Constraints on the depths and temperatures of basaltic magma generation on Earth and other terrestrial planets using new thermobarometers for mafic magmas
Introduction
Active basaltic volcanism is the surest indication that a planet is still dynamically active (BVSP, 1981). The melting that drives volcanism signifies that primordial or radioactively generated heat is being lost from the interior of the planet. Melting itself is one of the fundamental processes that drive planetary differentiation, that is, the gradual segregation of a planet into different compositional layers. For example, Earth's oceanic and continental crusts are ultimately the by-products of partial melting of the mantle. Because the compositions of melts are influenced by the thermal state of a planet, magmas can be used as records of the thermal and dynamic evolution of planetary interiors. In this paper, we use thermobarometric approaches to constrain the temperatures and pressures of melt extraction on Earth and different planetary bodies. Our results are used to discuss the origins of basaltic magmatism in the solar system.
Section snippets
Estimating pressure and temperature of mantle melting
Attempts to estimate the temperatures and pressures of melting have been relatively successful for the Earth's mid-ocean ridges (Klein and Langmuir, 1987, Langmuir et al., 1992, Plank and Langmuir, 1992). For example, the temperatures of mid-ocean ridge basalt (MORB) source regions in the mantle are determined by the distribution of Fe and Mg between olivine (a common liquidus phase of basaltic magmas) and basaltic melt (Roeder and Emslie, 1970, Beattie, 1993, Gudfinnsson and Presnall, 2001,
Melting Earth's mantle
In this section, we apply these thermobarometers to different tectonic environments on Earth. Earth is unique among the planets in our solar system because it has an active interior and a mobile surface manifested in the form of plate tectonics. Below, we discuss how melting occurs in the various geologic environments found on our planet.
Melting on other terrestrial planetary bodies
To place Earth in context, we now explore basaltic magmatism on other planetary bodies. Meteorite and spectral data on other planetary bodies are limited, but the data are sufficient for developing first-order constraints on the thermal states of the Earth's Moon, Venus, Mars, and meteorite parent bodies.
Epilogue
We end our tour of planetary magmatism with robust conclusions and open questions. Earth is a dynamic planet, resulting in a diversity of mantle melting environments and lithospheric thicknesses. The present average temperature of the Earth's mantle, as represented by passively upwelling mantle beneath mid-ocean ridges, is between 1300–1400 °C. At subduction zones where hot and young oceanic lithosphere descends back into the mantle, temperatures of the asthenospheric mantle wedge are nearly as
Acknowledgments
We thank F. Albarede, J. Blichert-Toft, A. Lenardic, Q.-Z. Yin, B. Jacobsen, M. Manga, R. Dasgupta, P. Asimow, M. Collier, M. Blondes, J. H. Jones, Z.-X. A. Li, M. Hirschmann, S. Hart, and M. Jackson for discussions over the years it took us to start and finish this manuscript. We are grateful to P. Asimow and K. Putirka for their thoughtful and constructive reviews. This work was supported by NSF grants to Lee and Plank and a Packard Fellowship to Lee, but the views expressed here are
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