K. Gopalan

SmNd ages of Archaean metavolcanics of the Dharwar craton, South India

Abstract Well constrained SmNd arrays of whole rocks are reported for the first time from three important stratigraphic units of the Dharwar volcano-sedimentary sequence of south India. If interpreted as isochrons, these arrays will correspond to the following ages and initial Nd ratios relative to CHUR: 2.911±0.049 Ga and −0.20 ± 0.40 for the Kalasapura mafic basal metavolcanics of the Bababudan greenstone belt; 2.848±0.070 Ga and −0.88 ± 0.82 for the Santaveri Formation comprising mafic to felsic metavolcanics overlying the Kalasapura rocks; and 2.747±0.015 Ga and +1.72 ± 0.10 for the Ingaldhal mafic-felsic metavolcanic suite of the Chitradurga greenstone belt. The possibility that these SmNd arrays are mixing lines between unrelated components depleted and enriched, respectively, in light rare earth elements has been discussed, leading to the conclusion that the ages closely correspond to their time of eruption: the older suites from the Bababudan belt from a mantle source of CHUR-like composition, the younger suite from the Chitradurga belt from one with a prior history of LREE depletion.

Temperature dependence of oxygen isotope fractionation of CO2 from magnesite-phosphoric acid reaction

Abstract New experimental results are reported on oxygen isotope fractionation factors, α T , between the δ 18 O compositions of carbon dioxide liberated by phosphoric acid in the temperature interval of 323 to 373K and that of total oxygen from a natural magnesite (MgCO 3 ). These results are distinctly different from some previously published mutually inconsistent data, and can be expressed as a linear relationship: 10 3 lnα T = [{(6.845 ± 0.475)∗10 5 }/T 2 ] + (4.22 ± 0.08), where 10 3 lnα T refers to fractionation at different temperatures T in Kelvin. Fractionation factors have also been determined at 323 and 368K on a natural calcite. The results on calcite are in excellent agreement with previously published data and can be written as:10 3 lnα T = [{(5.608 ± 0.151)∗10 5 }/T 2 ] + (3.89 ± 0.08). The combined results on magnesite and calcite yield a computed value of α = 1.01117 for dolomite at 298K, assuming equal proportion of 0.5 mole of magnesium and calcium in dolomite, the previously reported experimental values being 1.01109 and 1.01110.

A long-lived enriched mantle source for two Proterozoic carbonatite complexes from Tamil Nadu, southern India

We report new neodymium and strontium isotopic data for two Proterozoic carbonatites and related alkalic rocks, at Hogenakal and Sevathur in southern India. These complexes were emplaced into the crust at 2.4 Ga (Hogenakal) and 0.77 Ga (Sevathur). Their initial strontium and neodymium isotopic compositions, together with oxygen isotope data, suggest the involvement of a single long-lived enriched mantle source in their origin. The isotopic evolution of this source indicates that it formed approximately contemporaneously with the accretion and metamorphism of the overlying crust at the southern margin of the Dharwar craton and survived convective disruption in the mantle from early Proterozoic until at least 770 Ma ago. The older of the two carbonatites was intruded into young crust that was not older than about 150 Ma at the time of emplacement. The isotopic data contrast with those from carbonatites of the Canadian Shield, for which isotopic evidence also suggests origin from a long-lived lithospheric source, but one with a depleted chemical signature. They, therefore, indicate that there is no geochemically unique lithospheric source for carbonatites.

PB, SR AND ND ISOTOPE SYSTEMATICS OF URANIUM MINERALISED STROMATOLITIC DOLOMITES FROM THE PROTEROZOIC CUDDAPAH SUPERGROUP, SOUTH INDIA: CONSTRAINTS ON AGE AND PROVENANCE

Abstract The Pb, Sr and Nd isotopic compositions were determined on uranium mineralised and barren stromatolitic dolomite samples from the Vempalle and Tadpatri Formations of the Cuddapah Supergroup in southern India. Subsamples from individual handspecimens were analyzed to minimize the potential of heterogeneous initial Pb compositions. All of the U-mineralised samples gave Pb–Pb ages close to 1800 Ma but with high MSWD values, presumably due to local redistribution of U and Pb in the samples. The best Pb–Pb age of 1756±29 Ma is interpreted as the time of U-mineralisation and as a minimum age for carbonate sedimentation and dolomitisation within the Cuddapah Supergroup. The Sr isotopic compositions of the samples vary with the mineralised samples being more radiogenic. Even the lowest 87 Sr / 86 Sr of the barren sample exceeds the seawater 87 Sr / 86 Sr about 1800 Ma ago. On the other hand, the Nd systematics of the samples does not appear to have been disturbed by processes subsequent to carbonate sedimentation. The calculated initial Nd isotopic compositions suggest a dominant continental crustal source that was at least 600 Ma old at the time of deposition of the carbonate sediment.

