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Oxidation states of continental arc magmas are raised by assimilation of the plutonic roots of the arc


IPGP - Îlot Cuvier


Séminaires Systèmes Volcaniques

Salle 310

Michael Dungan

University of Oregon

Subduction-related magmas (arc magmas) are distinguished from magmas at mid-ocean ridges and in intra-plate settings by generally higher oxidation states. This and other particular features of arc magmas are attributed primarily to water-rich contributions from subducted lithologies. Nonetheless, the precise nature of the oxidizing agents in these melts and fluids has not been definitively identified. The topic of this presentation is whether such magmas undergo significant changes in oxidation state during intra-crustal evolution. Shallow degassing of a mostly H2O-CO2 volatile phase does not seem to impose large changes, but the presence of either oxidized or reduced sulfur probably has an impact. Attempts to determine if fractional crystallization leads to systematic oxidation or reduction have found relatively minor effects. Mantle-derived magmas almost inevitably interact with the crust, and open-system processes involving assimilation of the roots of continental arcs have the potential to contribute ferric iron to such magmas by incorporating some combination of amphibole, mica, and oxides. Hydrous minerals are vulnerable to breakdown upon digestion in mafic magmas, and if Fe-rich oxides are involved in melting reactions the Fe3+/ΣFe in host melts will increase substantially, thereby raising the magmatic oxidation state. An investigation of a suite of rapidly erupted basaltic magmas (~235 ka) at the Tatara-San Pedro complex (TSPC) has identified increases in ƒO2 recorded by groundmass Fe-Ti oxide pairs that correlate with whole-rock indices of crustal additions (e.g., Rb/Y). Parental magmas are near NNO, and the most oxidized sample is at NNO +1.4 (average of 5 pairs). This also has revealed rarely appreciated features such as partly equilibrated xenocrystic Fe-Ti oxide grains inherited from plutonic lithologies, which may have contributed to the continued stability of two oxides in mafic magmas (~5.5-8.5 wt.% MgO), crystallization along both limbs of the rhombohedral solidus down to ~560°C, and the impacts of non-binary substitutions (particularly Mg and Mn) on values of T and ƒO2 calculated with the Ghiorso and Evans (2008) model. Some potentially problematic issues have arisen with respect to model results. I will present an integration of field observations, paleomagnetic data, petrography and mineral chemistry, whole-rock compositions, including radiogenic isotopes, and will emphasize variations manifested by ~120 Fe-Ti oxide pairs from 21 samples. This large dataset allows for the detailed examination of many facets of Fe-Ti oxide petrology that are not readily accessible with smaller datasets. To attend online (Zoom meeting): Meeting ID : 820 5033 5445 Password : 696019 Biography: Native of Minnesota: BA Macalester College (1969), PhD University of Washington, Seattle (1974) Postdoctoral Appointments: Rice University (1974-75) & Johnson Space Center (1975-78) Houston TX Studies of diverse lunar samples, including Apollo 12 Ilmenite Basalts, plus magma mixing in MORB 22°N MAR. Professor at Southern Methodist University, Dallas, TX (1978-1992) Professeur de Volcanologie, Université de Genève, Genève CH (1993-2012) Continuing studies of: (1) The TSPC, Longaví, Llaima, and other SVZ volcanoes 36-41°S, Chilean Andes (2) Central San Juan caldera complex, with a strong emphasis on the Fish Canyon magmatic system Courtesy Professor, University of Oregon, Eugene OR (2012 - present day).