Goldschmidt 2015 (Prague)

This Goldschmidt I will be talking about how modern basalts might preserve a chemical record of Precambrian redox evolution. Eleanor Jennings has an invited talk on some fantastic new Thermocalc results showing how the major element chemistry of basalts is affected by lithological heterogeneity.

Solid-Earth Oxygen Cycling Revealed by Enriched Mantle Domains

Authors: Oliver Shorttle, Yves Moussallam, Margaret Hartley, John Maclennan, Marie Edmonds & Bramley Murton

Session: 22c Precambrian redox evolution

When/where: Tuesday 18th August, 11:15 in Panorama Hall

Basaltic seafloor has been forming at mid-ocean ridges and being recycled back into the mantle for billions of years. During its ~100Myr residence at the Earth’s surface this basaltic crust is subject to significant alteration, as heat from its cooling drives vigorous hydrothermal circulation. The alteration involves chemical exchange between the basalt and circulating seawater, in addition to affecting the isotopic and trace element chemistry of the crust, this exchange could change the crust’s oxidation state if oxidised species such as sulfate in seawater are reduced during interaction with the crust. The recycling of this altered crust back into the mantle, followed by its 100-1000Myr residence time in the mantle means that modern basalts that sample ancient recycled material could provide a window into ancient ocean-crust redox processes.

Here I present XANES iron speciation data from basalts erupted along the Reykjanes Ridge south of Iceland. The basalts show a trend of increasing oxidation towards Iceland, concurrent with increasing proportions of recycled oceanic crust in their source (see also Shorttle et al. 2014). If this signal can be linked to alteration at a ridge axis, then we have evidence for the oxidising nature of ancient seafloor-hydrosphere interaction.

The Composition of Melts from a Heterogeneous Mantle: Application of a Thermodynamic Model

Authors: Eleanor S Jennings, Tim J Holland, Oliver Shorttle, Sally A Gibson & John Maclennan

Session: 20e Magma generation and evolution – A symposium in honor and memory of Michael J. O’Hara for his life-long contributions to understanding the working of the Earth and other planets by means of petrology and geochemistry

When/where: Tuesday 18th August 10:45 – 11:00 in Meeting Hall IA

Despite the inevitability of plate recycling having created a highly lithologically heterogeneous mantle, it has been notoriously difficult to infer the presence of these mineralogically diverse mantle domains from the chemistry of erupted basalts. A major challenge has been in having an accurate forward model to predict how the major element chemistry of a basalt reflects the pressure, temperature and mineralogy of its mantle source. Eleanor has used Thermocalc to rigorously investigate the consequences of varying mantle major element composition on the composition of fractional melts. A key finding is that as the source composition is changed the concomitant shifts in mineral abundance partly buffer the composition of fractional melts, such that basalts can significantly under represent variation in source mineralogy.