ACCESSORY MINERALS IN VOLCANIC SEQUENCES: ELUCIDATING MAGMA CHAMBER PROCESSES AND THE LINKAGE BETWEEN PLUTONS AND VOLCANISM

The accessory minerals zircon and sphene provide a record of the histories of the systems in which they reside and manifest a link between volcanic and plutonic realms. Analysis by SHRIMP permits evaluation of age (U-Pb, U-Th) and environment (elemental analysis) of growth zones. Zircon yields a more comprehensive record because it is more widespread, generally saturates at higher T, and can survive fluctuating conditions. Sphene, where present, provides very accessible information because of its large size and abundance and high concentrations of important elements and mineral and melt inclusions. Its presence or absence provides a useful environmental constraint because of its restricted occurrence, and its crystallization leaves a profound 'fingerprint' in evolving melt REE patterns.

Examples of implications from ongoing studies of zircon and sphene from a variety of volcanic settings:

(1) Arc volcano (Mt St Helens): Zircons from samples spanning the 300 ky volcanic history record growth 10's to 100's of ky before eruptions and Ts well below those of erupting magma . They thus reflect the plutonic realm beneath the volcano and reveal repeated interaction between stored 'plutonic' and invading 'volcanic' magma.

(2) Supereruption (Peach Spring Tuff, AZ-CA-NV): Sphene and zircon from intracaldera Peach Spring Tuff show strong 'reverse' zoning, with less evolved rim compositions (higher MREE in sphene, very low U in zircon; higher rim Ts). Zoning appears to be 'normal' and recorded Ts lower in distal outflow.

(3) Exposed plutonic- volcanic system (Searchlight -Highland Range, NV): Sphene is restricted to evolved rhyolites but has an extreme effect on REE in coexisting melt and evolution trends. In most samples it is normally zoned (lower T and REE in rims), but with extensive magma mingling there is reverse zoning and reaction to ilmenite. Identical REE fractionation is shown in rhyolites and comagmatic leucogranites in Searchlight pluton.

(4) Rhyolite from oceanic hot spot-rift setting (Iceland): Preliminary data suggest that zircon in historic Icelandic eruptions crystallized at high but variable T and reveals simple fractionation trends. The rhyolites are poor in zircon and phenocrysts in general and appear to reflect portions of magmatic systems that did not have well-established 'plutonic' zones.