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Snyder, D.C., 2005

Processes and time scales of differentiation in silicic magma chambers: chemical and isotopic investigations

Bibliographic Reference

Snyder, D.C., 2005, Processes and time scales of differentiation in silicic magma chambers: chemical and isotopic investigations: Oxford, Ohio, Miami University, Ph.D. dissertation, 216 p., illust.


This dissertation investigates the processes responsible for the chemical and isotopic evolution of silicic magmas, and the time scales over which these processes are operative through the application of chemical and isotopic data from plutonic (Alaska) and volcanic (Azores) rocks. The White Mountain granitoid suite represents a window into Cretaceous-age magma intruded into Wrangellia terrane basement. Three 40Ar/39Ar analyses provide cooling ages between 113.3 +/- 1.3 and 117.38 +/- 0.54 Ma, suggesting at least two pulses of magmatism are represented in the granitoid suite. Approximately 20% of the total exposed granitoid is enclave-bearing. Enclave rare-earth-element patterns and isotopic characteristics, and the lack of petrographic evidence for quenched margins, suggest that they are cumulates from liquids chemically similar to but isotopically distinct from their host materials. The granitoid suite exhibits a narrow range in whole-rock isotopic compositions ([epsilon] Nd (115 Ma) 7.2 to 9.1 and 87Sr/86Sr(i) 0.7032 to 0.7043). These values indicate a limited role for ancient, evolved continental lithosphere in the White Mountain magmatic system(s), with the dominant source reservoir being isotopically primitive material (depleted mantle). Chemical and isotopic data suggest that the Fogo 1563 A.D. magma evolved via closed system fractional crystallization. Strontium isotope variations in the Fogo A whole rocks (Azores) are proposed to be the result of three distinct processes: Contamination of the Fogo A magma by assimilation of radiogenic seawater-altered syenite wall rock, to explain the Sr and Th isotopic compositions of the glass separates; incorporation of xenocrysts into the trachytic magma, required to explain the range in feldspar Sr isotopic compositions; and post-eruptive surface alteration. We assert that continuous differentiation models best explain the observed 226Ra-230Th variations in the Fogo deposits, suggesting magma residence times prior to eruption of ? 50 - 80 years for Fogo 1563, and ? 5.5 ka for the larger volume Fogo A eruption. These time scales represent liquid residence ages rather than the crystallization ages documented in most previous magmatic time scale studies, and allow constraints to be placed on the time scales necessary for the development of chemical zonation in the Fogo magma chamber.

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