On the Source of Early Syn-rift Magmatism: Potential Contributions from Mafic, Ultramafic, and Hydrous Phases in the Lithosphere and Asthenosphere

Abstract:

Magma produced during the early stages of continental rifting is often enriched in incompatible elements compared to later stage magmas, which trend toward the trace element and isotopic compositions of a depleted mantle (midocean ridge basalt) source. These early-rift enriched magmas are suggested to originate from the melting of fertile domains in either the asthenosphere or the lithospheric mantle, which undergo decompression melting after only minor amounts of extension. Fertile hydrous, mafic, or ultramafic phases have been suggested to be introduced into the upper mantle from rising plumes, ancient subducted lithosphere, or previous subduction related fluid or melt metasomatism. We use a 1D decompression melting model to examine the behavior of hydrous (wet lherzolite), ultramafic (dry lherzolite, pyroxenite) and mafic (eclogite) phases in the mantle during rifting. The models show that fertile phases (pyroxenite, eclogite, and wet lherzolite) in the mantle begin melting almost immediately after rifting begins if the pre-extension lithosphere is ca. 120 km thick or less, whereas dry lherzolite doesn’t begin melting until the lithosphere has thinned to roughly 60 km (100% extension). Wet lherzolite is capable of producing early syn-rift melt even if the pre-rift lithosphere is very thick (200 km). In comparison, the lithosphere must thin to roughly 130 km before pyroxenite and eclogite phases in the mantle start to melt. The melt volumes and rates for all phases increase with hotter geotherms, however the eclogite source is less affected. All phases melt earlier when the rift rate is accelerated, because the lithosphere thins faster and reaches the necessary thickness for melting sooner. However, the rift rate has little effect on the total melt volume produced by the fertile and non-fertile phases as the rate of extension appears to only quicken or slow the melting.