RE-OS, SM-ND, AND RB-SR ISOTOPE EVIDENCE FOR THICK ARCHEAN LITHOSPHERIC MANTLE BENEATH THE SIBERIAN CRATON MODIFIED BY MULTISTAGE METASOMATISM

Citation
Dg. Pearson et al., RE-OS, SM-ND, AND RB-SR ISOTOPE EVIDENCE FOR THICK ARCHEAN LITHOSPHERIC MANTLE BENEATH THE SIBERIAN CRATON MODIFIED BY MULTISTAGE METASOMATISM, Geochimica et cosmochimica acta, 59(5), 1995, pp. 959-977
Citations number
78
Categorie Soggetti
Geosciences, Interdisciplinary
ISSN journal
00167037
Volume
59
Issue
5
Year of publication
1995
Pages
959 - 977
Database
ISI
SICI code
0016-7037(1995)59:5<959:RSARIE>2.0.ZU;2-3
Abstract
A suite of peridotite xenoliths from kimberlites intruding the Siberia n craton indicate the pressence of lithospheric mantle over 150 km thi ck at 350 Ma. We report Sr-Nd isotope data for minerals from the perid otite xenoliths together with osmium isotopic compositions for whole-r ocks and two olivine separates. Additionally, the osmium isotopic comp osition of a carbonatite from Fort Portal, Uganda, has been measured i n order to evaluate the effect of carbonatite metasomatism on mantle R e-Os systematics. Osmium isotope compositions of peridotite xenoliths from the Mir and Udachnaya kimberlites vary from those characteristic of the oceanic mantle, to considerably less radiogenic values (Os-187/ Os-188, 0.16469 to 0.10812), comparable to those previously found in o ther crationic peridotites. In contrast, two eclogite xenoliths from U dachnaya have extremely radiogenic Os, Os-187/Os-188, up to 9.67. The lowest peridotite osmium isotopic compositions require Re depletion in the mid-Archaean (3.2 Ga) and this age is interpreted as the time of differentiation of the Siberian cratonic lithospheric mantle. Archaean depletion ages for spinel peridotites of relatively shallow origin an d garnet peridotites and dunites containing diamond indicate that the depleted lithosphere reached from the Moho to 150 Km depth at this tim e and has been stable for 3 Ga. Re-Os and Sm-Nd model ages for two ecl ogite xenoliths are also in the range of 2.7 to 3.1 Ga and support an ancient origin for the Siberian lithosphere. The oldest peridotite dep letion ages and the eclogite model ages correspond to the oldest crust al ages obtained from the Anabar Shield of the Siberian craton, and su ggest that the initiation of major crust formation and stabilisation o f a thick cratonic keel were coeval. In general, the Siberian low-temp erature peridotites are not as enriched in incompatible elements as th ose from the Kaapvaal craton yet their diopsides possess similar, low Sm/Nd. The low incompatible element concentrations but LREE/MREE enric hment seen in some Siberian lherzolites suggest they may be the produc ts of disequilibrium melting. Neodymium and strontium isotopic composi tions of minerals from the peridotites are extremely heterogeneous (ep silon(Nd(350)), -55.1 to 491; Sr-87/Sr-86, 0.70253 to 0.72235). Subcal cic garnets of diamond inclusion-like composition within megacrystalli ne peridotites have epsilon(Nd(350)) values varying from -55.1 to -12. 1. Depleted mantle model Nd ages are as old as 3.2 Ga permitting an an cient, enriched origin similar to that suggested for diamond inclusion s (Richardson et al., 1984). Alternatively, consideration of the compl ex garnet Sm-Nd isotope systematics and the presence of unsupported ra diogenic Sr together with marked trace element zonation (Shimizu et al ., 1994) suggest that these subcalcic garnets crystallised recently (c lose to the time of kimberlite eruption) from ancient, LREE-enriched, high Rb/Sr precursors. We propose that the isotope systematics of subc alcic garnet diamond inclusions can also be interpreted in terms of a recent origin.