Initial Sr Compositions Research Articles

Chemical and Sr–Nd isotopic compositions and zircon U–Pb ages of the Birimian granitoids from NE Burkina Faso, West African Craton: Implications on the geodynamic setting and crustal evolution

The West African Craton was the site of massive juvenile crustal addition in the early Proterozoic during the Eburnean orogeny (2.1Ga). In this study, we determined chemical and isotopic compositions as well as zircon U–Pb ages of granitoid samples from NE Burkina Faso. These data were then used to constrain the chemical characteristics, emplacement age and tectonic environment of these rocks in the context of the West African Craton evolution. Birimian (Paleoproterozoic) granitoids of NE Burkina Faso are associated with volcanic and volcano-sedimentary sequences of the greenstone belts. Analyses indicate that the Na-rich Birimian granitoids of tonalite–trondhjemite–granodiorite (TTG) composition were generated by partial melting of a garnet-bearing amphibolite crust. Granite intrusions derived from partial re-melting of the deep basement of felsic calc-alkaline nature with contribution of metasediments. Zircon U–Pb isotopic analyses yielded ages between 2122±15Ma and 2181±7Ma for the Birimian Na-rich granitoids and 2151±10Ma for a biotite granite (IJ10). Sr–Nd isotopic data show very low initial Sr composition (ISr (T)≤0.7007), positive epsilon neodymium values (ɛNd (T)=+0.7 to +2.5) and Sm–Nd model ages (TDM) of 2363–2558Ma). These data are similar to those of mafic rocks of the Pissila greenstone belt: (ISr (T)=0.7003–0.7015, ɛNd (T)=+3.2 to +4.1, TDM=2336–2552Ma), and further suggest that the Na-rich granitoids are of juvenile nature. The present study together with published data on the Birimian volcanics and meta-sedimentary rocks of the West African Craton preclude contamination of the juvenile Birimian rocks of NE Burkina Faso by reworked Archean materials. The Birimian granitoids of NE Burkina Faso were mostly emplaced during the early stage (2.19–2.15Ga) of a large scale crustal growth event (2.2–2.0Ga) where large amount of juvenile materials were added to the crust from the depleted mantle in a tectonic environment comparable to modern volcanic arcs.

Late Cretaceous ultramafic lamprophyres and carbonatites from the Delitzsch Complex, Germany

The Delitzsch Complex consists of late Cretaceous ultramafic lamprophyres and carbonatitic rocks. They form dikes and diatremes, emplaced into Palaeozoic to lower Permian volcanic and sedimentary rocks, and are covered by up to 120m thick sequences of Tertiary sedimentary rocks. The complex includes a diversity of magmatic and subvolcanic rocks. The lithologies range from monchiquites and alkali picrites to dolomite– and calcite–carbonatites (rauhaugites, beforsites, and alvikites), and ultramafic lamprophyres (alnöites, aillikites). Contact relationships and the distribution of xenolithic material indicate that phases of carbonatitic and ultramafic lamprophyre magmatism overlapped. New U–Pb ages (72±1Ma on baddeleyite) from a dolomite–carbonatite (beforsite), Rb–Sr ages (73±2Ma on phlogopite) from an ultramafic lamprophyre (alnöite) in combination with modeling of the effect of the initial 87Sr/86Sr of phlogopite on the isochron ages of dolomite– and calcite–carbonatites demonstrate: (1) a short duration of magmatic activity for the main phases of the subvolcanic emplacement of the Delitzsch Complex; (2) phlogopite crystals in carbonatites of the Delitzsch Complex are xenocrysts; (3) the calculated initial 87Sr/86Sr composition of xenocrystic phlogopite equals the initial Sr composition of phlogopite from the alnöite.

Late Archaean intrusive complexes in the Olekma granite-greenstone terrain (eastern Siberia): geochemical and isotopic study

