In search of a hidden long-term isolated sub-chondritic (142)Nd/(144)Nd reservoir in the deep mantle: Implications for the Nd isotope systematics of the Earth

• Published in: Geochimica et cosmochimica acta Vol. 74; no. 2; pp. 738 - 750
• Main Authors: Murphy, David Thomas, Brandon, Alan D, Debaille, Vinciane, Burgess, Ray, Ballentine, Chris
• Format: Journal Article
• Language: English
• Published: 15.01.2010
• ISSN: 0016-7037
• DOI: 10.1016/j.gca.2009.10.005

Here we search for evidence of the existence of a sub-chondritic (142)Nd/(144)Nd reservoir that balances the Nd isotope chemistry of the Earth relative to chondrites. If present, it may reside in the source region of deeply sourced mantle plume material. We suggest that lavas from Hawai'i with coupled elevations in (186)Os/(188)Os and (187)Os/(188)Os, from Iceland that represent mixing of upper mantle and lower mantle components, and from Gough with sub-chondritic (143)Nd/(144)Nd and high (207)Pb/(206)Pb, are favorable samples that could reflect mantle sources that have interacted with an Early-Enriched Reservoir (EER) with sub-chondritic (142)Nd/(144)Nd. High-precision Nd isotope analyses of basalts from Hawai'i, Iceland and Gough demonstrate no discernable (142)Nd/(144)Nd deviation from terrestrial standards. These data are consistent with previous high-precision Nd isotope analysis of recent mantle-derived samples and demonstrate that no mantle-derived material to date provides evidence for the existence of an EER in the mantle. We then evaluate mass balance in the Earth with respect to both (142)Nd/(144)Nd and (143)Nd/(144)Nd. The Nd isotope systematics of EERs are modeled for different sizes and timing of formation relative to e(143)Nd estimates of the reservoirs in the k(142)Nd = 0 Earth, where k(142)Nd is ((measured (142)Nd/(144)Nd/terrestrial standard (142)Nd/(144)Nd)-1 * 10(-6)) and the k(142)Nd = 0 Earth is the proportion of the silicate Earth with (142)Nd/(144)Nd indistinguishable from the terrestrial standard. The models indicate that it is not possible to balance the Earth with respect to both (142)Nd/(144)Nd and (143)Nd/(144)Nd unless the k(142)Nd = 0 Earth has a e(143)Nd within error of the present-day Depleted Mid-ocean ridge basalt Mantle source (DMM). The 4567 Myr age (142)Nd-(143)Nd isochron for the Earth intersects k(142)Nd = 0 at e(143)Nd of +8 +/- 2 providing a minimum e(143)Nd for the k(142)Nd = 0 Earth. The high e(143)Nd of the k(142)Nd = 0 Earth is confirmed by the Nd isotope systematics of Archean mantle-derived rocks that consistently have positive e(143)Nd. If the EER formed early after solar system formation (0-70 Ma) continental crust and DMM can be complementary reservoirs with respect to Nd isotopes, with no requirement for significant additional reservoirs. If the EER formed after 70 Ma then the k(142)Nd = 0 Earth must have a bulk e(143)Nd more radiogenic than DMM and additional high e(143)Nd material is required to balance the Nd isotope systematics of the Earth.