Origin and differentiation of picritic arc magmas, Ambae (Aoba), Vanuatu
Origin and differentiation of picritic arc magmas, Ambae (Aoba), Vanuatu
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DOI：
10.1007/bf00307867
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发表时间：
1993-05
影响因子：
3.5
通讯作者：
S. Eggins
中科院分区：
文献类型：
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作者：
S. Eggins
俯冲带岩浆生成和岩浆演化的关键方面是解决在瓦努阿图的Ambassador（Aoba）火山的研究。两个主要的熔岩套房（低钛套房和高钛套房）的斑晶矿物学，地球化学和地层学的基础上确认。在较原始的低钛岩套中，斑晶组合以镁橄榄石（mg = 80 ~ 93.4）和单斜辉石（mg = 80 ~ 92）为主，并包括副富铬尖晶石（mg = 50 ~ 84）。钙斜长石和钛磁铁矿是高钛套岩和最演化的低钛套岩中重要的附加斑晶相。低钛套熔岩跨越连续的成分范围，从苦橄质（高达20%重量MgO）的高铝玄武岩（<5%重量MgO），并与分化涉及观察到的斑晶是一致的。熔体组合物（无定形熔岩和基质）在低钛套件形成的液体下降线对应于岩相学确定的结晶顺序：橄榄石+铬尖晶石，其次是单斜辉石+橄榄石+钛磁铁矿，然后斜长石+单斜辉石+橄榄石+钛磁铁矿。一个初级熔体的低钛套件已被估计通过校正最镁熔体组合物（一个无定形熔岩与10.5重量% MgO）的晶体分馏，在氧化条件下确定的橄榄石尖晶石对（fo 2 FMQ + 2.5日志单位），直到在平衡与最镁橄榄石斑晶。所得组合物具有15wt%的MgO和15wt%的MgFe 2值。它需要橄榄岩地幔楔在与对流上地幔（Tp = 1300°C）的当前估计一致的潜在温度下进行深（1300 GPa）熔融。至少有三个地球化学上不同的源组件是必要的，占地球化学差异，地球化学异质性内，主要的熔岩套房。两个组分，一个是富含LILE的，另一个是富含LILE和LREE的，可能都来自俯冲洋壳，可能分别是水流体和硅酸盐熔体。第三种成分是由于橄榄岩地幔楔的熔融程度不同，或不相容元素亏损的程度不同。
Key aspects of magma generation and magma evolution in subduction zones are addressed in a study of Ambae (Aoba) volcano, Vanuatu. Two major lava suites (a low-Ti suite and high-Ti suite) are recognised on the basis of phenocryst mineralogy, geochemistry, and stratigraphy. Phenocryst assemblages in the more primitive low-Ti suite are dominated by magnesian olivine (mg∼80 to 93.4) and clinopyroxene (mg∼80 to 92), and include accessory Cr-rich spinel (cr∼50 to 84). Calcic plagioclase and titanomagnetite are important additional phenocryst phases in the high-Ti suite lavas and the most evolved low-Ti suite lavas. The low-Ti suite lavas span a continuous compositional range, from picritic (up to ∼20 wt% MgO) to high-alumina basalts (<5 wt% MgO), and are consistent with differentiation involving observed phenocrysts. Melt compositions (aphyric lavas and groundmasses) in the low-Ti suite form a liquid-line of descent which corresponds with the petrographically-determined order of crystallisation: olivine + Cr-spinel, followed by clinopyroxene + olivine + titanomagnetite, and then plagioclase + clinopyroxene + olivine + titanomagnetite. A primary melt for the low-Ti suite has been estimated by correcting the most magnesian melt composition (an aphyric lava with ∼10.5 wt% MgO) for crystal fractionation, at the oxidising conditions determined from olivine-spinel pairs (fo2∼FMQ + 2.5 log units), until in equilibrium with the most magnesian olivine phenocrysts. The resultant composition has ∼15 wt% MgO and anmgFe2value of ∼81. It requires deep (∼3 GPa) melting of the peridotitic mantle wedge at a potential temperature consistent with current estimates for the convecting upper mantle (Tp∼1300°C). At least three geochemically-distinct source components are necessary to account for geochemical differences between, and geochemical heterogeneity within, the major lava suites. Two components, one LILE-rich and the other LILE- and LREE-rich, may both derive from the subducting ocean crust, possibly as an aqueous fluid and a silicate melt respeetively. A third component is attributed to either differnt degrees of melting, or extents of incompatible-element depletion, of the peridotitic mantle wedge.