美国伯里亚学院Birner, Suzanne K.小组近日取得一项新成果。他们的研究报道了由橄榄岩中的超低氧逸度记录的深、热和古熔融。相关论文于2024年7月24日发表于国际顶尖学术期刊《自然》杂志上。

据研究人员介绍，洋脊橄榄岩记录的对流上地幔氧逸度(f_O2)变化超过四个个数量级。尽管人们对不同构造背景下地幔f_O2变化的机制以及现代岩石与古老地幔衍生岩石之间的f_O2比较给予了大量关注，但对现代洋中脊背景下橄榄岩记录的f_O2高可变性的来源了解较少。

本文报道了Gakkel山脊和东太平洋海隆(EPR)的橄榄岩的岩石学和地球化学数据，包括16个新的f_O2高精度测定数据。Gakkel山脊的难熔橄榄岩记录的f_O2比地幔平均值低四个数量级以上。通过热力学和矿物分配模型，研究人员证明了超低f_O2的偏移可以由高电位温度(Tp)下的最大程度熔融产生，从石榴子石区开始，持续到尖晶石区——这些条件在古代科马提岩而不是现代玄武岩的形成过程中得到满足。

但这并不意味着太古代时期对流的上地幔具有比今天更低的三价铁和二价铁比率，也不意味着热点地区石榴石子区中的现代熔融产生减少的岩浆。相反，它意味着古老的、难熔的、超还原的地幔筏继续在现代地幔中循环，而对现代山脊火山作用的贡献很小。

附：英文原文

Title: Deep, hot, ancient melting recorded by ultralow oxygen fugacity in peridotites

Author: Birner, Suzanne K., Cottrell, Elizabeth, Davis, Fred A., Warren, Jessica M.

Issue&Volume: 2024-07-24

Abstract: The oxygen fugacity (f_O2) of convecting upper mantle recorded by ridge peridotites varies by more than four orders of magnitude. Although much attention has been given to mechanisms that drive variations in mantle f_O2 between tectonic settings and to comparisons of f_O2 between modern rocks and ancient-mantle-derived rocks, comparatively little has been done to understand the origins of the high variability in f_O2 recorded by peridotites from modern mid-ocean ridge settings. Here we report the petrography and geochemistry of peridotites from the Gakkel Ridge and East Pacific Rise (EPR), including 16 new high-precision determinations of f_O2. Refractory peridotites from the Gakkel Ridge record f_O2 more than four orders of magnitude below the mantle average. With thermodynamic and mineral partitioning modelling, we show that excursions to ultralow f_O2 can be produced by large degrees of melting at high potential temperature (Tp), beginning in the garnet field and continuing into the spinel field—conditions met during the generation of ancient komatiites but not modern basalts. This does not mean that ambient convecting upper mantle had a lower ferric to ferrous ratio in Archaean times than today nor that modern melting in the garnet field at hotspots produce reduced magmas. Instead, it implies that rafts of ancient, refractory, ultrareduced mantle continue to circulate in the modern mantle while contributing little to modern ridge volcanism.

DOI: 10.1038/s41586-024-07603-w

Source: https://www.nature.com/articles/s41586-024-07603-w