CARBON and OXYGEN ISOTOPE COMPOSITION OF MANTLE CARBONATES FROM THE NAVAJO VOLCANIC FIELD (COLORADO PLATEAU, USA)

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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