The effects of composition and temperature on chalcophile and lithophile element partitioning into magmatic sulphides
The effects of composition and temperature on chalcophile and lithophile element partitioning into magmatic sulphides
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DOI：
10.1016/j.epsl.2015.05.012
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发表时间：
2015-08-15
影响因子：
5.3
通讯作者：
Wood, Bernard J.
中科院分区：
文献类型：
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作者：
Kiseeva, Ekaterina S.;Wood, Bernard J.
我们开发了一个全面的模型来描述硫化物液体和无水硅酸盐液体的玄武岩组成的微量元素和微量元素之间的分配。因此，我们能够完全解释温度和硫化物组成对Ag、Cd、Co、Cr、Cu、Ga、Ge、In、Mn、Ni、Pb、Sb、Ti、Tl、V和Zn的分配的影响。该模型是从分区实验中进行的活塞缸装置在1.5 GPa和1300至1700摄氏度的硫化物组合物覆盖的四元FeS-NiS-CuS0.5-FeO。大多数元素的分区是一个很强的功能的氧（或FeO）含量的硫化物。这随硅酸盐熔体的FeO含量线性增加，随硫化物的Ni含量降低。正如预期的那样，随着硫化物氧含量的增加，亲石元素更强烈地分配到硫化物中，而随着氧含量的增加，亲硫元素更不容易进入硫化物中。我们参数化的影响，通过使用8模型的非理想相互作用在金属液体。由此得到的元素M在硫化物和硅酸盐液体之间的分配系数方程可以表示为logD（M）（硫/硅）= A + B/T-n/2 log [FeO]（corr）+1673/T [Mn（FeO）（MSn/2）log（1-x（FeO））+NiS（MSn/2）log（1-x（NiS））+NiS（CuS0.5）（MSn/2）log（1-x（CuS 0.5））]其中A是与分配反应的熵变相关的常数，B是与其焓相关的常数，n是感兴趣的元素的化合价。相互作用参数α（FeO）、β（NiS）和β（NiS）（CuS 0.5）是指微量元素和基质之间的非理想相互作用，xFeO、xNis、xcusos和xFes是硫化物中FeO、NiS和CuS 0.5的摩尔分数，FeOcorr是硅酸盐液体的FeO含量（wt %），其针对硫化物中FeS的理想活性校正如下：[FeO]（校正）=[FeO]（硅酸盐）/[Fe/（Fe + Ni + Cu）]（硫化物）我们发现，对于大多数元素，Ni和Cu对分配的影响明显小于氧的影响。温度对Ni、Cu和Ag的影响最大。我们用我们的模型计算了MORB熔体下降液线上沿着析出的硫化物量，发现70%的硅酸盐结晶伴随着0.23%的硫化物析出。后者足以控制熔体中Cu、Ag和Pb等亲铜元素的浓度。我们的分配系数和观察到的亲硫元素浓度在MORB玻璃被用来估计硫在MORB液体中的溶解度。我们获得了类似于800 ppm（原始MORB）和类似于2000 ppm（进化MORB）之间，与实验推导的模型合理的协议值。实验数据也使我们能够重新考虑MORB中的Ce/Pb和Nd/Pb比值。我们发现，在亏损地幔10%熔融所产生的岩浆的分离结晶过程中，只要后者含有> 100ppm的S和约650ppm的Ce，550ppm的Nd和27.5ppb的Pb，就可以获得恒定的Ce/Pb和Nd/Pb比值，分别为25和20。我们研究了地幔中亲铜元素丰度的模式是由晚期硫化物冰铜的分凝建立的假设。以Cu、Ag、Pb和Zn为例，我们发现地幔的Pb/Zn和Cu/Ag比值原则上可以用0.4%的硫化物冰铜向核部的分凝来解释。(C)2015作者由爱思唯尔公司出版
We develop a comprehensive model to describe trace and minor element partitioning between sulphide liquids and anhydrous silicate liquids of approximately basaltic composition. We are able thereby to account completely for the effects of temperature and sulphide composition on the partitioning of Ag, Cd, Co, Cr, Cu, Ga, Ge, In, Mn, Ni, Pb, Sb, Ti, Tl, V and Zn. The model was developed from partitioning experiments performed In a piston-cylinder apparatus at 1.5 GPa and 1300 to 1700 degrees C with sulphide compositions covering the quaternary FeS-NiS-CuS0.5-FeO.Partitioning of most elements is a strong function of the oxygen (or FeO) content of the sulphide. This increases linearly with the FeO content of the silicate melt and decreases with Ni content of the sulphide. As expected, lithophile elements partition more strongly into sulphide as its oxygen content increases, while chalcophile elements enter sulphide less readily with increasing oxygen. We parameterised the effects by using the 8-model of non-ideal interactions in metallic liquids. The resulting equation for partition coefficient of an element M between sulphide and silicate liquids can be expressed aslogD(M)(sulph/sil) = A + B/T - n/2 log[FeO](corr) + 1673/T [epsilon(FeO)(MSn/2) log(1 - x(FeO))+ epsilon(NiS)(MSn/2) log(1 - x(NiS)) + epsilon(CuS0.5)(MSn/2) log(1 - x(CuS0.5))]where A is a constant related to the entropy change of the partitioning reaction, B is a constant related to its enthalpy and n is the valency of the element of interest. Interaction parameters epsilon(FeO), epsilon(NiS) and epsilon(CuS0.5) refer to non-ideal interactions between trace element and matrix, xFeo, xNis, xcusos and xFes are mole fractions of FeO, NiS, and CuS0.5 in sulphide and FeOcorr is FeO content of silicate liquid (wt%) corrected for the ideal activity of FeS in the sulphide as follows:[FeO](corrected) = [FeO](silicate)/[Fe/(Fe + Ni + Cu)](sulph)We find, for most elements, that the effect of Ni and Cu on partitioning is significantly smaller than the effect of oxygen. The effects of temperature are greatest for Ni, Cu and Ag.We used our model to calculate the amount of sulphide liquid precipitated along the liquid line of descent of MORB melts and find that 70% of silicate crystallisation is accompanied by similar to 0.23% of sulphide precipitation. The latter is sufficient to control the melt concentrations of chalcophile elements such as Cu, Ag and Pb. Our partition coefficients and observed chalcophile element concentrations in MORB glasses were used to estimate sulphur solubility in MORB liquids. We obtained between similar to 800 ppm (for primitive MORB) and similar to 2000 ppm (for evolved MORB), values in reasonable agreement with experimentally-derived models. The experimental data also enable us to reconsider Ce/Pb and Nd/Pb ratios in MORB. We find that constant Ce/Pb and Nd/Pb ratios of 25 and 20, respectively, can be achieved during fractional crystallisation of magmas generated by 10% melting of depleted mantle provided the latter contains >100 ppm S and about 650 ppm Ce, 550 ppm Nd and 27.5 ppb Pb.Finally, we investigated the hypothesis that the pattern of chalcophile element abundances in the mantle was established by segregation of a late sulphide matte. Taking the elements Cu, Ag, Pb and Zn asexamples we find that the Pb/Zn and Cu/Ag ratios of the mantle can, in principle, be explained by segregation of 0.4% sulphide matte to the core. (C) 2015 The Authors. Published by Elsevier B.V.