53 / 2023-04-11 11:39:14

Planetary evolved magmatism constrained by partition coefficients between zircon and silicate melt

zircon, partition coefficient, evolved melt, early Earth, early solar system

高压物理与材料科学 > 高压地球科学-海报

Partition coefficients between zircon and melt can provide insights into the evolved magmatism of early planets. However, trace elements such as P, Mg, and Ca can be incorporated into zircon through alteration processes, the element P among which potentially affecting the partitioning behavior of other elements between primary zircon and melt. To investigate this effect, we conducted high-temperature experiments examining the partition coefficients of trace elements between zircon and silicate melt at varying P concentrations. Our results show that increasing P content negatively affects the partition coefficients of alkaline elements and positively affects Al, without significant influence on Ti or P itself. Zircons in this study typically have concentrations of Mg, Ca, Al, P, and La below 150 ppm, 100 ppm, 500 ppm, 1000 ppm, and 1 ppm, respectively. These values can serve as screening criteria to distinguish primary from altered zircon, in combination with other literature-based criteria. We found that lunar and Vesta zircons have mostly retained their primary compositions, while some mesosiderite zircons and all Martian zircons have been altered. By filtering out altered zircon and using the experimentally updated partition coefficients for REE, we can better understand the characteristics of the evolved melt equilibrated with early protogenetic zircon, shedding light on early magmatism in the solar system. Notably, our findings indicate that P content has a negligible effect on REE partitioning, implying that the REE partition coefficients we derived can be applied to extraterrestrial zircon even with varying P concentrations.

        Here we have calculated the log fO₂ and (La/Yb)_N values of melts equilibrated with screened primary zircon, using our updated partition coefficients in combination with a previous log fO₂ model. Our results show that the log fO2 of the equilibrium melt was -3.33±1.14 for Hadean Earth, -9.26±1.20 for the Moon, -7.32±2.07 for Vesta, -11.23 for H5 chondrite, and -11.19 for mesosiderite. This indicates that the early solar system had a range of log fO₂ environments. The  (La/Yb)_N values of the melts equilibrated with early primary zircon were below 10, similar to the 4.02 Ga Acasta gneiss on Earth and typical extraterrestrial evolved rocks, but significantly smaller than Earth’s TTG. Consistently, both the 4.02 Ga Acasta gneiss and extraterrestrial evolved rocks have higher content of mafic major elements than young Earth’s TTG. Therefore, it appears that early evolved magmatism occurred under conditions without plate tectonics in the early solar system, including Hadean Earth. The high  (La/Yb)_N values and low content of mafic components in younger TTG suggest later delamination of denser, garnet-containing material from Earth’s crust into the mantle.