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| Titel |
Baffin Island picrites contain normal terrestrial 142Nd/144Nd: Implications for the source of high 3He/4He in deep Earth |
| VerfasserIn |
Goverdina A. M. de Leeuw, Richard W. Carlson, Robert M. Ellam, Finlay M. Stuart |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250036243
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| Zusammenfassung |
| The short-lived (t1-2 = 103 My) radiogenic isotope system 146Sm-142Nd has been used to
infer early differentiation of the Earth. Compared to chondrites, most terrestrial rocks are
enriched in 142Nd by about 20 ppm. One explanation for this is that bulk Earth did not start
off with a chondritic Sm/Nd. However, the magnitude of the enrichment requires bulk Earth
Sm/Nd beyond the range observed in chondrites. An alternative explanation is that the Earth
experienced differentiation during the lifetime of 146Sm that isolated a complementary low
Sm/Nd reservoir (the early enriched reservoir (EER)) which is able to balance the
apparent 142Nd excess. Thus, in a chondritic bulk Earth, non-chondritic 142Nd/144Nd
signatures require that early enriched and early depleted reservoirs separated from the
primitive undifferentiated mantle. The highest 3He/4He in basalts is conventionally
interpreted as requiring the existence of a mantle reservoir that has been convectively
isolated for over 3 billion years, and it has previously been proposed that the EER
might be the source of elevated 3He/4He in the deep Earth. As there is uncertainty
about the relative partition coefficients of U and He there is the possibility that
high 3He/4He may also be hosted in the early depleted reservoir (EDR). Here we
present 142Nd/144Nd for a suite of proto-Iceland plume picrites from Baffin Island
that have the highest mantle 3He/4He (c. 50 Ra) yet measured. For all samples
142Nd/144Nd is identical, within analytical precision, to the terrestrial standard giving a
mean ε142Nd value of -0.03 ± 0.05 (n = 11). Thus, the high 3He/4He source has a
super-chondritic 142Nd/144Nd signature which does not appear to be derived from primitive
undifferentiated mantle. The data are consistent with the addition of primordial
He into the Icelandic plume source which contains modern 142Nd/144Nd values. |
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