Apparent oxygen isotope equilibrium during progressive foliation development and porphyroblast growth in metapelites: Implications for stable isotope and conventional thermometry

C. A. Berg, D. P. Moecher

Research output: Contribution to journalArticlepeer-review

4 Scopus citations

Abstract

The assumption of oxygen isotope and major element equilibrium during prograde metamorphism was tested using staurolite-grade pelitic schists that have undergone sequential porphyroblast growth and multiple episodes of recrystallization of matrix minerals and foliation development. Textural relationships are used to infer a metamorphic history that involves garnet growth followed by staurolite growth, with each porphyroblast growth event followed by at least one period of recrystallization of matrix minerals. Conventional geothermobarometry using Qtz-Grt-Pl-Ms-Bt ± St equilibria yields peak P-T conditions of c. 625 °C at 9-11 kbar, consistent with KMnFMASH petrogenetic grid predictions for stability of the assemblage Grt + St + Bt. Qtz-Grt oxygen isotope fractionations yield apparent temperatures of c. 590 °C and Qtz-St fractionations yield an apparent temperature of c. 595 °C. Diffusional modelling indicates that quartz isotopic compositions were reset by c. 30 °C via retrograde isotopic diffusional exchange with micas. The isotopic temperatures appear to be in excellent agreement with one another, and suggest oxygen isotope equilibrium was attained between garnet and staurolite at c. 625 °C. However, the agreement of Qtz-Grt and Qtz-Str isotopic temperatures is not consistent with petrographic observations (garnet grew before staurolite) and petrogenetic grid constraints that predict that garnet grows over a temperature interval of c. 525-550 °C. Given that: (i) oxygen diffusion rates in staurolite and garnet are slow enough to render an individual porphyroblast effectively closed to exchange after it forms; and (ii) matrix minerals are able to exchange isotopes via recrystallization during each period of deformation; garnet and staurolite could not have simultaneously achieved oxygen isotope equilibrium with each other or with minerals in the recrystallized matrix. Thus, the Qtz-Grt fractionations, which yield apparent temperatures that are in apparent agreement with peak metamorphic temperature and apparent temperatures for Qtz-St fractionations, cannot be fractionations resulting from equilibrium isotopic exchange. Instead, they are apparent fractionations between porphyroblasts formed at different temperature and times in the prograde P-T-D path, and quartz that recrystallized and exchanged with micas and plagioclase during several phases of deformation.

Original languageEnglish
Pages (from-to)471-487
Number of pages17
JournalJournal of Metamorphic Geology
Volume23
Issue number6
DOIs
StatePublished - Aug 2005

ASJC Scopus subject areas

  • Geology
  • Geochemistry and Petrology

Fingerprint

Dive into the research topics of 'Apparent oxygen isotope equilibrium during progressive foliation development and porphyroblast growth in metapelites: Implications for stable isotope and conventional thermometry'. Together they form a unique fingerprint.

Cite this