocryst groups are also distinct. The Al-rich phenocrysts are either euhedral and strongly zoned, commonly displaying strong sector zoning (e.g. OB93-173), or rounded (resorbed) and zoned (e.g. OB93-114; Fig. 9a). The Al-poor phenocrysts are euhedral or subhedral and weakly zoned (Fig. 9b). Some Al-poor phenocrysts contain plagioclase, oxide, and melt inclusions. In several samples, glomerocrysts of rounded small clinopyroxene were observed (e.g. OB93-136).
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Olivine
As with clinopyroxene, olivine is also a minor phenocryst phase (0–6 vol. %). Many of the olivine phenocrysts in the Crozier basalts are partially to completely iddingsitized, reflecting the effects of low-temperature alteration, so that only 38 analyses were acquired on 27 phenocrysts and microlites from six samples. Most of the olivine phenocrysts are euhedral and slightly skeletal (Fig. 9c). Where present, zonation is normal and limited. The forsterite content of the majority of the olivine phenocrysts is in the range of Fo70–80 and most microphenocrysts and groundmass grains are in the range of Fo40–70 (Fig. 3b). The most Mg-rich olivine (Fo86) occurs in a partially resorbed olivine phenocryst from OB93-173. Sample OB93-136 also contains slightly resorbed olivine phenocrysts, but with lower Fo contents (Fo74). Strikingly euhedral olivine phenocrysts from a doleritic-textured sample (OB93-180) are also relatively Mg rich (Fo79).
Fe–Ti oxides
Euhedral Fe–Ti oxide microphenocrysts occur in >50% of the studied samples from the Crozier section, which is consistent with the low-MgO compositions of the Crozier lavas and the saturation in titanomagnetite or ilmenite in Fe–Ti-enriched basalts. No quantitative analyses of the Fe–Ti oxides were carried out in this study, but semi-quantitative EDS analyses of Fe–Ti oxide microphenocrysts revealed the presence of both titanomagnetite and ilmenite in each of the examined samples.
Amphibole
Amphibole occurs as reacted (destabilized) phenocrysts in two samples from the Crozier section: OB93-173, and OB93-133, a trachybasalt. Each of the reacted amphibole phenocrysts, which were probably kaersutitic, now consists of a black mixture of clinopyroxene and titanomagnetite (Fig. 9d). These phenocrysts evidently became unstable during ascent. The decomposition reaction may be similar to that proposed by Giret et al. (1980) for the destabilization of amphibole that crystallized from alkalic magmas in some of the intrusions found on the archipelago: amphibole + liquid1 = clinopyroxene + Fe–Ti oxide + liquid2 + H2O. Some amphibole phenocrysts enclose plagioclase and large, fibrous apatite crystals.
Apatite
Apatite is a minor phenocryst phase that is found in both the amphibole-bearing samples (OB93-133 and -173) and in four other relatively evolved samples (OB93-114, -174, -194 and -201). The apatite phenocrysts are relatively large (up to 3 mm) and are characterized by a fibrous texture and a light pink color in plane-polarized light. Some apatite grains occur as inclusions in plagioclase or amphibole phenocrysts.
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MINERAL–MELT EQUILIBRIA |
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A critical criterion to establish for the phenocrysts in the Crozier lavas is whether or not individual crystals were in equilibrium with their host rocks at the time of eruption. If mineral–melt equilibrium can be demonstrated then specific mineral–melt pairs can be used to constrain certain intensive parameters of crystallization (e.g. clinopyroxene–melt equilibria and pressure—see discussion to follow). The demonstration of disequilibrium between specific mineral–melt pairs yields equally important information, as disequilibrium phenocrysts indicate abrupt changes in intensive parameters during magma evolution. In the following discussion, we evaluate the Fe/Mg mineral–melt equilibria for the two major ferromagnesian phenocryst phases in the Crozier lavas: olivine and clinopyroxene. The whole-rock compositions used form the basis of a comprehensive overview of the geochemical and isotopic evolution of the Crozier basaltic section (Weis et al., in preparation). The whole-rock mg-number [mg-number = Mg2+/(Mg2+ + Fe2+)] is calculated assuming Fe3+/Fe2+ = 0·1, a value that corresponds to crystallization conditions in the region of the FMQ – 1 oxygen buffer (where FMQ is fayalite–magnetite–quartz).
