The mobility of W and Mo in subduction zone fluids and the Mo-W-Th-U systematics of island arc magmas
The mobility of W and Mo in subduction zone fluids and the Mo-W-Th-U systematics of island arc magmas
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DOI：
10.1016/j.epsl.2012.07.032
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发表时间：
2012-10-15
影响因子：
5.3
通讯作者：
Audetat, A.
中科院分区：
文献类型：
--
作者：
Bali, E.;Keppler, H.;Audetat, A.
我们已经研究了在2.61 GPa，600至800摄氏度的可变控制的氧逸度条件下，有关俯冲带的水流体中的W-和Mo-氧化物的溶解度。我们观察到，W-氧化物的溶解度仅略微依赖于由Co-Co和Re-ReO 2缓冲的条件之间的fO（2）。相比之下，钼氧化物的溶解度是强烈依赖于在相同条件下的氧逸度和流体盐度。在2.61 GPa下，在Co-COO和Re-ReO_2之间的fO（2）范围内，它们的溶解度可用下列方程描述：logW = 0.07 logfO（2）-4.72 × 1000/T+4.43andlogMo = 0.44 logfO（2）+ 0.42 logNaCl-1.8 × 1000/T+4.80其中W、Mo和NaCl均为摩尔浓度。fO（2）是氧逸度，T是以开尔文为单位的温度。我们还研究了Mo和W在与Mo-和W-氧化物共存的地幔矿物中的溶解度，并将这些数据与流体溶解度相结合来计算W和Mo的流体/矿物分配系数。在水溶液存在下，W和Mo在榴辉岩主要矿物（石榴石和单斜辉石）的结构中不相容，但在金红石中相容。因此，英里的存在或不存在强烈影响这些元素在俯冲带的流动性。从分配系数，我们计算了从N-MORB榴辉岩源释放的含水流体的组成。我们发现弧岩浆的Mo-Ce-W-Th-U系统可以用亏损地幔与含金红石的N-MORB源在fO（2）-条件介于FMQ-1.6和FMQ+3之间（最大盐度为20wt%NaClequiv）释放的含水流体相的混合来模拟。如果残余榴辉岩中的金红石含量是独立确定的，则氧化还原条件、盐度和加入熔融源区的流体量可以通过绘制Mo/Ce、W/Th和Mo/W与U/Th的关系图来严格约束。特别是，W/Th比是流体量的敏感指标，因为W总是非常强烈地分配到流体中，几乎与氧逸度和盐度无关。Mo/W比对氧化还原条件特别敏感，而U/Th反映盐度和氧化还原状态。(C)2012爱思唯尔有限公司版权所有。
We have studied the solubility of W- and Mo-oxide in aqueous fluids at 2.61 GPa, 600 to 800 degrees C at variable controlled oxygen fugacity conditions, relevant to subduction zones. We observed that the solubility of W-oxide is only slightly dependent on fO(2) between conditions buffered by Co-Co and Re-ReO2. In contrast, Mo-oxide solubility is strongly dependent on both oxygen fugacity and fluid salinity under the same conditions. At 2.61 GPa, in the fO(2) range between Co-COO and Re-ReO2 their solubility can be described by the following equations:logW = 0.07 logfO(2)-4.72 x 1000/T+4.43andlogMo = 0.44 logfO(2) + 0.42 logNaCl-1.8 x 1000/T+4.80where W, Mo and NaCl are concentrations in molalities. fO(2) is oxygen fugacity and T is temperature in Kelvin. We also studied the solubility of Mo and W in mantle minerals coexisting with Mo- and W-oxide and combined these data with the fluid solubilities to calculate fluid/mineral partition coefficients of W and Mo. Both W and Mo are incompatible in the structure of major eclogite minerals (garnet and clinopyroxene) in the presence of aqueous fluid, but they are compatible in rutile. Thus, the presence or absence of mile strongly affects the mobility of these elements in subduction zones. From the partition coefficients we calculated the composition of aqueous fluid released from an N-MORB eclogite source. We found that the Mo-Ce-W-Th-U systematics of arc magmas can be modeled by the mixing of depleted mantle with an aqueous fluid phase that was released from a rutile-bearing N-MORB source in fO(2)-conditions between FMQ-1.6 and FMQ+3 having a maximum salinity of 20 wt% NaClequiv. If rutile contents in residual eclogite are independently determined, redox conditions, salinity and amount of fluid added to the source region of melting can be tightly constrained by plotting Mo/Ce, W/Th, and Mo/W vs. U/Th. In particular, the W/Th ratio is a sensitive indicator of the amount of fluid, since W always very strongly partitions into the fluid, nearly independent of oxygen fugacity and salinity. The Mo/W ratio is particularly sensitive to redox conditions, while U/Th reflects both salinity and redox state. (C) 2012 Elsevier B.V. All rights reserved.