| It is widely accepted that at subduction zones,mass and energy transfer from the subducted slab to the overlying mantle wedge is mediated by hydrous fluids.The nature of fluids changes with subduction depth,namely,pressure and temperature,as increasing H2O ionization along subduction paths can enhance dissolved solute concentrations via formation of polymerized silicate network.Therefore,the solubility of both H2O in silicate melt and silicate in aqueous fluid increase with pressure and temperature.A single miscible supercritical fluid with solute-rich composition is allowed to form when P-T conditions are above the Second Critical End-point.Aqueous fluids-like viscosity and melt-like wetting and element-carrying capability make supercritical fluids an ideal agent for chemical transport at subduction zones.To better understand High field strength elements?HFSE?mobility and Nb/Ta fractionation in subduction zones,we experimentally determined rutile and zircon solubilities in supercritical fluids and Nb and Ta partitioning between rutile and supercritical fluids using high P-T apparatus.The first part of this thesis determines rutile?TiO2?solubility and Nb and Ta partition coefficients?Drt/sf Nb and Drt/sf Ta?in rutile?rt?-silica-rich supercritical fluid?Sf?systems at 1.5-2.5GPa and 920-1150?over variable fluid chemistries including solute?SiO2±albite component?,H2O,Cl,and F contents.Under the investigated conditions,TiO2 solubility in the supercritical fluids varies from 761±107 to 9795±448 ppm;Dru/sf Nb and Dru/sf Ta vary from 12±1 to 208±30and 34±5 to 2464±140,respectively.Higher solute,Cl,and F contents in the systems and higher temperatures result in higher TiO2 solubilities and lower Dru/sf Nb and Dru/sf Ta,and thus,fluid chemistry and temperature exert main controls on Ti,Nb,and Ta mobility.In all cases,Dru/sf Nb/Dru/sf Ta<0.70,suggesting that supercritical fluids released from subducting slabs are higher in Nb/Ta relative to their protoliths.Therefore,such fluids are expected to result in higher Nb/Ta ratios in mantle wedges and arc magmas.Although Ti,Nb,and Ta could be mobile in supercritical fluids,Nb/Ta ratios in primitive arc basalts compared to those in mid-ocean ridge basalts show that only10%of arc basalts are possibly disturbed by solute-rich supercritical fluids during their generation.The second part of this thesis determines zircon?zircon?solubility in solute-rich supercritical fluids in KAlSi3O8?±K2O±Al2O3?–H2O systems at 2.0-6.0 GPa and 800-1000oC,close to the top-slab conditions at sub-arc depths.The experiments at 2.0 GPa and 900-1000oC produced zircon+supercritical fluid?Sf?,and those at 2.0 GPa,800oC and 4.0 and 6.0GPa,1000oC produced zircon+Sf±sanidine±muscovite±kyanite.ZrO2 content in the supercritical fluids increases with temperature and solute content and alkalinity?molar K/Al ratio?but decreases with pressure.At 2.0 GPa,ZrO2 content in KAlSi3O8–H2O fluids?K/Al=1.0?increases from<100?ppm at 800-900oC and solute content<50 wt%to 500-700ppm at 1000oC and solute content>70 wt%.At 2.0 GPa and 1000?°C?30wt%H2O?,addition of K2O to the KAlSi3O8–H2O system strongly enhances ZrO2content to10000?ppm at K/Al of 1.67.Therefore,solute content and alkalinity exert primary controls on zircon solubility.At 4.0 and 6.0 GPa,the crystallization of muscovite and kyanite results in K/Al higher than 1.0 in the KAlSi3O8–H2O fluids and the negative effect of pressure on zircon solubility was offset and even reversed due to the enrichment of K2O.Comparison with literature data shows that zircon solubility in solute-rich supercritical fluids is at least 10 times higher than that in dilute aqueous fluids.During subduction,the supercritical fluid released from slab will become higher in solute and alkali/Al due to increasing P-T and formation of Al-rich residual phases.Rutile and zircon solubilities and element carrying capacity of supercritical fluid thus increase with subduction depth.Together these results we demonstrate that solute-rich supercritical fluids may serve as an effective agent for transfer of HFSE in subduction zones as rutile and zircon solubilities in solute-rich supercritical fluids are intermediate between dilute aqueous fluids and hydrous melts at given conditions.However,in most cases supercritical fluids are too short-lived to directly transmit slab-derived signals to arc magmas because they separate immiscible hydrous melts and aqueous fluids at slab-mantle interface in response to pressure drop and abrupt changes in bulk rock compositions.HFSE partition favorably into the hydrous melts during aqueous fluids and hydrous melts separation,and the majority of HFSE will be discharged by subsequent rock-melts interaction via precipitation of HFSE-bearing minerals,whereas relative high partition coefficients of HFSE between aqueous fluids and hydrous melts still enable aqueous fluids to enrich arc magma source regions in HFSE. |
PDF Full Text Request