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Fractionation of noble gases (He, Ar) during MORB mantle melting: a case study on the Southeast Indian Ridge

Burnard, Pete and Graham, David and Farley, Ken (2004) Fractionation of noble gases (He, Ar) during MORB mantle melting: a case study on the Southeast Indian Ridge. Earth and Planetary Science Letters, 227 (3-4). pp. 457-472. ISSN 0012-821X. http://resolver.caltech.edu/CaltechAUTHORS:20121119-104626471

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Abstract

New measurements of the He, Ar and CO_2 abundances trapped in basaltic glasses from the Southeast Indian Ridge (SEIR) show that volatile concentrations in the SEIR magmas were controlled by fractional degassing. Fractionation between volatile species is consistent with their solubilities in silicate melts. As a result, there are linear relationships between (for example) ln(^4He/^(40)Ar^*) vs. ln[^(40)Ar^*] and between ln(^4He/^(40)Ar^*) vs. ln(^(40)Ar^*/CO_2) (where ^(40)Ar^* is the ^(40)Ar corrected for atmospheric contributions). The slopes of these correlations permit the relative He/Ar and Ar/CO_2 solubilities to be estimated; these are generally consistent with experimentally determined noble gas solubilities in basaltic melts. However, there are systematic differences in the degassing trajectories. For example, in a plot of ln(^4He/^(40)Ar^*) vs. ln(^(40)Ar^*/CO_2), samples from the deepest portions of the ridge consistently plot at lower ^4He/^(40)Ar^* for a given ^(40)Ar^*/CO_2, compared to shallower sections of ridge. These variations in ^4He/^(40)Ar^* likely reflect variations in He/Ar in the primary melt, i.e. their relative abundances prior to degassing. We estimated the variation in ^4He/^(40)Ar^* in the initial melts (i.e. the ^4He/^(40)Ar^* prior to degassing) by extrapolating the degassing trend to a constant mantle-like ^(40)Ar^*/CO_2 ratio and assuming that the relative He–Ar–CO_2 solubilities do not vary between samples. The ^4He/^(40)Ar^* corrected for degassing in this manner varies by a factor ≈10 and correlates positively with the ^3He/^4He ratio. It is possible that the correlation between “degassing corrected” ^4He/^(40)Ar^* ratios and the ^3He/^4He ratio results from preferential diffusion of ^3He relative to ^4He and of 4He relative to ^(40)Ar from the solid mantle into primary melts during melting. However, modeling this diffusive process fails to reproduce the comparatively large variations in ^3He/^4He found in the basalts; therefore, it seems likely that mantle heterogeneities, in combination with diffusive fractionation, resulted in coupled He isotope and He/Ar variations.


Item Type:Article
Related URLs:
URLURL TypeDescription
http://dx.doi.org/10.1016/j.epsl.2004.08.021DOIUNSPECIFIED
http://www.sciencedirect.com/science/article/pii/S0012821X04005333PublisherUNSPECIFIED
Additional Information:© 2004 Elsevier B.V. Received 21 January 2004; received in revised form 22 April 2004; accepted 18 August 2004. Available online 13 October 2004. Editor: B. Wood. This work was supported by the Marine Geology and Geophysics Division of the National Science Foundation (OCE99-12359 and OCE99-11418). Dave Christie (OSU) helped with sample selection and shared major and trace element data. The authors are extremely grateful for the thorough and insightful reviews by Dave Hilton, Manuel Moreira, Phillipe Sarda and an anonymous reviewer.
Funders:
Funding AgencyGrant Number
NSF Marine Geology and Geophysics DivisionOCE99-12359
NSF Marine Geology and Geophysics DivisionOCE99-11418
Subject Keywords:noble gas; degassing; diffusion; mid-ocean ridge basalt; Southeast Indian Ridge; mantle melting
Record Number:CaltechAUTHORS:20121119-104626471
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20121119-104626471
Official Citation:Pete Burnard, David Graham, Ken Farley, Fractionation of noble gases (He, Ar) during MORB mantle melting: a case study on the Southeast Indian Ridge, Earth and Planetary Science Letters, Volume 227, Issues 3–4, 15 November 2004, Pages 457-472, ISSN 0012-821X, 10.1016/j.epsl.2004.08.021. (http://www.sciencedirect.com/science/article/pii/S0012821X04005333)
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:35540
Collection:CaltechAUTHORS
Deposited By: Ruth Sustaita
Deposited On:19 Nov 2012 19:22
Last Modified:27 Dec 2012 03:02

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