SPHENE (TITANITE) AS TRACER OF PROCESSES IN FELSIC MAGMAS: EXAMPLES FROM MIOCENE ROCKS OF THE COLORADO RIVER EXTENSIONAL CORRIDOR (CREC), NV-AZ-CA

In the CREC, sphene is abundant in quartz monzonites and granites and as phenocrysts in rhyolite lavas and tuffs, and as a late interstitial phase in some gabbros and diorites. Phenocryst rim/glass measurements yield extreme REE partition coefficients, especially for the MREE (La 70-120; Sm 550-1200; Eu 500-650 [negative anomaly]; Gd 600-1300; Lu 110-240), demonstrating sphene’s potential for influencing absolute abundances and patterns of REE in evolving melt once it saturates (cf. Bachmann et al. CMP 2005). Application of the Zr-in-sphene thermometer (Hayden et al CMP 2008) yields temperatures mostly between 715 and 760 C. These relatively low temperatures, restriction of sphene in volcanic rocks to those with high-Si rhyolite glass, and limitation of evident sphene effect on REE in plutonic rocks to leucogranites and aplites, all suggest that sphene saturates and crystallizes abundantly in evolved melts, and that its occurrence in less felsic plutonic rocks records growth from evolved interstitial melt.

Two CREC examples demonstrate behavior of sphene and its use as a magmatic tracer. (1) In the Highland Range, high-Si rhyolite lavas and tuffs have strong MREE depletion and muted Eu anomalies, indicating sphene fractionation. Sphene phenocrysts are normally zoned (REE and Zr depletion toward rims), except in a mafic enclave-rich lava where they are reversely zoned and have ilmenite reaction rims. REE patterns (as well as concentrations of other trace and major elements) of these rocks match those of leucogranites in the nearby Searchlight pluton, thought to be the intrusive counterpart of the Highland Range volcanic sequence. (2) Sphene in outflow of the Peach Spring Tuff (PST) supereruption is normally zoned, but in intracaldera tuff it is reversely zoned. Sphene abundance, crystal size distributions, and morphology revealed by X-ray tomography confirm this distinction and indicate that the portion of the PST chamber from which intracaldera tuff erupted experienced a major heating event, accompanied by sphene resorption, that had little influence on the shallower(?) portion that yielded outflow.

Further insight into the behavior of sphene in magmas is likely to be gained from an experimental program underway to determine saturation as a function of melt composition, P, and water content.