Crystal mush remobilization in the Bandelier magmatic system: evidence from compositional zonation in clinopyroxene

Abstract

Compositionally zoned crystals can record changing melt composition and trace element partitioning behavior during magmatic differentiation. Diffusive reequilibration between compositionally distinct zones in crystals can also produce compositional gradients. Here, we compare the length scales of concentration gradients for different elements in clinopyroxene that originate from the Tshirege Tuff and late Valle Toledo Member rhyolites of the Bandelier magmatic system in New Mexico to determine what petrogenetic information is recorded in the zonation. Within these rhyolites there are unzoned ferrohedenbergite crystals, as well as less common normally-zoned clinopyroxene with ferrohedenbergite rims and ferroaugite cores. Compared to the ferroaugite cores, the ferrohedenbergite rims are enriched in Dy and Yb, but depleted in Co, Ti, Sc, Ce, and Nd. The length scales for fast and slow diffusing elements for most gradients measured are indistinguishable, which argues that the gradients emerged predominantly from changing magmatic composition during crystallization, with diffusion having little to no role in establishing the concentration gradients. Fractional crystallization of the phases present in the rhyolites fails to reproduce all trace-element zonation that occur in the clinopyroxene, however, indicating a more complex origin. Based on the compositional similarity of the ferroaugite cores with pyroxene from rhyolites that erupted ≥ 165 kyr earlier, we interpret the ferroaugite cores as antecrysts scavenged from crystal-rich mush during magmatic rejuvenation. The magmatic rejuvenation that remobilized the parent mush of the ferroaugite antecrysts was likely initiated near the end of a > 100 kyr eruption hiatus that preceded the final runup to the catastrophic Tshirege eruption.