Aluminum‐26 in calcium‐aluminum‐rich inclusions and chondrules from unequilibrated ordinary chondrites

Abstract— In order to investigate the distribution of ²⁶A1 in chondrites, we measured aluminum‐magnesium systematics in four calcium‐aluminum‐rich inclusions (CAIs) and eleven aluminum‐rich chondrules from unequilibrated ordinary chondrites (UOCs). All four CAIs were found to contain radiogenic ²⁶Mg (²⁶Mg*) from the decay of ²⁶A1. The inferred initial ²⁶Al/²⁷Al ratios for these objects ((²⁶Al/²⁷Al)₀ ? 5 × 10^?5) are indistinguishable from the (²⁶Al/²⁷Al)₀ ratios found in most CAIs from carbonaceous chondrites. These observations, together with the similarities in mineralogy and oxygen isotopic compositions of the two sets of CAIs, imply that CAIs in UOCs and carbonaceous chondrites formed by similar processes from similar (or the same) isotopic reservoirs, or perhaps in a single location in the solar system. We also found ²⁶Mg* in two of eleven aluminum‐rich chondrules. The (²⁶Al/²⁷Al)₀ ratio inferred for both of these chondrules is ～1 × 10^?5, clearly distinct from most CAIs but consistent with the values found in chondrules from type 3.0–3.1 UOCs and for aluminum‐rich chondrules from lightly metamorphosed carbonaceous chondrites (～0.5 × 10^?5 to ～2 × 10^?5). The consistency of the (²⁶Al/²⁷Al)₀ ratios for CAIs and chondrules in primitive chondrites, independent of meteorite class, implies broad‐scale nebular homogeneity with respect to ²⁶Al and indicates that the differences in initial ratios can be interpreted in terms of formation time. A timeline based on ²⁶Al indicates that chondrules began to form 1 to 2 Ma after most CAIs formed, that accretion of meteorite parent bodies was essentially complete by 4 Ma after CAIs, and that metamorphism was essentially over in type 4 chondrite parent bodies by 5 to 6 Ma after CAIs formed. Type 6 chondrites apparently did not cool until more than 7 Ma after CAIs formed. This timeline is consistent with ²⁶Al as a principal heat source for melting and metamorphism.     

‘Thermohaline front’ off the east coast of India and its generating mechanism

Physical oceanography measurements reveal a strong salinity (0.18 psu km^?1) and temperature (0.07 °C km^?1) front off the east coast of India in December 1997. T–S diagrams suggest lateral mixing between the fresh water at the coast and the ambient warmer, saltier water. This front seems to be the result of southward advection of fresh and cool water, formed in the northern Bay of Bengal during the monsoon, by the East Indian Coastal Current, as suggested by the large-scale salinity structure in the SODA re-analysis and the anti-cyclonic gyre in the northwestern Bay of Bengal during winter. The data further reveals an offshore front in January, which appears to be the result of a meso-scale re-circulation around an eddy, bringing cold and freshwater from the northern Bay of Bengal further away from the shore. Our cruise data hence illustrates that very strong salinity fronts can appear in the Bay of Bengal after the monsoon, as a result of intense coastal circulation and stirring by eddies.     

Rb-Sr age of Godhra and related granites,Gujarat, India

Rubidium and strontium determinations are reported for Godhra and geographically related granites from central Gujarat. The whole rock data define a Rb-Sr isochron corresponding to a common age of 955±20 m.y. and initial Sr ratio of 0·7130±0·001. This age is distinctly older than the age of 735 m.y. reported for the Erinpura suite of rocks from Mount Abu in western Rajasthan and from Idar in northern Gujarat. There are at least two generations of post-Delhi intrusive rocks in the Gujarat precambrian. Biotites associated with these granites have the same age as the whole-rocks within experimental error indicating the absence of significant metamorphic heating since the time of emplacement. It is significant that rocks of similar age occur in the Rajasthan Precambrian mainly in the axial zone of the Aravalli Mountains.     

RARE GASES IN BANSUR,UDAIPUR AND MADHIPURA CHONDRITES

We have analysed three Indian meteorites — Bansur, Udaipur and Madhipura — for the elemental and isotopic composition of Ne, Ar, Kr and Xe and determined their radiation and gas-retention ages. It is found that Udaipur belongs to the group of unequilibrated ordinary chondrites and Madhipura probably belongs to the group of shocked hypersthene chondrites.     

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

New experimental results are reported on oxygen isotope fractionation factors, α_T, between the δ¹⁸O 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 (MgCO₃). These results are distinctly different from some previously published mutually inconsistent data, and can be expressed as a linear relationship: 10³ lnα_T = [{(6.845 ± 0.475)∗10⁵}/T²] + (4.22 ± 0.08), where 10³ 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:10³ lnα_T = [{(5.608 ± 0.151)∗10⁵}/T²] + (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.     

