AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Clumped-Isotope Thermometry: A New Tool to Delineate Diagenetic Temperatures and Fluids of Calcite Phases
(1) Geological Sciences, University of Colorado, Boulder, CO.
(2) Earth and Space Sciences, University of Washington, Seattle, WA.
(3) Subsurface Technology, ConocoPhillips, Houston, TX.
Clumped-isotope thermometry (Eiler 2007) is a new technique that provides a precise measure of calcite’s precipitation temperature. The paleotemperature, coupled with conventional δ18O values, is then used to calculate the parent fluid’s δ18O. The technique is based on the temperature dependence of 13C and 18O to ‘clump’ into bonds with each other (Δ47 value) instead of being distributed randomly throughout the crystal lattice. We present two studies that illustrate the potential of this technique, with both illustrating diagenetic interpretations that would not have been possible with just δ18O data.
Permian syndepositional fractures in the Guadalupe Mountains, New Mexico, have an initial orange luminescent calcite cement with wide ranging δ18O (-10.5 to -15.5‰) and δ13C values (+2 to -9‰), suggesting multiple cementation events in meteoric fluids at varied temperatures. Overlying non-luminescent calcites contain hydrocarbon inclusions, a narrow range of δ18O values (-12 to -14‰), and quite negative δ13C values (-11 to -17.4‰). These phases are seemingly the product of warm basinal fluids. Both phases have ample secondary two-phase fluid inclusion. The Δ47 values yield a more refined interpretation as the initial cements formed at 26°C to 32°C in waters with δ18Osmow values of -6 to -10.4‰, whereas the non-luminescent cements formed at 54°C to 68°C in waters with δ18Osmowvalues of -3.5 to -5.3‰. There is no continuum between the two, and less diagenetic heterogeneity is indicated by the clumped isotope results.
The second example is a Paleogene lacustrine wackestone exposed near Long Point, Arizona that lies 36 m below a basalt flow dated at 6.8 Ma. Gastropod shells (δ18O of -13.6 to -14.3‰) are calcite with excellent preservation of shell microstructure. Calcite cements in gastropod body cavities have δ18O values range from -14.5‰ (earliest) to -17.3‰ (most recent). The similarity of shell and earliest cements δ18O values suggest shell alteration concurrent with initial cementation. The lack of any significant burial and similarity in δ18O values suggests all diagenesis in meteoric ground waters at near-surface temperatures. However, Δ47 values show cements formed from 19°C to 64°C in waters with δ18Osmow values of -14 to -6‰, respectively, and the gastropod shells recrystallized at 94°C to 122°C in waters with δ18Osmow values of -0.4 to 3.9‰. A thermal pulse from to the late Miocene basalt flow is the only way to explain the higher temperatures.