Two granitoid intrusions, the Amnunnakta and Oldongso massifs in the Olekma granite-greenstone terrain of the Aldan shield have been studied using Sm-Nd, Rb-Sr and Pb-Pb whole rock (WR) and U-Pb zircon methods. The isotopic composition of Pb in plagioclase, as well as whole-rock oxygen isotope compositions and REE patterns were also measured. The following ages were obtained for the Amnunnakta massif: U-Pb zircon 2984±22 Ma (2σ), Pb-Pb WR 2991±57 Ma, Sm-Nd WR 3094±430 Ma and for the Oldongso massif: U-Pb zircon 2999±51 Ma, Pb-Pb WR 2855±170 Ma, Sm-Nd WR 2923±144 Ma. The similar ages suggest penecontemporaneous formation of the two intrusive series. The observed resetting of the Rb-Sr WR systems (2767±116 Ma for Amnunnakta massif and 2597±82 Ma for the Oldongso massif) and opening of the plagioclase Pb isotopic systems are associated with superimposed metamorphic processes of the greenschist and epidote-amphibolite facies. Source regions of the massifs are characterized by long-term depletion in incompatible elements: the initial ϵ Nd(T) value is +2.4±0.4 for both massifs. The μ 1 values are mantle-like at 7.68 for the Amnunnakta and 7.71 for the Oldongso massifs and the initial Sr composition is low (Sr i=0.7015±5) for the Oldongso massif. Basic rocks of the Amnunnakta massif are characterized by low δ 18O values (2.7–4.7) and elevated Sr i=0.7052±7 which may be caused by both a primary low δ 18O, high rm 87Sr 86Sr magma or later interaction with a hydrothermal system. There is no evidence for recycled sedimentary material in the granitoids of either massif. One of the rocks, a fine-grained garnet-bearing trondhjemite, previously assigned to the Amnunnakta massif, yielded a T Nd model age of 3700 Ma and reflects the presence of older sialic material. Moderately fractionated REE patterns with well pronounced negative Eu anomalies in low-alumina trondhjemites of the Amnunnakta massif suggest their formation by a process of differentiation of high-alumina basaltic melts with fractionation of plagioclase. High-alumina tonalite-trondhjemites of the Oldongso massif are characterized by fractionated and depleted HREE patterns suggesting the residual presence of garnet and hornblende during partial melting of a basic source.

RbSr and SmNd systematics of cherts and other siliceous deposits

We have analyzed a suite of marine and lacustrine cherts to determine the concentration of Rb, Sr, Sm and Nd and the isotopic composition of Sr and Nd. The samples ranged from Pliocene to Silurian in age and included nodular cherts in carbonates as well as bedded cherts. Massive samples were leached in acid to remove carbonate and both the leach and residue analyzed. The residues contained 0.1 to 17 ppm Rb and 0.5 to 20 ppm Sr. All samples have high Rb Sr ratios (~0.1 to 0.8) and thus show enrichment of Rb relative to Sr of 6 to 60 times that of seawater. The Rb and Sr in pegmatitic quartz are much lower by factors of 10 to 10 3, although these also may have high Rb Sr . The site of Rb and Sr in the cherts was not established but appears to be correlated with Al content. The high Rb Sr ratios permit the use of cherts for dating in favorable circumstances. The Nd contents range from 9 to 1000 ppb with Sm Nd ~ 0.16 to 0.19, typical of continental sources. The initial 87Sr 86Sr ratio for a Pacific deep sea chert is equal to that of associated carbonates and hence reflects the seawater environment. In shelf and marginal basin environments, the initial 87Sr 86Sr ratios of chert are distinctly more radiogenic than seawater. This is most obvious where a substantial clay component is present, but is observed even in samples with Al 2O 3 contents less than 0.1%. These results show the Rb-Sr system in chert to be rather isolated from the associated carbonate with very little Sr exchange. This separate behavior appears to persist through both early and late recrystallization stages. These data suggest that, in many cases, cherts are formed by “dissolved” SiO 2 nucleating on clay-like material which preserves the isotopic characteristics of the detrital source. The detrital component required is less than 1% of the total chert. In some cases, it is possible that the cherts were formed from silica-saturated waters from a nonmarine source, possibly in coastal environments or during uplift. These waters contained radiogenic Sr in solution that was not exchanged with the carbonate matrix. This latter mechanism is also indicated by oxygen isotopic studies ( Knauth and Epstein, 1976; Kolodny and Epstein, 1976). The Nd isotopic signature of the deep sea samples reflects the seawater composition in agreement with the initial 87Sr 86Sr data on the same samples. Samples with excesses of 87Sr relative to seawater show low ϵ Nd, both sets of data thus indicating inheritance of an old detrital component. The results demonstrate that the Rb-Sr and Sm-Nd systems may be used in elucidating the origin and evolution of cherts and that the Rb-Sr system may be used for dating, if consideration is given to the initial Sr composition and to diagenetic changes. This opens up numerous possibilities for sedimentological-geochemical studies.

