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首页> 外文期刊>Contributions to Mineralogy and Petrology >Evidence for adiabatic decompression melting in the Southern Mariana Arc from high-Mg lavas and melt inclusions
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Evidence for adiabatic decompression melting in the Southern Mariana Arc from high-Mg lavas and melt inclusions

机译：高镁熔岩和熔体包裹体在南马里亚纳弧上绝热减压融化的证据

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摘要

Unusually magnesian (Mg# ∼76) basalts have been sampled from a small submarine volcano situated on the Mariana arc magmatic front. Total alkalis range from 1.7 to 1.94%, Al2O3 from 9.09 to 10.3% and CaO from 13.9 to 14.09%. These lavas can be classified based on mineralogy as picrite and ankaramite. Olivine-hosted melt inclusions (MIs) have median MgO contents of 17.17–17.86 wt%, 0.35–0.5% TiO2, 42–50% SiO2 and 1.66–3.43% total alkalis, which suggest that the parental magmas were primitive mantle melts. Trace element concentrations for both MIs and lavas are arc-like, although more depleted than most arc lavas. Chlorine (182–334 ppm) and H2O contents (0.11–0.64 wt%) in the MIs are consistent with the estimated median oxygen fugacities (log ΔFMQ of + 1.53–1.66) which lie at the low end of the range estimates for arc basalts and picrites (ΔFMQ = + 1 to + 3). Isotopic compositions of Sr, Nd, Hf and Pb are similar to those of other Mariana arc lavas and indicate derivation from an Indian Ocean mantle domain. The averaged magmatic temperature estimate from several geothermometers was 1,367°C at 1–1.5 GPa. We propose that high-Mg magmagenesis in this region results from the adiabatic decompression melting of relatively anhydrous but metasomatized mantle wedge. This melting is attributed to enhanced upwelling related to unusual tectonics on the over-riding plate related to a tear or other discontinuity on the subducted slab.
机译：异常的镁质玄武岩（Mg＃〜76）是从位于马里亚纳弧岩浆前缘的小型海底火山中取样的。总碱度为1.7％至1.94％，Al2 O3 为9.09％至10.3％，CaO为13.9％至14.09％。可以根据矿物学将这些熔岩分类为苦味岩和玄武岩。橄榄石型熔体夹杂物的MgO含量中位数为17.17–17.86 wt％，TiO2为0.35–0.5％，SiO2为42–50％和总碱值为1.66–3.43％。父母的岩浆是原始的地幔融化。 MIs和熔岩的痕量元素浓度都呈弧形，尽管比大多数弧形熔岩更容易消耗。 MI中的氯（182–334 ppm）和H2O（0.11–0.64 wt％）含量与估计的中值氧逸度（logΔFMQ为+ 1.53–1.66）一致，位于低端。弧形玄武岩和微尘的范围估计（ΔFMQ= + 1至+ 3）。 Sr，Nd，Hf和Pb的同位素组成与其他马里亚纳弧熔岩的同位素组成相似，表明其源于印度洋地幔区域。几个地热仪的平均岩浆温度估计值在1–1.5 GPa时为1,367°C。我们提出，该区域的高镁成岩作用是由相对无水但交代的地幔楔的绝热减压融化引起的。这种熔化归因于与上覆板块上异常的构造有关的上升流，该异常构造与俯冲板片上的裂缝或其他不连续性有关。

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