Experimental investigation of the composition of incipient melts in upper mantle peridotites in the presence of CO2 and H2O
Experimental investigation of the composition of incipient melts in upper mantle peridotites in the presence of CO2 and H2O
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DOI：
10.1016/j.lithos.2021.106224
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发表时间：
2021-09
期刊：
影响因子：
3.5
通讯作者：
Z. Pintér;S. Foley;G. Yaxley;A. Rosenthal;R. Rapp;A. Lanati;T. Rushmer
中科院分区：
文献类型：
--
作者：
Z. Pintér;S. Foley;G. Yaxley;A. Rosenthal;R. Rapp;A. Lanati;T. Rushmer
地幔岩浆的组成表明，上地幔中挥发分的丰度有很大的变化。CO2和H2O显著降低了地幔橄榄岩的熔点，形成了早期熔融的压温区，在大熔融开始前的较大温度范围(~300℃)内存在少量熔体。然而，由于淬火过程中普遍存在的亚稳定相形成的分析不确定性，这些熔体在高压实验中的化学表征在低熔体分数(熔体腔可能仅占据10-50μm~3的体积)时是具有挑战性的。这项系统的部分熔融研究呈现了在2.5GPa2.5-7 GPaCO2+H2O存在的情况下仔细确定的一系列橄榄岩的初始熔体的组成。使用了四种不同的富饶橄榄岩和贫化橄榄岩：夏威夷火成岩、富含K2O的火成岩、MORB火成岩和贫化二辉橄榄岩。为了获得准确的熔体组成，我们引入了熔体层析方法，该方法集成了多个区域扫描的熔体口袋，抛光到几个深度。结果表明，在2.5 GPa时，初始熔体和低度熔体从碳酸盐岩突然(在25℃以内)演化为黄泥质-霞石岩组分，而在4-5 GPa时则从富碳酸盐逐渐演化为碳酸盐硅酸盐(斜长岩)组分。在近固相线条件下，熔体组成主要受碳酸盐和水合相(如辉石和金云母)的分解控制，在给定的熔体分数下，随着压力的增加，熔体成分变得较少硅质，而略有较多的镁质。随着温度的升高，熔体中的SiO_2含量显著增加(2.75~44wt%)，而TiO_2、Na_2O和K_2O含量降低。富挥发、初始和低度熔体普遍具有较强的钾质(K2O≤6.63wt%)和钠质(Na2O≤3.06wt%)特征，表明这些熔体可能作为活动性交代剂，可能输送大量能量并在大量上地幔中引发化学变化。
The compositions of mantle-derived magmas indicate a substantial variety in the abundances of volatiles in the upper mantle. CO2and H2O depress the melting point of mantle peridotites considerably, delineating a pressure-temperature region of incipient melting where small degrees of melt exist over a large temperature range (~300 °C) before major melting begins. However, the chemical characterization of these melts in high-pressure experiments is challenging at low melt fractions (melt pockets may occupy volumes of only 10–50 μm3) because of analytical uncertainties related to the ubiquitous formation of metastable phases during quenching.This systematic partial melting study presents carefully determined compositions of incipient melts of a range of peridotites in the presence of CO2+ H2O mixtures at 2.5 to 7 GPa. Four different fertile and depleted peridotites were used: Hawaiian pyrolite, K2O-enriched pyrolite, MORB pyrolite and depleted lherzolite. To arrive at accurate melt compositions, we introduce themelt tomographymethod that integrates multiple area scans of melt pockets polished to several depths. Results confirm that incipient and low degree melts progress abruptly (within 25 °C) from carbonatitic towards melilititic-nephelinitic compositions at 2.5 GPa, whereas they progress gradually from carbonate-rich to carbonated silicate (aillikitic) compositions at 4–5 GPa. Melt compositions at near-solidus conditions are mainly controlled by the breakdown of carbonate, and hydrous phases such as pargasite and phlogopite, and become less siliceous and slightly more magnesian with increasing pressure at given melt fractions. Melts exhibit strong increases in SiO2(2.75 to 44 wt%) with increasing temperature, whereas TiO2, Na2O and K2O decrease. The generally strongly potassic (K2O ≤ 6.63 wt%) and sodic (Na2O ≤ 3.06 wt%) character of the volatile-rich, incipient and low-degree melts indicate that these would act as reactive metasomatic agents that may transport large amounts of energy and induce chemical changes in large volumes of the upper mantle.