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   Pacific seamount ages

Ages of seamounts, islands and plateaus on the Pacific plate

version 2.1 - 15 May 2004

compiled by

Valérie Clouard1 & Alain Bonneville2

1Departamento de Geofisica, U. de Chile, Blanco Encalada 2002,Casilla 2777 ,Santiago, Chile, valerie@dgf.uchile.cl

2Lab. Géosciences Marines, Institut de Physique du Globe, CNRS, 4, Place Jussieu,75252 Paris Cedex 05, France, bonnevil@ipgp.jussieu.fr

Click here to go to Discussion of this page

The report made available here presents a compilation of reliable dating of seamounts and islands on the Pacific plate from 1,300 samples. Paleomagnetic ages from seamount magnetism are not included. Furthermore, we do not consider foraminifer ages which only give the minimum ages of seamounts. Only radiometric ages are included, and Ar-Ar ages are preferred over K-Ar ages when both are available from the same location.

The table gives our compilation along with references to the selected data. An average age [1, 2] is given when several ages have been determined for the same volcanic stage. In this case the geographical coordinates are given only once for the line. A similar procedure is followed when several volcanic stages exist for the same seamount or island. NB: an ASCII file (ages_pacific_v2.1.dat) with only average ages and geographical coordinates is also available for plotting purposes.

Click here to download the average ages and geographical coordinates (Excel spreadsheet).

Click here to download the article Ages of seamounts, islands and plateaus on the Pacific plate (PDF).

References

  • Clouard, V., Etude géodynamique et structurale du volcanisme de la Polynésie française de 84 Ma à l'actuel, Ph.D., Université de la Polynésie française, 2000.
  • Clouard, V. and A. Bonneville, How many Pacific hotspots are fed by deep-mantle plumes?, Geology, 21, 695-698, 2001.

Discussion

Wed July 21st, 2004: Ajoy Baksi
A look at this effort shows that data from over 50 different papers were examined. It would appear that the original authors assertions as to the reliability of ages, have been accepted without question. Looking at some of the references, I know that some of the ages therein are totally untrustworthy, (almost) as bad as the ones I took up in my page (see ArAr.html).

What is the criterion for a reliable age? Should the radiometrically determined “number” gauge the crystallization time within (say) ± 10% or should it be to within ± 2% (1-sigma estimate)? The latter should be the preferred way for purposes of “tracking hotspots” properly; except for the youngest ages, where high atmospheric contamination levels would produce large analytical errors. However, I note that a 40Ar/39Ar determination of the age of the most recent geomagnetic reversal (Matuyama to Brunhes), gave an age of 783 ka with an error estimate of less than 2% (Baksi et al., Science, 1992). It is clear that in most cases, K-Ar dates cannot be taken to be accurate estimates of crystallization age for whole-rock basalts. Thus the careful K-Ar work (a large part on whole-rock basalts), setting up the geomagnetic polarity time scale for 5-1 Ma, was shown to have systematic errors averaging 5-7% (Baksi, GRL, 1995), resulting from alteration and partial loss of radiogenic argon (40Ar*). This was for samples critically screened for freshness by thin section examination! 40Ar/39Ar total fusion “ages” should not normally be taken as accurate estimates of crystallization age (see Baksi, Jour. Geol., 1999). We are then left only with 40Ar/39Ar stepheating results; and all authors have not been equally scrupulous in interpreting plateau and isochron data (see Baksi, 1999, and my webpage on this site: ArAr.html).

Many authors have been very careful in interpreting 40Ar/39Ar data, e.g. Brent Dalrymple and Marvin Lanphere. I wouldn’t be surprised to see that in cases, what they may have tentatively estimated to be a crystallization age, has now become entrenched in the literature as “reliable” or “very reliable”. Argon dating of altered whole-rock mafic material – as with almost all of the samples from “Pacific Seamounts” - is always a tricky business. On what basis is one to assess their freshness? Is it possible to gauge this from the 40Ar/39Ar raw data? I believe this may be possible, and I am working on some manuscripts. My suspicion is that on critical examination, a number of “accepted” ages for Pacific Seamounts may bite the dust; my fear is that it may hit epidemic proportions, with > 50% of the “ages” having to be rejected!

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