(2020) Effects of Farallon Slab Subduction on the Halogen Budget of the North American Lithosphere
Lassiter J, Barnes J, Holmes A, Segee-Wright G, Beaudoin G, Hoffmann E & John T
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03c: Room 1, Wednesday 24th June 08:15 - 08:18
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Hi John, I enjoyed your presentation - great to see all of the halogen data for the Colorado Plateau & Rio Grande Rift xenoliths. In my presentation in session 3D I modelled some of Mike Rowe's olivine-hosted melt inclusions for the central Rio Grande Rift, including the F and Cl contents, and found that the early phase F rich basalts could be modelled by enrichment from a small fraction melt with no need for a subduction component. I wasn't sure of the age of the host lavas that brought the xenoliths to the surface that you studied from the Rio Grande Rift - are they late (< 5 Ma) or early phase (>20 Ma)? Regardless, it's interesting that neither the xenoliths or lavas in the RGR show a significant flux of halogens from the Farallon plate. I suspect F is not being released from the slab and may be most of the Cl has been released (at shallower depths) from the slab further west.
Hi Sally. The Elephant Butte basalts and xenoliths from the central RGR are Pliocene or younger, so < 5 Ma. Cerro Chato is also quite young - it's estimated at < 700 Ka. In contrast, the Navajo Volcanic Field samples are Oligocene in age if I remember correctly, definitely much older (>20 Ma). So that could explain some of the difference in halogen enrichment we see in the different suites - the "subduction" signature in terms of Cl enrichment is restricted to the older suite. I think small-F melt enrichments for the younger suites could make a lot of sense - it would explain why we see no fractionation in terms of halogen/lithophile ratios compared to other MORB/OIB suites. I'll definitely take a look at your presentation. Thanks.
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