Mid-Crustal Magmatic Assimilation In Arc Magmas: their effect on Sr/Y and La/Yb ratio in different P and T conditions

Abstract:

Recent studies have observed an empirical correlation between Sr/Y and La/Yb ratios in whole-rock data to crustal thickness. Due to the association, these studies have suggested that these trace element ratio “pseudobarometers” can be used indirectly to calculate the depth of arc magmas, finding the thickness of the crust. Changes of trace element ratios are assumed to be due to partitioning of various pressure-dependent phases assemblages during melting. However, several articles argue the validity of pseudeobarometers as a proper way to calculate crustal thickness due to discrepancies between the two ratios, as well as geologically unrealistic model crustal thicknesses. To test the validity of these pseudeobarometers, we simulated partial melt formation in an arc magma setting using the thermodynamic modeling software PELE. The composition of the depleted mantle was collected from the GERM database. The fluid slab component is based on experimental data of a theorized fluid released at subducting. Melt is simulated to form at pressures between 1.5 Gpa-3.0 Gpa and temperatures between 1150°C-1300°C. The partial melts are then assimilated with a mid-crustal source, taken as the average continental crust composition from GERM. This is done to test how Sr/Y and La/Yb are affected by the assimilation of a crustal source and how the trace element ratios are comparable to real-world data. Therefore, we created a mixing model of assimilation with fractions of 5%, 10%,20%, and 30% crustal material, representing the amount of mid-crustal rock added to the resulting arc magmas. Our results show an increase in Sr/Y ratios and a decrease in La/Yb but are not comparable to real-world data seen in the literature, even with the assimilation of 30% of the crustal source. Failing to produce data seen elsewhere, this implies that there are additional processes involved in forming Sr/Y and La/Yb trace element ratios, suggesting that pseudobarometers are not solely an indicator of crustal thickness, but also of other geochemical processes.