V046-08
Modification of the halogen budget of the North American lithospheric mantle from subduction

Wednesday, 16 December 2020: 20:58
Virtual
Jaime Barnes1, John C Lassiter2, Alexandra Kali Holmes1, George Segee-Wright3, Grace Beaudoin3, J Elis Hoffmann4 and Timm John4, (1)University of Texas at Austin, Austin, TX, United States, (2)University of Texas at Austin, Dept. of Geological Sciences, Jackson School of Geosciences, Austin, TX, United States, (3)University of Texas at Austin, Austin, United States, (4)Free University of Berlin, Berlin, Germany
Recorded Presentation
Abstract:
The halogen budget of the Earth remains highly uncertain, chiefly because the only output at convergent margins that currently is well characterized is that from arc volcanoes; however, additional output pathways and/or reservoirs are likely. One such reservoir is that of the sub-continental lithospheric mantle (SCLM). In particular, SCLM that has been metasomatized by slab-derived fluids may have high concentrations of halogens compared to typical depleted upper mantle due to enrichment of halogens in slab-derived fluids.

We analyzed bulk rock halogen concentrations (F, Cl, Br, I) in ~35 mantle xenoliths from the southwestern United States to constrain the effects of metasomatism from slab-derived fluids from the subducting Farallon slab on halogen abundances. In general, mantle xenoliths from Cerro Chato (CC) along the eastern Colorado Plateau margin and Elephant Butte (EB) from the central Rio Grande Rift have low Cl (average ≈ 5 ppm) and F abundances (average ≈ 25 ppm). Average Cl/Nb (~11) and F/Nd (~31) ratios are similar to MORB/OIB and to olivine-hosted melt inclusions from these locations (Rowe et al., 2015). In contrast, mantle xenoliths from the Navajo Volcanic Field (NVF) in the Colorado Plateau have high Cl concentrations (average ≈ 205 ppm), but show no enrichment in F (average ≈ 14 ppm), suggesting a decoupling of Cl from F possibly due to Cl being more readily mobilized by in the slab-derived fluid and F retained in the subducting slab.

In contrast, Br and I are enriched in all xenoliths studied, with particularly high concentrations of I in some samples compared to MORB/OIB. I/Cl and I/Br ratios are also higher than MORB/OIB. These high Br and I concentrations are comparable to those from western Antarctic xenoliths (Broadley et al, 2016), and may reflect halogen addition from slab-derived fluids. Interestingly, no correlations are observed in halogen abundances or ratios with indices of metasomatism or melt depletion. Future work will investigate the decoupling among the halogens and with indices of metasomatism, and the role mineralogical hosts play in these trends.

Rowe et al., G3, 2015; Broadley et al., GCA, 2016

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