Highly siderophile elements mobility in the subcontinental lithospheric mantle beneath southern Patagonia

Abstract

Peridotite xenoliths collected from alkali basalts in the Argentinian Patagonia reveal the existence of an ancient depleted Paleoproterozoic mantle that records a subsequentmultistage metasomatic history.Metasomatismis associatedwith carbonatite-likemelts that evolved, after variable melt/rock ratio interaction, towards CO2-rich and Na-bearing Mg-rich (mafic) silicate, and volatile-rich alkali silicate melts. High degrees of partial melting produced strongly depleted mantle domains devoid of base-metal sulphides (BMS). Moderate degrees of partial melting and later unrelated metasomatism produced a range of slightly depleted, slightly enriched, and strongly enrichedmantle domains that preserve different types of BMS. Thus, six different BMS populationswere identified including typical residual Type 1A BMS enriched in Os, Ir, and Ru relative to Pt, Pd, and Au locatedwithin primary olivine and clinopyroxene, andmetasomatic Type 2A BMS that are relatively enriched in Pt, Pd, Au occurring as interstitial grains. Reworking of these two types of BMS by later metasomatism resulted in the formation of a new generation of BMS (Type 1B and Type 2B) that are intimately associatedwith carbonate/apatite blebs and/orempty vesicles, aswell aswith crypticallymetasomatised or metasomatic clinopyroxene. These newly formed BMSwere re-enriched in Os, Pd, Au, Re and in semi-metal elements (As, Se, Sb, Bi, Te) compared to their Type 1A and Type 2A precursors. A third generation of BMS corresponds toNi-Cu immiscible sulphidemattes entrainedwithinNa-bearing silica under-saturated alkali melt. They occur systematically related to intergranular glass veins and exhibit distinctively near flat CI-chondrite normalised highly siderophile element patterns with either positive Pd (Type 3A) or negative Pt (Type 3B) anomalies. Our findings indicate that Os, Pd, Re and Au can be selectively transported by volatile-rich alkali silicate melts in the subcontinental lithospheric mantle.Moreover, the transport of sulphide mattes entrained in silicate melts is also an effective mechanismto produce HSE endowment in the SCLMand play an important role as precursors of fertile, metal-rich magmas that form ore deposits in the overlying crust.