Oxidised phase relations of a primitive basalt from Grenada, Lesser Antilles
Oxidised phase relations of a primitive basalt from Grenada, Lesser Antilles
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DOI：
10.1007/s00410-013-0954-6
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发表时间：
2014-01
影响因子：
3.5
通讯作者：
C. C. Stamper-C.;E. Melekhova;Jon D. Blundy;R. Arculus;M. Humphreys;R. Brooker
中科院分区：
文献类型：
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作者：
C. C. Stamper-C.;E. Melekhova;Jon D. Blundy;R. Arculus;M. Humphreys;R. Brooker
本文报道了小安的列斯群岛格林纳达的一种原始弧玄武岩（MgO 15.4 wt %, SiO2 45.5 wt %）在1 atm ~ 1.7 GPa的无水、欠饱和和饱和条件下的液体密度测定。μXANES测量的高压实验玻璃的氧逸度范围为0.44 ~ 0.86，对应于镍-镍氧化还原缓冲液（NNO）上方的氧逸度（fO2）在3.2 ~ 7.8 log单位之间。在NNO−2.5 ~ + 3.8范围内进行的1-atm实验表明，增加fo2主要增加橄榄石的forsterite含量（Fo），对相关系影响不大。在较低的地壳压力下，所有含水量的结晶顺序为：橄榄石+富铬尖晶石+斜辉石。在2.9 wt % H2O和3.8 wt % H2O的存在下，无水液相分别降低100℃和120℃。在NNO + 3.2 ~ + 4.5的欠饱和条件下，实验得到的橄榄石成分与格林纳达苦荞岩中最原始的橄榄石斑晶相当（Fo91.4）。我们得出结论，源自格林纳达地下的直接地幔熔体可能被氧化为~NNO + 3，这与高镁玄武岩的橄榄尖晶石氧压测定的最高估计一致。μXANES对含橄榄石的实验玻璃进行了分析，根据假设所有铁都处于Fe2+状态时的值（），建立了半经验氧气压计。氧气晴雨表在独立的数据集上进行测试，能够将实验fo2复制到≤1.2 log单位。实验还表明，岛上具有地球化学和岩石学特征的M系列和c系列熔岩可能是由一种共同的苦荞质源的水熔融形成的。低压扩大了橄榄石稳定场，以牺牲斜辉石为代价，丰富了CaO中不断演变的熔体，并迫使分化沿着c系列液体下降线进行。较高的压力条件允许早期和丰富的斜辉石结晶，迅速耗尽CaO和MgO中的熔体，从而形成m系列。
A series of liquidus determinations is reported for a primitive arc basalt (15.4 wt % MgO, 45.5 wt % SiO2) from Grenada, Lesser Antilles, at anhydrous, H2O-undersaturated and H2O-saturated conditions in the pressure range 1 atm to 1.7 GPa.of high-pressure experimental glasses as measured by μXANES ranges from 0.44 to 0.86, corresponding to oxygen fugacities (fO2) between 3.2 and 7.8 log units above the nickel–nickel oxide redox buffer (NNO). 1-atm experiments conducted from NNO − 2.5 to + 3.8 show that increasingfO2mainly increases the forsterite content (Fo) of olivine and has little effect on phase relations. The crystallisation sequence at lower crustal pressures for all water contents is forsteritic olivine + Cr-rich spinel followed by clinopyroxene. The anhydrous liquidus is depressed by 100 and 120 °C in the presence of 2.9 and 3.8 wt % H2O, respectively. H2O-undersaturated experiments at NNO + 3.2 to + 4.5 produce olivine of equivalent composition to the most primitive olivine phenocrysts in Grenadan picrites (Fo91.4). We conclude that direct mantle melts originating beneath Grenada could be as oxidised as ~NNO + 3, consistent with the uppermost estimates from olivine–spinel oxybarometry of high Mg basalts. μXANES analyses of olivine-bearing experimental glasses are used to develop a semi-empirical oxybarometer based on the value ofwhen all Fe is assumed to be in the Fe2+state (). The oxybarometer is tested on an independent data set and is able to reproduce experimentalfO2to ≤1.2 log units. Experiments also show that the geochemically and petrographically distinct M- and C-series lavas on the island can be produced from hydrous melting of a common picritic source. Low pressures expand the olivine stability field at the expense of clinopyroxene, enriching an evolving melt in CaO and forcing differentiation to take place along a C-series liquid line of descent. Higher pressure conditions allow early and abundant clinopyroxene crystallisation, rapidly depleting the melt in both CaO and MgO, and thus creating the M-series.