Mathematics – Logic
Scientific paper
Dec 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufm.u21b0404j&link_type=abstract
American Geophysical Union, Fall Meeting 2007, abstract #U21B-0404
Mathematics
Logic
1009 Geochemical Modeling (3610, 8410), 1030 Geochemical Cycles (0330), 1037 Magma Genesis And Partial Melting (3619), 1040 Radiogenic Isotope Geochemistry, 1060 Planetary Geochemistry (5405, 5410, 5704, 5709, 6005, 6008)
Scientific paper
Mid-ocean ridge basalts (MORBs) that make up the bulk of the oceanic crust (OC) exhibit a relatively uniform isotopic composition. In particular, Sm-Nd isotopic results, suggests that their mantle source had been depleted (the depleted mantle or DM) by melt extraction to form the continental crust (CC) over geologic time with a mean age of extraction of 1.8 Ga. With reservoir models that take into account mantle heterogeneities by keeping track of all sub-reservoirs in DM we have established that the most frequent MORB isotopic compositions faithfully record the average isotopic composition of DM. Mass balance considerations based on Nd and Sr isotopic thus still suggest that mass of the DM is only about 30 % of the mantle. The lower mantle could thus in principle have a composition close to the bulk silicate Earth. We have a good model for sampling the DM but need a better understanding of its location/distribution in the mantle. It is now well established that 146Sm was live in the early Solar System and that samples from Isua, W. Greenland (~ 3.8 Ga old) have high 142Nd/144Nd values when compared to normal terrestrial Nd. This demonstrates that the earliest mantle differentiation (likely in a magma ocean) happened within 100 Myr of Solar System formation and has opened up possibilities for better understanding the earliest part of mantle evolution. Basalts from ocean islands show a much wider range in isotopic composition than MORBs. Their (and MORB) Nd, Sr and Pb isotope data can be described rather well by four isotopic components: DMM, HIMU, EMI, EMII. Many of the OIB data arrays point toward an isotopic component called FOZO or C. Our current analysis shows that these apparent isotopic components are non-existent or fictitious components/reservoirs. The real components are never seen because of the averaging that happens when basaltic melts sample the mantle. In particular, the isotopic composition of FOZO corresponds closely to the average composition of the matrix in the DM, which consists of small length- scale sub-reservoirs (<15km). Part of OIB or plume isotopic signatures are likely to originate in the lower mantle or the D" layer. The isotopic evidence suggests that the lower mantle was completely molten during part of the early magma ocean stage. Recent suggestions that the D" layer formed by foundering of a very early enriched crust is incompatible with the fact that the mean age of differentiation of the DM-CC system is on the order of 2 Ga (as shown by long-lived chronometers). Long-lived chronometers are of course much more reliable in giving us the long-term evolution of major earth reservoirs. However, extinct nuclides can give us a view into early processes that are not at all or only poorly recorded by the long-lived chronometers. We need better models for how plumes sample the mantle to make good use of the isotopic data on plume basalts.
Jacobsen Stein B.
Kellogg James B.
O'Connell Rirchard J.
Ranen Michael C.
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