Global variations in abyssal peridotite compositions
Global variations in abyssal peridotite compositions
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DOI：
10.1016/j.lithos.2015.12.023
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发表时间：
2016-04
期刊：
影响因子：
3.5
通讯作者：
J. Warren
中科院分区：
文献类型：
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作者：
J. Warren
深海橄榄岩是从大洋中脊收集的超镁铁质岩石，是绝热减压熔融的残留物。它们的成分提供了有关大洋岩石圈形成过程中熔融和熔融-岩石相互作用程度的信息，并对先前存在的地幔非均质性提供了限制。本文介绍了6个主要海脊系统53个地点1200多个样品的深海橄榄岩地球化学数据（模式、矿物主元素和单斜辉石微量元素）的汇编。根据成分和岩石学，橄榄岩可分为五个岩石组之一：（1）残留橄榄岩，（2）纯橄榄岩，（3）辉长脉和/或含斜长石的橄榄岩，（4）辉石岩脉橄榄岩，和（5）其他类型的熔融添加橄榄岩。几乎三分之一的深海橄榄岩是脉状的，这表明海洋岩石圈地幔平均比仅基于残余橄榄岩的估计更肥沃。所有脉体似乎都是在洋脊下的熔体输送过程中最近形成的，尽管有些辉石岩可能来自再循环洋壳的熔融。在缓慢和超缓慢的扩展速度，残留的深海橄榄岩定义了一个大的（0-15%模态单斜辉石和尖晶石Cr# = 0.1-0.6）的组成范围。这些变化与熔化程度应如何随铺展速率变化的预测不匹配。相反，残余橄榄岩的成分范围来自熔融、熔体-岩石相互作用和预先存在的成分变化的组合，其中熔体-岩石相互作用在这里用作通用术语，指的是在地幔中熔体输送期间可能发生的各种过程。在全球范围内，约10%的深海橄榄岩是难熔的（0%单斜辉石，尖晶石Cr# > 0.5，块状Al 2 O3 < 1重量%）部分脊段以方辉橄榄岩为主，但缺乏明显的玄武质地壳。因此，深海超镁铁质样品表明，地幔是多组分的，可能至少由三种组分（二辉橄榄岩，方辉橄榄岩，辉石岩）。总体而言，大的成分范围之间的残留和熔融添加橄榄岩意味着海洋岩石圈地幔是不均匀的，这将导致进一步的不均匀性的产生后俯冲回地幔。
Abyssal peridotites are ultramafic rocks collected from mid-ocean ridges that are the residues of adiabatic decompression melting. Their compositions provide information on the degree of melting and melt–rock interaction involved in the formation of oceanic lithosphere, as well as providing constraints on pre-existing mantle heterogeneities. This review presents a compilation of abyssal peridotite geochemical data (modes, mineral major elements, and clinopyroxene trace elements) for > 1200 samples from 53 localities on 6 major ridge systems. On the basis of composition and petrography, peridotites are classified into one of five lithological groups: (1) residual peridotite, (2) dunite, (3) gabbro-veined and/or plagioclase-bearing peridotite, (4) pyroxenite-veined peridotite, and (5) other types of melt-added peridotite. Almost a third of abyssal peridotites are veined, indicating that the oceanic lithospheric mantle is more fertile, on average, than estimates based on residual peridotites alone imply. All veins appear to have formed recently during melt transport beneath the ridge, though some pyroxenites may be derived from melting of recycled oceanic crust.A limited number of samples are available at intermediate and fast spreading rates, with samples from the East Pacific Rise indicating high degrees of melting. At slow and ultra-slow spreading rates, residual abyssal peridotites define a large (0–15% modal clinopyroxene and spinel Cr# = 0.1–0.6) compositional range. These variations do not match the prediction for how degree of melting should vary as a function of spreading rate. Instead, the compositional ranges of residual peridotites are derived from a combination of melting, melt–rock interaction and pre-existing compositional variability, where melt–rock interaction is used here as a general term to refer to the wide range of processes that can occur during melt transport in the mantle. Globally, ~ 10% of abyssal peridotites are refractory (0% clinopyroxene, spinel Cr# > 0.5, bulk Al2O3< 1 wt.%) and some ridge sections are dominated by harzburgites while lacking a significant basaltic crust. Abyssal ultramafic samples thus indicate that the mantle is multi-component, probably consisting of at least three components (lherzolite, harzburgite, and pyroxenite). Overall, the large compositional range among residual and melt-added peridotites implies that the oceanic lithospheric mantle is heterogeneous, which will lead to the generation of further heterogeneities upon subduction back into the mantle.