Helium-oxygen-osmium isotopic and elemental constraints on the mantle sources of the Deccan Traps

TitleHelium-oxygen-osmium isotopic and elemental constraints on the mantle sources of the Deccan Traps
Publication TypeJournal Article
Year of Publication2017
AuthorsPeters B.J, Day JMD, Greenwood R.C, Hilton DR, Gibson J., Franchi I.A
JournalEarth and Planetary Science Letters
Date Published2017/11
Type of ArticleArticle
ISBN Number0012-821X
Accession NumberWOS:000413380200025
Keywordscanary island lavas; continental-crust; Deccan Traps; garnet; geochemical evidence; he-3/he-4 ratios; helium; midocean ridge; oceanic-crust; Olivine; osmium; oxygen; phase-relations; picrite basalts; pyroxenite; pyroxenite-peridotite; terrestrial primordial helium

The Deccan Traps, a 65 million-year-old continental flood basalt province located in western India, is the result of one of the largest short-lived magmatic events to have occurred on Earth. The nature and composition of its mantle source(s), however, have been difficult to resolve due to extensive assimilation of continental crust into the ascending Traps magmas. To circumvent this issue, using high-precision electron microprobe analysis, we have analyzed olivine grains from MgO-rich (up to 15.7 wt.%) lavas that likely erupted before substantial crustal assimilation occurred. We compare olivine, pyroxene and plagioclase mineral chemistry and He-O-Os isotope compositions with bulk rock major and trace-element abundances and Os-187/Os-188 for both bulk-rocks and mineral separates. Helium isotope compositions for the olivine grains generally show strong influence from crustal assimilation (<3 R-A), but one ankaramite from the Pavagadh volcanic complex has a He-3/He-4 ratio of 10.7 RA, which is slightly lower than the range of He-3/He-4 measured for present-day Reunion Island volcanism (similar to 12-14 R-A). Olivine-dominated mineral separates span a more restricted range in Os-187/Os-188 (0.1267 to 0.1443) compared with their host lavas (0.1186 to 0.5010), with the separates reflecting a parental magma composition less affected by lithospheric or crustal interaction than for the bulk-rocks. Despite significant He-Os isotopic variations, Delta O-17 is relatively invariant (-0.008 +/- 0.014 parts per thousand) and indistinguishable from the bulk mantle, consistent with high-He-3/He-4 hotspots measured to-date. Compositions of olivine grains indicate the presence of up to 25% of a pyroxenite source for Deccan parental magmas, in good agreement with similar to 20% predicted from isotopic data for the same samples. Modeled pyroxenite signatures are similar to geochemical signatures expected to arise due to other types of mantle differentiation or due to assimilation of continental crust; however, we show that crustal assimilation cannot account for all of the compositional features of the olivine. Weak correlations exist between a global compilation of X-px (Deccan: 0.2-0.7) and He-3/He-4, delta O-18 (Deccan olivine: 4.9-5.2 parts per thousand) and Os-187/Os-188. Robust relationships between these parameters may be precluded due to a lack of two-reservoir source mixing, instead involving multiple mantle domains with distinct compositions, or because Xpx may reflect both source features and crustal assimilation. Notwithstanding, geochemical similarities exist between Deccan Traps olivine (He-3/He-4 = 10.7 R-A; 1(187)Os/Os-188(i) = 0.1313 +/- 45, 2 sigma) and Reunion igneous rocks (He-3/He-4 = 12-14 R-A; Os-187/Os-188(i) = 0.1324 +/- 14). These relationships imply that a characteristic geochemical 'fingerprint' may have persisted in the mantle plume that fed the Deccan Traps, since its inception at 65 Ma, to ongoing eruptions occurring on Reunion up to the present-day. (C) 2017 Elsevier B.V. All rights reserved.

Short TitleEarth Planet. Sci. Lett.
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