Evaluating formation fluid models and calibrations using clumped isotope paleothermometry on Bahamian dolomites

Sean T. Murray, Peter K Swart

Research output: Contribution to journalArticle

11 Citations (Scopus)

Abstract

The use of stable oxygen isotopes to understand the mechanisms of dolomite formation has been hampered by the inability to precipitate well-ordered dolomite under normal Earth surface conditions. Several studies have attempted to address this problem, either by precipitating high-temperature ordered dolomites and extrapolating the data to low temperatures or by using more disordered very-high Mg-calcites as a proxy for low temperature dolomites. The result is eight equations that disagree significantly from each other (by as much as ∼3.6‰ in the δ18O value of the precipitating fluid at 25 °C), and produce differences which can greatly affect the interpretation of the formation mechanisms for low temperature dolomites. However, by combining the recently developed clumped-isotope paleothermometer, an independent isotopic measurement (∆47) that directly relates to the temperature of formation, to Late Miocene to Pleistocene aged dolomites from the Bahamas with a well-constrained thermal and fluid history, we have attempted to narrow down the viable equations used to interpret the δ18O values of dolomites. The clumped-isotope temperatures measured on the Bahamian dolomites (16–37 °C) agrees with the range of temperatures expected in the Bahamas. Pairing these temperatures with geological and mineralogical arguments, we favor the equation suggested by Matthews and Katz (1977), as it is the only one that produces realistic δ18O fluid values across the range of clumped-isotope temperatures. Both the clumped-isotope temperatures and δ18O values of the precipitating fluid show a strong positive covariance that we have interpreted as reflecting the mixing of surface brines that have undergone varying amounts of evaporation and normal seawater. The different mechanisms driving these fluids included formation by normal marine seawater driven by the compensatory flow of the mixing zone, bank wide Kohout convection, and evaporative brine reflux.

Original languageEnglish (US)
Pages (from-to)73-93
Number of pages21
JournalGeochimica et Cosmochimica Acta
Volume206
DOIs
StatePublished - Jun 1 2017

Fingerprint

Isotopes
dolomite
Calibration
isotope
calibration
Fluids
fluid
Temperature
temperature
Seawater
seawater
Oxygen Isotopes
formation mechanism
Brines
Calcium Carbonate
brine
oxygen isotope
stable isotope
calcite
evaporation

Keywords

  • Bahamas
  • Clumped isotope
  • Dolomite
  • Fluid composition

ASJC Scopus subject areas

  • Geochemistry and Petrology

Cite this

Evaluating formation fluid models and calibrations using clumped isotope paleothermometry on Bahamian dolomites. / Murray, Sean T.; Swart, Peter K.

In: Geochimica et Cosmochimica Acta, Vol. 206, 01.06.2017, p. 73-93.

Research output: Contribution to journalArticle

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