Zircon petrochronology and 40Ar/39Ar sanidine dates for the mesa falls tuff: Crystal-scale records of magmatic evolution and the short lifespan of a large yellowstone magma chamber

Abstract

The 1·3Ma Mesa Falls Tuff (MFT), the second and volumetrically smallest of the Yellowstone caldera-forming eruptions, was examined using a joint zircon petrochronological and sanidine 40Ar/39Ar approach to constrain the thermal and chemical evolution, autocrystic growth, antecrystic recycling, and eruptive age of the host magma. A total of 451 laser ablation inductively coupled plasma mass spectrometry in situ spot analyses collected from 323 zircon crystals from five pumice blocks and two welded ash-flow tuff samples provide trace element and Ti-in-zircon thermometry data, which are in turn complemented by high-precision 206Pb/238U dates from over 50 of those grains. Sanidine grains from two of the pumices were analyzed by incremental step-heating or total fusion 40Ar/39Ar dating techniques performed on single crystals using a multi-collector mass spectrometer, yielding an eruption age of 1·300 6 0·001 Ma. Zircon dates range from 1·57 to 1·30 Ma. Rare grains older than 1·37Ma may contain inherited cores recycled from the Huckleberry Ridge Tuff (HRT) or other associated smaller volume, effusive Yellowstone magmas; however, the bulk of the Mesa Falls Tuff crystal load cannot be attributed to a long-lived, residual Huckleberry Ridge Tuff magma body. Zircon compositions define trends of strengthening negative europium anomaly and increasing incompatible trace element concentrations over ~150 °C of cooling. Crystals defining this full compositional spectrum range in age from 1·33 to 1·30 Ma; the dominant mode of 19 grains yields a mean crystallization age of 1·303 6 0·002 Ma, within uncertainty of the sanidine 40Ar/39Ar age, attesting to the rapidity of magma accumulation, differentiation and crystallization prior to eruption. A subset of composite grains composed of extremely differentiated core compositions overgrown by Mesa Fall Tuff-like rims probably represents earlier solidified sidewall or roof accumulations later remobilized within the main Mesa Falls Tuff magma. Fractional crystallization modeling utilizing temperature-dependent zircon-melt partition coefficients is successful in reproducing the trends in incompatible trace element enrichment within zircon grains as a function of decreasing temperature and increasing europium anomaly. Zircon geochemistry thus provides a robust proxy for magma evolution, from the time of zircon saturation through differentiation and eruption. Integrated sanidine and zircon dates coupled with the thermochemical trends indicate that the Mesa Falls Tuff magmatic system differentiated over a period of < 30 kyr, with the bulk of zircon crystal nucleation and growth occurring within 10 kyr of eruption. These petrochronological studies of the MFT and HRT clearly illustrate that the long-term volumetric extrusive rate at Yellowstone (~2 × 10-3 km3 a-1) is punctuated by episodes of much higher magmatic flux (~2 × 10-2 to ~2 × 10-1 km3 a-1).