The Meaning of Pressure for Primary Magmas: New Insights From PRIMELT3‐P

Abstract

This paper reports new software, PRIMELT3-P, for computing the pressure and temperature range over which a primary basaltic magma was formed by adiabatic decompression and fractional melting of fertile mantle peridotite. The underlying model was developed to explore the meaning of magmatic pressure and its connection to the physics of mantle melting, melt extraction, and melt migration. We present a comparison of the results of the batch melting model FractionatePT3 (Lee et al., 2009, https://doi.org/10.1016/j.epsl.2008.12.020) and the new fractional melting model PRIMELT3-P, each applied to basalts from various localities, with a particular focus on Hawai'i and Iceland. The unexpected result is that pressures for batch melting and fractional melting are quite similar across a broad range of potential temperatures, agreeing to within ±0.29 GPa (1σ) when applied to magmas that pass tests for derivation from peridotite sources. Although samples from both Hawai'i and Iceland yield a range of final pressures of melting, the minimum final pressure in each location coincides with independent geophysical constraints—the depth to the lithosphere-asthenosphere boundary beneath Hawai'i and to the Moho below Iceland. These results clarify the meaning of primary magma pressure, but the concept remains complex. We suggest that computed temperature and pressure arrays arising from heterogeneous peridotite-source primary magmas may reflect melt transport and mixing in a thermally zoned mantle plume with a complex network of branching melt channels that nucleate over a range of depths.

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