GEOCHEMICAL STRATIGRAPHY IN THE YELLOWSTONE LAVA CREEK TUFF MAGMA CHAMBER: IMPLICATIONS FOR ERUPTION TRIGGERING MECHANISMS FROM MAJOR AND TRACE ELEMENT ZONING IN FELDSPAR

The Lava Creek Tuff (LCT) consists of members A and B that erupted about 640ka from the Yellowstone caldera and involved the evacuation of ~1000km³ of pyroclastic material from the magma chamber. Whole rock incompatible trace element concentrations (XRF and ICP-MS analyses) reconstruct the geochemical stratigraphy of the pre-eruptive LCT chamber. The results define compositional zonation trends that clearly distinguish between members A and B, as well as identification of the lower and upper sub units of each member (e.g., upper A Zr=185-300ppm, Nb=49-51ppm; lower A Zr=162-166ppm, Nb =67-74ppm; upper B Zr=280-365ppm, Nb=57-62ppm; lower B Zr=170-190ppm, Nb =53-69ppm). Such signatures could denote the presence or absence of the accessory mineral phases, chevkinite, allanite and zircon. Chevkinite and allanite are exclusive to member A, and zircon varies in abundance between the subunits of both member A and B. The chemistry of individual feldspar phenocrysts acquired by electron microprobe and laser ablation analyses help to further characterize the subunits of members A and B in the pre-eruptive magma chamber. Large (usually up to 4mm but exceptionally to 2cm) zoned feldspar crystals (Or₅₇–An₂ to Or₀₈–An₁₉) exhibit variable Ca and decreasing trace-element (Sr, Mg, Ti, Pb) concentrations towards the rim. Small-scale Sr variations in the core could indicate early recharge of low-Sr mafic magma with a short residence time that would have limited diffusion. Sanidine phenocrysts from the upper and lower parts of members A and B exhibit different zonation patterns for compatible trace elements such as Sr and Ba. Lower member B is more enriched in Ba in the cores (690 ppm in the cores, 380 ppm in rims) and upper member A is more enriched with Ba in the rims (2980 ppm in the cores, 5900 ppm in rims), suggesting a role for rejuvenation. The influx of heat at the base of the magma chamber that accompanies mafic recharging events affects the trace element zonation in minerals by fluctuating the parameters associated with the activation energy needed to facilitate diffusivity.