CO2-induced small water solubility in olivine and implications for properties of the shallow mantle
CO2-induced small water solubility in olivine and implications for properties of the shallow mantle
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DOI：
10.1016/j.epsl.2014.06.025
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发表时间：
2014-10
影响因子：
5.3
通讯作者：
Xiaozhi Yang;D. Liu;Q. Xia
中科院分区：
文献类型：
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作者：
Xiaozhi Yang;D. Liu;Q. Xia
摘要H2O和CO2是∼30-150 km深度地幔流体的重要组成部分，对名义上无水的橄榄石中的水溶解有很大影响；然而，现有的橄榄石加氢实验几乎都是在与H2O(无CO2)共存的情况下进行的。在橄榄岩和流体饱和条件下，通过对天然橄榄石进行H-退火处理，研究了CO2对橄榄石水溶度的影响。实验在1.5-5 Gpa和1100-1300°C下进行，氧逸度由Ni-NiO控制，缓冲液为H2O或H2O-CO2。在实验过程中，橄榄石的成分没有变化。水合橄榄石的红外光谱特征是在∼3650到3000 cm−1的所有运行中，在高频(>；3450 cm−1)和低频(<；345 0 cm−1)，与H2O共存的橄榄石的H2O溶解度为∼12 0~370ppm，与H2O-CO2共存的橄榄石的H2O溶解度为∼6 5~180ppm。当缓冲液中存在CO2时，由于影响水在矿物和共存流体之间的分配，橄榄石的H2O溶解度降低了∼2倍，测得的H2O溶解度与流体组成(∼0.2~0.5时CO2与CO2+H2O的摩尔比)无关。在浅地幔中平衡的橄榄石可能主要由波数∼36 5 0-3 0 0 0 cm−1中的OH基团主导，低频带的强度可能高于或相当于高频带的强度。如果观测到的二氧化碳对水溶解度的影响是正确的，那么以前估计的浅地幔中水的储存量可能被高估了至少∼4倍。我们的结果对认识浅地幔的部分熔融、电导率异常和交代作用具有深远的影响。
Abstract H 2 O and CO 2 are important components of fluids in the mantle at∼ 30–150 km depth, and may affect strongly water dissolution in nominally anhydrous olivine; however, available experimental hydrogenation of olivine has been nearly exclusively carried out in coexistence with H 2 O (CO 2-free). In this study, the effect of CO 2 on water solubility in olivine has been investigated by H-annealing natural olivine under peridotite-and fluid-saturated conditions. Experiments were conducted at 1.5–5 GPa and 1100–1300° C, with oxygen fugacity controlled by Ni–NiO and with either H 2 O or H 2 O–CO 2 as buffering fluid. The olivine shows no change in composition during the experiments. The infrared spectra of the hydrated olivine are characterized by prominent OH bands from∼ 3650 to 3000 cm− 1 in all the runs, at both high frequency (> 3450 cm− 1) and low frequency (< 3450 cm− 1), and the H 2 O solubility is∼ 120–370 ppm for the olivine in coexisting with H 2 O, and∼ 65–180 ppm for the olivine in coexisting with H 2 O–CO 2. When CO 2 is present in the buffering fluid, the H 2 O solubility of olivine is reduced by a factor of∼ 2, due to effect on the partitioning of water between minerals and coexisting fluid, and the measured H 2 O solubility shows independence on fluid composition (the molar ratio of CO 2 to CO 2+ H 2 O at∼ 0.2–0.5) given pressure, temperature and oxygen fugacity. Olivine equilibrated in the shallow mantle is probably dominated by OH groups in the wavenumber∼ 3650–3000 cm− 1, and the intensity of OH bands at low frequency may be higher than or comparable to those at higher frequencies. The storage capacity of water in the shallow mantle in previous estimates may have been overestimated by a factor of at least∼ 4 if the observed effect of CO 2 on water solubility is correct. Our results have profound influence on understanding partial melting, electrical conductivity anomalies and metasomatism in the shallow mantle.