Argon isotope constraints on modification of oxygen isotopes in Iceland Basalts by surficial processes

• Published in: Chemical Geology
• Main Authors: Burnard, Pete, Harrison, Darrell
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: Elsevier 2005
• Subjects: Basalt; Iceland; Helium; Argon; Oxygen; Mantle; Reykjanes
• Online Access: https://doi.org/10.1016/j.chemgeo.2004.11.001

New data from the Reykjanes Peninsula, Iceland, show that ^3He/^4He ratios of subglacial basaltic glasses are well correlated with CaO/Al_2O_3. High CaO/Al_2O_3 ratios are likely due to assimilation of lower crustal clinopyroxene and the large variations in ^3He/^4He likely result from addition of crustal ^4He to the magma during this assimilation. Low ^(40)Ar/^(36)Ar ratios occur only in samples with low δ^(18)O, consistent with a near-surface origin (i.e., alteration or assimilation of altered volcanics) of both the low δ^(18)O and low ^(40)Ar/^(36)Ar in most samples. However, high ^(40)Ar/^(36)Ar ratios (up to 6500) measured in some samples—the samples that have the highest δ^(18)O—is consistent with little or no alteration of these samples. These high ^(40)Ar/^(36)Ar samples have δ^(18)O ±4.6‰, significantly lower than that of the MORB source region (5.2‰). It seems likely that, while alteration by meteoric fluids does influence oxygen isotopic composition, δ^(18)O of the mantle source to these basalts is ±4.6‰. Low δ^(18)O in the Icelandic mantle source could be the result of recycling lithospheric mantle, although this is not likely the source of high ^3He/^4He ratios in Iceland. © 2004 Elsevier B.V. Received 19 March 2004, Accepted 3 November 2004, Available online 14 January 2005. The oxygen isotope analyses were performed in John Eiler's lab at Caltech, and the authors are extremely grateful for the equipment access and subsequent discussions. Reviews by Philippe Sarda and Mark Kurz considerably improved the final manuscript. Sujoy Mukhopadhyay, Don Anderson, and David Graham freely shared their ideas. Neils Oskarsson's help was invaluable during sample collecting. Support for PB was provided by the Caltech Postdoctoral Fellows program. DH was supported by NERC grant no. GR3/11637. Reviews by Chris Hawkesworth, Eleanor Dixon, Manuel Moreira, and an anonymous reviewer considerably improved an earlier version of this manuscript.