In an effort to further constrain genetic models of kimberlites and lamproites we have analysed a suite of well characterised southern African kimberlites (fresh, hypabyssal facies) and a varied suite of lamproites from the western U.S.A. and Australia.
Re (20-680 ppt) and Os (700 ppt to 3.1 ppb) contents of the kimberlites (n=16) are highly variable with no distinction between Group I, Group II or transitional variants. Lamproites and associated rocks (n=12) have systematically lower Re contents (15 to 398 ppt), and generally lower Os contents (113 ppt to 2.23 ppb) than either group of kimberlites but their Re/Os values are similar. The Re contents of some kimberlites and lamproites are surprisingly low. This may be the result of sulphide breakdown and oxidation during eruption.
Mesozoic kimberlites have 187Os/188Os values from 0.11405 to 0.13918. Calculated initial Os isotopic compositions, expressed as gOs values (percent deviation from Bulk Earth at the time of eruption) for the Mesozoic kimberlites range from -10 to +6.7. There are no systematic differences between initial gOs values for Mesozoic Group I and Group II kimberlites. The least radiogenic sample is from the Group II Bellsbank kimberlite which contains a very high abundance of olivine macrocrysts. The gOs value of -10 for the bulk sample is remarkably close to the mean gOs value of -10.0 ± 4.7 for 21 Low-T Kaapvaal peridotite xenoliths (Pearson et al., 1995). Removing a significant amount of the macrocrysts from a replicate sample resulted in a 35% lower Os concentration and more radiogenic isotope composition (gOs -7). This and a good correlation between gOs and 1/Os for Group II kimberlites indicates the possibility that the unradiogenic Os isotopic signatures in some kimberlites is largely dominated by incorporation of comminuted lithospheric peridotite into the kimberlite rather than specifically requiring a depleted lithospheric mantle source to melt. A further possibility is the contamination of kimberlite magma by siderophile element enriched "nuggets" within the lithospheric mantle (McDonald et al., 1995).
If low gOs signatures in kimberlites are a result of incorporation of lithospheric peridotites, the more radiogenic samples have similar Os isotopic compositions to modern oceanic magmas. These characteristics are difficult to reconcile with the notion that kimberlites are melts of depleted lithospheric mantle sources unless the xenolith inventory is not representative. Additionally, it seems that the old, incompatible element enriched component of Group II kimberlites does not contribute ancient Os. This contrasts with xenoliths which may have v. unradiogenic Os and highly enriched Sr and Nd isotope signatures. The identity of the kimberlite sources is not yet resolved using Os. Parental kimberlitic magmas may originate either directly from the asthenosphere, or from major element depleted subducted oceanic lithosphere and then pick up enriched lithospheric components. Alternatively they are melts of lithosphere not significantly represented by the xenolith inventory.
The Os isotope compositions of the lamproites are much more variable than those of kimberlites, most being more radiogenic than Bulk Earth. 2 W. Australian olivine lamproites have similar Os systematics to kimberlites. The remaining, more radiogenic values, cannot realistically be explained by contamination with crust and appear to require highly heterogeneous sources. Samples from throughout the Wyoming Craton have consistently radiogenic Os isotope compositions that resemble metasomatised peridotites from that region (Carlson and Irving, 1994). The isotopic variability of lamproites and lamprophyres may be explained if they originated from complex sources consisting of veined lithospheric mantle (Foley, 1992).
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