Partitioning of light lithophile elements during basalt eruptions on Earth and application to Martian shergottites
Partitioning of light lithophile elements during basalt eruptions on Earth and application to Martian shergottites
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地球玄武岩喷发过程中轻亲石元素的分配及其在火星六角辉石中的应用
DOI：
10.1016/j.epsl.2014.11.034
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发表时间：
2015
影响因子：
5.3
通讯作者：
Edmonds M
中科院分区：
文献类型：
--
作者：
Edmonds M
一个神秘的记录轻亲石元素（LLE）分带辉石在玄武岩shergottite陨石，其中LLE浓度急剧下降，从核心的边缘，已被解释为由于分配的LLE到含水蒸汽在岩浆上升到火星表面。这些趋势被用作火星玄武岩熔体富含水的证据（McSween等人，2001年）。锂和硼是轻亲石元素（LLE），它们可以分解成火山矿物和硅酸盐熔体的蒸汽，使它们成为岩浆上升到地球和其他行星表面期间脱气过程的潜在示踪剂。虽然LLE脱气行为是相对较好的理解富硅熔体，其中水和LLE浓度相对较高，非常少的数据存在的LLE丰度，异质性和玄武岩熔体脱气。数据的缺乏阻碍了对shergottite陨石趋势的解释。通过对夏威夷基拉韦厄火山中橄榄岩熔融包裹体的LLE、挥发分和微量元素的地球化学研究，可以证明锂在熔融、岩浆混合和分异过程中的行为与轻至中稀土元素相似。锂和硼的不均匀性是从幔源原始熔体中继承下来的，这种不均匀性对分馏和脱气信号起主导作用。锂和硼在基拉韦厄火山喷发的玄武岩熔体中只有很弱的挥发性，汽-熔体分配系数<0.1。在高温下进一步抑制LLE的脱气。辉石和相关的熔体包裹体LLE浓度从一系列的火山被用来量化锂辉石熔体分配系数，这与熔体H2O含量呈负相关，范围从0.13在低水含量到<0.08 at H2O contents >4重量%。观测到的陆地LLE分配行为外推到火星原始熔体通过建模。在shergottite辉石中观察到的分区是唯一一致的脱气LLE从火星熔体接近其液相线温度，如果汽-融分配系数是一个数量级大于在地球上观察到的。LLE和微量元素的范围内观察到的shergottite辉石，而不是一致的并发混合和分馏的异质熔体从地幔。
An enigmatic record of light lithophile element (LLE) zoning in pyroxenes in basaltic shergottite meteorites, whereby LLE concentrations decrease dramatically from the cores to the rims, has been interpreted as being due to partitioning of LLE into a hydrous vapor during magma ascent to the surface on Mars. These trends are used as evidence that Martian basaltic melts are water-rich (McSween et al., 2001). Lithium and boron are light lithophile elements (LLE) that partition into volcanic minerals and into vapor from silicate melts, making them potential tracers of degassing processes during magma ascent to the surface of Earth and of other planets. While LLE degassing behavior is relatively well understood for silica-rich melts, where water and LLE concentrations are relatively high, very little data exists for LLE abundance, heterogeneity and degassing in basaltic melts. The lack of data hampers interpretation of the trends in the shergottite meteorites. Through a geochemical study of LLE, volatile and trace elements in olivine-hosted melt inclusions from Kilauea Volcano, Hawaii, it can be demonstrated that lithium behaves similarly to the light to middle rare Earth elements during melting, magma mixing and fractionation. Considerable heterogeneity in lithium and boron is inherited from mantle-derived primary melts, which is dominant over the fractionation and degassing signal. Lithium and boron are only very weakly volatile in basaltic melt erupted from Kilauea Volcano, with vapor-melt partition coefficients <0.1. Degassing of LLE is further inhibited at high temperatures. Pyroxene and associated melt inclusion LLE concentrations from a range of volcanoes are used to quantify lithium pyroxene-melt partition coefficients, which correlate negatively with melt H2O content, ranging from 0.13 at low water contents to <0.08 at H2O contents >4 wt%. The observed terrestrial LLE partitioning behavior is extrapolated to Martian primitive melts through modeling. The zoning observed in the shergottite pyroxenes is only consistent with degassing of LLE from a Martian melt near its liquidus temperature if the vapor-melt partition coefficient was an order of magnitude larger than observed on Earth. The range in LLE and trace elements observed in shergottite pyroxenes are instead consistent with concurrent mixing and fractionation of heterogeneous melts from the mantle.
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