40Ar–39Ar age of Siberian basaltic volcanism

Abstract Five of six samples representing the entire stratigraphic sequence (∼3700 m thick) of basaltic flows in the Norilsk section in the northwestern part of Siberian flood basalt province, Russia yield 40 Ar– 39 Ar plateau ages between 248.3±1.7 Ma and 246.9±2.5 Ma relative to an age of 520.4 Ma for the hornblende standard, MMhb-1. These results clarify some discrepancies in existing data and confirm that eruption of the bulk of Siberian basalts was close to 248 Ma and possibly lasted less than 2 Ma. Although the violence of this volcanism may have caused the massive extinctions at the Permo–Triassic boundary, the onset of changes across this boundary seems to have been distinctly earlier.

2.0 GA OLD PYROXENITE-CARBONATITE COMPLEX OF HOGENAKAL, TAMIL-NADU, SOUTH-INDIA

The Hogenakal carbonatites form a series of discontinuous bodies within two pyroxenite dykes. Each body forms veins and lenses emplaced in pyroxenite. Based on field relationships, the sequence of formation is inferred to be pyroxenite first, syenite later, and carbonatites last. The carbonatitic rocks are classified into three types: mica-apatite-calcite (MAC) carbonatite, mica-pyroxene-apatite-calcite (MPAC) carbonatite, and carbonate mica (CM) pyroxenite. Two whole-rock mineral RbSr isochrons for MPAC carbonatites yield ages of 1.984±0.078 Ga and 1.994±0.076 Ga, respectively, which makes this carbonatite not only the oldest yet known in the Indian subcontinent but also one of the few oldest carbonatites known in the world. The carbonatites are poor in alkalies and rich in Sr and to a certain extent in Ba. REE abundances and LaYb ratios vary in the order pyroxenite < CM pyroxenite < MPAC carbonatite < MAC carbonatite; all rocks show LREE-enriched patterns with steep slopes. It is suggested that the pyroxenites represent intrusions of crystal mush formed by separation of pyroxenes from an ijolitic magma. Increase in the concentration of CO2 in such a magma led to the separation of a carbonatitic melt which subsequently intruded the pyroxenite. Mixing of this carbonatite melt with fragmented pyroxenite led to the formation of the MPAC carbonatite and the CM pyroxenite in which pyroxenite dominates over carbonatite. In the closing stages a second pulse of carbonatite was intruded which formed the MAC carbonatite. K-metasomatism is restricted essentially to phlogopitization of pyroxenes and K-feldspar. The association of albite/oligoclase-rich fenites with the carbonatite complex is indicative of NaK metasomatism. The near equality of the initial Sr ratios of the three samples analysed, together with their high Sr contents, argue against any significant crustal contamination. The initial ratio of 0.70169 must therefore represent the Sr composition of the mantle source of the ijolitic magma. This ratio corresponds to an ϵSr of −6.3 ± 0.6, which implies that the subcontinental mantle which gave rise to the Hogenakal pyroxenite/carbonatite was depleted in incompatible elements even prior to 2 Ga ago, presumably due to an earlier event of crustal formation.

147sm–143Nd and 87Rb–87Sr ages of the eucrite Piplia Kalan

Abstract Prompted by the finding that the eucrite Piplia Kalan could have retained Pu fission Xe earlier than the eucrites so far studied and hence be very ancient, we have measured a precise internal 147 Sm– 143 Nd isochron for this meteorite. The age and initial Nd ratio relative to CHUR are 4.570 ± 0.023 Ga and −1.3 ± 0.7 ϵu, respectively. A Rb–Sr whole rock (clast) isochron for this meteorite corresponds to an age of 3.963 ± 0.119 Ga and initial 87 Sr/ 86 Sr ratio of 0.69902 ± 3. But initial 87 Sr/ 86 Sr ratio calculated for an age of 4.57 Ga is 0.698956 ± 25, which is indistinguishable from 0.698970 ± 15 reported for the angrites LEW and ADOR dated at 4.5578 ± 0.0005 Ga. These results indicating that Piplia Kalan could have formed within only a few million years of the earliest condensates in the Solar System are strongly supported by the recent discovery of live 26 Al in it.

A simple chemical resistant hotplate for geochemical applications

Hotplates in geochemical laboratories are most susceptible to chemical corrosion and hence a major source of corrosion-generated particles as possible contamination in trace-element analysis of rocks and minerals. Teflon-coated hotplates are now commercially available to alleviate this problem, but are expensive and need to be imported. This report describes the design of a Teflon-covered rather than coated hotplate which is inexpensive and easy to fabricate from readily available materials.