Olivine–liquid equilibrium
The Fe/Mg exchange partition coefficient between olivine and basaltic liquid is well constrained by experiments (0·30 ± 0·03; Roeder & Emslie, 1970) and is constant for a wide range of conditions, except at pressures higher than 10 kbar (Ulmer, 1989) and for Fe-enriched, mg-number < 0·25, compositions (Toplis & Carroll, 1995). Mineral–melt equilibrium relations of this kind are best shown in a plot of whole-rock mg-number vs the forsterite content of olivine (e.g. Garcia, 1996), with the equilibrium field shown as the shaded band that extends across the diagram (Fig. 10a). Figure 10a shows the relationship between mg-number and the Fo content of olivine phenocrysts, microphenocrysts and groundmass grains for seven Crozier lavas. Core compositions for OB93-152, -133 and -180 fall within the equilibrium field. The two points from OB93-136 fall just below the equilibrium field, suggesting that true equilibrium core compositions were not analyzed. For the two samples that fall significantly below the equilibrium field, OB93-188 and -147, it is clear that the analyzed olivines are not phenocrysts, but late-crystallizing groundmass grains. The remaining sample, OB93-173, reveals more complex relations suggesting the presence of xenocryst olivine: one phenocryst core–rim pair plots within the equilibrium field, whereas a second core–rim pair is much more Mg rich (Fo84–86) and was probably inherited at depth by the alkalic magma during ascent. This Mg-rich olivine is also strongly resorbed, thus confirming an exotic origin.
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Clinopyroxene–liquid equilibrium
The Fe/Mg exchange partition coefficient between clinopyroxene and basaltic liquid is less well constrained than that for olivine–liquid, principally because of the presence of ferric iron in clinopyroxene and in the melt. Although there may be a slight compositional effect (Hoover & Irvine, 1977), a value of 0·23 ± 0·05 appears to be consistent with experimental results (Grove & Bryan, 1983; Toplis & Carroll, 1995). Figure 10b shows the mg-numberwhole rock–mg-numberclinopyroxene relations for clinopyroxene phenocrysts, microphenocrysts and groundmass grains for 16 samples from the Crozier section. The majority of the analyzed clinopyroxenes are characterized by mg-number values of 0·70–0·85. Some of the grains that were identified petrographically as groundmass are clearly equilibrium microphenocrysts (e.g. OB93-114, -197, -202, -204 and -180). Whereas nearly all of the samples contain equilibrium-composition clinopyroxene (except OB93-192, which has only groundmass grains), it is notable that about a third of the samples contain phenocrysts that are too Mg rich to be considered as equilibrium compositions. Most of these Mg-rich clinopyroxenes show textural evidence for resorption (e.g. Fig. 9a) suggesting that they crystallized before the equilibrium phenocrysts that occur in the same samples, perhaps at depth from more Mg-rich magmas. It should be noted that the mixed sample (OB93-111) contains two populations of clinopyroxene—an Mg-rich group (large grains) that probably crystallized from the more mafic magma and an equilibrium group (microphenocrysts) that probably crystallized from the more evolved magma that produced the groundmass ternary feldspars discussed above. No distinction can be made on the basis of Fe–Mg mineral–melt partitioning between the high-Al augites (OB93-111, -126 and -73) and the low-Al augites.
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PRESSURE AND TEMPERATURE CALCULATIONS |
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The primary goal of this study is to constrain the evolution of the magma conduit system that led to the formation of the mildly alkalic basaltic lavas in the Mont Crozier section on the Kerguelen Archipelago and to evaluate the role of the anomalously thick crust beneath the archipelago on magma ascent and differentiation. Thermobarometry based on phenocryst compositions in lavas can be used to determine at what depth magmas stalled and crystallized. There are a variety of methods to estimate the pressures and temperatures of crystallization of basaltic magmas, especially for MORB compositions (e.g. Weaver & Langmuir, 1990
; Danyushevsky et al., 1996; Yang et al., 1996). We have chosen to use two methods (Putirka et al., 1996; Nimis, 1999) that are both calibrated for mildly alkalic compositions and that explicitly use clinopyroxene phenocryst compositions.
Clinopyroxene–liquid geothermobarometry
Putirka et al. (1996) calibrated a series of thermodynamic expressions based on experimental work relating temperature and pressure to equilibrium constants that allowed for the construction of clinopyroxene–liquid thermobarometers. The calibrations are valid for a wide range of alkalinity variations in basaltic melts (tholeiites to ankaramites), for pressures from 8 to 30 kbar, and nominally anhydrous conditions. They have been used to constrain magma transport at Hawaii (Putirka, 1997) and the formulation has been extended to 100 kbar and 2350 K to examine the role of clinopyroxene during melt extraction (Putirka, 1999). We calculated clinopyroxene–melt equilibrium temperatures and pressures for the Crozier basaltic lavas using mineral compositions presented here and whole-rock compositions from Weis et al. (in preparation). We used only clinopyroxene core and rim compositions that fell well within the Fe–Mg equilibrium field of Fig. 10b, unless otherwise specified, and avoided samples with demonstrable mineral accumulation and evidence for magma mixing (i.e. OB93-111). The pyroxene components for the Crozier clinopyroxene phenocrysts were recalculated following the procedure of Putirka et al. (1996), which maximizes the jadeite component relative to the procedure of Lindsley (1983). We used models P1 and T2 from Putirka et al. (1996) to calculate pressure and temperature, respectively, and used an iterative technique to converge on final values (pressure and temperature are interrelated). Estimated errors for the pressure calculations are ±1·4 kbar (Putirka et al., 1996).
Clinopyroxene–liquid thermobarometry for samples from the Crozier section reveals that the magmas crystallized over a wide range of pressure, from 1 kbar to 11–12 kbar (Fig. 11a). The low-pressure temperature estimates are in accord with the low-MgO contents of the Crozier lavas, based on comparison with experimental studies of similar compositions (Juster et al., 1989), except for a few samples where the exchange thermometer may be overestimating equilibrium temperatures (i.e. OB93-136, -173 and -197). Clinopyroxene phenocrysts present in several samples (OB93-167 and -152) clearly crystallized at pressures equivalent to those of sub-volcanic magma chambers (1 kbar). The core compositions of OB93-133, -136 and -147 indicate mid-crustal equilibrium pressures (3 kbar). Core compositions for OB93-204 and OB93-126 provide even higher pressure estimates, 6 and 8·5 kbar, respectively. Finally, the sample OB93-173, which contains equilibrium clinopyroxene phenocrysts (Fig. 10a), yields an equilibrium pressure estimate of 11–12 kbar.
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Clinopyroxene structural geobarometry
An alternative approach to determining the pressures of crystallization of clinopyroxene-saturated basaltic rocks involves the crystal structural response of clinopyroxene to pressure changes (Nimis, 1995). Originally based on experiments from dry systems and tholeiitic to alkaline basalts, this geobarometer has been extended to hydrous conditions and to mildly alkalic compositions ranging from trachybasalts to trachyandesites (Nimis & Ulmer, 1998; Nimis, 1999), both of which are appropriate for the Crozier lavas. The advantage of the structural barometer is that it does not require knowledge of the melt composition and thus problems of disequilibrium can be avoided. However, this method does require an independent estimate of temperature as calculated pressures will rise by 1 kbar per 20°C underestimation of temperature and the uncertainty on calculated pressures is considered to be ±1·75 kbar (Nimis, 1999). We initially used the anhydrous pressure-independent temperature calibration (T1) of Putirka et al. (1996) for the temperature values, but given the acute temperature sensitivity of the structural barometer and the potential water contents of the Crozier magmas, we chose to employ the empirical thermometer for augite-saturated hydrous liquids of Sugawara (2000). The thermometer of Sugawara (2000), which relates the temperature of a hydrous augite-saturated liquid to SiO2, FeO, MgO and CaO, requires an estimation of H2O contents. We arbitrarily set H2O contents to equal whole-rock K2O contents based on the near 1:1 ratio of K2O to H2O observed for ocean island basalts (Wallace & Anderson, 2000) and the recent work by Wallace (2002) on tholeiitic to transitional basaltic glasses from ODP Site 1140 on the Northern Kerguelen Plateau, where H2O values of 0·7 wt % were determined for the most alkali-rich compositions.
