Partitioning of halogens between mantle minerals and aqueous fluids: implications for the fluid flow regime in subduction zones
Partitioning of halogens between mantle minerals and aqueous fluids: implications for the fluid flow regime in subduction zones
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DOI：
10.1007/s00410-012-0799-4
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发表时间：
2013-01-01
影响因子：
3.5
通讯作者：
Keppler, Hans
中科院分区：
文献类型：
--
作者：
Bernini, Diego;Wiedenbeck, Michael;Keppler, Hans
我们在1,100 A ℃和2.6 GPa条件下，用MgCl 2或MgF 2对镁橄榄石-顽火辉石-辉石-H2O体系进行了相平衡实验，以限制卤素在橄榄岩矿物组合中的溶解度和流体-矿物分配系数。氯在镁橄榄石、顽火辉石和镁铝榴石中的溶解度非常低，分别为2.1-3.9和4.0-11.4 ppm，并且与流体盐度（0.3-30重量% Cl）无关，这表明在非常低的氯浓度下已经达到晶体中的某些固有饱和极限。因此，氯在上地幔矿物中是极其不相容的。氟在顽火辉石中的溶解度为170-336 ppm，在镁铝榴石中的溶解度为510- 1，110 ppm，同样与流体盐度无关。镁橄榄石溶解1，750 - 1，900 ppm，直到1.6重量% F的流体盐度。在较高的氟含量的系统中，镁橄榄石被取代的矿物的腐殖酸组。与氟相比，氯的溶解度低三个数量级，这与晶格应变的增加是一致的。流体-矿物分配系数F = 10（0）-10（2），Cl = 10（3）-10（5）。由于后者的值是数量级高于那些为羟基分区，流体流从俯冲板通过地幔楔将导致一个有效的封存的H2O到名义上无水矿物的楔，而氯成为富集在残余流体。简单的质量平衡计算表明，要产生在某些原始弧岩浆中观察到的Cl/H2O比值升高，需要高达3，000以上的岩石-流体比值。因此，流体从俯冲板片进入地幔楔中的熔融区不仅在狭窄的通道中迅速发生，而且至少在某些俯冲带中，流体渗透到地幔橄榄岩中并与大体积的地幔楔相互作用。再加上原始弧岩浆的Cl/H2O比值，因此，我们的数据约束火山弧下的流体流动制度。
We have performed phase equilibrium experiments in the system forsterite-enstatite-pyrope-H2O with MgCl2 or MgF2 at 1,100 A degrees C and 2.6 GPa to constrain the solubility of halogens in the peridotite mineral assemblage and the fluid-mineral partition coefficients. The chlorine solubility in forsterite, enstatite and in pyrope is very low, 2.1-3.9 and 4.0-11.4 ppm, respectively, and it is independent of the fluid salinity (0.3-30 wt% Cl), suggesting that some intrinsic saturation limit in the crystal is reached already at very low chlorine concentrations. Chlorine is therefore exceedingly incompatible in upper-mantle minerals. The fluorine solubility is 170-336 ppm in enstatite and 510-1,110 ppm in pyrope, again independent of fluid salinity. Forsterite dissolves 1,750-1,900 ppm up to a fluid salinity of 1.6 wt% F. At higher fluorine contents in the system, forsterite is replaced by the minerals of the humite group. The lower solubility of chlorine by three orders of magnitude when compared to fluorine is consistent with increasing lattice strain. Fluid-mineral partition coefficients are 10(0)-10(2) for fluorine and 10(3)-10(5) for chlorine. Since the latter values are orders of magnitude higher than those for hydroxyl partitioning, fluid flow from the subducting slab through the mantle wedge will lead to an efficient sequestration of H2O into the nominally anhydrous minerals in the wedge, whereas chlorine becomes enriched in the residual fluid. Simple mass balance calculations reveal that rock-fluid ratios of up to > 3,000 are required to produce the elevated Cl/H2O ratios observed in some primitive arc magmas. Accordingly, fluid flow from the subducted slab into the zone of melting in the mantle wedge does not only occur rapidly in narrow channels, but at least in some subduction zones, fluid pervasively infiltrates the mantle peridotite and interacts with a large volume of the mantle wedge. Together with the Cl/H2O ratios of primitive arc magmas, our data therefore constrain the fluid flow regime below volcanic arcs.