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Melt Inclusions of Sulfide in Mantle-Driven Amphibole Megacrysts
详细信息   
摘要
Sulphide deposits are of considerable economic importance but the origin of metals and sulphur in these deposits is commonly ambiguous and raises questions.  Much attention has been focused on a primary mantle origin for sulphides after extensive research on sulphide melt inclusions （SMI） in mantle-derived xenoliths in basaltic and kimberlitic rocks.  Most studies treated the evolution of magma and SMI separately by numerical simulations based on isotopic geochemistry or petrochemistry,  few have attempted to use detailed chemical analysis and petrographic evidence for the origin of the sulphide,  let alone describing how the SMI evolve in mantle-derived magmas.  Using ToF-SIMS,  we have analyzed the detailed element compositions,  obtaining positive and negative ion maps of the SMI and surrounding host amphibole megacrysts Amp-M.  SMI enclosed in mantle-derived Amp-M that represents melt trapped in the minerals provide important clues as to the behavior of immiscible sulphide liquids during the evolution of magmas and the formation of sulphide deposits.  Temperature and pressure during formation of the Amp-M were estimated using the TiO2-Al2O3 geothermometer and geobarometer formula has two main interval distribution： The upper-mantle Amp-M： T： 850~900℃（temperature）, P： 0.70×109～0.82×109Pa（pressure）,  corresponding to a depth of D： 23.10～27.06 km; and the lower-crust Amp-M： T： 900～950℃,  P： 1.09×109～1.17×109Pa,  D： 35.97～38.61 km.  Observations and researches indicate that the upper-mantle Amp-M was formed from the crystallization of the upper-mantle alkaline basaltic magma,  which resulted from previous partial melting of the upper mantle,  and the lower-crust Amp-M were formed from the fractional crystallization and evolution basaltic magma in magma chambers at the base of the continental crust.  At the mantle-crust boundary,  basaltic magma upwelled and pooled in the lower crust.  During fractional crystallization and the sima contamination,  S （and Ni,  Cu and Cr） were released and were eventually trapped as droplets within growing amphibole megacrysts.  As part of the general process of mantle metasomatism,  this could lead to a significant concentration of sulphur and chalcophile elements in the lowermost crust,  making these components available for remobilization during episodes of crustal thinning at 140Ma,  resulting in the MLYR large-scale mineralization belt.  Our study suggests that this process is more common than previous thought.

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