Date of Award

5-2015

Degree Name

Master of Science (MS)

Department

Earth Sciences

Advisor(s)

Suzanne L. Baldwin

Keywords

Eclogite, Papua New Guinea, Phengite, Thermobarometry, (U)HP, Ultrahigh-pressure

Subject Categories

Earth Sciences

Abstract

The D’Entrecasteaux Islands of Papua New Guinea (PNG) contain the youngest known ultrahigh- and high pressure ((U)HP) terrane in the world (Davies and Warren, 1992; Baldwin et al., 2008). Conventional peak pressure-temperature (P-T) estimates for the (U)HP terrane do not confirm (U)HP conditions, despite the presence of coesite in a sample from the terrane (Baldwin et al., 2008). The objective of this project was to apply phengite geobarometers to samples from the (U)HP terrane in PNG, providing new pressure estimates. Si-in-phengite (Massonne and Schreyer, 1987) and Ti-in-phengite (Auzanneau et al., 2010) barometry were applied to two samples from the PNG (U)HP terrane to constrain P-T paths. The coesite-bearing eclogite (~8-7 Ma, Monteleone et al., 2007; Baldwin et al., 2008; Baldwin and Das, 2013) was used to constrain peak pressure conditions, whereas a felsic gneiss (~ 1.5 Ma, S. Baldwin, pers. comm.) was used to constrain the final stages of exhumation to the surface. ‘Equilibrium assemblage diagrams’ calculated for both samples do not indicate equilibrium in the samples on the whole-rock scale. Whole-rock scale disequilibrium demonstrates the need for the application of new geobarometers that require equilibrium of the sample on a small (mm) scale, as opposed to throughout the whole rock.

Results of the phengite geobarometers show that peak pressures were in the coesite stability field (27-38 kbar), on the basis of Ti-in-phengite barometry of the coesite-bearing eclogite. In contrast, Si-in-phengite barometry of the felsic gneiss yielded estimated pressures of ~ 2-6 kbar, interpreted to indicate exhumation from shallow-mid crustal depths (~6-18 km). Though these two samples have different bulk compositions and were collected from different localities, and thus do not necessarily record the same P-T path, they do confirm a general exhumation trend from (U)HP depths (~ 80-100 km) between ~8-7 to ~1.5 Ma (~6-18 km) for the PNG (U)HP terrane. Additionally, this is the first time that application of a geobarometer has yielded pressure estimates for the coesite-bearing eclogite entirely within the coesite field (based on Ti-in-phengite). Results indicate the success in application of single-mineral barometers in cases where the scale of equilibrium is limited, on the order of single mineral grains, as opposed to the whole rock.

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Open Access

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