CO 2 fluid and silicate glass as monitors of alkali basalt/peridotite interaction in the mantle wedge beneath Gobernador Gregores, Southern Patagonia

Abstract

A suite of mantle-wedge amphibole + phlogopite-bearing spinel peridotite xenoliths in Plio-Pleistocene alkali basalts from Southern Patagonia (Gobernador Gregores, Santa Cruz Province, Argentina) contains carbonic fluid inclusions, glass and carbonate in several textural domains. Here we present a microstructural and fluid inclusion study showing that fluid (corresponding to pure CO 2) and glass post-date the hydrous mantle assemblage and formed soon before and/or during xenolith entrainement in the host alkali basalt. The high densities preserved by a number of CO 2 inclusions indicate that fluid infiltration took place at mantle depths. The low densities pertaining to the majority of analyzed fluid inclusions derive from inclusion re-equilibration during xenolith ascent. Glass occurs in reaction haloes around clinopyroxene, amphibole and phlogopite, where it hosts microlites of new pyroxene, olivine and locally carbonate. Glass veins cut the mantle minerals and locally contain primary carbonate. Glasses vary widely in composition depending on the textural domains and attain Si- and alkali-rich compositions (SiO 2 = 47.0–68.3 wt.%; Na 2O + K 2O = 5.8–12.2 wt.%). Incompatible trace element patterns of glasses in anhydrous xenoliths are closely similar to those of the host alkali basalts, whereas the compositions of interstitial and vein glasses in the hydrous xenoliths indicate that a compositional control has been exerted by the local mineral assemblage (mainly amphibole). The δ 18O values of carbonate from the glass pockets and veins in the xenoliths, as well as of carbonate globules and amygdales in the host basalts are in the range 19.62 to 21.04‰. Corresponding δ 13C values are − 9.25 to − 10.12‰ and − 7.59 to − 9.32‰, respectively. These values are very different from those of primary carbonatites and the δ 18O values clearly exceed those expected for minerals and glasses from mantle assemblages. The similarity of isotopic ratios of carbonates from both xenoliths and host lavas and their shift towards low δ 13C and high δ 18O values may be the result of basalt-peridotite interaction during ascent of the mantle xenoliths. Our study points to a close relationship between the infiltration of carbonic fluid together with fractions of the host alkali basalt, and melting of hydrous peridotite-forming minerals. Assuming an initial average content of 1.5–2 wt.% CO 2 in the primary alkaline melt and considering that the dissolved amounts of CO 2 and H 2O in such a melt at 400 MPa can be in the order of 0.3 wt.% and 3.5% respectively, approximately 75 to 50% of the total carbon dioxide load was released by the uprising host magma. This process led to infiltration and entrapment of high-density CO 2 inclusions in the GG mantle rocks, to hydrous phase breakdown and to carbonate precipitation in veins and at some reaction sites after the primary mantle minerals. We propose that formation of CO 2 inclusions, glass and carbonate in hydrated mantle xenoliths is not unique of carbonatite metasomatism: comparable effects can be produced by degassing alkaline magmas.