Physics
Scientific paper
May 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agusm.v41a..03h&link_type=abstract
American Geophysical Union, Spring Meeting 2005, abstract #V41A-03
Physics
8147 Planetary Interiors (5430, 5724)
Scientific paper
Can thermochemical models of mantle convection be tested by geochemical data? The answer is: Yes, but with real difficulty. The bulk of the mantle is inaccessible to direct laboratory analysis: mantle samples are limited to xenoliths (lithosphere), abyssal peridotites (MORB), and alpine peridotites (MORB or arc). An indirect sample of the mantle is provided by melts (basalts, picrites, etc.). The major breakthroughs in Mantle Geochemistry of the past 3-4 decades have come from measurements of isotope ratios and incompatible trace element abundances in melts. The links between major element variations (Fe/Mg, Si/Mg) that control the geophysical observables of the mantle and the geochemical observables (FOZO, etc.) are weak. Combined geodynamic and tomographic models of mantle convection indicate variations of 10% in the Fe content of the mantle. The previously published range of Fe/Mn in mantle peridotites is 50-70, and showed no correlation with MgO. We have developed a significantly more precise method of Fe/Mn ratio determination by ICP mass spectrometry. New data for mantle xenoliths show a strong positive correlation between Fe/Mn and MgO, and indicate that the primitive mantle has Fe/Mn 61. Hawaiian basalts and picrites have significantly higher Fe/Mn (67) than MORB (56), or Icelandic picrites (58). Assuming that the mantle source beneath Hawaii has a higher Fe content, these results imply an iron excess of 15-20% relative to ambient mantle. However, the Fe/Mn ratio depends on both the molar Fe/Mg ratio and on the Si/Mg ratio, because of the compatibility of Fe and Mn in olivine vs. pyroxenes. Recent studies of Ni in olivine in Hawaiian basalts have been taken to imply a higher Si content for their mantle source. Thus, it is not yet possible to uniquely distinguish between source variations of Fe/Mg and Si/Mg from the Fe/Mn ratios of Hawaiian melts. Either interpretation implies that the sources of some plumes (Hawaii, but not Iceland) are higher in Fe or Si than ambient mantle (MORB). Mantle geochemistry is finally coming to grips with the single most important question that geophysicists may ask of it: major element variation in mantle plumes that reach the surface to form volcanic islands.
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