Publications (2)0 Total impact
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Article: P-T-Fluid evolution and graphite deposition during retrograde metamorphism in Ribeira Fold Belt, SE Brazil: oxygen fugacity, fluid inclusions and C-O-H isotopic evidence
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ABSTRACT: Combined fluid inclusion (FI) microthermometry, Raman spectroscopy, X-ray diffraction, CeOeH isotopes and oxygen fugacities of granulites from central Ribeira Fold Belt, SE Brazil, provided the following results: i) MagnetiteeHematite fO2 estimates range from 10�11.5 bar (QFM þ1) to 10�18.3 bar (QFM �1) for the temperature range of 896 �Ce656 �C, implying fO2 decrease from metamorphic peak temperatures to retrograde conditions; ii) 5 main types of fluid inclusions were observed: a) CO2 and CO2eN2 (0e11 mol%) high to medium density (1.01e0.59 g/cm3) FI; b) CO2 and CO2eN2 (0e36 mol%) low density (0.19e0.29 g/cm3) FI; c) CO2 (94e95 mol%)eN2 (3 mol%)eCH4 (2e3 mol%)eH2O (water 4v (25 �C) ¼ 0.1) FI; d) low-salinity H2OeCO2 FI; and e) late low-salinity H2O FI; iii) Raman analyses evidence two graphite types in khondalites: an early highly ordered graphite (Tw450 �C) overgrown by a disordered kind (Tw330 �C); iv) d18O quartz results of 10.3e10.7& imply high-temperature CO2 d18O values of 14.4e14.8&, suggesting the involvement of a metamorphic fluid, whereas lower temperature biotite d18O and dD results of 7.5e8.5& and �54 to �67&, respectively imply H2O d18O values of 10e11& and dDH2O of �23 to �36&, suggesting d18O depletion and increasing fluid/rock ratio from metamorphic peak to retrograde conditions. Isotopic results are compatible with low-temperature H2O influx and fO2 decrease that promoted graphite deposition in retrograde granulites, simultaneous with low density CO2, CO2eN2 and CO2eN2eCH4eH2O fluid inclusions at T ¼ 450e330 �C. Graphite d13C results of �10.9 to �11.4& imply CO2 d13C values of �0.8 to �1.3&, suggesting decarbonation of Cambrian marine carbonates with small admixture of lighter biogenic or mantle derived fluids. Based on these results, it is suggested that metamorphic fluids from the central segment of Ribeira Fold Belt evolved to CO2eN2 fluids during granulitic metamorphism at high fO2, followed by rapid pressure drop at Tw400e450 �C during late exhumation that caused fO2 reduction induced by temperature decrease and water influx, turning carbonic fluids into CO2eH2O (depleting biotite d18O and dD values), and progressively into H2O. When fO2 decreased substantially by mixture of carbonic and aqueous fluids, graphite deposited forming khondalites.Journal of South American Earth Sciences. 01/2011; 31:93-109. -
Chapter: Jazigos filonianos hidrotermais e aplitopegmatíticos espacialmente associados a granitos (norte de Portugal).
01/2006: pages 123 - 138; , ISBN: 972-778-094-6
