Initial Sr-isotope dating and ancillary geochemical studies on kimberlite and associated inclusions

Abstract

Abstract provided by author:

An interesting outcome of the age work is the postulated existence of phlogopite calcite masses in the mantle. Incomplete (partial) Sr-isotopic equilibration probably occurred between constituent minerals. Since the petrogenetic significance of the results hinges on the freshness of the samples this parameter was carefully assessed using petrographic, acid leaching and electron probe methods:-

1) Whole rock peridotite and eclogite nodules (and separated eclogitic garnets) are contaminated to varying and uncertain extents: they cannot be used for genetic studies. However, the characteristics and possible sources of the contaminants are discussed. Peridotitic contamination appears tohave carbonatitic (or fresh kimberlitic) affinities. Eclogitic K, Rb and Sr characteristics should be seen in terms of three contributors; omphacite, but dominantly kelyphite and mica, both of which are secondary

2) Since the isochrons for the micas define emplacement time, no significant in situ contamination of the micas has occurred and their initial Sr isotope ratios should be of genetic significance. However, K, Rb and Sr relative and absolute abundance studies indicate that Sr-rich impurities, possibly of carbonatitic origin, are associated with some micas. This material was in isotopic equilibrium with the co-existing mica but it did not significantly affect the mica K/Rb ratio

The micas analysed fall into different groups on the basis of K/Rb and Rb parameters. Each group is composed of micas from a particular source and although each possesses a different mean initial Sr isotope ratio, (Sr 87/Sr 86) o, the uncertainties in the isotope ratios overlap in many cases. The mean (Sr87/Sr86) o values show no systematic variation with K/Rb ratios. The significance of the mica K/Rb and (Sr87/Sr86) o parameters cannot be adequately explained in terms of existing theories of geochemistry but different groups of micas probably have different histories. The high K/Rb ratios, (up to 380), of some eclogite and peridotite nodule micas is noteworthy

3) Kimberlite whole rocks were assessed according to empirical petrographic criteria. The initial Sr ratios of primary (basaltic), fresh kimberlite magma of Cretaceous age is considered to be ~ 0, 704. Higher ratios of 0, 705 - 0. 708 occur in fresh micaceous kimberlites and in samples assessed as altered. Of possible processes considered for augmenting isotope ratios above ~ 0, 704, a ground water mechanism may best account for the high initial Sr ratios of typical basaltic kimberlite. Assimilation of wall rock may be significant locally. Incorporation of high Sr87/Sr86 ratio, xenolithic, calcitic phlogopite nodular material from the mantle may be of importance in producing high Sr isotope ratios in apparently fresh micaceous kimberlite. Incorporation of xenolithic minerals typical of peridotites, eclogites and less calcitic phlogopite nodules could account for the range in isotope ratios in fresh basaltic kimberlites and for many of the K and Rb characteristics of in situ kimberlite. There is a similarity in initial Sr isotope and K/Rb ratios of fresh basaltic kimberlite and carbonatites

4) The clinopyroxenes have very low K and Rb contents. The extent of possible contamination is uncertain, however it seems feasible that some of the clinopyroxene impurities may originate within the pyroxene system. K/Rb ratios of peridotite diopsides (~ 230) and of eclogite omphacites and discrete diopsides (600- 8000) are distinct. Neither the Sr isotope ratios nor Sr contents were affected by possible alteration

Initial Sr isotope ratios of clinopyroxenes from different hosts and whole rock kimberlite increase in the following order: discrete diopside nodules (0, 703 - 0, 7O4) most fresh basaltic kimberlites analysed (0. 704 - 0. 705) eclogite omphacites (0, 7047-0, 7062) granular garnet peridotite diopsides (0, 7060-0, 7075). Isotopic evidence (and petrographic assessment) indicates that peridotite micas are primary but that eclogite micas are probably secondary. The high (Sr87/Sr86) o ratios of the peridotite diopsides may result from isotopic exchange with the coexisting mica. Omphacites resided (for at least part of their history) in a less Rb-rich environment: discrete diopsides were associated with still less Rb. However, the low K and Rb abundances and high K/Rb ratios of both omphacites and discrete diopsides indicate that these minerals may have formed in a similar (possibly cumulate or residual) manner

It is shown that garnet peridotite of the type analysed cannot be a recent parent of eclogite nor can it be directly parental to kimberlite. Both eclogite and peridotite nodules are accidental inclusions and the association of garnet peridotite with eclogite in kimberlite (unless the association formed very many millions of years before emplacement) is considered fortuitous. Eclogite represented by omphacite (and garnet) cannot be parental to kimberlite. Discrete diopside nodules could have been associated with the source of primary basaltic kimberlite or be megacrysts accidentally incorporated

The similarity between isotopic and certain other parameters eclogite and Stormberg volcanics is noted. The nature of possible associations of the two rock types is considered