Abstract
Carbonates are ubiquitous in the rock record and provide a broad array of stable isotope-based paleoclimatic proxies (i.e., δ18O, δ13C, ∆17O, ∆47, ∆48) that provide information on stratigraphy, carbon cycling, temperature, hydrology, and the altitude of ancient land surfaces. Thus, carbonates are an essential archive of environmental and topographic histories of continental terranes. However, carbonate minerals are highly susceptible to post-depositional alteration of primary isotopic values via fluid-mediated and solid-state reactions. We propose a hierarchical suite of techniques to comprehensively assess alteration in carbonates, from essential and readily accessible tools to novel, high-resolution techniques. This framework provides a means of identifying preserved textures in differentially altered samples that contain high-value environmental information. To illustrate this progressive approach, we present a case study of Tethyan nearshore carbonates from the Paleocene Tso Jianding Group (Tibet). We demonstrate the utility of each technique in identifying chemical and crystallographic indicators of post-depositional alteration at progressively finer spatial scales. For example, secondary ionization mass spectrometry (SIMS) oxygen isotope maps of micrite and bioclasts reveal significant isotopic heterogeneity due to grain-scale water-rock exchange in textures that were labeled “primary” by optical inspection at coarser spatial resolution. Optical and cathodoluminescence microscopy should be the minimum required assessment of carbonate samples used in stable isotope analyses, but supplemented when necessary by SIMS, PIC mapping, and other yet untapped technologies that may allow distinction of primary and altered fabrics at finer spatial resolutions.
Highlights
Carbonate rocks provide an archive of geochemical information related to the climatic and tectonic setting of their depositional environment
We present a hierarchical framework for assessing alteration in carbonates used for environmental and tectonic reconstructions
We built on the exchange-diffusion modeling and texture-specific isotopic measurements of Ingalls (2019) to construct a more detailed alteration history for the Jialazi Formation
Summary
Carbonate rocks provide an archive of geochemical information related to the climatic and tectonic setting of their depositional environment. The shallow marine Paleocene-Eocene Jialazi Formation of the Tso Jianding Group (southern Tibet) is ideal for investigating the expression of water-rock and solid-state alteration in micritic and biogenic carbonate at a variety of spatial scales because of the incongruity of its isotope values and textural characteristics. Thermal maturation indices of organic matter and benchmark values for marine carbonate δ18O (∼0‰ VPDB) clearly identified these rocks as strongly impacted by isotopic exchange reactions Had these samples been derived from a terrestrial environment whose expected values could have been similar to the values obtained, and only screened by optical petrography, the altered δ18O values might have been used erroneously to make stable isotope-based calculations of ancient elevations. Alteration of high-altitude samples can result in inaccurate paleo-elevations by similar magnitudes
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