Paper No. 37-8
Presentation Time: 3:30 PM
INCREASED OXIDATION STATES OF ARC BASALTS DURING REACTIVE ASSIMILATION OF THE PLUTONIC ROOTS OF THE QUATERNARY ANDEAN SOUTHERN VOLCANIC ZONE, CHILE: THE UPPER PLACETA SAN PEDRO SEQUENCE (UPSPS) OF THE TATARA-SAN PEDRO COMPLEX (36 S)
Open systems are the standard-state of magmatic differentiation. This contribution builds on Dungan and Davidson (2004) and Costa and Dungan (2005): assimilation of diverse plutonic lithologies was nearly the sole process that created compositional variability in UPSPS magmas (~240 ka; ~50-52.5 wt.% SiO2; 67 samples). A Holocene San Pedro lava (Costa et al., 2002) contains a large suite of highly diverse crustal xenoliths, many of which contain modally abundant amphibole and phlogopite as replacement products of olivine and other phases. These underpin our interpretation. Post-incorporation, incongruent melting of hydrous minerals facilitated disaggregation of nominally refractory xenoliths and added incompatible elements to evolving UPSPS melts. Another consequence was increased Fe3+/ΣFe in melts, hence higher ƒO2, which was partly linked to high Fe3+/ΣFe in decomposed hydrous minerals (up to ~0.5) relative to mafic melts with oxygen fugacities near the NNO buffer assemblage (~0.20-0.27). Some of this oxidized iron was added to oxide xenocrysts, and such grains continued to undergo diffusive Fe2+⇌Fe3+exchange with host melts, thereby modifying both as they approached equilibrium. 130 Mgt-Ilm pairs (21 samples) yield correlations between increasing ƒO2 (NNO to NNO +1.8 log units) and decreasing temperature (~850 to 560ºC) with increasing indices of assimilation, such as Rb/Zr (0.14-0.35; ~8-28 ppm Rb), at nearly constant Mg# (~55-58; but, up to 64 in high-Rb, xenocryst-rich samples). The only impact of crystal fractionation appears to be partial losses of olivine and augite xenocrysts, leaving behind elemental fingerprints of crustal inputs in melts. Unusually low Mgt-Ilm equilibration temperatures in some thick basaltic-lava interiors are due to multiple factors, including the thermal cost of ingesting xenolithic debris. Quasi-rhyolitic melts, generated during assimilation, are manifested by alkali-feldspar rims on plagioclase and rare groundmass SiO2. Such components are inferred to have contributed to anomalously high volume-fractions of evolved near-solidus liquid, thereby facilitating low-T melt-oxide equilibration. Oxidation states of arc magmas cannot be assumed to exclusively reflect mantle-source characteristics plus minor modifications by crystal fractionation.