Trace element partitioning between MgAl2O4-spinel and carbonatitic silicate melt from 3 to 6 GPa, with emphasis on the role of cation order-disorder

Abstract

To better understand the partitioning of trace elements between spinels and silicate melts at high P, a series of experiments have been conducted in the system CaO–MgO–Al2O3–SiO2–K2O–CO2 at the P-T conditions of 3–6 GPa and 1500–1650 °C. These experiments successfully produced large MgAl2O4-spinel crystals coexisting with large carbonatitic silicate melt pools (plus large crystals of olivine and garnet in some cases). The experimental products were analyzed by using an electron microprobe in the wavelength-dispersive mode and an Agilent 7500 Ce laser ablation inductively-coupled plasma mass spectrometry, which led to high-P partition coefficients for 34 elements. The results suggest that pressure may have nominal significant impacts on the partition coefficients of some trace elements such as Nb, Ta, Ti, and nearly all rare earth elements (REEs), with the partition coefficients varying in several orders of magnitudes, but may play negligible roles in the partitioning of U, Mn, Fe, Sr, Ba, Na, K, Rb, and etc. Further analyses have revealed that much of the nominal P effects can be assigned to the Mg–Al cation order-disorder reaction. Cation exchange reactions are common in many geologically important minerals such as olivine, garnet, pyroxene, and plagioclase, and their effects on trace element partitioning should be extensively investigated in the near future before better understanding of the trace element partitioning systematics can be achieved. As one example, our experimental results have clearly demonstrated that due to much larger degrees of the Mg–Al cation order-disorder triggered by high P, spinel can strongly fractionate the REEs (with some heavy REEs compatible in the spinels and thus depleted in the relevant magmas), a phenomenon conventionally attributed to the presence of garnet. Some other important conclusions from our experiments include that Cr (or Ni) in the spinel structure at Cr-poor (or Ni-poor) conditions may be not as compatible as conventionally thought for Cr-rich (or Ni-rich) conditions, and Fe isotopes may be fractionated by spinels at high P.