GSA Connects 2021 in Portland, Oregon

Paper No. 232-13
Presentation Time: 4:55 PM


GUILD, Meghan and BARNES, Jaime D., Jackson School of Geosciences, University of Texas at Austin, Austin, TX 78712

The shallow, conductively cooled mantle lithosphere is believed to be a significant reservoir for carbon (e.g., Lee et al., 2019; Kelemen & Manning, 2015; Dasgupta, 2013). However, the residence time and long-term fate of carbon within this reservoir is poorly constrained due, in part, to the lack of detailed petrologic and geochemical studies of carbonate in lithospheric mantle samples. Here, we pair petrologic studies with carbon and oxygen isotope analyses of carbonate in mantle xenoliths from the Navajo Volcanic Field to better characterize the carbon stored beneath the Colorado Plateau. Petrologic investigations of carbonated peridotites show carbonates are dominantly calcitic and magnesitic and are found in textural equilibrium with mantle minerals or as veins. In ophicarbonates, carbonates are found intergrown with serpentine. Micro-drilled carbonates and bulk powders were analyzed for δ13C and δ18O compositions. Carbonate in peridotite falls into two distinct δ13C populations, with ~70% of the samples averaging around –4.5‰ (near the mantle values of ~ –5.5‰) and a 12C-enriched population, averaging –9.0‰. Carbonated peridotite with low δ13C carbonate and mantle-like δ13C carbonate have δ18O values averaging at +21.0‰ and +19.4‰, respectively. Ophicarbonates, however, record lower average δ18O values (+15.6‰) and higher δ13C (–2.4‰). The variation of δ13C and δ18O values observed in the mantle xenoliths suggests interaction with multiple fluids/melts in the mantle lithosphere. Enrichments in 18O are attributed to interaction with subducted sediments/carbonates, or carbonatite melts, as suggested in Perkins et al. (2006). Whereas the low δ13C observed in the mantle lithosphere may be due to the devolatilization of the subducting Farallon slab.

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Kelemen, P. B. & Manning, C. E. Reevaluating carbon fluxes in subduction zones, what goes down, mostly comes up. PNAS 112, E3997–E4006 (2015).

Lee, C.-T. A., Jiang, H., Dasgupta, R. & Torres, M. A Framework for Understanding Whole-Earth Carbon Cycling. in Deep Carbon (eds. Orcutt, B. N., Daniel, I. & Dasgupta, R.) 313–357 (Cambridge University Press, 2019). doi:10.1017/9781108677950.011.

Perkins, G. B., Sharp, Z. D. & Selverstone, J. Oxygen isotope evidence for subduction and rift-related mantle metasomatism beneath the Colorado Plateau–Rio Grande rift transition. CMP 151, 633 (2006).