科研之友
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摘要
The isotope Sm-146 undergoes alpha-decay to Nd-142, with a half-life of 103 million years. Measurable variations in the Nd-142/Nd-144 values of rocks resulting from Sm-Nd fractionation could therefore only have been produced within about 400 million years of the Solar System's formation ( that is, when Sm-146 was extant). The Nd-142/(144) Nd compositions of terrestrial rocks(1-7) are accordingly a sensitive monitor of the main silicate differentiation events that took place in the early Earth. High Nd-142/(144) Nd values measured in some Archaean rocks from Greenland(1-5) hint at the existence of an early incompatible-element-depleted mantle. Here we present measurements of low Nd-142/(144) Nd values in 1.48-gigayear-(Gyr)old lithospheric mantle-derived alkaline rocks from the Khariar nepheline syenite complex in southeastern India(8). These data suggest that a reservoir that was relatively enriched in incompatible elements formed at least 4.2 Gyr ago and traces of its isotopic signature persisted within the lithospheric root of the Bastar craton until at least 1.48 Gyr ago. These low Nd-142/(144) Nd compositions may represent a diluted signature of a Hadean (4 to 4.57 Gyr ago) enriched reservoir that is characterized by even lower values. That no evidence of the early depleted mantle has been observed in rocks younger than 3.6 Gyr (refs 3, 4, 7) implies that such domains had effectively mixed back into the convecting mantle by then. In contrast, some early enriched components apparently escaped this fate. Thus, the mantle sampled by magmatism since 3.6 Gyr ago may be biased towards a depleted composition that would be balanced by relatively more enriched reservoirs that are 'hidden' in Hadean crust(6), the D '' layer(9-11) of the lowermost mantle or, as we propose here, also within the roots of old cratons.
- 出版日期2009-6-25
