Changes in the Thermal and Compositional Structure of the Earth’s Mantle, High-Mg LIP Magmatism, and Magmatic Ni-Cu-PGE Deposits Through Time
Changes in the Thermal and Compositional Structure of the Earth’s Mantle, High-Mg LIP Magmatism, and Magmatic Ni-Cu-PGE Deposits Through Time
Previously Published Material: Gave a talk on the temperature changes at the "Great Plume Debate" a few years ago, and have published some of the ideas on the ore deposits. This is an update and overview requested by the convenors.
Abstract ID#: 35943
English Abstract:
There was an abrupt change in the maximum MgO content of mantle-derived magmas at the Archean-Proterozoic boundary: from Archean high-Mg komatiites with up to 33% MgO erupted at up to 1660oC (or more if superheated), to Proterozoic low-Mg komatiites/ferropicrites with up to 22% MgO erupted at up to ~1440oC and Phanerozoic low-Mg komatiites/high-Mg picrites with up to 23% MgO erupted at up to ~1460oC. The ~10% MgO change in composition corresponds to a ~200oC change in eruption T and a 170oC change in the potential temperature of the mantle source regions. The abrupt decrease in maximum magma temperature, along with a wide range of other fundamental changes that occurred at the end of the Archean, indicate a fundamental and irreversible change in the thermal and/or compositional structure of the mantle at that time from one that could produce much hotter plumes but that involved only nominally hotter upper mantle. The origin is still being debated, but the most widely discussed scenarios are: 1) generation of a thermal/compositional boundary layer along the core-mantle boundary, 2) a change from whole-mantle to 2-layered convection, and 3) a change from 2-layered to whole-mantle convection. Regardless of the precise origin, the change profoundly influenced the viscosity of the magmas, their ability to ascend through the crust and therefore their volcanic/subvolcanic settings of emplacement, and the composition of associated magmatic Ni-Cu-PGE mineralization. Archean deposits are predominantly predominantly volcanic to very high level subvolcanic with higher Ni/Cu and lower Pd/Ir, Proterozoic deposits are volcanic with intermediate Ni/Cu and Pd/Ir, and Phanerozoic deposits are subvolcanic to plutonic with lower Ni/Cu and higher Pd/Ir. Most deposits, regardless of age, exhibit relatively flat mantle-normalized metal patterns and appear to be derived from primarily peridotitic mantle sources, but some are enriched in Co-Ni-Cu relative to Au-PGE and appear to be derived from pyroxenitic sources.