Monazite solubility in hydrous silicic melts at high pressure conditions relevant to subduction zone metamorphism

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• 作者：Susanne Skora ; ^{Susanne.Skora@bristol.ac.uk} ; Jon Blundy
• 关键词：monazite ; solubility ; dissolution mechanism ; subduction zone ; element recycling ; geothermometry
• 刊名：Earth and Planetary Science Letters
• 出版年：2012
• 期刊代码：18_0012821x
• 类别：ph
• 出版时间：1 March, 2012
• 卷：321-322
• 期：Complete
• 页码：104-114
• 文件大小：830 K

摘要

Critical to any application of accessory phase stability to subduction zone thermal structure and processes is knowledge of the thermodynamic stability of these minerals in different types of subducted rock, their solubility in the presence of fluids, and the extent to which they fractionate trace element ratios of interest. This study focuses on monazite, which is the principal carrier of light rare earth elements (LREE) and thorium (Th) in CaO-poor subducted sediments. Relatively little is known about the mechanism of monazite dissolution in high-pressure hydrous melts (or supercritical fluids), yet monazite and allanite (the principal carrier of LREE and Th in oceanic basalts and some CaO-rich sediments) solubility has been used recently to quantify subducted slab-top temperatures (Plank, T., Cooper, L.B., Manning, C.E., 2009. Emerging geothermometers for estimating slab surface temperatures. Nature Geosci. 2, 611-615).

We have studied monazite solubility at subduction zone conditions (3 GPa, T 鈮?#xA0;800 掳C) in hydrous sediment-melting experiments. Experimental results highlight the important role that phosphorous exerts on monazite solubility in hydrous silicic melts at high pressure. Thermodynamically this corresponds to a case where monazite dissolves predominantly as its dissociated constituent ions (LREE^3 + and PO₄^{3 鈭?/sup>). This is in contrast to monazite solubility in granitic melts at low pressures (0.2 GPa) where it appears to dissolve predominantly as associate LREEPO₄ species, such that its solubility is essentially independent of dissolved phosphorous. Our results have implications for monazite-based thermometry, as the error introduced by not taking phosphorous into account in high-pressure fluids can amount to > 100 掳C.}