C. Brooks

The geochemistry and evolution of early precambrian mantle

Seven high-purity cumulate clinopyroxenes from 2.7 b.y. maficultramafic rock associations from the Abitibi belt, Superior Province, Canada, have been analyzed for major elements and K, Rb, Cs, Ba, Sr and 87Sr/86Sr ratio. Attempts to reconstruct the trace element patterns of the original parent magmas were partially successful; Sr contents (140 ppm), K/Rb (470) and K/Ba (16) ratios are similar to those of modern low-K island arc tholeiites. K/Cs ratios (2700) are significantly lower than island arc tholeiites (17,000) or oceanic island and oceanic ridge basalts (> 30,000); the presentday mantle seems to be more depleted in Cs than in Archean times.Initial Sr isotope ratios of the 7 Archean clinopyroxenes average 0.70114±13(2σ) with relatively little variation; this value is in good agreement with initial ratios published for felsic and mafic rocks of the same age, though the latter show much larger variations and uncertainties. The pyroxene Sr isotope data, in conjunction with data for rocks of other ages, defines the following simple model for mantle evolution:a)starting with primordial Sr, a short period of relatively rapid 87Sr/86Sr growth, followed by Rb depletion;b)a period between ≧ 3.5 b.y. and ∼ 1.7 b.y. when closed-system Sr isotope evolution occurred (with Rb/Sr ∼ 0.023);c)development of large-scale Rb/Sr heterogeneities in the mantle at ∼ 1.7 b.y., leading to a present-day mantle with 87Sr/86Sr ranging from 0.7023 to 0.7065 and Rb/Sr ranging from ∼ 0 to 0.065.

Strontium and its isotopes in Canadian rivers: Fluxes and global implications☆

Systematic sampling of the 39 largest Canadian rivers shows that the weighted average 87Sr86Sr ratio in the dissolved load is 0.7111, similar to previous measurements on such large rivers as the Amazon and Mississippi. Consequently, we believe that the above estimate is likely representative of the global average. This imposes a limit of 6.5 × 1011g yr−1 on the amount of Sr exchanged with basalts in hydrothermal cells on mid-oceanic ridges. Evaluation of geological information suggests that Sr from silicate sources is of considerable importance for all but the largest Canadian rivers. The latter have chemical and isotopic composition consistent with ~4:1 carbonate vs. silicate derivation of Sr, but such interpretation is not unique. In terms of their water discharge, the 39 Canadian rivers studied account for 4.2% of the world total and their weighted average concentrations for other dissolved solutes are: TDS 176 ppm, Ca 18 ppm, Cl 6.8 ppm and Sr 84 ppb.

Clinopyroxene-matrix partitioning of K, Rb, Cs, Sr and Ba

Abstract Extremely pure samples of clinopyroxene phenocrysts from two volcanic rocks have been analyzed for K, Rb, Cs, Sr and Ba. In conjunction with matrix concentrations, partition coefficients are obtained which are in the range 0.001–0.004 for K, Rb, Cs and Ba. These values are lower than those in the literature by factors of 6–100 but are in good agreement with values determined experimentally at pressures of 15–30 kb by Shimizu (Geochim. Cosmochim. Acta38, 1974). Values for partition coefficients measured on separates of impure or cloudy pyroxenes from these same rocks were higher and similar to those in the literature. We suggest this effect is related to ‘trapping,’ during crystal growth, of liquid which is enriched in the larger ions (such as Rb and Cs) due to lack of diffusion equilibrium in the liquid. Partition coefficient values for olivine and plagioclase from one of these same rocks were also determined.

Determination of initial 87Sr86Sr and 143Nd144Nd in primary minerals from mafic and ultramafic rocks: Experimental procedure and implications for the isotopic characteristics of the Archean mantle under the Abitibi greenstone belt, Canada

In order to evaluate the isotopic characteristics of the Archean mantle we have analysed primary minerals from mafic and ultramafic rocks from the Abitibi greenstone belt (Canada). In the first part of this study, we show the effects of washing, leaching and partial dissolution on the mobility of K, Rb, Ba, Sr, Nd and Sm and on the Sr isotopic composition of Archean and modern clinopyroxenes. These experiments lead to the establishment of an analytical procedure allowing the measurement of primary 87Sr86Sr and 143Nd144Nd. Following this procedure, we analysed six clinopyroxenes and a plagioclase from five localities in the Abitibi belt The results show that the mantle source of these rocks was homogeneous and depleted in RbSr and NdSm over the entire belt, with average 87Sr86Sr = 0.70105 ± 0.00006 and ϵNd = 2.49 ± 0.27.

Andean cenozoic volcanism: magma genesis in the light of strontium isotopic composition and trace-element geochemistry

Subduction of the oceanic Nazca plate beneath the western edge of the South American plate is thought by most proponents of plate theory to have generated the tectonic forces and magmatic activity that have structured the Andean orogen. By this view, the Cenozoic andesitic magmas of the Andean stratovolcanoes originated within the oceanic crust of the descending plate or within the mantle adjacent to that plate. Accordingly, these Andean volcanic rocks should exhibit chemical and isotopic compositions similar to those of island-arc andesite of like origin. However, while major- and minor-element analyses reveal that the late Cenozoic volcanic rocks are chemically akin to those of the calc-alkalic suite of circum-Pacific island arcs, they exhibit significantly higher Sr/sup 87//Sr/sup 86/ ratios. The purpose of this paper is to examine these anomalously high isotopic ratios in the light of trace-element geochemistry and to develop from this a model for the genesis of the Cenozoic magma.

