Experimental Evidence for Polybaric Differentiation of Primitive Arc Basalt beneath St. Vincent, Lesser Antilles
Experimental Evidence for Polybaric Differentiation of Primitive Arc Basalt beneath St. Vincent, Lesser Antilles
复制标题
DOI：
10.1093/petrology/egu074
复制
发表时间：
2015
影响因子：
3.9
通讯作者：
E. Melekhova;J. Blundy;R. Robertson;M. Humphreys
中科院分区：
文献类型：
--
作者：
E. Melekhova;J. Blundy;R. Robertson;M. Humphreys
对圣文森特苏弗里埃原始高镁玄武岩（HMB）进行了平衡结晶实验，初始水含量为0.6，2.3，4.5wt%，压力为0.4，0.7，1.0，1.3GPa，温度为1350 ~ 950°C。通过对实验玻璃中Fe 3+的µXANES分析确定的氧化还原条件比镍-氧化镍（NNO）缓冲液高1-4个对数单位。这项研究的目的是探索的分化条件，引起了所观察到的地球化学变化的熔岩和深成（堆积）捕虏体从圣文森特。初始H2O含量为4.5wt%的实验在接近其液相线（1180°C和1.3GPa）时多次饱和，具有尖晶石二辉橄榄岩组合，这与地幔楔中HMB的主要起源一致。未观察到HMB与2.3wt% H2O的多重饱和，但推断在> 1.3GPa的压力下发生。实验结果表明，初始H2O含量对HMB岩浆的分异路径有重要影响，不同的岩浆类型是在离散的、严格约束的P-T-H2O条件下形成的。低镁玄武岩（LMB）可在1.0 - 1.3GPa压力下由HMB与2.3 - 4.5wt%的H2O生成，对应于圣文森特岛下莫霍面深度。LMB的CaO含量对分异压力敏感，当压力>0·5GPa时产生高CaO LMB。在0.6 - 2.3wt%H_2O条件下，HMB在0.7 - 1.0GPa压力下可生成玄武安山岩（BA）。高铝玄武岩（HAB）是在中地壳至上地壳（≤0·4GPa）条件下，通过斜长石结晶的延迟和橄榄石、单斜辉石和尖晶石的主要分馏作用，由具有高初始H2O（≥ 4wt%）的HMB分异而形成的。圣文森特安山岩只能在下地壳条件下从相对干燥（≤ 0.6wt%H2O）的HMB中产生。这表明部分熔融起源于前体HMB，其在深度处固化以产生具有约30%角闪石（即约0.6wt%结构结合H2O）的辉长岩。实验确定的分化条件是一致的，从莫霍面和上地幔延伸到中地壳或上地壳的热区内的多压分化。在热区内的分化发生的组合的结晶HMB与2-5重量%的H2O和部分熔融的祖先HMB辉长岩。虽然实验熔体提供了一个很好的匹配喷发的熔岩成分，实验晶体成分不匹配斑晶或堆积晶体，保存在捕虏体。未能再现自然晶体成分表明，这些形成的差异岩浆上升，并达到其H2O饱和liquidi在较浅的压力。因此，有一个断开之间的高压相组成和组合，产生液体成分的多样性和低压组合和组合，发生斑晶和堆积捕虏体。这一发现支持了弧岩浆生成过程中存在隐性分馏的观点。
Equilibrium crystallization experiments have been performed on a primitive high-MgO basalt (HMB) from Soufriere, St. Vincent, with three initial H2O contents (0·6, 2·3 and 4·5 wt %), at pressures of 0·4, 0·7, 1·0 and 1·3 GPa and temperatures from 1350 to 950°C. Redox conditions, as determined by µXANES analysis of Fe3+ in experimental glasses, were 1–4 log units above the nickel–nickel oxide (NNO) buffer. The aim of the study was to explore the differentiation conditions that gave rise to the observed geochemical variation in lavas and plutonic (cumulate) xenoliths from St. Vincent. An experiment with 4·5 wt % initial H2O is multiply saturated close to its liquidus (1180°C and 1·3 GPa) with a spinel lherzolite assemblage, which is consistent with a primary origin for HMB in the mantle wedge. Multiple saturation of HMB with 2·3 wt % H2O was not observed, but is inferred to occur at pressures >1·3 GPa. The experimental results show that initial H2O content has significant influence on differentiation paths of primary HMB magma, with different lava varieties generated under discrete, well-constrained P–T–H2O conditions. Low-magnesian basalts (LMB) can be generated from HMB with 2·3–4·5 wt % H2O at pressures of 1·0–1·3 GPa, corresponding to Moho depths beneath St. Vincent. The CaO contents of LMB are sensitive to differentiation pressure: high-CaO LMB are produced at pressures >0·5 GPa. Basaltic andesites (BA) can be generated at 0·7–1·0 GPa from HMB with 0·6–2·3 wt % H2O. High-alumina basalts (HAB) are produced at mid- to upper-crustal conditions (≤0·4 GPa) by differentiation of HMB with high initial H2O (≥4 wt %) through delay of plagioclase crystallization and dominant fractionation of olivine, clinopyroxene and spinel. St. Vincent andesites could be produced from relatively dry (≤0·6 wt % H2O) HMB only at lower-crustal conditions. This is suggestive of a partial melting origin from precursor HMB that had solidified at depth to produce gabbros with ∼30% hornblende (i.e. ∼0·6 wt % structurally bound H2O). The experimentally determined differentiation conditions are consistent with polybaric differentiation within a hot zone that extends from the Moho and uppermost mantle to the mid- or upper crust. Within the hot zone differentiation occurs by a combination of crystallization of HMB with 2–5 wt % H2O and partial melting of ancestral HMB gabbros. Although the experimental melts provide an excellent match to erupted lava compositions, experimental crystal compositions do not match either phenocrysts or cumulate crystals, as preserved in xenoliths. The failure to reproduce natural crystal compositions suggests that these are formed as differentiated magmas ascend and attain their H2O-saturated liquidi at shallower pressures. Thus there is a disconnect between the high-pressure phase compositions and assemblages that generate liquid compositional diversity and the low-pressure composition and assemblages that occur as phenocrysts and in cumulate xenoliths. This finding lends support to the idea of cryptic fractionation in the generation of arc magmas.