N. Bhandari

Impact did not trigger Deccan volcanism: Evidence from Anjar K/T Boundary intertrappean sediments

Many hypotheses including asteroidal and cometary impacts, Deccan volcanism, impact induced volcanism and coincidental impact and volcanism have been put forth to explain the observed enhancement of iridium and mass extinction at the K/T boundary (KTB). The identification of KTB layer within the Deccan intertrappean sediments at Anjar, about half way between Flow III and Flow IV provides new constraints on some of these hypotheses. The chemical characteristics of this layer show high concentrations of Ir, Os and other siderophiles accompanied by enrichment of chalcophiles and depletion of lithophiles. The Os/Ir ˜1.1, close to the meteoritic value and other chemical and stratigraphic criteria indicate that it may be the ejecta fallout layer, resulting from a bolide impact at the KTB. Presence of three basalt flows below this layer implies that the volcanism was already active when this layer was deposited and impact of the K/T bolide did not trigger Deccan volcanism.

Behavior of the Heliosphere over Prolonged Solar Quiet Periods by 44Ti Measurements in Meteorites

The heliospheric magnetic field (HMF) is controlled by solar activity, as established by measurements over the last few decades, but its characteristics when the sun was quiet for prolonged periods, such as during Gleissberg or Maunder minima, are not known. Titanium-44, produced in meteorites, provides a monitor of the galactic cosmic ray (GCR) flux and allows estimation of the modulation effect of the sun for the period 1883 to 1992. The titanium-44 activity is consistent with the expected value, but the increase, due to the last Gleissberg minimum, is four times greater than expected for a GCR modulation based solely on sunspot numbers. This result implies that the HMF was weaker than at present and as a result the GCR flux (for energy greater than 1 gigaelectron volt) was higher between 2.2 to 3.6 protons per square centimeter per second per 4π steradians at 1 to 3 astronomical units in solar cycles 12 to 15.

Geochemistry of some K/T sections in India

Abstract A number of marine as well as continental K/T sections in India, particularly around the Deccan volcanic regime, have been analyzed to obtain concentration profiles of some platinum group and other trace elements, specifically to search for anomalous levels of iridium. Except for the Um Sohryngkew River (USR) section in Meghalaya, where iridium is found to occur at an anomalously high level in the K/T boundary layer, all other sections show low levels in the range of 10 to ∼120 pg/g. The iridium profile in the USR section is characterized by a sharp peak of 12.1 ng/g in a 1.5 cm thick limonitic layer, superimposed on a broad band of ∼0.2 ng/g extending over a 50±20 cm thick section, beyond which Ir decreases to the base level of ∼0.02 ng/g, both in the Tertiary as well as in the Cretaceous. High Ir is accompanied by high Os but the Os/Ir ratio in the peak (0.37) is different from the Os/Ir ratio of ∼2 measured in the broad band. The Ir profile in the USR section seems to be consistent with fallout of cometary debris preceding and following the main impact. The measurements at other sites in the Deccan suggest that the iridium inventory of the Deccan is too low to account for the high iridium observed in the KTB layer all over the world, but may be sufficient to give rise to its enhancement adjacent to the KTB peak.

Cosmogenic effects in shergottites

Heavy nuclei tracks, induced radioactivity and thermoluminescence have been studied in Shergotty. ALHA 77005 and EETA 79001 achondrites with a view to understanding their thermal and cosmic ray exposure history. The track data indicate low ablation, on the average 1 to 3 cm. for all three, indicative of low geocentric velocities and small meteoroid masses (1–25 kg). The 26A1 activity of 91 dpm/kg in Shergotty, when compared with the expected production rates, suggest a small (effective radius ~12 cm) size of the meteoroid in space, consistent with the track data. Nuclear track, 26A1 and other cosmogenic effects in the three shergottites are consistent with their single stage exposure in space as small objects. The TL sensitivity of the shergottites is low, about 10−2 to 10−4 of the value measured in Dhajala (H3) chondrite. This is probably an effect of severe shock suffered by the shergottites. The TL data indicate low equivalent dose (< 30 krads) at low glow curve temperature (T∗ ⋍ 250°C) for all the shergottites, whereas for ALHA 77005 even the high glow curve temperature (T∗ ~ 380°C) equivalent dose is low, 35 krads compared to ~ 100 krads found in Shergotty, EETA 79001 and also in ordinary chondrites. Two plausible causes for the unusually low value of the equivalent dose include anomalous TL characteristics of shock-induced phases and solar heating in meteoroid orbit with small perihelion distance (0.70-0.85 A.U.).

