Crustal magmatism under a hydrothermal system, and the imprints of assimilation of hydrothermally altered protolith: an investigation of geochemical signatures in rhyolitic magmas at Yellowstone caldera

Abstract:

Yellowstone caldera, Wyoming, hosts one of the largest hydrothermal systems on Earth, fueled by heat and volatiles released from hotspot-derived basalt magmas that stall in the crust. Prolonged hydrothermal activity has pervasively altered the subsurface and such altered material is presumed to have acted as a source for magmas erupted after the two largest caldera eruptions, as evidenced by low-δ¹⁸O signatures in these magmas. This study focuses on the youngest Yellowstone volcanic units, the ~ 255 ka to ~ 70 ka large volume (~ 360 km³) Central Plateau Member (CPM) rhyolites. New laser-ablation ICP-MS whole rock, glass and mineral trace element data were obtained in order to refine existing constraints on CPM petrogenesis. Small temporal increases in elements such as As (3.1-4.1 ppm), Rb (170-200 ppm), Cs (3.6-4.3 ppm), Pb (26-31 ppm), Th (23-27 ppm) and U (5.4-6.8 ppm) contrast with increases of ~ 40-50 % in HFSE and REE in the same samples. The highest observed temporal increase is that of Zn, from 65 to 105 ppm. Caesium is highly incompatible with mineral/glass partition coefficients K_D < 0.05 measured in all investigated mineral phases. Rubidium is also incompatible but its sanidine/glass K_D ~ 0.4 results in a larger bulk distribution coefficient D_Rb ~ 0.2. For Pb, sanidine/glass K_D ~ 0.8 leads to D_Pb > 0.4. Zinc is observed to be compatible in clinopyroxene, fayalite, zircon, chevkinite (K_D ~ 5-12), and Fe-Ti oxides (K_D ~ 40), such that D_Zn may approach 1. Fractional crystallization or partial melting processes alone cannot explain the same small increase rate of elements with diverse degrees of incompatibility (Rb, Cs and Pb), nor a larger increase rate in nearly compatible Zn. Assimilation by the juvenile CPM magmas of a crustal material of distinct composition appears to be required, and hydrothermally altered rhyolites, comprising much of the Yellowstone subsurface represent the most likely assimilant. Lower Rb, Cs, Pb (perhaps also As and U) and higher Zn concentrations in this altered rock horizon would explain the unusual temporal variations in these fluid-mobile elements in CPM lavas. As the CPM magma bodies developed and evolved, fractionating some mineral phases, they also assimilated these altered rhyolites, modifying incompatible trace element increase rates.