Syn‐Rift Magmatism and Sequential Melting of Fertile Lithologies in the Lithosphere and Asthenosphere

The timing and rate of decompression melting of a compositionally heterogeneous mantle during continental rifting are assessed with a new one‐dimensional geodynamic code, MELT1D. MELT1D computes pressure and temperature in the extending lithosphere and rising asthenosphere and calculates the resulting melt fraction and eruption rates for different lithologies. A series of models simulate syn‐rift melt production from (a) dry and wet depleted lherzolite similar to the mantle that underlies most mid‐ocean ridges, (b) dry and wet relatively fertile ultramafic compositions representing plume or primitive mantle material, (c) pyroxenite representing recycled ultramafic oceanic crust or magmatic metasomes, and (d) basalt representing recycled mafic crust or metasomes. The models predict sequential melting of the different compositions that is broadly consistent with basalt eruption histories in many Phanerozoic rifts. Results show a progressive transition in magma sources as the lithosphere thins, beginning with melting of wet mantle and compositionally fertile mafic components near the lithosphere‐asthenosphere boundary during the earliest stages of extension. This transitions to magmatism dominated by melting of relatively fertile ultramafic components (pyrolitic and pyroxenitic compositions) as extension progresses, and finally to melting of ambient lherzolite asthenosphere as lithosphere thinning approaches breakup. Mantle composition, pre‐rift lithosphere thickness, and mantle temperature exert the greatest controls on the timing and volumes of magmas produced from each lithology. In general, a cool or thick lithosphere has a greater capacity to sequester fertile lithologies than thin or warm lithosphere, and thus has a greater capacity to produce early syn‐rift magmas without requiring a hot mantle plume. Rifting is the process by which continents are broken apart and is generally associated with the production of magma. Magmas that are produced during early rifting are chemically different than those produced during later rifting. To understand which rocks deep in the earth may be melting to generate the different magmas at different times, we developed new computer code to model rifting scenarios in different places. Using this code, we estimate to what extent different rocks would melt during rifting and how much magma they would produce. We tested four different rock types: basalt, dry peridotite, wet peridotite, and pyroxenite. Our findings show that there is a relative order regardless of the rifting scenario in which each rock type starts to melt and when they produce the most magma. If all the rocks tested are present in the earth before rifting, then the earliest magmas produced during rifting will be from the melting of basalts and wet peridotite, followed by the melting of pyroxenite, and finally dry peridotite. These findings help to inform us of the sources of early rift magmas and what rocks may have been present deep in the earth before rifting took place. Melt produced from a compositionally heterogeneous mantle during rifting is assessed with a new 1‐D geodynamic modeling code, MELT1DLithosphere thickness, mantle composition, and temperature are the primary controls on the timing of melt production from each lithologyModels predict a transition from melting of wet and/or mafic components to fertile ultramafic components and finally to ambient mantle Melt produced from a compositionally heterogeneous mantle during rifting is assessed with a new 1‐D geodynamic modeling code, MELT1D Lithosphere thickness, mantle composition, and temperature are the primary controls on the timing of melt production from each lithology Models predict a transition from melting of wet and/or mafic components to fertile ultramafic components and finally to ambient mantle