Other
Scientific paper
Jun 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994gecoa..58.2509f&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 58, Issue 11, pp.2509-2517
Other
9
Scientific paper
40 Ar/ 36 Ar and helium and argon concentrations have been repeatedly measured on olivine phenocrysts in a single tholeiitic basalt (PIN-12) from the Juan Fernandez hotspot. Forty olivine splits were analyzed by crushing of bulk samples or laser fusion of single crystals. The measured 40 Ar/ 36 Ar ratios span a very large range (400-7700) and are consistent with binary mixing of two argon components. Unlike argon, helium in repeated measurements of this single basalt flow has a reproducible isotopic ratio, 17 times the air 3 He/ 4 He value. It is unlikely that such large variations in 40 Ar/ 36 Ar represent microscale mantle heterogeneity. Rather the results indicate highly variable mixing proportions of a mantle-derived radiogenic argon component ( 40 Ar/ 36 Ar >> 7700) and an isotopically air-like endmember that is almost certainly an atmospheric contaminant. This air-like constituent cannot be removed by physical and chemical treatments of the olivines. Analysis of individual crystals by laser fusion shows that both the radiogenic and the contaminant components are in fluid inclusions. Siting of the contaminant in inclusions requires the addition of airderived noble gases to hotspot magmas prior to or during emplacement, a process that may occur by assimilation of altered crust during crustal storage or, alternatively, by direct addition of air or seawater to the magma. In either case the olivines must continue to trap argon, presumably by fracture annealing and/or bubble enclosure, after the contaminating event. If atmospheric contamination is a general phenomenon, the 40 Ar/ 36 Ar composition of olivines (and possibly of basalt glasses as well) must be only a lower limit for the mantle source ratio. With the exception of helium, the other noble gases may be similarly compromised. Our results support contentions that lavas with near-atmospheric noble gas compositions reflect severe atmospheric contamination, rather than the air-like signature of an undegassed primitive mantle reservoir.
Craig Harmon
Farley Ken A.
No associations
LandOfFree
Atmospheric argon contamination of ocean island basalt olivine phenocrysts does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.
If you have personal experience with Atmospheric argon contamination of ocean island basalt olivine phenocrysts, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Atmospheric argon contamination of ocean island basalt olivine phenocrysts will most certainly appreciate the feedback.
Profile ID: LFWR-SCP-O-1864960