The Shabogamo Intrusive Suite, Labrador: Sr and Nd isotopic evidence for contaminated mafic magmas in the Proterozoic

Abstract The Shabogamo Intrusive Suite comprises numerous bodies of variably metamorphosed gabbro which intrude Archean and Proterozoic sequences at the junction of the Superior, Churchill, and Grenville structural provinces in western Labrador. Combined Sm-Nd and Rb-Sr systematics in two bodies, ranging from unmetamorphosed to lightly metamorphosed, document a crystallization age of about 1375 m.y., and suggest that both bodies crystallized from magmas with similar Nd and Sr isotopic compositions. This age is in accordance with the existence of a regional magmatic event in the Churchill Province at approximately 1400 m.y. Rb-Sr systematics in two bodies of amphibolite-grade gabbro suggest a regional metamorphic event at about 950 m.y., corresponding to the waning stages of Grenville activity. Sm-Nd systematics in these high-grade bodies are affected to a much lesser degree than Rb-Sr. Initial ratios for143Nd/144Nd and87Sr/86Sr are lower and higher, respectively, than bulk earth values at 1375 m.y. Both these displacements are in the direction of older crustal material at 1375 m.y., and a model is proposed to produce the Shabogamo magma by mixing a mantle-derived magma with a partial melt of crustal rocks (approximately 4: 1 by volume). Young volcanic rocks with anomalous Nd and Sr isotopic ratios, which have previously been taken as evidence for “enriched” mantle, may be interpreted similarly.

The local chronology and regional implications of a Rb‐Sr investigation of granitic rocks from the Corryong District, southeastern Australia

Abstract A Rb‐Sr analysis of granitic rocks in the Corryong District reveals that the regional aureole granites of that area (Corryong Batholith) are 430 ± 8 m.y. old. This age is some 15 m.y. older than the regional aureole Cooma Granite and if both granites are the ultimate products of the metamorphism, then the locus of this metamorphism progressed eastwards. Unlike other continents (e.g. S. America) the migratory nature of this trend of metamorphism and granite emplacement is not readily explainable in terms of possible Palaeozoic plate‐tectonic mechanisms. The initial Sr87/Sr86 ratio for the Corryong Batholith is 0.716 ± 0.001. This value is similar to the value for the Cooma Granite and is compatible with the generation of the extensive Corryong Batholith by partial or complete anatexis of Ordovician‐type metasediments. The Rb‐Sr ages do not allow us to preclude the possibility that the Ordovician‐Silurian Benambran Orogeny was responsible for this anatexis. Indeed they could be interpreted to favou...

Geochemical constraints on the magmatic evolution of the pre- and post-Oligocene volcanic suites of southern Peru: Implications for the tectonic evolution of the Central Volcanic Zone

In southern Peru, the pre-Oligocene volcanic suite (the Late Cretaceous to early Paleocene silicic tuff, agglomerate, and ignimbrite of the Toquepala Group) have positive ϵ Nd i (+0.6 to +5.3) and commonly negative ϵ Sr i values (-7.0 to +8.0), with 206 Pb/ 204 Pb ratios comparable to those ascribed to arc magmas. Although the former generally display unfractionated La/Yb (2 to 15) and low Th/U (0.1 to 4.2), the post-Oligocene volcanic suite, comprising early Miocene to Pleistocene calc-alkaline andesite flows and ignimbrite sheets with minor basaltic intercalations, has elevated La/Yb (9 to 34) and Th/U (3 to 8), which are correlated with negative ϵ Nd i (-0.9 to -12.0), positive ϵ Sr i values (+13 to +64) and low 206 Pb/ 204 Pb. The principal trace-element and isotopic characteristics that strongly characterize the post-Oligocene volcanic suite are precisely those that confer the unusual signature to the Precambrian basement granulite and gneiss exposed in the area of study (the Arequipa Massif). The latter commonly display high Th/U (20-70) and La/Yb (14 to >57), low 206 Pb/ 204 Pb (16.11 to 17.10) but present an extreme enrichment in radiogenic Sr (ϵ Sr +396 to +999). The contrasting trace-element and isotopic signatures exhibited by both suites can be best explained if the pre- and post-Oligocene volcanic suites originated from an isotopically depleted subcontinental mantle wedge, with the ignimbrite (rhyolite) being derived by crustal fractionation from andesitic parents. In this hypothesis, the post-Oligocene magmas must have undergone substantial crustal contamination (mainly by a thick Andean crust having a large Precambrian component) during fractionation, whereas the pre-Oligocene volcanic rocks underwent little contamination, perhaps because they ascended through a thinner late Mesozoic crust. We suggest, as one possibility, that the increasing degree of crustal contamination experienced by the post-Oligocene volcanic suites is partly related to the tectonic thickening of the Precambrian basement which took place during late Mesozoic-early Cenozoic time, a process which would promote larger degrees of contamination of mantle-derived magmas upon ascent. If this assumption is correct, then the rapid Cenozoic crustal thickening of the southern Peruvian Andean crust could result from a combination of massive introduction of juvenile magmas in the lower crust, underplating of a buoyant subducted oceanic plate, and crustal shortening, the latter mechanism being perhaps similar to that actually occurring in the sub-Andes where the Brazilian Shield is underthrusting the Eastern Cordillera.