Physics
Scientific paper
Sep 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994georl..21.1927d&link_type=abstract
Geophysical Research Letters (ISSN 0094-8276), vol. 21, no. 18, p. 1927-1930
Physics
22
Argon Isotopes, Basalt, Magma, Mid-Ocean Ridges, Radioactive Age Determination, Tectonics, Crystallization, Earth Mantle, Geochronology, Mass Spectrometers, Outgassing, Sea Floor Spreading
Scientific paper
Determination of reliable crystallization ages by K-Ar methods for young (is less than 1 Ma), fresh basalts from the seafloor (mid-ocean ridge basalts (MORBs)) has been frustrated by several effects. The small amounts of radiogenic Ar-40 developed over these timescales in such low-K rocks are difficult to resolve from predominantly atmospheric Ar-40. An additional concern is that mantle-derived Ar-40 may not be totally outgassed when magmas quench at seafloor hydrostatic pressures. We have developed a successful strategy for partially separating atmospheric from radiogenic Ar-40 in (40)Ar-(39)Ar incremental heating experiments on mid-ocean ridge basalts, from the East Pacific Rise near the Clipperton Fracture Zone. Pre-heating samples to 400 C removes surficial atmospheric Ar-40, while subsequent heating at 600-1000 C produces favorable proportions of radiogenic Ar-40. Experiments using 'zero-age' glassy and holocrystalline samples show that mantle-derived Ar-40 is retained only in the outermost few cm of rapidly cooled pillow basalts.
Duncan Roy A.
Hogan L. G.
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