Mg isotope systematics during magmatic processes: Inter-mineral fractionation in mafic to ultramafic Hawaiian xenoliths
Mg isotope systematics during magmatic processes: Inter-mineral fractionation in mafic to ultramafic Hawaiian xenoliths
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DOI：
10.1016/j.gca.2018.02.002
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发表时间：
2018-04
影响因子：
5
通讯作者：
A. Stracke;E. Tipper;S. Klemme;M. Bizimis
中科院分区：
文献类型：
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作者：
A. Stracke;E. Tipper;S. Klemme;M. Bizimis
所观察到的块状岩浆岩之间的镁同位素比值的差异很小，往往在一个子每轧机的水平。深成岩中矿物间26 Mg/24 Mg比值（以δ 26 Mg表示）的差异具有相似的规模，并且主要归因于岩浆温度下的平衡同位素分馏。本文报道了夏威夷奥陶洛斯岛和考艾岛火山作用再生阶段尖晶石橄榄岩和石榴辉石岩捕虏体中矿物的Mg同位素数据。新的数据进行了比较，文献数据和理论预测，调查负责矿物间镁同位素分馏在岩浆温度的过程。理论预测，在岩浆温度下，橄榄石和尖晶石之间的δ 26 Mg的差异高达每密尔水平，并且Δ 26 Mgolivine-尖晶石（=δ 26 Mgolivine- δ 26 Mg尖晶石）随着温度的升高而普遍降低，但也随着尖晶石中Cr#的增加而降低。对于具有简单熔融亏损成岩历史的橄榄岩，其中熔融亏损的增加与熔融温度的增加有关，因此Δ 26 Mgolivine尖晶石应随尖晶石中Cr#的增加而系统地减少。然而，大多数天然橄榄岩，包括夏威夷尖晶石橄榄岩在这项研究中的调查，是由不同程度的熔融岩石反应，干扰系统的原生温度和成分相关的橄榄石尖晶石镁同位素系统叠加。扩散、亚固相线再平衡或表面蚀变可能进一步影响橄榄岩中观察到的橄榄石-尖晶石Mg同位素分馏，使Δ 26 Mgolivine-spinelin橄榄岩成为难以应用的地质温度计。单斜辉石和石榴子石的Mg同位素数据表明，这对矿物是一个更有前途的地质温度计，但它的应用仅限于含石榴子石的火成岩（石榴辉石岩）和变质岩（榴辉岩）。虽然在块状岩浆岩中观察到的δ 26 Mg变化在百万分之一的范围内，但矿物间Mg同位素的明显可分辨差异意味着，同位素不同的矿物（如石榴石、尖晶石和单斜辉石）的结晶或优先熔融应导致块状熔体和残渣之间的Mg同位素分馏。计算镁同位素的变化，在部分地幔熔融确实预测熔体和残留物之间的差异，但这些是分析解析熔融的镁铁质岩性，即石榴石辉石岩。因此，石榴石辉石岩熔体的贡献可能解释了在洋岛玄武岩中观察到的一些同位素轻δ 26 Mg和微量岩石地幔不均匀性。因此，高温镁同位素分馏的应用是有前途的和多样化的，最近在分析精度方面的进展可能允许充分利用岩浆岩中δ 26 Mg的百万分之一变化所固有的成岩潜力。
Observed differences in Mg isotope ratios between bulk magmatic rocks are small, often on a sub per mill level. Inter–mineral differences in the26Mg/24Mg ratio (expressed as δ26Mg) in plutonic rocks are on a similar scale, and have mostly been attributed to equilibrium isotope fractionation at magmatic temperatures. Here we report Mg isotope data on minerals in spinel peridotite and garnet pyroxenite xenoliths from the rejuvenated stage of volcanism on Oahu and Kauai, Hawaii. The new data are compared to literature data and to theoretical predictions to investigate the processes responsible for inter–mineral Mg isotope fractionation at magmatic temperatures. Theory predicts up to per mill level differences in δ26Mg between olivine and spinel at magmatic temperatures and a general decrease in Δ26Mgolivine-spinel(=δ26Mgolivine– δ26Mgspinel) with increasing temperature, but also with increasing Cr# in spinel. For peridotites with a simple petrogenetic history by melt depletion, where increasing depletion relates to increasing melting temperatures, Δ26Mgolivine-spinelshould thus systematically decrease with increasing Cr# in spinel. However, most natural peridotites, including the Hawaiian spinel peridotites investigated in this study, are overprinted by variable extents of melt-rock reaction, which disturb the systematic primary temperature and compositionally related olivine–spinel Mg isotope systematics. Diffusion, subsolidus re-equilibration, or surface alteration may further affect the observed olivine–spinel Mg isotope fractionation in peridotites, making Δ26Mgolivine-spinelin peridotites a difficult–to–apply geothermometer. The available Mg isotope data on clinopyroxene and garnet suggest that this mineral pair is a more promising geothermometer, but its application is restricted to garnet–bearing igneous (garnet pyroxenites) and metamorphic rocks (eclogites). Although the observed δ26Mg variation is on a sub per mill range in bulk magmatic rocks, the clearly resolvable inter–mineral Mg isotope differences imply that crystallization or preferential melting of isotopically distinct minerals such garnet, spinel, and clinopyroxene should cause Mg isotope fractionation between bulk melt and residue. Calculated Mg isotope variations during partial mantle melting indeed predict differences between melt and residue, but these are analytically resolvable only for melting of mafic lithologies, that is, garnet pyroxenites. Contributions from garnet pyroxenite melts may thus account for some of the isotopically light δ26Mg observed in ocean island basalts and trace lithological mantle heterogeneity. Consequently, applications for high-temperature Mg isotope fractionations are promising and diverse, and recent advances in analytical precision may allow the full petrogenetic potential inherent in the sub per mill variations in δ26Mg in magmatic rocks to be exploited.