HR: 0800h
AN: V51B-0544    [Abstracts]
TI: Critical Evaluation of Radiometric Ages Used for Tracking Hotspots in the Pacific Ocean
AU: * Baksi, A K
EM: abaksi@geol.lsu.edu
AF: Department of Geology, Louisiana State University, Baton Rouge, LA 70803 United States
AB: One of the pillars supporting the plume hypothesis, is the progression of ages for numerous hotspot tracks in oceans. These ages should be based on radiometric measurements. The argon dating methods have been the tool most commonly used. Since most of the rocks selected for dating have suffered (considerable) alteration, K-Ar dates should not be used as accurate measures of the age of crystallization. 40Ar/39Ar total fusion ages, though better than K-Ar dates in general, do not pinpoint samples that (a) contain excess argon or (b) have suffered partial loss of 40Ar* due to alteration. Hence 40Ar/39Ar incremental heating studies remain as the (only) tool of choice. From such experiments, at a minimum, ages must be based on plateau and/or isochron sections that meet the necessary statistical requirements to be considered crystallization ages. Earlier (Baksi, 1999, Jour. Geol.) it has been shown that almost all the purported crystallization ages for hotspot tracks in the Atlantic and Indian Oceans, are invalid (see also www.mantleplumes.org/ArAr.html). Herein, I apply the tests outlined therein, to evaluate ages available in the literature for hotspot tracks in the Pacific Ocean. These can be divided into five main groups. (1) Those with reliable age data (e.g. Dalrymple and Garcia,1980; Dalrymple et al., 1980, DSDP 55, Hawaiian-Emperor Chain); the authors use care in selecting valid ages from their data sets. (2) Others (e.g. Pringle, 1993, AGU Monograph 77, Musicians Seamounts), most ages are statistically valid, though some fail the requisite test. In addition, many samples show high levels of atmospheric argon, suggesting the samples are (quite) altered; this could lead to incorrect plateau ages. (3) The next set (e.g. Winterer et al., 1993, AGU Monograph 77, Cretaceous guyots in the Northwest Pacific; Ozima et al., 1977, JGRAS, Western Pacific guyots; Saito and Ozima, 1977, EPSL, Western Pacific area) have few, if any, valid ages. Most plateaux/isochrons clearly fail the statistical test of reliability; many steps show high levels of atmospheric argon - the samples are (badly) altered. (4) A set of papers (e.g. Gripp and Gordon, 2002, Geophys. J. Int., young hotspot tracks; Duncan, 1985 - New Hebrides-Samoa lineament) make use of K-Ar dates, wholly or in the main. These dates should be treated as minimum estimates of the crystallization age. (5) A final set of papers (Sager et al., 1993, AGU Monograph 77, Japanese and Marcus-Wake Seamounts; Lincoln et al., 1993, AGU Monograph 77, Marshall Islands), quote ages without listing the relevant analytical data. These results are to be treated as suspect, and not used for quantitative tracking of hotspot trails. In conclusion, the purported progression of ages for numerous hotspot tracks in the Pacific Ocean does not withstand critical scrutiny.
DE: 9355 Pacific Ocean
DE: 9604 Cenozoic
DE: 9609 Mesozoic
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
DE: 1035 Geochronology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting