Cumulate xenoliths from St. Vincent, Lesser Antilles Island Arc: a window into upper crustal differentiation of mantle-derived basalts
Cumulate xenoliths from St. Vincent, Lesser Antilles Island Arc: a window into upper crustal differentiation of mantle-derived basalts
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DOI：
10.1007/s00410-011-0665-9
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发表时间：
2012-02
影响因子：
3.5
通讯作者：
Peter Tollan;I. Bindeman;Jon D. Blundy
中科院分区：
文献类型：
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作者：
Peter Tollan;I. Bindeman;Jon D. Blundy
为了阐明俯冲带背景下幔源玄武岩浆的上地壳分异，我们测定了小安的列斯群岛圣文森特深成块体中单个积云矿物的矿物化学和氢氧同位素组成。深成岩石类型在矿物学上表现出很大的变化，从橄榄辉长岩到长角闪岩和角闪石，具有相应的各种堆晶结构。矿物成分不同于圣文森特爆发的玄武岩熔岩和已发表的圣文森特高镁玄武岩高压（4-10 kb）实验运行产品，具有较高的Anglagioclass与lowerFoolivine共存。堆晶橄榄石（4.89-5.18‰）、斜长石（5.84-6.28‰）、单斜辉石（5.17-5.47‰）和角闪石（5.48-5.61‰）的氧同位素组成（δ 18 O）以及角闪石的氢同位素组成（δD = −35.5-−49.9‰）均符合幔源玄武岩熔体的封闭岩浆分异作用。我们采用了一些建模练习，以限制报告的化学和同位素组成的起源。δ 18 O我们把这归因于同位素不平衡之间的积云矿物结晶在不同的温度下，与平衡延迟缓慢的氧气扩散橄榄石在长期的地壳存储。我们使用熔体包裹体和斜长石成分来确定母体岩浆水含量（水饱和，4.6 ± 0.5重量% H2O）和结晶压力（173 ± 50 MPa）。将这些值应用到以前报道的玄武质和玄武质安山岩熔岩成分中，我们可以重现积云斜长石和橄榄石成分及其相关趋势。我们的结论是原始含水玄武岩圣文森特涉及结晶的橄榄石和富铬尖晶石在地壳内的深度，降低MgO和Cr2 O3和提高Al 2 O3和CaO的残余熔体由于斜长石的抑制。低密度，含水玄武质和玄武安山岩熔体，然后迅速上升通过地壳，停滞在水饱和后，在本研究中观察到的化学性质不同的积云阶段结晶浅深度可以发生。沉积的晶体为浅岩浆房提供了装甲，在那里，矿物之间的氧同位素慢慢接近平衡，然后被后来的岩浆注入重新活化和夹带。
In order to shed light on upper crustal differentiation of mantle-derived basaltic magmas in a subduction zone setting, we have determined the mineral chemistry and oxygen and hydrogen isotope composition of individual cumulus minerals in plutonic blocks from St. Vincent, Lesser Antilles. Plutonic rock types display great variation in mineralogy, from olivine–gabbros to troctolites and hornblendites, with a corresponding variety of cumulate textures. Mineral compositions differ from those in erupted basaltic lavas from St. Vincent and in published high-pressure (4–10 kb) experimental run products of a St. Vincent high-Mg basalt in having higherAnplagioclase coexisting with lowerFoolivine. The oxygen isotope compositions (δ18O) of cumulus olivine (4.89–5.18‰), plagioclase (5.84–6.28‰), clinopyroxene (5.17–5.47‰) and hornblende (5.48–5.61‰) and hydrogen isotope composition of hornblende (δD = −35.5 to −49.9‰) are all consistent with closed system magmatic differentiation of a mantle-derived basaltic melt. We employed a number of modelling exercises to constrain the origin of the chemical and isotopic compositions reported. δ18OOlivineis up to 0.2‰ higher than modelled values for closed system fractional crystallisation of a primary melt. We attribute this to isotopic disequilibria between cumulus minerals crystallising at different temperatures, with equilibration retarded by slow oxygen diffusion in olivine during prolonged crustal storage. We used melt inclusion and plagioclase compositions to determine parental magmatic water contents (water saturated, 4.6 ± 0.5 wt% H2O) and crystallisation pressures (173 ± 50 MPa). Applying these values to previously reported basaltic and basaltic andesite lava compositions, we can reproduce the cumulus plagioclase and olivine compositions and their associated trend. We conclude that differentiation of primitive hydrous basalts on St. Vincent involves crystallisation of olivine and Cr-rich spinel at depth within the crust, lowering MgO and Cr2O3and raising Al2O3and CaO of residual melt due to suppression of plagioclase. Low density, hydrous basaltic and basaltic andesite melts then ascend rapidly through the crust, stalling at shallow depth upon water saturation where crystallisation of the chemically distinct cumulus phases observed in this study can occur. Deposited crystals armour the shallow magma chamber where oxygen isotope equilibration between minerals is slowly approached, before remobilisation and entrainment by later injections of magma.