Aradhna Tripati, PhD, University of California (Los Angeles)
Specification of the generation mechanisms and timing of diagenetic carbonate minerals is a longstanding problem in the geosciences. These diagenetic phases can often precipitate within open pore spaces and sometimes occlude them completely, greatly diminishing the fluid-flow potential of a given sedimentary unit. Of course, such effects have vast implications for hydrocarbon and formation water migration. Therefore, the study of these carbonates represents an interdisciplinary avenue of research spanning the fields of petroleum geology, hydrology and carbonate petrology.
Motivated by these 'big-picture' interests, we have characterized late-stage calcites that formed in the well-studied Permian El Capitan Formation. The main tool used, clumped isotope thermometery, provides a unique opportunity to characterize the formation conditions of these carbonate phases and therefore place them within the paragenesis of the host unit with a high degree of certainty.
Diagenetic carbonate cements can comprise a significant component of sedimentary rock units and in most cases act as the primary agent of lithification. In many scenarios, post-depositional cementation seemingly follows a relatively straightforward genetic sequence from early marine-phase precipitation to progressively later-stage mineral formation. The final phases precipitated in such a paragenetic sequence are often blocky calcite spars which fill-in and occlude the remaining pore spaces, restricting subsequent fluid flow. The timing of this final stage of cementation is generally difficult to constrain using traditional petrographic and geochemical analyses alone.
A hallmark example of equivocal late-stage cement precipitation involves blocky calcites of the El Capitan Formation. Previous research has led to a variety of hypotheses concerning blocky calcite emplacement timing, ranging from deep burial to post-uplift. The lasting debate stems largely from the ambiguities associated with traditional proxies, in particular oxygen isotope compositions in carbonates. Over the past year, we have collected clumped isotope data to clearly define the conditions associated with the formation of different generations of cements.
We reconstructed temperature and fluid oxygen isotope compositions, and constructed a water/rock model that demonstrates the requirement of late-stage, meteoric fluid influence on mineral formation. Furthermore, with these data we argue that cementation occurred during exhumation – and not during burial. The research results have been presented at multiple meetings and a manuscript has been prepared for submission to Science for consideration for publication (see below).
Additional samples were also acquired through collaboration with Peter Scholle and Dana Ulmer-Scholle at New Mexico Tech through a field collection undertaken this past year. Analyses of these samples are underway. Results will be the subject of a second manuscript we anticipate writing in 2013, likely for the Journal of Sedimentary Research. This extensive sample set includes not only the late-stage phases studied previously but also primary framework carbonates, early marine cements and isopachous rim cements. Analyses of these samples will help 1) confirm our original results and 2) further establish the complete paragenesis of the El Capitan Formation.
Loyd, S.J., Dickson, J.A.D., Hudson, J.D., Eiler, J.M., and Tripati, A.K., 2011, Assessing cementation in the El Capitan Reef Complex and Lincolnshire Limestone using 13C-18O bond abundances in carbonates: Goldschmidt, Prague, Czech Republic..
Loyd, S.J., Dickson, J.A.D., Hudson, J.D., Eiler, J.M., and Tripati, A.K., 2011, Assessing cementation in the El Capitan Reef Complex and Lincolnshire Limestone using 13C-18O bond abundances in carbonates: 2nd Annual Clumped Isotope Workshop, Imperial College, London, UK..
Loyd, S.J., Dickson, J.A.D., Hudson, J.D., and Tripati, A.K., 2011, Assessing cementation in the El Capitan Reef Complex and Lincolnshire Limestone using 13C-18O bond abundances in carbonates: ACS-PRF Annual Western Regional Meeting.
Loyd, S.J., Dickson, J.A.D., and Tripati, A.K., in preparation, On the way up? Evidence for uplift-related diagenetic mineralization: Science.
A larger sample set has been compiled after careful petrographic work and microdrilling by undergraduates and postdoctoral researchers. This includes roughly 100 samples and will require extensive laboratory analysis. In addition to clumped isotopes, carbonates will be analyzed for more traditionally utilized carbonate-bound Sr, Mn and Fe. Integrated geochemical analyses should provide better constraints on carbonate-yielding fluid chemistry and help in characterizing the overall paragenesis of the formation. As data are acquired, we will collaborate extensively with the New Mexico Tech group (who have conducted an unparalleled assessment of petrographic and field relationships of the unit) and ultimately compose a longer manuscript for publication.
Impact on Career Advancement
The PI considers this project to be one of the key contributions she has made in the application of clumped isotope geochemistry to the geosciences. This opportunity has expanded her breadth of research to include a new field: diagenesis. In addition, the PI has expanded her collaboration circle to include the group at New Mexico Tech. She hopes this collaboration will be a longstanding one and thinks that both groups will continue to exchange petrographic, field, and geochemical datasets and experience. Such collaborative efforts promote development of scientific infrastructure among institutions and across disciplines and therefore are beneficial to the geosciences as a whole. The exciting findings of the work will help strengthen the case for her tenure.
Furthermore, this research project has supported a number of members of her group, from postdoctoral researchers to undergraduates. Postdoc Sean Loyd was originally going to undertake this project but received a fellowship. He was able to continue working on the project with the support of other members of Tripati's research group. Tripati has given Loyd the opportunity to act as first author. This work is being prominently featured in his job applications, and we hope he will receive a faculty position this year. It has also supported two postdoctoral researchers who have assisted with conducting analyses, maintaining the mass spectrometer for clumped isotope measurements, and learned about applications of clumped isotope geochemistry to the study of diagenesis. Both postdoctoral researchers are continuing in academic research. Finally it has supported five undergraduates who have assisted with sample and standard preparation, mass spectrometric analyses, and thin-section preparation. All of these undergraduates are applying for graduate school and are interested in sedimentary petrology.
I would like to gratefully acknowledge the American Chemical Society's Petroleum Research Fund for its support.