Carbonate rocks are an ideal recorder of environmental conditions on our planet throughout Earth history because they have been forming throughout Earth's history across many environments on land and in the sea. However, throughout the long history of a carbonate rock, which can be millions of years or more, the rock can be buried to many kilometers depth, which exposes the rock to extreme temperatures and/or bring the rock into contact with water that can chemically alter the original minerals. In other words, the chemical records of Earth's environmental conditions within a carbonate mineral can alter under various conditions, and therefore the chemical values measured may not reflect the ancient environment as expected. Using 60 million year old marine rocks that were deposited in the Tethys Sea that existed at that time between India and Asia, the investigators have demonstrated that traditional tools may not identify all types of chemical alteration. The team's initial data and observations show that it is not yet understood how a carbonate rock can be chemically altered without leaving a visual trace of that alteration. In this study, the PIs will test their ability to use a novel chemical tool that determines the temperature at which a carbonate mineral forms by using other chemical tools that determine the age of that carbonate rock and are also sensititive to changes with burial. This technique could prove valuable for the oil and gas industry by providing additional tools for assessing the thermal maturity of oil-producing rocks in otherwise inaccessible basins where traditional tools cannot be used. Second, the PIs will test the capability of many chemical techniques that look for changes on extremely small scales to better learn how alteration happens and what fingerprints that alteration leaves behind. The result of the second part of this project will be a new tool for Earth scientists to use to identify altered geochemical signals from other carbonate minerals. This will improve reconstructions of modern and ancient environments based on carbonate minerals.
Carbonates are highly susceptible to biological, physical, and chemical alteration after deposition. Carbonate 'clumped' isotope thermometry, T(delta cap 47), allows to more accurately and completely reconstruct ancient environments by providing measurements of carbonate formation temperature. High delta cap 47 values (low temperature) are often viewed as indicative of primary mineral formation temperature because clumping (delta cap 47) is typically thought to decrease during burial in the absence of later water-mediated recrystallization at Earth's surface. Preliminary data from buried Paleocene marine carbonates suggests this is an oversimplification; rather, marine carbonates show evidence of extensive oxygen exchange with non-marine 18O-depleted water and deep burial temperatures (>150 degree C), but appear unaltered using conventional diagenetic screening metrics (i.e. optical petrography) and yield apparent low delta cap 47-derived temperatures. These observations highlight a knowledge gap in the isotopic effects of optically undetected carbonate alteration. The PIs will (a) test the capability of clumped isotope measurements paired with calcite U/Pb ages to reconstruct paragenetic sequences for sedimentary basin analysis and (b) optimize a suite of micro-analytical techniques to identify products of different alteration conditions, building a predictive framework for geochemical, textural, and isotopic fingerprints of 'stable mineral recrystallization.' The PIs will map crystallographic, elemental, and isotopic heterogeneity within carbonates to guide selection of higher spatial resolution delta cap 47 measurements.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
|Effective start/end date||1/2/20 → 8/31/22|
- National Science Foundation: $184,321.00