Reports: DNI251182-DNI2: Hot and Heavy or Cool and Fresh: Resolving Meteoric from Burial Diagenesis Using Clumped Isotopes

Aradhna Tripati, PhD, University of California (Los Angeles)

Sedimentary rocks of all ages, lithologies and depositional settings exhibit cements. The timing of cementation within a given sedimentary unit, however, is generally poorly constrained. 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. Such effects have vast implications for hydrocarbon and formation water migration. Therefore, the study of cements represents an interdisciplinary avenue of research spanning the fields of petroleum geology, hydrology and carbonate petrology. The formation conditions of the youngest of cement generations are particularly difficult to characterize. Late-stage spar calcites of the well-studied Capitan Formation have been the subject of ongoing debate, with interpreted origins ranging from burial- to post-uplift-related processes. The main tool used, clumped isotope thermometry, 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. Results/Highlights We have found evidence for extensive, uplift-related late-stage spar formation in the succession, studying the fore-reef slope and shelf facies to constrain mineralization temperatures, provide previously unattainable information concerning precipitation environment, and explore the spatial extent of precipitation events. Spar precipitation temperatures range from ~30 to 75 degrees C and show strong positive correlation with reconstructed pore water oxygen isotope values, indicating rock- buffered behavior. Evaluation of the data using a water-rock (W-R) model indicates that fluid(s) involved in diagenesis must have had a significant meteoric component, exhibiting fluid oxygen isotope values approaching -12 per mil (VSMOW). These fluid oxygen isotope values correlate well with temperature, evidence for spar precipitation from two end-member fluids. These data along with petrographic, outcrop, and core relationships indicate that spar precipitation occurred across back-reef, reef, and fore-reef slope facies when fluids with such light isotopic signatures would have infiltrated the basin, and not during burial as generally assumed. Therefore it must have occurred during recent (Tertiary or later) uplift. Meteoric fluids may have been delivered locally through fault and fracture networks but also must have infiltrated less fractured facies of McKittrick Canyon and produced spar cements. Results indicate that extensive spar precipitation can occur very late in the diagenetic sequence. In the case of the Capitan reef complex, spar precipitation occurred some 200 million years after initial deposition. At this point, it is unclear whether or not modern spar formation is occurring. The fact that the Capitan Formation acts as a modern aquifer for much of the surrounding region suggests that spar precipitation potential exists. 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. 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: Clumped Isotope Workshop. 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., Scholle, P. Tripati, A.K., in press, Extensive, uplift-related late- stage spar formation in the Permian Capitan Formation, Journal of Sedimentary Research. Impact on Career Advancement The PI considers this project on diagenesis to be one of the key contributions she has made in the application of clumped isotope geochemistry. The exciting findings of the work will help strengthen the case for her tenure this year. The PI has expanded her collaboration circle to include the group at New Mexico Tech, 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. Furthermore, this research project has supported a number of members of her group. Postdoc Sean Loyd was originally going to undertake this project but received a fellowship. He continued working on the project with support of other members of Tripati's group, and Tripati has given Loyd the opportunity to be first-author. This work was featured in his job applications, and he has begun a faculty position at California State University, Fullerton. It has also supported postdoctoral researchers who have assisted with conducting analyses, maintained the mass spectrometer for isotope measurements, and learned about applications of clumped isotope geochemistry to the study of diagenesis. All postdoctoral researchers are continuing in academic research. Finally it has supported multiple 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.