WORK ON THE ZIRCON, HIGHLIGHT THE ALLANITE (Invited Presentation)

Allanite is an accessory mineral in silicic systems that provides complementary temporal and chemical perspectives about magma assembly and storage to those of zircon. For the voluminous (~2800 km³) 74 ka Youngest Toba Tuff (YTT), a significant fraction of zircon cores may have been carried over from an earlier magma reservoir and mantled for up to 400 kyr by younger growth [1]. In contrast, compositions and ages show allanite to be cognate to the YTT. Moreover, allanite populations generally have compositional affinities to host pumices for some fraction of their growth. Dates for domains within individual YTT allanite crystals are interpreted to represent those of crystallization. They predate eruption by 10s of kyr to ~150 kyr (compared to 100s of kyr for zircon) and show that crystallization occurred for a significant fraction of the repose interval before YTT eruption. Allanite from the most evolved pumices have the greatest range in composition as well as an apparent crystallization temperature range of ~700-780°C, based on a temperature-dependent MnO/MgO exchange coefficient [2]. Compositionally-discrete domains persisted within the YTT for >100 kyr, judging by contemporaneous growth of allanites with affinities to host pumices. In the final ~40 kyr leading up to eruption, allanites from most pumices partially overlap in composition, and the compositional range overall expands to the most and least evolved compositions. Based on our results, we infer that the YTT magmatic system was composed of at least two relatively homogeneous, non-communicating rhyolitic magmas during the interval between ~200 ka and 115 ka. The compositionally different magmas subsequently coalesced and mingled, accompanied by intrusion and crystallization of rhyodacitic magmas. The younger age distribution of YTT allanites compared to zircons is favored at the cool and wet eutectoid conditions that characterize at least half of the YTT. Our results suggest that periodic heat ± water fluxes dissolved allanite but had only a minor effect on larger crystals in the zircon population. They also enhanced growth of younger allanites when allanite saturation reoccurred [3].

[1] Reid and Vazquez (2017) G-cubed; [2] Vazquez and Reid (2004) Science; [3] Tierney et al. (submitted) CMP.