Geochemical and geophysical constraints on the volatile inventory of Earth's lithospheric mantle
Abstract
The subcontinental lithospheric mantle represents one of Earth's most long-lived and ancient chemical reservoirs. It potentially acts as both 'sink' and 'source' for volatiles (CO2, H2O, S, F, Cl, Li) and thereby plays a critical role in modulating their flux from our planet's deep interior to its atmosphere (via volcanism). Nevertheless, the volatile inventories of this major reservoir, and their variability beneath the continents and oceans, are poorly constrained. Geophysical studies have revealed large differences in the thickness and thermal structure of the lithosphere whereas mantle peridotites -- found as xenoliths, in orogenic massifs or dredged from the ocean floor - show systematic variations in composition between oceanic, continental on- and off-craton settings. We combine high-precision in-situ analyses of volatiles in the most abundant, nominally volatile-free phases (olivine & pyroxenes) with their respective modal proportions to estimate concentrations of H2O, F and Li in lithospheric mantle from oceanic, continental on- and off-craton settings. We use these analyses together with new estimates of global volumes of lithospheric mantle (from surface wave tomography) to calculate volatile inventories at different tectonic settings.
Our findings reveal, for the first time, that the H2O, F, Li (and CO2) budgets of the lithospheric mantle at each major tectonic setting are all on the order of 107 to 109 Mt. For mantle with only nominally volatile-free phases these amounts vary according to incompatibility of the volatiles in percolating metasomatic melts, i.e. CO2>H2O>F>Li. When modal estimates of phlogopite and amphibole are included the volatile inventory of H2O and F increases by an order of magnitude. Our calculations show that the volume of lithospheric mantle beneath the global cratons exceeds that beneath continental off-craton and oceanic regions and is the greatest repository for H2O (8.55 x 109 Mt), CO2 (3.53 x 109 Mt), F (1.77 x 109 Mt) and Li (8.13 x 107 Mt). Our internally-consistent estimates for volatile inventories provide important constraints on the potential of different regions of Earth's lithospheric mantle to act as a source of H2O, F, Li and CO2 for melts generated during major rifting and heating events (e.g. Large Igneous Provinces).- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V51I0158G
- Keywords:
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- 1038 Mantle processes;
- GEOCHEMISTRY;
- 1060 Planetary geochemistry;
- GEOCHEMISTRY;
- 8430 Volcanic gases;
- VOLCANOLOGY;
- 8450 Planetary volcanism;
- VOLCANOLOGY