The nature of the basement in the Archaean Dharwar craton of southern India and the age of the Peninsular Gneiss

The Archaean Peninsular Gneiss of southern India is considered by a number of workers to be the basement upon which the Dharwar supracrustal rocks were deposited. However, the Peninsular Gneiss in its present state is a composite gneiss formed by synkinematic migmatization during successive episodes of folding (DhF₁, DhF_1a and DhF₂) that affected the Dharwar supracrustal rocks. An even earlier phase of migmatization and deformation (DhF_*) is evident from relict fabrics in small enclaves of gneissic tonalites and amphibolites within the Peninsular Gneiss. We consider these enclaves to represent the original basement for the Dharwar supracrustal rocks. Tonalitic pebbles in conglomerates of the Dharwar Supergroup confirm the inference that the supracrustal rocks were deposited on a gneissic basement. Whole rock Rb-Sr ages of gneisses showing only the DhF₁ structures fall in the range of 3100–3200 Ma. Where the later deformation (DhF₂) has been associated with considerable recrystallization, the Rb-Sr ages are between 2500 Ma and 2700 Ma. Significantly, a new Rb-Sr analysis of tonalitic gneiss pebbles in the Kaldurga conglomerate of the Dharwar sequence is consistent with an age of ～2500 Ma and not that of 3300 Ma reported earlier by Venkatasubramanian and Narayanaswamy (1974). Pb-Pb ages based on direct evaporation of detrital zircon grains from the metasedimentary rocks of the Dharwar sequence fall into two groups, 3300–3100 Ma, and 2800–3000 Ma. Stratigraphic, structural, textural and geochronologic data, therefore, indicate that the Peninsular Gneiss of the Dharwar craton evolved over a protracted period of time ranging from > 3300 Ma to 2500 Ma.     

Rb-Sr age of the Sivamalai alkaline complex,Tamil Nadu

The Sivamalai alkaline complex comprises ferro-, pyroxene-hornblende- and nepheline-syenites. Field relations show that the nepheline syenites followed the emplacement of non-feldspathoidal syenites. Mineralogical data on the syenite suite have been reviewed. The Sivamalai alkaline rocks are not strongly enriched in rare-earth elements like most miaskites. Rb-Sr isotopic analyses of a suite of six samples from the various members of the complex define an isochron corresponding to an age of 623 ±21 Ma (2σ) and initial Sr ratio of 0.70376 ±14 (2σ). This is consistent with a model of fractional crystallization of a parent magma derived from an upper mantle source with apparently no isotopic evidence for more than one magma source for the complex. The Sivamalai alkaline complex represents a Pan-African alkaline magmatic event in the southern granulite terrane of Peninsular India.     

Present status of the geochronology of the Precambrian rocks of Rajasthan

Age determinations, mostly by Rb---Sr analyses, of the Precambrian rocks of Rajasthan by us and by others are summarized and discussed. Broad periods of acid magmatism at (1) 3000−2900 m.y., (2) 2600−2500 m.y., (3) 2000−1900 m.y., (4) 1700−1500 m.y. and (5) 850−750 m.y. were identified. The oldest rocks in the area are the yet undated banded gneisses (BGC) east of Udaipur, intruded by the Untala granite dated at 2950 m.y. and hence of mid-Archean age. The basal status hitherto attributed to the Berach granite dated at 2600 m.y. is no longer tenable. The radiometric control on the beginning and duration of the overlying Aravalli Supergroup is not yet satisfactory, though a lower limit at 2000 m.y. is indicated. Heron's original Delhi rocks have recorded two magmatic events widely separate in space and time. While the earliest granitic activity at 1600 m.y. is recorded only in the Alwar basin in the northeast, the younger activity between 850-750 m.y. is widespread, as shown by the nearly concordant ages of “Erinpura-type” granites along the Aravalli mountain Range and the Malani rhyolites in the western plains of the Aravalli Range.     

Statistical analysis of dynamic fibrils observed from NST/BBSO observations

We present the results obtained from the analysis of dynamic fibrils in NOAA active region(AR) 12132, using high resolution Hα observations from the New Solar Telescope operating at Big Bear Solar Observatory. The dynamic fibrils are seen to be moving up and down, and most of these dynamic fibrils are periodic and have a jet-like appearance. We found from our observations that the fibrils follow almost perfect parabolic paths in many cases. A statistical analysis on the properties of the parabolic paths showing an analysis on deceleration, maximum velocity, duration and kinetic energy of these fibrils is presented here. We found the average maximum velocity to be around 15 kms~(-1) and mean deceleration to be around 100 ms~(-2). The observed deceleration appears to be a fraction of gravity of the Sun and is not compatible with the path of ballistic motion due to gravity of the Sun. We found a positive correlation between deceleration and maximum velocity. This correlation is consistent with simulations done earlier on magnetoacoustic shock waves propagating upward.