Coevolution of sedimentary and strongly peraluminous granite phosphorus records

Abstract

Phosphorus plays an important role in both surface biological cycles as a critical macronutrient and in magmatic systems as the backbone for trace element-rich phosphate minerals. Phosphorus cycling between igneous and sedimentary systems throughout Earth history has had potentially profound effects on biogeochemical cycles. Previous studies have focused on the impacts of enhanced contribution of phosphorus from igneous rocks on marine phosphorus budgets. In contrast, the effect of changing sedimentary phosphorus abundances on the phosphorus budget of magmas has not been investigated. Here I present a compilation of phosphorus concentrations in strongly peraluminous granites (SPGs), which are derived through the partial melting of sedimentary rocks, spanning the past ∼3.5 billion years. Maximum phosphorus concentrations of SPGs increase abruptly in the Phanerozoic, closely following a similar, previously documented increase in phosphorus concentrations in marine siliciclastic sediments deposited after 720 Ma. The effect of metamorphic, magmatic, and subsolidus processes on SPG bulk-rock phosphorus concentrations are considered in turn. Lower average bulk-rock phosphorus concentrations in SPGs derived from metasedimentary rocks deposited prior to 720 Ma likely reflect lower phosphorus concentrations of their metasedimentary sources. The SPG compilation both confirms previous observations of phosphorus abundances in the marine sedimentary record and provides an alternative archive in that it integrates large volumes of continental shelf to slope sedimentary rocks. A clear shift in the crustal phosphorus cycle is documented with the initiation of enhanced recycling of sedimentary phosphorus into the igneous crust, where it can be again weathered and returned to the ocean to fuel further photosynthesis. Although phosphorus concentrations in zircon have been used to distinguish between derivation from strongly peraluminous "S-type" from metaluminous "I-type" granites, the results presented here suggest that phosphorus concentrations in SPGs have a time dependency and that phosphorus concentrations in zircon from SPGs may vary similarly in time.

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