Paper No. 14-6
Presentation Time: 3:10 PM
EVIDENCE FOR CRYSTAL-MUSH MOBILIZATION AND AN INTEGRATED MODEL FOR THE UPPER-CRUSTAL, OPEN MAGMATIC SYSTEM BENEATH HISTORICALLY HYPERACTIVE LLAIMA VOLCANO (38.7 S, CHILEAN ANDES)
Llaima is a large, hyperactive volcano that may have produced >50 historic eruptions. Eight extensively-sampled (n = 244) eruptions survive as flank lavas. A low preservation-probability is due to rapid burial (tephra, lava, landslides, ice) and local flank erosion by lahars. Crystal-rich historic lavas are mafic (~51-56 wt.% SiO2), as is most of the late Holocene cone, and they are heterogeneous to varying degrees for major and trace elements, and for levels of U-series disequilibrium. These must be manifestations of intra-crustal processes: mantle heterogeneity at Llaima corresponds to the minimum along the SVZ. Decreases from high Pa- and U-excesses (mantle-source signals) in least-evolved samples correlate negatively with Rb, K, U, and Th concentrations across a spectrum of ‘more-evolved’ samples, and reflect minor assimilation of U- and Th-rich crustal components in secular equilibrium. Low H2O and negligible CO2 in olivine-hosted melt inclusions from historic tephra deposits require entrapment of variably degassed melts at pressures down to 40 MPa. These signals plus variable compositions of MIs in single samples, multiple resorption surfaces that separate alternating growth zones of ~An85-80 and ~An65-60 in phenocrysts, and trends toward anomalously low Al2O3 in the most-evolved glasses (12-13 wt.% at 54-58 wt.% SiO2) are manifestations of plagioclase-rich phenocryst assemblages in hybrid crystal mushes that were stored just beneath the edifice. This presentation is focused on remarkably diverse olivine-zoning patterns in lavas. In some thin sections, reversely- and normally-zoned grains, with and without reversely-zoned shoulders, feature abrupt terminations of divergently-zoned grain margins, and ranges of core and edge compositions, thereby requiring mixing of new magma batches with multiple bodies of resident crystal mush on short time-scales. Repeated magma recharge and mixing during repose periods prevents complete solidification of mush bodies in sub-volcanic reservoirs located near the base of the edifice, despite a tendency for high crystal-growth rates related to degassing during shallow storage. The Llaima magmatic system tends to remain in a precariously perched state between eruption-triggering events, thereby creating conditions favorable to high eruption-frequency.