Apatite as an indicator of fluid salinity in subduction zone settings : implications for the deep earth chlorine cycle




Li, Huijuan

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In search of a parameter to monitor the variation in the salinity of subduction zone fluids, piston-cylinder experiments have been conducted to investigate chlorine partitioning behaviour in subducted sediment systems at subduction zone conditions. Apatite was found to be a prime candidate for monitoring fluid Cl content, as it represents the most chlorine rich phase. Chlorine partition coefficients between apatite and melt/aqueous fluid (DCl) show a general increase with increasing temperature, and a decrease with increasing pressure. DCl values were found to be negatively correlated with the bulk F concentration/ F contents in apatite; this was attributed to non-ideal mixing of F-Cl and F-OH in apatite. A thermodynamic framework was developed and applied to describe the OH-Cl-F exchange equilibrium between apatite and melt. Ideal mixing for the binary Cl-OH apatite solution is valid for the investigated P and T range of 2.5-4.5 GPa and 600-900 degree C. Through least square linear regression of our experimental data, values for the interaction parameters W^{Ap}F-Cl and W^{Ap}F-OH were determined. Combining all derived thermodynamic data yields an equation to express melt Cl content (wt%) as a function of pressure, temperature and apatite composition. This expression (27) in addition to measured Cl partition coefficients was utilized to calculate the salinity of fluids in equilibrium with selected natural samples. Case studies of apatites from subducted oceanic sediment samples and UHP metapelites from the Dora Maira and Kokchetav massifs demonstrate that aqueous fluids present during prograde subduction have low element mobility, and a low salinity that is << seawater salinity. Conversely, hydrous melts display salinities which are (equivalent or higher than){u2265} seawater salinity, which further enhances their trace element uptake. According to the determination of trace element concentrations in glasses from our 2.5 GPa, 800 degree C experiments, the F and Cl content in the melt increased the concentrations of REE, Th, U, Y, Zn, Sc and Mn, had no effect on Ti, Zr, Hf, P, Sr and LILE; while the concentrations of Nb and Ta were found to decrease. Therefore our study suggests that the presence of F and Cl in melt can contribute to the fractionation of REE/LFSE relative to HFSE during subduction recycling. Our experiments indicate that the addition of fluid with seawater equivalent salinity does not produce a significant change in the phase and melting relations compared to the F, Cl-free system. Apatite appears incapable of fractionating the H{u2082}O/Cl ratio in either aqueous fluid or hydrous melt even though it possesses a high Cl/H{u2082}O ratio. The extensive variation in the H{u2082}O/F fluid ratio suggests that hydrous minerals present in the slab residue have the potential to fractionate H{u2082}O and Cl from F in subduction fluids. -- provided by Candidate.






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