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(2020) Slab Temperature Control on Volatile Budgets of Arc Magmas Tracked from Melt Inclusion Halogen Contents
Brahm R, Kuritani T, Sakamoto N, Yurimoto H, Zellmer G, Nakagawa M & Sato E
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05d: Room 2, View in program
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Hi Raimundo - great study! Amazing how distinct those 7 centres are in their trace element contents. On your figure of Pb/Ce vs. F/Y what is the difference between 'slab fluid flux signal' (vertical arrow) and '%slab fluid in melt' (horizontal arrow)? Also, are there any other cases where you get slab melting way out in the back arc? Or is that a very unique scenario? Thanks and best wishes! Lucy
Hi Lucy, thank you! By slab fluid signal I refer to the flux of slab fluid added to the mantle source, whereas %slab fluid in melt refers to the final proportion of the slab component in the generated magma. I think we are seeing a dilution effect, where a low degree of fluid flux into the source mantle produces low melt volumes or low degree partial melts. This process results in high fluid concentrations in the primary melt. Rishiri’s tectonic setting is distinct. It is very far from the trench, and the slab surface under it reaches almost 8GPa. I would not know if it is comparable to any other back-arc volcano. Hydrous silicate melts have been invoked to explain particular geochemical signals of other volcanic systems, but I don’t think that any of it is directly comparable to this particular rear-arc setting. As I said in the presentation, Rishiri’s halogen compositions are remarkably similar to those found in an olivine-hosted melt inclusion group from Mt. Shasta (Le Voyer et al., 2010), which is not a back-arc setting. Slab temperature conditions might be closer to a back-arc setting as subduction rate is very low in the Cascades.
Could it be possible that there is phlogopite in the mantle wedge under Rishiri volcano? if so, it would affect the F/Cl ratio. Have you considered that?
Hi Diego, thanks for the question. I have not yet explored the possibility of phlogopite involvement in the F/Cl ratio shift in the rear-arc. Nonetheless, as this change is accompanied with enrichment of incompatible but immobile elements like Zr, I am inclined to think that the most probable cause of higher F/Cl is a change in the nature of the slab liquid (silicate melt or supercritical fluid). The somewhat narrow range of F/Cl ratios from arc to back-arc (excluding Rishiri volcano) makes me think that the process of fluid release should be a continuous dehydration reaction of a particular lithology, which I believe to be serpentinites. Serpentinites can be present and continuously dehydrated as subduction progresses. Kendrick et al. (2020) and their presentation in this conference, found that the halogen signals of arc and back-arc magmas (using F, Cl, I and Br) are probably inherited from the serpentinites from the metasomatized slab lithospheric mantle. Again, the change towards the rear-arc is indicating a shift on the controlling reaction of fluid release, which needs to start transporting more immobile elements from a source already depleted of mobile elements through progressive dehydration.
Hello Raimundo, Excellent study! The very high Cl and F are interesting, and yes, similarly high F and Cl are found at Shasta, but not elsewhere in the Cascades (to my knowledge) although slab Ts remain quite high along the arc. What about the possibility of a subducted fracture zone or seamounts that could contribute volatiles? Thanks again for a great talk!
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