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Título

GARNET-SPINEL PERIDOTITES FROM THE GOIÁS ALKALINE PROVINCE REVEAL COMPLEX ENRICHMENT BY He-Ne-Ar ISOTOPES

Texto do resumo

The Goiás Alkaline Province (GAP) represents an important occurrence of alkaline rock intrusions and lava flows throughout central Brazil. In this study, garnet-spinel peridotites, hosted by olivine nephelinites were collected from a volcanic plug near the town of Paraúna, Goiás state. As of now, these xenoliths represent the deepest ever reported for the GAP. The noble gas isotopic composition (He, Ne, Ar) of the samples was determined at the University of Tokyo in a high vacuum mass spectrometer coupled to a gas line (MS-IV). Due to reduced sample size, only two were analyzed for whole-rock (PAR12-WR and PAR21-WR) and another for orthopyroxene concentrate (PAR12-OPX). Individual aliquots for each sample were analyzed applying sequential strokes (stepwise crushing at 100, 500, 1000 and 2000 times) to extract the gases trapped in fluid inclusions. Except for the first extraction of each sample (100x: 3He/4He = 5.15-6.45 Ra, where Ra represents the atmospheric ratio of 1.410-6), helium isotopic ratios are homogenous and vary between 3.66 and 4.38 Ra. The results for the first extraction are compatible with those defined for the subcontinental lithospheric mantle (SCLM = 6.0  1.0 Ra), while the others are a product of subduction-related components (<5 Ra). We interpret that the helium signature shown in the first extraction is a consequence of the interaction between the SCLM and an alkaline silicate melt. This compositional difference is attributed to the fact that minerals, chiefly olivine and orthopyroxene, show well developed coronae, characterized by aggregates of fine crystals surrounding their macrocrystal counterparts. The Ne results with sufficient analytical quality and that differ from atmospheric composition, considering the analytical uncertainties (1) display SCLM composition. Because Ne results are not associated with the first extraction, we assume that they were preserved, representing the original SCLM. Compared to mid-ocean ridge basalts (40Ar/36ArMORB ~40000), the low 40Ar/36Ar isotope ratios (984-2065) reveal strong atmospheric contribution. The mixing between a radiogenic component (subduction-related) and the SCLM is corroborated by the relationship between 3He/4He versus 21Ne/22Ne(E) (where (E) stands for extrapolated ratios to the Solar 21Ne/22Ne of 12.5). Finally, the low 4He/40Ar* ratios (0.31-1.05), in which Ar* was calculated aiming to avoid the atmospheric contribution, suggests that the SCLM beneath the GAP represents the residue after high degrees of partial melting. Therefore, we conclude that the lithospheric mantle beneath the GAP records a complex evolution involving partial melting, subduction-related metasomatism and alkaline silicate metasomatism.

Palavras Chave

Noble gases; Isotope Geochemistry; Mantle xenoliths; Goiás Alkaline Province