Constraints on the depths and temperatures of basaltic magma generation on Earth and other terrestrial planets using new thermobarometers for mafic magmas

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Abstract

Basaltic magmatism is a common feature of dynamically active terrestrial planets. The compositions of basalts reflect the temperatures and pressures of magma generation, providing windows into a planet's thermal state. Here, we present new thermobarometers based on magma Si and Mg contents to estimate the pressures and temperatures of basaltic magma generation on Earth and other terrestrial planets. Melting on Earth is intimately tied to plate tectonics and occurs mostly at plate boundaries: mid-ocean ridges and subduction zones. Beneath ridges, melting is driven by adiabatic decompression of passively upwelling mantle at 1300–1400 °C. Similar temperatures of melting are found for some arcs, suggesting that decompression melting is also important in arcs and that enhanced melting by hydrous fluxing is superimposed on this background. However, in arcs where melting temperatures are low (1200 °C), hydrous fluxing is required. Temperatures hotter than ridges (> 1400 °C) are primarily found away from plate boundaries: beneath thick continental lithosphere and oceanic “hotspots” like Hawaii. Oceanic “hotspots” are thought to derive from deep thermal upwellings (“plumes”), but some hot anomalies beneath continents are not associated with deep-seated plumes and hence must have different origins, such as thermal insulation or radioactive heating of metasomatized zones. Melting on Venus, as constrained from spectral data of its surface, occurs at higher temperatures (1500 °C) and pressures than on Earth, perhaps because Venus is characterized by a thick and stagnant upper thermal boundary layer that retards convective heat loss. In this regard, Venus' upper thermal boundary layer may be analogous to thick continents on Earth. Mars appears to have cooled off to < 1300 °C within its first billion years, but considerable controversy exists over the interpretation of young (< 500 My) basaltic meteorites that record temperatures of 1550 °C. As for the first billion years of Earth's history, its upper mantle was hotter than 1700 °C, hence melting commenced at pressures greater than 7 GPa, where melts could have been denser than residual solids, resulting in downward fertilization of the Earth's mantle.

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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