Isotopic age determinations on granitic and gneissic rocks from the Ubendian-Usagaran system in southern Tanzania

Isotopic age measurements on granites and gneisses are reported from the Ubendian-Usagaran System in southern Tanzania. For the Lukumburu-Wino Granites a Rb-Sr whole-rock isochron (9 points) of 1771 ± 145 Ma with initial 87 Sr 86 Sr of 0.704 ± 0.003 was obtained ( λ 87 Rb = 1.42 · 10 −11 a −1; errors 95% confidence level). Two samples from neighbouring granites and three gneisses from the contact zone of the Lukumburu-Wino Granites also fit to this isochron; inclusion into the isochron calculation yields an age of 1747 ± 85 Ma with the same initial Sr composition. Five parts of a banded Usagaran gneiss sample produce an isochron of 589 ± 70 Ma with initial 87 Sr 86Sr of 0.7194 ± 0.0005. This internal isochron is interpreted as reflecting Sr isotopic equilibration through the sample during the main phase (or one of the main phases) of the Pan-African thermotectonic episode. Rb-Sr and K-Ar ages of 17 micas show a general resetting by the Pan-African thermotectonic episode. The Rb-Sr ages of the muscovites and the biotites average 528 Ma and 463 Ma, respectively, and are interpreted as reflecting subsequent stages in the cooling history after the termination of the event. Biotites have K-Ar ages persistently higher than the corresponding Rb-Sr ages, possibly reflecting a high partial 40Ar pressure through the crust during the resetting of the biotite K-Ar systems.

A 2.9-b.y. event in the Rhodesian Archaean

Rb-Sr whole rock isochrons corresponding to ages of2.87 ± 0.07 b.y. and2.93 ± 0.16 b.y. (2σ errors) have been obtained on granitic rocks from southern Rhodesia. Tuffs from the Lower Greenstones succession at Belingwe gave a Rb-Sr whole rock age of2.51 ± 0.10 b.y. with a high initial 87Sr/ 86Sr ratio of0.7056 ± 4. Samples from two localities within the Chibi granite yield sub-parallel whole rock isochrons but they demonstrate significant differences in their initial Sr compositions 2.62 b.y. ago (0.7047 and 0.7014). Rb-Sr mineral results from six localities on the Rhodesian craton indicate that biotites last cooled through their blocking conditions 2.65–2.51 b.y. ago, although co-existing muscovites sometimes preserve ages closer to the whole rock age of the host rock. Biotites from three localities along the southern margin of the craton yield ages of 2.10–1.87 b.y. reflecting the 2.0-b.y. thermal event that appears to be restricted to the Limpopo mobile belt in this area. The available data as to the nature of the 2.9 b.y. event are reviewed with particular attention given to whether greenstone belts of that age can be recognised. Problems such as Archaean K-rich adamellites with fairly low initial Sr ratios, and the relationship between high-grade granulite and low-grade granite-greenstone terrains are re-assessed in the light of our knowledge of the 2.9-b.y. event.

Age of an Apollo 15 mare basalt; Lunar crust and mantle evolution

An internal Rb-Sr isochron for the large basalt boulder 15555 returned from the edge of Hadley rille by the Apollo 15 mission yields an age 3.32 ± 0.06AE and an initial 88Sr/ 86Sr, I = 0.69934 ± 5. This age and I value fall well within the range obtained for the Apollo 12 basalts from the Ocean of Storms and may indicate that extensive lava flows occurred at ∼ 3.3 AE over widespread areas of the moon. The Sr composition of the anorthosite 15415 is as low as that of plagioclase extracted from the Apollo 11 low K rocks. The initial Sr composition of 15415 for an assumed age of 3.3 to 4.6 AE is extremely primitive and provides further evidence for an extremely short formation interval (3 to 1 × 10 6 yr) of a nonchondritic moon with respect to an origin in time defined by BABI. The initial 87Sr/ 86Sr for crystalline rocks returned from all lunar missions is correlated with the concentration of Rb and correspondingly K, U and Th. This correlation places distinctive constraints on the evolution of lunar magmas and the internal structure of the moon.

Rb-Sr ages and elemental abundances of K, Rb, Sr, and Ba in samples from the Ocean of Storms

