John Blenkinsop

Nd and Sr isotopic compositions of East African carbonatites: Implications for mantle heterogeneity

Sr and Nd isotopic data from some young carbonatites from East Africa exhibit an excellent negative correlation on an ϵSr - ϵNd diagram. A simple mixing model involving continental crust was tested and found not to fit the isotope patterns. The mixing of two mantle sources is preferred, one being depleted and the other slightly enriched in the large ion lithophile elements relative to bulk earth. The presence of a depleted source below the Tanzanian Shield is consistent with findings from some Canadian carbonatites.

Archean depleted mantle: Evidence from Nd and Sr initial isotopic ratios of carbonatites

Abstract Initial 87 Sr 86 Sr and 143 Nd 144 Nd ratios of some carbonatites and related alkalic rocks intruded into the Canadian Shield between 2700 Ma and 110 Ma ago define development lines on a plot of initial ratio versus age. The isotopic data show that these complexes have been derived from a mantle source with a Rb Sr ratio of 0.020 ± 0.002 and a Sm Nd ratio of 0.358 ± 0.008, which are lower and higher respectively than the values for the silicate portion of the Earth. An estimate of the mean age of formation of this LIL-depleted reservoir, based on the intersection of the development lines with those of bulk Earth, is about 2900 Ma, a figure similar to the age of most Superior Province rocks. A model is proposed that involves differentiation, ca. 2900 Ma ago, of mantle similar in chemical composition to bulk Earth, into granitoid rocks (the Superior Province), and a depleted, residual upper mantle. The source for the Nd and Sr contained in the carbonatites is thought to lie within this depleted sub-continental lithosphere. Measured Sm Nd ratios of about 0.15 for the carbonatites are much lower than the calculated Sm Nd ratio of their source, and support enrichment of the light REE either slightly before or at the same time as carbonatite magma generation.

Lead and strontium isotope relationships in the Oka carbonatite complex, Quebec

Abstract Lead isotopic compositions are presented for 8 calcite, 1 pyrite and 1 melilite separates from the Oka, Quebec, carbonatite complex. Pyrite, melilite and two calcite samples with small in situ decay corrections yield similar initial isotope ratios, averaging 206 Pb 204 Pb = 19.65 , 207 Pb 204 Pb = 15.56 , 208 Pb 204 Pb = 39.00 . This composition shows a close affinity for Pb isotope ratios from ocean island basalts (OIB) and indicates an origin of the Pb in large-ion lithophile (LIL)-element depleted mantle sources. The Pb probably results from a mixture from a LIL-element depleted source and possibly, metasomatic fluids. Four calcites fail to yield accurate initial ratio information due to large in situ decay corrections. In addition three samples exhibit high initial 207 Pb 204 Pb ratios indicative of probable crustal contamination. The silicate fraction of one okaite and a sample of country rock gneiss near Oka contain Pb with isotopic compositions that illustrate the kinds of contamination that could be involved. Sr isotope data are reported for 2 whole-rock sovites, plus 1 nepheline, 4 calcite and 2 apatite separates. All samples yield nearly identical isotopic compositions, with a mean 87 Sr 86 Sr ratio of 0.70331 ± 0.00002 at the 2 sigma level. This result verifies an earlier value reported by Bell et al. (1982) and supports their suggestion of a LIL-element depleted mantle source for the Sr. The Sr data do not show the apparent crustal contamination detected by some of the Pb data, presumably because of the high Sr contents of the carbonate samples. The Oka data are compared with results of a similar study by Lancelot and Allegre (1974) on carbonatite complexes from eastern Uganda. We conclude that the African data, as presently known, are compatible with the model developed for the mantle-derived Oka suite.

Differential diagenesis of sedimentary components and the implication for strontium isotope analysis of carbonate rocks

Geochemical analyses of various components foraminifera, coccoliths and siliciclastic fractions of limestone and marl . samples from the marine Trubi Formation Early Pliocene of southern Italy revealed subtle diagenetic contamination. The coccolith fraction is altered from its original value both in its trace element Sr rCa, MgrCa, FerCa, MnrCa, NarCa all

Nd and Sr isotope systematics of the Oka complex, Quebec, and their bearing on the evolution of the sub-continental upper mantle

A detailed isotopic study of minerals and whole rocks from the Cretaceous Oka complex, Quebec, Canada, shows a very small variation in initial Nd and Sr isotopic compositions. Assuming an age of 109 Ma for the complex, apatite, calcite, garnet, melilite, monticellite, olivine and pyroxene and whole rocks yield a range for initial 87Sr/86Sr of 0.70323–0.70333; and for initial 143Nd/144Nd of 0.51271–0.51284 (εSR(T)= −14.8 to −16.2; εNd(T)=+4.1 to +6.6). The negative εSR and positive εNd indicate derivation of the Nd and Sr from a source with a time-integrated depletion in the large-ion lithophile (LIL) elements. This agrees with data from other Canadian carbonatites and confirms that a large part of the Canadian Shield is underlain by a source region depleted in the LIL elements. The new data from Oka suggest that the depleted source may have remained coupled to the continental crust until recent time.

Age relationships along the Hermitage Bay–Dover Fault System, Newfoundland

WE present here geochronological evidence that parts of the Central Mobile Belt and the Avalon Zone of Newfoundland have been contiguous since the Ordovician or earlier, and that some time after about 4 Myr, significant movement took place along part of the boundary.

Extreme variations in strontium initial ratios in ore-related fluids

Magmato-hydrothermal cassiterite-topaz ore at the Carboniferous East Kemptville (EK) greisenhosted tin deposit (Nova Scotia, Canada) is cross-cut by veins containing apatite, triplite, vivianite and fluorite. Initial 87Sr/86Sr ratios of these minerals have an extreme range (0.7135 to 0.8284). The initial ratios of the host rocks, EK quartz-topaz rock and Davis Lake biotite monzogranite (0.729±0.001, 0.727±0.004), are also high. The adjacent Meguma Group metasedimentary rocks are more typical of crustal material (0.712–0.719 at 330 Ma). Rb and Sr contents of EK fluorite (max.: 13.0, 1420 ppm) and apatite (max.: 88.1, 6660 ppm) are unusually high and variable. Unexpectedly, high Sr contents correlate positively with the high initial 87Sr/86Sr ratios. Fluorite and phosphate minerals from the first set of post-greisen veins at East Kemptville reflect the unusual chemistry of a high 87Sr/86Sr fluid present in the deposit after ore formation. The most extreme composition of this fluid was characterized by 87Sr/86Sr>0.8284, high Rb/Sr, high P, Rb, Cu, Zn and Fe contents, but low abundances of Ca, Pb and Sn with respect to the Davis Lake monzogranite. Such a fluid could have been derived from the greisen fluid and modified by reaction with the overlying Meguma metawacke. A second alternative, which cannot be well constrained at present, is that an extremely radiogenic fluid entered the deposit after ore formation and mixed with the postore fluid. In either case, the modified fluid subsequently mixed with meteoric water and precipitated the minerals with much lower 87Sr/86Sr ratios and Rb and Sr contents.