[S-GC16] Volatile Cycles in the Deep Earth - from Subduction Zones to the Mantle and Core

[SGC16-P05] MORB-like and radiogenic/nucleogenic noble gas components in southern Patagonian subcontinental lithospheric mantle

Tiago Jalowitzki¹, *Hirochika Sumino², Rommulo V Conceição¹, Yuji Orihashi³, Keisuke Nagao⁴, Gustavo W Bertotto⁵, Eduardo Balbinot⁶, Manuel Schilling⁷, Fernanda Gervasoni⁸ (1.Universidade Federal do Rio Grande do Sul, 2.Dept. of Basic Science, Grad. School of Arts and Sciences, Univ. of Tokyo, 3.Earthquake Research Institute, Univ. of Tokyo, 4.Korea Polar Research Institute, 5.CONICET, Universidad Nacional de La Pampa, 6.Univ. of Surrey, 7.Universidad Austral de Chile, 8.Westfählische-Wilhelms-Universität Münster)

Keywords:noble gas, mantle xenolith, Patagonia, subduction, subcontinental lithospheric mantle

Southern Andean Patagonia is one of the few sites where interactions between oceanic and continental lithosphere due to the subduction of an active spreading ridge beneath continent can be investigated. In order to characterize the noble gas composition of Patagonian subcontinental lithospheric mantle (SCLM) we analyzed noble gas and lithophile (Sr–Nd–Pb) isotopes of mantle xenoliths from Pali–Aike Volcanic Field and Gobernador Gregores in southern Patagonia.
Noble gas composition of the mantle xenoliths reflects three-component mixing between air, SCLM and MORB-like. Pali-Aike mantle xenoliths represent the intrinsic local SCLM reservoir with higher (U+Th+K)/(³He, ²²Ne, ³⁶Ar) ratios than MORB source. This mantle reservoir is characterized by radiogenic ³He/⁴He_AVERAGE = 6.87 ± 0.04 R_A and nucleogenic mantle neon with ²¹Ne/²²Ne average of 0.090, with ³He/²²Ne ratios (up to 13.66 ± 0.37) higher than depleted MORBs (8.31–9.75). ⁴⁰Ar/³⁶Ar ratios vary from near–atmospheric ratio (510) up to 16400, with mantle ⁴⁰Ar/³⁶Ar reaching 54000. Mantle ¹²⁹Xe/¹³²Xe reach up to 1.11, whereas ¹³⁶Xe/¹³²Xe up to 0.40. Gobernador Gregores mantle xenoliths represent the SCLM metasomatized by MORB-like component with ³He/⁴He_AVERAGE = 7.24 ± 0.09 R_A, slightly less nucleogenic mantle neon with ²¹Ne/²²Ne = 0.065, ³He/²²Ne = 8.39 ± 0.14, and ⁴⁰Ar/³⁶Ar ratios usually less than 4000.
Based on these new data, we conclude that the highly radiogenic/nucleogenic signature of Pali–Aike mantle xenoliths compared to the MORB source represents an intrinsic feature of the SCLM reservoir beneath southern Patagonia. This signature could have been homogenized during the last 14 Ma, after rapid the passage and northward migration of the Chile Triple Junction and its slab window at this latitude. On the other hand, the less radiogenic/nucleogenic MORB–like component identified in Gobernador Gregores mantle xenoliths can be explained by recent metasomatism of the SCLM due to the asthenospheric mantle upwelling in response to the opening of a slab window beneath Patagonia because of South Chile Ridge subduction.