Sulfur isotope fractionation between fluid and andesitic melt: An experimental study
Sulfur isotope fractionation between fluid and andesitic melt: An experimental study
复制标题
DOI：
10.1016/j.gca.2014.07.015
复制
发表时间：
2014-10-01
影响因子：
5
通讯作者：
Knipping, Jaayke L.
中科院分区：
文献类型：
--
作者：
Fiege, Adrian;Holtz, Francois;Knipping, Jaayke L.
由Fiege等人进行的减压实验生产的玻璃。(2014A)研究了岩浆脱气过程中流体与安山岩熔体之间硫同位素的分馏特征。最初的材料是合成玻璃，其成分接近克拉喀托英安安山岩。玻璃含有4.55-7.95wt%的H2O，类似于140-2700ppm的硫(S)和0-1000ppm的氯(Cl)。实验在1030℃、氧逸度(f(O2))从Qfm+0.8个对数单位到Qfm+4.2个对数单位(QFM：石英铁闪石磁铁矿缓冲液)的内热式压力容器(IHPV)中进行。减压实验是通过连续释放压力(P)从接近400 Mpa到最终P为150、100、70和30 Mpa进行的。减压速率(R)为0.0 1~0.17 Mpa/S，样品在最终P处进行0~72h的退火(t(A))，然后从10 30℃快速淬火至室温(T)。由于S的流体熔体分配系数较大，减压后形成了含S的水相。用二次离子质谱仪(SIMS)测定了减压前后玻璃的同位素组成。用质量平衡计算估算了气体熔体S的同位素分馏因子α(g-m)，没有观察到r和t(A)对α(g-m)的影响。然而，SIMS数据显示，α(g-m)从类似于0.9985+/-0.0007的Qfm+3显著增加到类似于Qfm+1的1.0042+/-0.0042。值得注意的是，还原条件下的同位素分馏比前人预测的大一个数量级。根据我们的实验结果和前人对流体和硅酸盐熔体中S形态的研究结果，提出了一个新的模型来预测fO(2)对安山岩体系中1030℃的α(g-m)(或S-34(g-m))的影响。我们的实验结果和我们的模型对于解释自然样品(如熔融包裹体或火山气体)中的S同位素特征具有重要意义。(C)2014爱思唯尔有限公司。保留所有权利。
Glasses produced from decompression experiments conducted by Fiege et al. (2014a) were used to investigate the fractionation of sulfur isotopes between fluid and andesitic melt upon magma degassing. Starting materials were synthetic glasses with a composition close to a Krakatau dacitic andesite. The glasses contained 4.55-7.95 wt% H2O, similar to 140 to 2700 ppm sulfur (S), and 0-1000 ppm chlorine (Cl). The experiments were carried out in internally heated pressure vessels (IHPV) at 1030 degrees C and oxygen fugacities (f(O2)) ranging from QFM+0.8 log units up to QFM+4.2 log units (QFM: quartz fayalite magnetite buffer).The decompression experiments were conducted by releasing pressure (P) continuously from similar to 400 MPa to final P of 150, 100, 70 and 30 MPa. The decompression rate (r) ranged from 0.01 to 0.17 MPa/s. The samples were annealed for 0-72 h (annealing time, t(A)) at the final P and quenched rapidly from 1030 degrees C to room temperature (T). The decompression led to the formation of a S-bearing aqueous fluid phase due to the relatively large fluid melt partitioning coefficients of S. Secondary ion mass spectrometry (SIMS) was used to determine the isotopic composition of the glasses before and after decompression. Mass balance calculations were applied to estimate the gas melt S isotope fractionation factor alpha(g-m).No detectable effect of r and t(A) on alpha(g-m) was observed. However, SIMS data revealed a remarkable increase of alpha(g-m) from similar to 0.9985 +/- 0.0007 at >QFM+3 to similar to 1.0042 +/- 0.0042 at similar to QFM+1. Noteworthy, the isotopic fractionation at reducing conditions was about an order of magnitude larger than predicted by previous works. Based on our experimental results and on previous findings for S speciation in fluid and silicate melt a new model predicting the effect of fO(2) on alpha(g-m) (or Delta S-34(g-m)) in andesitic systems at 1030 degrees C is proposed. Our experimental results as well as our modeling are of high importance for the interpretation of S isotope signatures in natural samples (e.g., melt inclusions or volcanic gases). (C) 2014 Elsevier Ltd. All rights reserved.