Helium and neon isotopes in mantle-derived xenoliths from Southern Patagonia: MORB-like and radiogenic/nucleogenic noble gas components in the subcontinental lithospheric mantle

JALOWITZKI TIAGO; SUMINO, HIROCHIKA; CONCEIÇÃO ROMMULO V.; ORIHASHI, YUJI; NAGAO, KEISUKE; BERTOTTO GUSTAVO W.; SCHILLING MANUEL; GERVASONI FERNANDA

San Pablo

Simposio; 9th South American Symposium on Isotope Geology; 2014

Instituto de Geociencias-Universidade de São Paulo

Here we present new helium and neon isotopic compositions for mantle xenoliths from Gobernador Gregores (GG) and Pali-Aike Volcanic Field (PAVF), Southern Patagonia. Previous noble gas studies of mantle-derived xenoliths from Patagonia were carried out by Sumino et al. (2004), Conceição et al. (2007, 2008, 2009) and Jalowitzki et al. (2012). Whole rock and separated olivines were used for noble gas analysis, using single step heating and stepwise crushing extraction methods at the University of Tokyo. Only helium and neon data obtained by crushing will be considered to avoid secondary radiogenic or cosmogenic contributions. Mantle-derived xenoliths from GG and two different localities from PAVF show a narrow range of 3He/4He (6.7-7.7RA), indicating a major contribution of a MORB-like component (3He/4HeMORB = 8±1RA). These ratios partly overlap those defined to the subcontinental mantle worldwide (6.1 ± 0.9RA; Gautheron & Moreira, 2002) and indicate a slight radiogenic component compared to MORB. In a neon three-isotope diagram (20Ne/22Ne vs. 21Ne/22Ne), data for one sample from GG are plotted on the air-MORB source mixing line, whereas the others from PAVF and GG have higher 21Ne/22Ne relative to the MORB trend for a given 20Ne/22Ne, which is explained by a mixing between air and endmembers more nucleogenic than the MORB source. The MORB-like component observed in GG samples represents the replacement of the mantle wedge due to asthenospheric upwelling in response to the opening of Patagonian slab window, which is a consequence of Chile Ridge subduction. The slight radiogenic/nucleogenic components observed in PAVF and GG indicates higher (U+Th)/(3He, 22Ne) ratios for the source region of these samples and could be related to the modal (e.g. phlogopite and pargasite) and cryptic metasomatism. The presence of radiogenic/nucleogenic ratios shows that these component shave not been totally overprinted by the opening of slab window.