Positive europium anomaly at the Permo-Triassic Boundary, Spiti, India

A limonitic layer at the base of the Otoceras-Ophiceras zone in the Lalung section in Spiti valley shows a significant positive europium anomaly with maximum Eu/Eu* = 1.9. This layer defines the contact between the earliest Griesbachian and the Permian Productus shale sequence. The layer shows a small cerium anomaly having the maximum value of (Ce/La)N = 1.4. The iridium concentration in this layer, at 73 pg/g, is only marginally above the levels found in the adjacent shales although higher than that found in the overlying limestones. Another horizon having 114 pg/g of Ir occurs 70 cm below this limonitic layer in the shale sequence. These observations have important implications, both for the terrestrial (mainly volcanic) and extra-terrestrial impact hypotheses for the Permo-Triassic transition which are discussed.

Measurement of cosmogenic radionuclides in meteorites with a sensitive gamma-ray spectrometer

SummaryA large-volume HPGe gamma-ray spectrometer in a NaI(Tl) well has been set up underground at 70 m.w.e. depth for whole body counting of cosmogenic radionuclides in meteorites. The detectors are housed in a 20 cm thick lead shield with a lining of cadmium and OFHC copper. The scintillator is simultaneously operated in anticoincidence as well as in coincidence in selected energy channels to achieve low background levels (in the range of counts per day) and high specificity. In this way a large number of radionuclides such as26Al,44Ti,60Co,22Na,54Mn, and shorter-lived nuclides produced in extraterrestrial materials like meteorites and lunar rocks can be analysed. Results on Bouvante and Bereba achondrites and Dhajala and Torino chondrites are presented.

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.

Evidence of Impact at the Permian/Triassic Boundary from Mössbauer Spectroscopy

Sediment samples from Spiti valley, India, deposited about 250 Million years ago during the Permian-Triassic transition, have been analysed. 57Fe Mössbauer spectroscopic studies show that iron occurs mainly as nanoparticles of hematite and goethite with a broad size distribution. The results show a high degree of resemblance with the Cretaceous-Tertiary boundary (65 My) samples where an asteroidal impact is believed to have caused mass extinction.

Cosmogenic 44Ti in meteorites and century scale solar modulation.

The galactic cosmic ray (GCR) flux is modulated by solar activity: 11 and 22 year cycles are well established. Meteorites offer the possibility of studying longer cycles in the interplanetary space, due to GCR nuclear interactions. 44Ti cosmogenic isotope produced in meteorites is suitable for detecting a century scale modulation. So far reliable measurements of 44Ti in the samples are scarce, the radioisotope concentration being very low. A gamma-ray sensitive spectrometer has been set up in the underground laboratory of Monte dei Cappuccini. The system consists of a 2 kg HPGe detector inside a 28 kg NaI well. The system has a background of approximately 1 count/day (in coincidence mode) in the 44Ti region. We present here the results obtained on the Rio Negro meteorite, which fell in the year 1934.

Galactic cosmic ray variations in the last two centuries recorded by cosmogenic 44Ti in meteorites

We have previously shown that the very low activity of the cosmogenic 44Ti in meteorites records the galactic cosmic ray (GCR) flux over a century time-scale (Bonino et al., 1995). The heliospheric magnetic field (HMF) variations are the principal source of the GCR modulation in the heliosphere. We present here new measurements which extend our investigation over the last two centuries. The 44Ti activity at the time of fall has been obtained on the basis of the recently revised value of its half-life of 59.2 years. We basically confirm our earlier findings (Bonino et al., 1997). The 44Ti profile, with the time of fall of the meteorites, shows variations in phase in agreement with those expected. The magnitude of oscillations is ∼ 4 times higher than expected (∼ 6%) on the basis of the GCR flux over the past centuries deduced from sunspot number since 1700 and by the neutron monitor and balloon measurements in the last decades. The higher 44Ti production rate during prolonged solar quiet periods (Gleissberg minima) may imply that during these minima the HMF was weaker than during the recent minima of the 11-year solar cycle.