57th Annual Report on Research 2012 Under Sponsorship of the ACS Petroleum Research Fund

We have met with great success in the second year of the grant (Sept. 1, 2011 to Aug. 31, 2012).  As detailed below, the salient accomplishments include:  1)  Completion of experiments for determining kinetics of solid-state C-O bond reordering in calcite.  2) Development of the theory and mathematics relating the state of ¹³C-¹⁸O ordering in calcite to the rates of temperature decrease (cooling).  3)  Preparation and publication of a manuscript describing (1) and (2) (Passey and Henkes, EPSL 2012).  4)  Writing an NSF proposal in January 2012, which was awarded funding in August 2012, to extend our studies of C-O bond reordering to biogenic calcites (brachiopods, mollusks), and 5)  continued support of graduate student Greg Henkes, who has been applying the technique to burial / exhumation histories of brachiopod-bearing Carboniferous-age strata from North America and Asia.

1)  We completed our experimental characterization of the kinetics of C-O bond reordering in two different mineral specimens, an optical grade calcite, and a lower-grade groundwater spar calcite.  This involved more than 130 heating experiments at temperatures ranging from 130 to 800 °C, time durations from 30 minutes to ~40 days, and pressures up to 100 MPa.  The D₄₇ parameter (a measure of the state of ¹³C-¹⁸O ordering) was determined for all 130+ samples using our mass spectrometer specifically configured for measuring isotopic 'clumps' in CO₂liberated from the calcite samples by acid digestion.  As described in Passey and Henkes (2012), we find that the reordering kinetics follow a first-order rate law, but only after an initial period of rapid reordering that we attribute to the presence of nonequilibrium defects.  These defects are quickly annealed (or annihilated) during the initial moments of laboratory heating, with minerals soon reaching equilibrium defect concentrations, after which the first-order reordering kinetics become dominant.

2)  Dodson (1973) developed the classic 'closure temperature' concept and mathematics that describe how the apparent radiometric age of a mineral relates to the rate of cooling of the mineral through a window spanning temperatures where daughter isotopes are lost by diffusion, down to temperatures where daughters are quantitatively retained.  Intuitively we figured that bond ordering in carbonate minerals should have analogous behavior:  at high temperatures, the rates of ¹³C-¹⁸O bond reordering are essentially instantaneous, such that the actual clumped isotope composition of a mineral at some high temperature T_h is the same as the equilibrium clumped isotope composition predicted by thermodynamics for a mineral residing at the same temperature T_h.  As the mineral progressively cools (i.e., as after a metamorphic event, or during exhumation), the rate of ¹³C-¹⁸O bond reordering slows, and eventually the actual clumped isotope composition will no longer mirror the equilibrium clumped isotope composition of the mineral.  We spent late fall and early winter of 2011/2012 searching for the correct mathematics to describe the cooling-rate-dependence of 'closure temperatures' for carbonate clumped isotopes, and eventually discovered that a remarkably simple analogy could be drawn to Dodson's formulation for closure of radioisotope systems where daughters are lost according to first-order kinetics.  The basic idea is described in Passey and Henkes (2012).

3)  We prepared a manuscript describing the results described above during spring 2012.  The paper was submitted in May, and with a fast turnaround in review was accepted in mid-July 2012.  The paper was published in EPSL in fall 2012.  We envision that this paper will be a landmark because it is the first to describe the kinetics of C-O bond reordering in calcite, and it gives the mathematics that enable the kinetic parameters to be put to good geological use, both as a 'geospeedometer' (a method for determining rock cooling rates) and as a method to estimate the thermal histories of sedimentary packages (e.g., numerical modeling of changes in ¹³C-¹⁸O ordering during burial and exhumation).

4)  With collaborators Ethan Grossman (Texas A&M), and Alberto Perez-Huerta (U. Alabama), we submitted a proposal in January to NSF EAR Geobiology and Low Temperature Geochemistry to study C-O bond reordering kinetics of biogenic calcite, and to evaluate the method using well preserved Carboniferous age brachiopods that have been subjected to different burial and exhumation histories (i.e., different temperature -time histories).  The proposal was successful (NSF EAR 1227076), and we were awarded a total of $274,849 ($147,206 to Hopkins, $71,048 to Texas A&M, and $56,595 to Alabama) to conduct the proposed research.  The funding will support a graduate student each at Johns Hopkins and Texas A&M for the better part of two years.  Along with the PRF graduate student support this represents funding for the majority of my PhD student Gregory Henkes' tenure as a graduate student.  The PRF-DNI funding was absolutely essential for acquiring this support from the National Science Foundation.

5)  My student Greg Henkes is well into a study investigating the clumped isotope compositions of late Paleozoic brachiopods, and relating these to the burial and exhumation histories of the sedimentary packages that they come from.  He finds that brachiopods from deeply buried basins (~ > 4km) have clumped isotope compositions that reflect maximum burial temperatures (ca. 150 °C).  Brachiopods from shallowly-buried basins (< ~ 1 km) have temperatures that are plausible as original temperatures of growth of the organisms (ca. < 35-40 °C).  Finally, brachiopods from basins with intermediate burial have clumped isotope compositions reflecting partial alteration during heating.  Using the kinetics developed under support of the PRF-DNI and reported in Passey and Henkes (2012), Greg has developed numerical models that march the brachiopod shells through their ca. 300 M.y.r. time-temperature histories at something like 1 M.y.r. intervals, predicting the clumped isotope compositions of the shells after each time step, and importantly predicting the 'final' clumped isotope composition of the shells once they've returned back to the surface.  He is observing remarkably good correspondence between the model-predicted clumped isotope compositions and the actual clumped isotope compositions determined in the laboratory.  We foresee that clumped isotope thermometry will be an important part of evaluating the temperature histories of sedimentary basins, and Greg is currently well along into writing up the results for publication.