The results of clinopyroxene structural barometry for lavas from the Crozier section (Fig. 11b) are similar to those determined by clinopyroxene–melt equilibria (Fig. 11a) and show a wide range in estimated pressures for core compositions from 1 to 11 kbar. Three distinct groups can be observed: a low-pressure group (1 kbar; OB93-136, -152, -167 and -204), an intermediate-pressure group (5 kbar; OB93-111 and -126) and a high-pressure group (10–11 kbar; OB93-133 and -173). It is not clear whether these groups are real or an artifact of the less well-constrained variables (i.e. H2O, temperature) in the calculation, but as with the previous calculations the relative differences are assumed to be significant. Although both methods of calculation give similar results for many of the samples, there are some notable differences. Relative to the mineral–melt equilibria method (Fig. 11a), the clinopyroxene structural barometer gives much lower estimated structural pressures for OB93-126, -136 and -204 and much higher values for OB93-133.
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CRUSTAL STRUCTURE AND PHENOCRYST CRYSTALLIZATION ENVIRONMENTS |
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 TOPABSTRACTINTRODUCTIONGEOLOGIC SETTING OF THE...THE MONT CROZIER BASALTIC...PETROGRAPHY AND MINERAL...MINERAL-MELT EQUILIBRIAPRESSURE AND TEMPERATURE... CRUSTAL STRUCTURE AND PHENOCRYST... CONCLUSIONSREFERENCES |
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The important variation in phenocryst abundances, textures, and compositions for mildly alkalic basaltic lavas from the Mont Crozier section on the Kerguelen Archipelago indicates that significant variations in intensive parameters, especially pressures of crystallization, occurred throughout the
1 Myr required to build up this section of nearly 1000 m height. These variations reflect the existence of distinct phenocryst crystallization environments and require the existence of numerous interlinked magma reservoirs. These reservoirs represent levels at which ascending magmas stalled and fractionated before renewed ascent and range from sub-volcanic magma chambers, to mid-crustal storage areas, to zones that lie within the mantle beneath the archipelago.
Seismic refraction studies across the Kerguelen Archipelago indicate that the underlying crust is entirely oceanic in origin (Recq et al., 1994; Charvis et al., 1995), i.e. there are no low-velocity zones that could be interpreted as representing continental material. This is in accord with numerous geochemical studies of lava sections on the archipelago that show no evidence for contamination of the basaltic magmas by continental crust (e.g. Weis et al., 1993; Yang et al., 1998; Frey et al., 2000; Weis et al., 2002). The Moho reaches a depth of 19–20 km in the central part of the archipelago and thins to 16 km under the Courbet Peninsula (Charvis et al., 1995). The oceanic crust beneath the Courbet Peninsula, as well as beneath the entire Northern Kerguelen Plateau, has been divided into three major layers (Charvis et al., 1995): (1) the upper crust, which is 8–9 km thick and has velocities similar to those of oceanic layer 2; (2) the lower crust, which is 6–7 km thick with velocities comparable with oceanic layer 3; (3) a high-velocity-gradient crust-to-mantle transition zone that has been interpreted as 2–3 km of underplated material (Recq et al., 1994; Grégoire et al., 1998). On the basis of the thermobarometric results presented above and the results of recent experiments (0–15 kbar) on a relatively high-MgO (5 wt %) sample (OB93-147) from the Crozier section (Lo Cascio et al., 2001), we propose that this unique crustal structure played an important role in determining where magmas stalled and fractionated on their way to the surface. The experimental work of Lo Cascio et al. (2001) shows that, under slightly hydrous conditions (1 wt % H2O), high-Al clinopyroxene is the major fractionating phase at pressures >5 kbar. Thus, in the Crozier magmatic system, most of the higher-pressure phenocryst phases were probably left behind to form deep-seated cumulates.
