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SUMMARY:Volatiles in Earth’s mantle: primordial or subducted? Insights f
 rom new generation nitrogen and sulfur isotope systematics - Dr Jabrane La
 bidi (IPGP) and Dr James Dottin III
DTSTART:20210225T150000Z
DTEND:20210225T160000Z
UID:TALK154504@talks.cam.ac.uk
CONTACT:Oliver Shorttle
DESCRIPTION:Hydrothermal 15N15N abundances constrain the origins of mantle
  nitrogen\n\nNitrogen (N) is the main constituent of the Earth’s atmosph
 ere\, but its provenance in the Earth’s mantle is uncertain. In this pre
 sentation\, we discuss nitrogen enrichments in multiple mantle reservoirs.
  We show that subduction may not be as important as previously thought to 
 account for mantle nitrogen. We use the rare 15N15N isotopologue of N2 as 
 a novel tracer of air contamination in volcanic gas effusions. By correcti
 ng for air contributions in the gases using this tracer\, we derive new es
 timates for mantle 15N and N2/3He ratios from multiple volcanic regions. W
 e focus on Yellowstone\, a primitive hotspot\, and the central American su
 bduction zone. We show that subduction may cause elevated 15N and N2/3He v
 alues in a mantle source\, as the result of the accumulation of surface-de
 rived components. However\, our 15N15N-based analysis requires the Yellows
 tone plume to have some of the lowest N2/3He ratio. This is inconsistent w
 ith subducted volatiles in this mantle source\, and allows plume nitrogen 
 to be a primordial component. This result opens the possibility that the b
 udget of mantle volatiles was at least partly established during planetary
  formation\, rather than exclusively reflecting subduction and tectonic pl
 ate activity.\n\nIsotopic evidence of multiple sulfur sources delivered to
  the Samoan islands\n\nLike nitrogen\, sulfur isotopes can be used as a to
 ol in placing constraints on crustal recycling and the nature of volatiles
  in primordial mantle reservoirs. Basalts from the Samoan islands sample c
 ontributions from all classical mantle endmembers\, including extreme EM I
 I and high 3He/4He components\, as well as dilute contributions from the H
 IMU\, EM I\, and DM components. The geochemical heterogeneity of the Samoa
 n islands provides an opportunity to test whether distinct S-isotope compo
 sitions are delivered to the Samoan mantle plume and whether they are link
 ed to the various observed mantle components. Through high precision\, qua
 druple S-isotope analyses of Samoan Basalts we observe unique S-isotope co
 mpositions linked to the HIMU\, EM II\, and EM I components at Samoa. We a
 lso use relationships between sulfur and tungsten isotopes to show that th
 e primordial S-isotope composition of the mantle is within uncertainty of 
 the convective mantle\, suggesting S isotope compositions were well mixed 
 within 60 Ma of Earth’s accretion.\n
LOCATION:https://zoom.us/j/97120087330
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