Contrasting behaviours of CO₂, S, H₂O and halogens (F, Cl, Br, and I) in enriched-mantle melts from Pitcairn and Society seamounts

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• 作者：Mark A. Kendrick^a ; ^b ; ^{mark.kendrick@anu.edu.au" class="auth_mail} ; Matthew G. Jackson^c ; Adam J.R. Kent^d ; Erik H. Hauri^e ; Paul J. Wallace^f ; Jon Woodhead^a
• 关键词：Mantle ; Ocean island basalt ; EM1 ; EM2 ; Subduction ; Volatiles
• 刊名：Chemical Geology
• 出版年：26 March, 2014
• 年：2014
• 卷：370
• 期：Complete
• 页码：69-81
• 全文大小：1645 K

文摘

In order to improve characterisation of volatiles in the EM1 and EM2 mantle sources, which are interpreted to contain subducted sedimentary or lithospheric components, we report electron microprobe, FTIR and SIMS CO₂, H₂O, S, F and Cl concentrations of variably enriched glasses from Pitcairn and Society seamounts in Polynesia. The analyses complement previously published Cl, Br and I data for some of the same glasses and all the techniques show reasonable agreement of better than 5-9% for S, Cl and H₂O. The concentrations of H₂O and all the halogens increase as a function of melt evolution with the highest values of 1.6 wt.% H₂O, 2100 ppm F, 1500 ppm Cl, 3.7 ppm Br and 80 ppb I in melts with < 2 wt.% MgO. In contrast, CO₂ and S are strongly influenced by degassing and it is the least evolved melts that preserve the highest CO₂ concentrations, that indicate CO₂ oversaturation of some lavas. Comparison of volatile and non-volatile elements of similar compatibility (e.g. H₂O/Ce, Cl/K) as a function of equilibration depth and ⁸⁷Sr/⁸⁶Sr, suggests H₂O and halogens were not significantly degassed from melts equilibrated at > 100 bars and most of the variation in H₂O/Ce reflects source composition. Despite their overall enrichment in volatiles, both the Pitcairn and Society EM1 and EM2 sources are depleted in H₂O as well as Cl, Br, and I relative to Ce and K. Slightly different behaviour is suggested for F and a correlation between F/Cl and K/Cl in Pitcairn melts, could be explained by preferential subduction of F relative to Cl into the EM sources. The relative abundances of H₂O, halogens and lithophile elements in melts from different tectonic settings indicate subduction-related H₂O and Cl loss efficiencies are similar at ~ 90-96%; F is suggested to have a lower loss efficiency; whereas Br and I are lost more efficiently. Nonetheless, dehydrated lithosphere containing 5-10% of its original volatile content is interpreted as the most likely source of volatile enrichment in the Pitcairn and Society mantle sources.