Geochemistry of ODP Hole 183-1137A basalts

The Kerguelen Plateau and Broken Ridge in the southern Indian Ocean together represent one of the most voluminous large igneous provinces (LIPs) ever emplaced on Earth. A scientific objective of Ocean Drilling Program (ODP) Leg 183 was to constrain the post-melting magma evolution of Kerguelen Plateau magmas. In an effort to better understand this evolution, isotopic and trace element analysis of individual plagioclase crystals hosted within two Kerguelen Plateau basalts recovered from Elan Bank were undertaken. Previous whole-rock studies established that the two host basalts investigated in this study are samples of crustally contaminated (lower group) and relatively uncontaminated (upper group) basalt.
Plagioclase phenocrysts from the uncontaminated basalt are dominantly normal zoned and exhibit a 87Sr/86SrI range of 0.704845-0.704985, which overlaps uncontaminated group whole-rock values previously reported. Plagioclase crystals from the contaminated basalt are dominantly reverse zoned and exhibit a 87Sr/86SrI range of 0.705510-0.705735, which all lie within contaminated group whole-rock values previously reported. There are no systematic within crystal core to rim variations in 87Sr/86SrI from either group, with the exception that contaminated group crystal rims have overall less radiogenic 87Sr/86SrI than other zones. These observations indicate that crustal assimilation occurred before the formation of Unit 10 plagioclase phenocrysts, which is supported by parent magma trace element abundance data inverted using carefully calculated partition coefficients. Trace element diffusion modeling indicates that the upper group basalt (Unit 4) experienced a more vigorous eruptive flux than the lower group basalt (Unit 10).
We suggest that plagioclase phenocrysts in both the upper and lower group basalts originated from the shallowest section of what was likely a complex magma chamber system. We contend that the magmatic system contained regions of extensive plagioclase-dominated crystal mush. Crustal assimilation was not a significant ongoing process in this portion of the Elan Bank magmatic system. Both basalts exhibit compelling evidence for remobilization and partial resorption of crystalline debris (e.g., reverse zoned crystals, glomerocrysts). We suggest Unit 4 and 10 magmas ascended different sections of the Elan Bank magma system, where the Unit 10 magmas ascended a section of the magma system that penetrated a stranded fragment of continental crust.

南印度洋的凯尔盖朗海台（Kerguelen Plateau）与破碎海岭（Broken Ridge）共同构成了地球上已知规模最为宏大的大火成岩省（large igneous provinces, LIPs）之一。大洋钻探计划（Ocean Drilling Program, ODP）第183航次的科学目标之一，是限定凯尔盖朗海台岩浆的熔融后演化过程。为深入阐明这一演化路径，研究团队对从埃兰浅滩（Elan Bank）钻取的两块凯尔盖朗海台玄武岩中的单个斜长石（plagioclase）晶体开展了同位素与微量元素分析。此前的全岩研究已明确，本研究涉及的两块寄主玄武岩分别代表受地壳混染的（下组）与相对未受混染的（上组）玄武岩样品。
未受混染的玄武岩中的斜长石斑晶（phenocryst）以正环带结构为主，其87Sr/86SrI比值范围为0.704845~0.704985，与此前报道的未受混染组全岩同位素比值范围重合。受混染的玄武岩中的斜长石晶体则以反环带结构为主，其87Sr/86SrI比值范围为0.705510~0.705735，全部落在此前报道的受混染组全岩同位素比值范围内。两组样品的斜长石晶体内部从核部到边部均未呈现系统性的87Sr/86SrI变化，仅受混染组的晶体边部整体比其他环带具有更低的放射成因87Sr/86SrI比值。上述观测结果表明，地壳同化作用（crustal assimilation）发生于10号单元斜长石斑晶形成之前，这一结论得到了通过精确计算的分配系数（partition coefficient）反演得到的母岩浆微量元素丰度数据的支持。微量元素扩散模拟结果显示，上组玄武岩（4号单元）的喷发通量比下组玄武岩（10号单元）更为强烈。
研究认为，两组玄武岩中的斜长石斑晶均形成于复杂岩浆房（magma chamber）系统的最浅部区域。研究同时提出，该岩浆系统中存在以斜长石为主的大规模晶粥（crystal mush）区域。在埃兰浅滩这套岩浆系统的该区域中，地壳同化作用并非持续进行的重要过程。两块玄武岩样品均显示出显著的晶体碎屑再活化与部分溶蚀证据（例如反环带晶体、晶簇（glomerocryst））。研究认为，4号与10号单元的岩浆沿埃兰浅滩岩浆系统的不同通道上升，其中10号单元的岩浆所经通道穿透了滞留的大陆地壳碎块。