Rb-Sr internal isochron ages for three basaltic crystalline rocks from the Ocean of Storms are 300–600 m.y. younger than the ages of basaltic rocks from the Sea of Tranquillity. Rock 12063 has an age of 3.34 ± 0.10AE and [ 87Sr 86Sr ] I of 0.69914 ± 3. Internal isochron ages of two separate samples of rock 12004 yield ages of 3.01 ±0.11 and 3.03 ± 0.09AE indicating that this rock is distinctly younger than rock 12063 by about 300 m.y. A relatively imprecise age of 2.92 ± 0.18AE is obtained for rock 12052. The range in ages obtained by us indicate magmatic activity extending over a 300 m.y. period in the Ocean of Storms. Initial Sr compositions for the three rocks are distinctly higher than BABI value and provide evidence for the extreme alkali depletion in the source areas of these basalts, as also indicated by the trace element data. The differences in the initial Sr compositions and ages suggest that these lavas are produced from different source materials. An internal isochron age of 3.63 ± 0.11AE and a [ 87Sr 86Sr ] I of 0.69903 ± 5 has been obtained on the Apollo 11 Sample 10058. Model ages for the soil samples 12033 and 12070 are 4.3AE and 4.6AE respectively. The different model ages and the abundances of K, Rb, Sr and Ba in the soils indicate the heterogeneous nature and lack of mixing in the soils. The soils are highly radiogenic compared to the rocks. Both the trace element data and the Sr-isotopic data from leaching experiments show that the soil is not simply derived from the mare basalts, but contains a highly radiogenic and K, Rb, Ba rich component similar in composition to rock 12013. We infer that this exotic component is more ubiquitous on the lunar surface than present sampling indicates.

Lunar chronology and evolution from Rb [sbnd]Sr studies of Apollo 11 and 12 samples

Rb-Sr internal isochrons for a total of eight Apollo 12 crystalline rocks yield ages of 3.36 to 3.16 AE. The initial Sr compositions ( I) are relatively primitive and range from 0.69918 to 0.69957 as compared to BABI= 0.69898 ± 3. No clear groupings in I are observed, however, the wide range indicates that at least four different rock bodies were sampled. An Apollo 11 basalt (10024) yielded an age of 3.61 ± 0.07AE andI = 0.69935 ∓ 8 in agreement with previous results on other Apollo 11 high K rocks. Several Apollo 12 soil samples yield model ages which range from 4.4 to 4.6 AE and indicate that the special nature and older “age” of the lunar soil determined at the Apollo 11 site is a widespread phenomenon. Initial Sr compositions from Apollo 11 and Apollo 12 support our previous conclusions that the moon as a whole has a Rb/Sr much lower ( Rb/Sr≈ 0.008) than found in chondrites. We summarize the current status of Rb Sr lunar chronology and some implications regarding the melting and differentiation history of the moon.

Rb [sbnd]Sr ages from the ocean of storms

Rb Sr internal isochrons for two texturally and mineralogically distinct crystalline rocks returned by Apollo 12 yield similar ages of 3.36 ± 0.10and3.26 ± 0.10 × 10 9 years. The initial Sr compositions determined are relatively primitive but distinct so that these samples represent two different rock bodies. These ages are slightly but distinctly younger than the Mare Tranquillitatis Rb Sr ages and indicate that the flooding of the maria may have occurred only during a short interval of ∼ 300 × 10 6 years duration 3.6 × 10 9 years ago. The soil at the Ocean of Storms yields a well determined model age of 4.44 × 10 9 years and therefore the special nature and older age of the lunar soil determined at the Apollo 11 site is found to be a widespread phenomenon. Initial Sr compositions from Apollo 12 support our previous conclusion based on Apollo 11 measurements that the moon as a whole has a Rb/Sr ratio much lower than that in chondrites.

Rb-Sr ages of lunar rocks from the sea of tranquillity

87Rb- 87Sr internal isochrons have been determined for six crystalline rocks from the Sea of Tranquillity, and all yield ages within the narrow range3.65±0.06 × 10 9yr. Differences in initial ( 87Sr/ 86Sr) I define at least two main rock groups which coincide with the two groups indicated by alkali content of the rocks. This demonstrates that at least two magnetic reservoirs were involved in the 3.65 × 10 9 yr event on the moon. The soil yields a model Rb-Sr age of 4.6 × 10 9 yr which suggests that it consists of an average over a variety of rock systems of different ages which represent a closed total system 4.6 × 10 9 yr old or the widespread existence of rocks of 4.6 × 10 9 yr age. A single peculiar rock fragment from the soil gives a model age of 4.4 × 10 9 yr. Initial Sr compositions for the lunar rocks and for soil fragments define a narrow range (0.6990–0.6994) whose lower limit is equal to the basaltic achondrite initial Sr. The moon, earth and basaltic achondrites represent planets with Rb/Sr much lower than in the sun. The moon and the basaltic achondrites must have separated from the solar nebula within a time interval of less than 4 × 10 6 yr. From consideration of Sr-Rb relations, if the moon formed by fission from the earth, it must have occurred prior to 4.4 × 10 9 yr ago.