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(2020) Boron Isotopes: no 'Arc' in Archean?

Smit M, Scherstén A, Næraa T, Emo R, Scherer E, Sprung P, Bleeker W, Mezger K, Maltese A, Cai M, Rasbury T & Whitehouse M


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03e: Room 1, Wednesday 24th June 23:51 - 23:54

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Submitted by Capucine Albert on Tuesday 23rd June 10:26
Thank you for this, this is really interesting work! I was wondering if you considered the possibliity that seawater composition might not have been this isotopically heavy in the Archean?
Hi Capucine, Yes! Great question. We did consider this (see Smit et al., 2019, looong appendix) and the simple answer is that there are some estimates and they do not seem to be very different from the modern. The constraints are few and imprecise, though. The delta11B value of Archean seawater may well have been different and they may have fluctuated on short time scales as well, but that is definitely something that still needs to be investigated in detail. It is something we are now looking into! Many thanks for your question!

Submitted by Thomas Zack on Wednesday 24th June 13:59
Hi Matthijs, cool stuff. However, what about ground-truthing of some "safe" island arc intrusions? Or even going into the rock record and look at "likely" arcs that went through some slight tectonic squeezing, e.g., our 1.5 Ga Idefjorden terrane here in Gothenburg ;). Guess all boron isotope data from arc settings so far is volcanic- true? Potentially there could be a systematic bias due to some dehydration fractionation, although -I admit- the correlation with Zr/Hf speaks against it. Still, should be easy to test your predictions.
Hi Thomas! Yes, all B data that we evaluated as a reference frame were from proper arc lavas and intrusions from a variety of arcs in subduction zones with different geothermal gradients. There are indeed processes that could B signatures during devolatilization and dehydration of magmas, but the effects are furtunately relatively predictable and well-constrained; they can be "looked through" fairly well. Fractionation during magmatic crystallization in TTG intrusions is fairly minor and is indeed traced quite well through are data as we observe the correlations with Zr/Hf that one would expect for such processes. All in all, we are fairly confident of the composition of the source of these TTGs, which, at least in terms of B, seems very similar between the various terranes we investigated. Hope this helps! Ow, and if the Idefjorden rocks could help us further, I may consider hopping in the car and join you for a fun trip. At least that is still allowed (in Stockholm now). Thanks!

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