We envisage a complex conduit system beneath the Courbet Peninsula, which is shown in a simplified, schematic form in Fig. 12. The crustal structure is adapted from the seismic refraction profiles of Charvis et al. (1995), where layer 2 represents basaltic lava flows and sills and layer 3 represents cumulate gabbroic rocks. The highest pressures recorded in clinopyroxenes from the Crozier lavas are from OB73-173, which is isotopically similar to the other Crozier lavas (Weis et al., in preparation) and contains equilibrium high-Al clinopyroxene phenocrysts that crystallized at 11 kbar in the lithospheric mantle beneath the Kerguelen Archipelago (Fig. 11). Pressures in the range of 5–6 kbar were obtained from both geobarometric methods employed in this study and correspond to the depths near the base of layer 3 and the transition zone (Fig. 12). Only a few clinopyroxene phenocrysts were erupted that record these relatively high pressures of crystallization. However, both the observation that the geochemistry of the Crozier mildly basaltic magmas requires significant high-pressure fractionation of high-Al clinopyroxene (i.e. increasing Al2O3 and decreasing CaO for decreasing MgO) (Damasceno et al., 1999, 2000) and the experimental evidence cited above for the stability of high-Al clinopyroxene at relatively high pressures (Lo Cascio et al., 2001) strongly suggest that most of the Crozier magmas stalled and fractionated high-Al clinopyroxene near the base of the crust as a result of the relatively important density contrast between the underlying mantle and the overlying oceanic crust. Thus, high-Al clinopyroxene may be an important component of the seismic crust-to-mantle transition zone. High-pressure fractionation under hydrous conditions locally led to saturation in amphibole (kaersutite), which reacted with the surrounding melt during decompression and eruption. The preferential association of sieve-textured plagioclase in samples that contain reacted amphiboles suggests that dissolution of plagioclase also occurred during decompression and may also be related to volatile release during amphibole decomposition.
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The experimental work of Lo Cascio et al. (2001) also indicates that plagioclase is the major liquidus mineral at low pressures (<5 kbar). Thus, the majority of the plagioclase phenocrysts observed in the Crozier lavas probably formed under relatively low-pressure conditions in sub-volcanic magma reservoirs (Fig. 12). The low-pressure phenocryst assemblage in the lavas is characterized by euhedral plagioclase, clinopyroxene, olivine ± titanomagnetite and ilmenite depending on the degree of fractionation. Individual samples contain multiple populations of different phenocryst shapes and textures, particularly plagioclase (e.g. euhedral and unzoned, strongly zoned, sieve-textured). Simple euhedral, unzoned phenocrysts indicate equilibrium crystallization, whereas the strong oscillatory zoning as observed in some plagioclase phenocrysts (see Fig. 5b), and in some clinopyroxene phenocrysts, indicates that mineral growth locally occurred far from equilibrium, requiring diffusion-controlled feedback between the growing crystal and the adjacent melt (Shore & Fowler, 1996). The observed steady-state (i.e. limited compositional variation) oscillatory overgrowths in plagioclase are consistent with an early extended period of growth within a confined low-pressure magma reservoir (Stewart & Fowler, 2001), probably without turbulent convection (Singer et al., 1995). Local mixing of magmas in these high-level reservoirs yielded extreme compositions and strong disequilibrium textures and compositions such as those observed in OB93-111 (mixed basalt and trachyte).
