Steady growth or fits and starts: observing the style of and controls on carbonate crystallization in an Alpine fold and thrust belt
Abstract
Shallow-water carbonates are the principal archive of ancient earth-surface conditions, but all have been subjected to subsurface modification in the intervening time. In order to extract primary depositional signals in carbonate minerals, it is necessary to understand the mechanisms by which they are overprinted during post-depositional recrystallization. Open questions inlcude: Do carbonate grains grow continously when residing at elevated temperatures, or episodically, in response to discrete events? In addition to T, P, and t, how do confounding variables such as strain, lithology, mineralogy, or fluid content affect this process? We measured phase-specific clumped isotope (Δ47) temperatures from carbonate-bearing units in the Helvetic nappes and Infrahelvetic complex of the Glarus alps, Switzerland. Here, Mesozoic carbonates were metamorphosed to at most lower-greenschist facies in the Late Eocene, and discordantly thrust over Mesozoic-Tertiary flysch along the Oligocene-Miocene-aged Glarus fault. In broad agreement with conventional thermal maturity proxies, calcite Δ47-based temperatures increase to the south and with stratigraphic depth, from 45 °C at the northenmost terminus of the nappes to 210 °C in the southermost exposures of the Infrahelvetic complex. Within the colder sections, however, calcite Δ47 temperatures are highly heterogeneous and vary by up to 50 °C across m-scale lithological transitions. A strong positive correlation between crystallization temperature and fluid d18O values at these scales suggests that local fluid content is a primary control on the suceptability of carbonates grains to recrystallization and coarsening in shallow burial environments. The loss of outcrop-scale calcite Δ47 heterogeneity with increasing metamorphic grade suggests that variable fluid-rock ratios do not preclude pervasive recystallization in calcite above 100 °C. Dolomite Δ47-based temperatures are 50-150 °C colder than coexisting calcite temperatures in flysch units where peak conditions exceeded 200 °C. Barring a late addition of cold, retrograde dolomite, these discrepancies indicate that carbonate recrystallization is also phase-specific, and that dolomite is uniquely resistant to recrystallization during low-grade metamorphism in major fold-and-thrust belts.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2017
- Bibcode:
- 2017AGUFMEP11D..08L
- Keywords:
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- 0414 Biogeochemical cycles;
- processes;
- and modeling;
- BIOGEOSCIENCES;
- 1051 Sedimentary geochemistry;
- GEOCHEMISTRY;
- 1862 Sediment transport;
- HYDROLOGY;
- 4273 Physical and biogeochemical interactions;
- OCEANOGRAPHY: GENERAL