Partial melting and subduction-related metasomatism recorded by geochemical and isotope (He-Ne-Ar-Sr-Nd) compositions of spinel lherzolite xenoliths from Coyhaique, Chilean Patagonia

Abstract

Spinel lherzolite xenoliths from Coyhaique, Chilean Patagonia, are located ~320 km from the Chile Trench and, therefore, their geochemical and isotopic compositions may record slab-derived metasomatism from current and/or earlier subduction zones. We present new mineral and whole-rock major and trace elements that are discussed together with noble gas data (He, Ne, Ar) and Sr-Nd isotopic ratios. Partial melting is recorded by negative correlations between basaltic elements vs. MgO in whole-rock and minerals. In general, silicate minerals show depleted light-rare earth element (LREE) over heavy-REE (HREE) patterns. Whole-rock compositions and clinopyroxene record variable degrees of partial melting (1–15%), which is corroborated by the Sr-Nd depleted character of the lherzolites. Cryptic metasomatism is evidenced by: 1) enriched LREE patterns in whole-rock and type-2 clinopyroxenes; 2) negative anomalies of Nb-Ta-Ti in whole-rock samples coupled with enrichment of large ion lithophile elements and chalcophile elements over high field strength elements and HREE; 3) positive correlations of Pb and U vs. La and negative correlations of Nb/Nb* and Ti/Ti* vs. La in clinopyroxene, and 4) positive anomalies of Li in all silicate minerals. Geochemical differences between whole-rocks and their mineral constituents indicate selective enrichment of the whole-rock samples due to grain-boundary components. 3He/4He ratios reported here are the first strongly radiogenic values found in the Patagonian subcontinental lithospheric mantle (SCLM; 0.20–2.52 RA). Most Ne isotopic ratios are indistinguishable from air composition with few samples showing a nucleogenic component, whereas Ar isotopic ratios are extensively affected by an atmospheric component. The correlation between 3He/4He and 4He/40Ar* corroborates the metasomatism recorded by lithophile and chalcophile elements. Therefore, we identify a heterogeneous SCLM affected by low but variable degrees of partial melting and subsequent enrichment by melts or fluids from recycled subduction-related materials in the spinel stability field (1.06–1.90 GPa and 886–1150 °C).