Abstract
Application of the 147Sm-143Nd and 146Sm-142Nd
chronometers suggests that mantle depletion and related crust
extraction had started during the first hundred million years of
Earth history (Caro et al., 2006), but there is limited evidence
for the presence of an enriched silicate reservoir before 4.0 Ga
(Harrison et al., 2006). Here, we report new 147Sm-143Nd data
from low-grade metamorphosed Paleoarchean volcanic and
sedimentary rocks of the Warrawoona Group, Pilbara Craton,
Western Australia. A Sm-Nd isochron age of 3.48 ± 0.09 Ga is
in an agreement with the U-Pb zircon age from intercalated
volcanoclastic rocks and indicates the preservation of their
Sm-Nd isotope signature since the Archean. The initial HNd
value of -3.3 ± 0.5 is out of the range (+1< HNd < +3)
established for the Archean cratons based on whole-rock Sm-
Nd isochrons with ages agreeing with independent
chronological constraints. This data is best explained by the
assimilation of older continental crust, as shown by significant
Ta-Nb depletion associated with LILE, Th, U and LREE
enrichment (Green et al., 2000). Geochemical modeling
suggests the assimilation of 10% to 20% of crustal material
that may be either Archean granites, shales, or average upper
continental crust (Condie, 1993). Assimilation of this crustal
material with HNd values of –6 (granite), –8 (shales) and/or –10
(upper CC) by a mantle-derived melt accounts for the
observed Nd isotope composition of basalts. This crustal
component, with 147Sm/144Nd ratios of 0.09–0.12, would have
been isolated from the depleted mantle at 4.0–4.4 Ga, which is
consistent with the Nd model ages derived from the studied
rocks. Our observations, albeit indirect, clearly show the
existence of a an Hadean crustal reservoir. Its presence is
confirmed by negative HNd of Archean seawater, inferred from
carbonate rocks with seawater-like REE patterns. The
presence of a crustal basement involved in plume magmatism
indicates eruption of flood basalts onto a submerged
continental platform, analogous with the Phanerozoic
Kerguelen plateau (Van Kranendonk and Pirajno, 2004). This
could have been an important mode of crustal production in
the Archean.
References
Caro G. et al. (2006) Geochim.Cosmochim.Acta 70, 164-191.
Condie K.C. (1993) Chem. Geol. 104, 1-37.
Green M.G. et al. (2000) Tectonoph. 322, 69-88.
Harrison et al. (2006) Science 312, 1947-1950.
Van Kranendonk M. and Pirajno F. (2004) Geochemistry:
Exploration, Environment, Analysis 4, 253-278.
Original language | English |
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Pages (from-to) | A1015-A1015 |
Number of pages | 1 |
Journal | Geochimica et Cosmochimica Acta |
Volume | 71 |
Issue number | 15 supplement |
Publication status | Published - Aug 2007 |
Externally published | Yes |
Event | Goldschmidt Conference (17th : 2007) - Cologne, Germany Duration: 19 Aug 2007 → 24 Aug 2007 Conference number: 17 |
Keywords
- CONTINENTAL-CRUST
- PILBARA CRATON