Osmium mass balance in peridotite and the effects of mantle-derived sulphides on basalt petrogenesis
Osmium mass balance in peridotite and the effects of mantle-derived sulphides on basalt petrogenesis
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DOI：
10.1016/j.gca.2011.07.001
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发表时间：
2011-10
影响因子：
5
通讯作者：
J. Harvey;C. Dale;A. Gannoun;K. Burton
中科院分区：
文献类型：
--
作者：
J. Harvey;C. Dale;A. Gannoun;K. Burton
富集地幔（EM）玄武岩的分析，使用亲石元素为基础的同位素系统，长期以来提供了证据的离散地幔油藏的可变组成。在部分熔融时，地幔储集层将其同位素指纹赋予所产生的部分熔融。然而，越来越多的人认识到，这可能是不简单的划定这些以前定义良好的地幔水库;“地幔动物园”可能包含更多的水库比以前设想的。在这里，我们证明了一个简单的模型，从两个人口的组成不同的地幔硫化物的不同贡献可以很容易地解释所观察到的不均匀性，在没有额外的地幔水库，这种玄武岩的Os同位素系统学。从美国新墨西哥州基尔本霍尔尖晶石二辉橄榄岩中分离出的单个硫化物颗粒的锇元素和同位素分析表明，从Re-Os系统学和与共存硅酸盐的结构关系两个方面来看，地幔硫化物存在两个离散的群体。一个人口，与圆形形态，被保存在硅酸盐颗粒，通常具有高[Os]和低[Re]与非放射性，通常亚南极洲187 Os/188 Os归因于长期隔离在低Re环境。相比之下，不规则形状的硫化物，保存在沿着硅酸盐晶界，具有低[Os]，高[Re]和更广泛的范围，但通常超奥氏体187 Os/188 Os（[Os]通常为1- 2 ppm，187 Os/188 Os为0.3729;这项研究）。这一群体被认为是交代硫化物。未受污染的硅酸盐相含有可忽略不计的Os（<100 ppt），因此玄武岩的Os元素和同位素组成主要是体积上不显著的硫化物（[Os] ≤ 37 ppm;本研究）。在部分熔融的早期阶段，超方解石间隙硫化物被动员并纳入熔体，增加其放射性187 Os/188 Os签名。只有当硅酸盐内的硫化物通过持续的部分熔融暴露于熔体时，封闭的硫化物才会将其高[Os]和非放射性的187 Os/188 Os加入到聚合熔体中。全岩铂族元素数据也与这种情况相一致。（i）所有交代硫化物的加入，然后（ii）少量铠装硫化物的加入，这一顺序可以解释全世界EM玄武岩的[Os]和187 Os/188 Os的范围，而不需要额外的硅酸盐地幔储层的贡献。
Analyses of enriched mantle (EM)-basalts, using lithophile element-based isotope systems, have long provided evidence for discrete mantle reservoirs with variable composition. Upon partial melting, the mantle reservoir imparts its isotopic fingerprint upon the partial melt produced. However, it has increasingly been recognised that it may not be simple to delimit these previously well-defined mantle reservoirs; the “mantle zoo” may contain more reservoirs than previously envisaged. Here we demonstrate that a simple model with varying contributions from two populations of compositionally distinct mantle sulphides can readily account for the observed heterogeneities in Os isotope systematics of such basalts without additional mantle reservoirs. Osmium elemental and isotopic analyses of individual sulphide grains separated from spinel lherzolites from Kilbourne Hole, New Mexico, USA demonstrate that two discrete populations of mantle sulphide exist in terms of both Re–Os systematics and textural relationship with co-existing silicates. One population, with a rounded morphology, is preserved in silicate grains and typically possesses high [Os] and low [Re] with unradiogenic, typically sub-chondritic187Os/188Os attributable to long term isolation in a low-Re environment. By contrast, irregular-shaped sulphides, preserved along silicate grain boundaries, possess low [Os], higher [Re] and a wider range of, but generally supra-chondritic187Os/188Os ([Os] typically⩽1–2ppm,187Os/188Os⩽0.3729; this study). This population is thought to represent metasomatic sulphide. Uncontaminated silicate phases contain negligible Os (<100ppt) therefore the Os elemental and isotope composition of basalts is dominated by volumetrically insignificant sulphide ([Os]⩽37ppm; this study). During the early stages of partial melting, supra-chondritic interstitial sulphides are mobilised and incorporated into the melt, adding their radiogenic187Os/188Os signature. Only when sulphides armoured within silicates are exposed to the melt through continued partial melting will enclosed sulphides add their high [Os] and unradiogenic187Os/188Os to the aggregate melt. Platinum-group element data for whole rocks are also consistent with this scenario. The sequence of (i) addition of all of the metasomatic sulphide, followed by (ii) the incorporation of small amounts of armoured sulphide can thus account for the range of both [Os] and187Os/188Os of EM-basalts worldwide without the need for contributions from additional silicate mantle reservoirs.