RECHARGE AND MOBILIZATION OF CRYSTAL MUSH TO PRODUCE AND ERUPT A ZONED MAGMA CHAMBER – THE TSHIREGE MEMBER OF THE BANDELIER TUFF, VALLES CALDERA, NEW MEXICO, USA
Boro, Joseph Robert
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The 1.26 Ma Tshirege Member of the Bandelier Tuff is the second of two major (~400 km3, dense rock equivalent) compositionally zoned rhyolitic eruptions from the Valles caldera. This eruption was triggered by a dacitic recharge magma which mixed and mingled with the rhyolite prior to eruption contributing heat and H2O from second boiling. The recharge magma interacted with a heterogenous crystal mush pile which was produced by the progressive fractional crystallization of a rhyolitic magma body. The chemistry of this rhyolite was recorded in the form of melt inclusions found within quartz in the mush pile. Faceting of melt inclusions and modeling diffusive relaxation of [Ti] across growth boundaries in quartz shows that the Tshirege magma was fractionated over a period of ~1,000-10,000 years and then the mush was incrementally mobilized over the period of 100-1,000 years. This mobilization was mostly affected by the addition of H2O from second boiling of the invading recharge dacitic magma. Continued recharge of the system eventually over pressurized it to a point of assured eruption.