Document Type

Thesis

Date of Degree Completion

Spring 2016

Degree Name

Master of Science (MS)

Department

Geological Sciences

Committee Chair

Chris Mattinson

Second Committee Member

Audrey Huerta

Third Committee Member

Wendy A. Bohrson

Abstract

Ultrahigh-pressure (UHP, depths ≳100 km) eclogite samples from the Dulan area of the North Qaidam Terrane, western China, preserve evidence of fluid release along a prograde pressure-temperature (P-T) path. Eclogite sample D126A, Grt + Omp + Qtz + Zo + Amp + Phe + minor Rt + Hem, contains garnet porphyroblasts that preserve strong Ca zoning, recording higher core values (Grs34), lower inner mantle values (Grs25), increasing outer mantle values (Grs28), and lower rim values (Grs26). Isochemical phase diagrams (pseudosections), assuming H2O saturation, produce a P-T path constrained by multiple points. The P-T path begins at ~16 kbar, <400 °C and reaches peak P conditions at ~28.5 kbar, ~675 °C. A P reversal on the prograde path is interpreted to represent an episode of exhumation, followed by further subduction (yo-yo subduction), but, the P reversal may also be an artifact of chemical disequilibrium within garnet zones. The P-T path ends at retrograde conditions estimated between ~17-21 kbar, ~611-675 °C. Predicted fluid release rates from D126A are highly variable along the P-T path. Assuming a subduction rate of 14 km/Myr (Cascadia subduction rate), fluid release rates increase as the P-T path crosses the terminal stability boundary of hydrous minerals. Fluid release rates spike at ~560 kg/m3/Myr at the stability limit of amphibole and ~510 kg/m3/Myr at the stability limit of lawsonite. Such spikes in fluid release rates are interpreted to increase the availability of fluid, which could promote zircon recrystallization, and therefore zircon ages may be likely to record spikes in fluid release rates along the P-T path. The spikes in fluid release rates could promote the migration of fluids long distances, increase fluid pressures that result in seismic failure, and increase mass transfer into the mantle wedge.

Language

English

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