Although clinopyroxene is inferred to be an important fractionating phase, along with plagioclase ± olivine ± Fe–Ti oxides, in the older 29–26 Ma tholeiitic–transitional basaltic lavas from the central and northwestern parts of the Kerguelen Archipelago (Yang et al., 1998), there is no evidence for an important role of high-pressure high-Al clinopyroxene fractionation in these magmas (Gautier, 1987). The transition from tholeiitic–transitional basalts to mildly alkalic basalts at 25 Ma observed in lavas from the Courbet Peninsula, including the Crozier section, and from the Southeast Province appears to involve both a decrease in the extent of melting combined with a significant decrease in magma supply and an increase in the depth of melting of the Kerguelen mantle plume source (Frey et al., 2000). These changes in melting conditions may reflect the influence of progressive lithospheric thickening beneath the Kerguelen Archipelago as the archipelago moved from a ridge-centered position along the Southeast Indian Ridge at 40 Ma to an intraplate setting during emplacement of the major lava sequences. As the ascent of mantle plumes is limited by the presence of overlying, relatively cold, viscous lithosphere (e.g. Ribe & Christensen, 1994; Leitch & Davies, 2001), the evidence for deeper melting, for reduced extents of decompression melting, and for an important role of high-pressure high-Al clinopyroxene fractionation in the mildly alkalic basaltic lavas from the Kerguelen Archipelago is probably related to changing mechanical properties of the lithosphere and to the gradual deepening of the relevant interfaces with time—the lithosphere–asthenosphere limit as a result of cooling for melting and the crust–upper mantle limit as a result of continued addition of basaltic lavas and cumulate gabbros in the crust for clinopyroxene fractionation.
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CONCLUSIONS |
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Phenocryst compositions and mineral–melt equilibria place important constraints on the evolution of mildly alkalic basaltic magmas that formed the 25 Ma Mont Crozier volcanic section, of nearly 1000 m height, on the Kerguelen Archipelago. Basaltic lavas from the Crozier section are mostly low-MgO basalts to trachybasalts, with a few evolved trachyandesites, that underwent significant fractionation in magma reservoirs during ascent. The transition from the older, 29–26 Ma, tholeiitic to transitional basalts from the northern part of the archipelago to the younger 25–24 Ma mildly alkalic basalts in the east and SE is marked by an increase in the amount of phenocrysts in the lavas, especially plagioclase, and the presence of both low-Al and high-Al clinopyroxene phenocrysts. Clinopyroxene–liquid thermobarometry and clinopyroxene structural barometry on the Crozier lavas indicate that magmas crystallized at pressures ranging from 1 to 12 kbar. The majority of the plagioclase phenocrysts formed under low-pressure conditions in sub-volcanic magma reservoirs where magma mixing and vapor saturation were locally important processes. Most of the high-pressure high-Al clinopyroxene phenocrysts were left behind to contribute to the high-velocity-gradient crust-to-mantle transition zone at the base of the crust. Progressive thickening of both the Northern Kerguelen Plateau by continued magmatic activity related to the Kerguelen mantle plume and of the lithosphere by cooling as the plateau moved from a ridge-centered position at
40 Ma to an intraplate position at 25 Ma were the controlling factors that produced increased alkalinity in the erupted basalts, significantly decreased magma supply rates, and stabilized high-pressure, high-Al clinopyroxene.
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ACKNOWLEDGEMENTS |
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We thank Olivier Brisse, Eric Frappa, Jean-Yves Cottin, and Nobu Shimizu for their contributions to the fieldwork and sampling of the Mont Crozier section during the 1993 Austral summer field campaign. Fieldwork on the Kerguelen Archipelago would not have been possible without the support and assistance of the French Institute for Polar Research and Technology (IFRTP). Discussions with Nick Arndt, Stephanie Ingle, and Mauro Lo Cascio helped us significantly improve the presentation of our ideas. The assistance of Nilanjan Chatterjee for the microprobe work at MIT, Michelle Veschambre for microprobe work at Clermont-Ferrand, and Chantal Perrache for the preparation of polished thin sections at St. Etienne is gratefully acknowledged. We thank Kirsten Nicolaysen for performing additional phenocryst analyses. We thank Keith Putirka for discussions and getting us started on calculating clinopyroxene–melt thermobarometry, and Paolo Nimis for sending us a copy of his CpxBar program. D.D. thanks the Belgian Grant FRIA for funding his dissertation research. This project was supported by funding from the Communauté Française in Belgium (ARC Convention 98/03-233). Finally, many thanks are due to Keith Putirka and Nick Arndt for their helpful reviews, and to Paul Wallace for efficient editorial handling of the manuscript.
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FOOTNOTES |
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*Corresponding author. Telephone: (322) 650-4714. Fax: (322) 650-3748. E-mail: jscoates{at}ulb.ac.be
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