24 research outputs found

    Late Eocene 3He and Ir anomalies associated with ordinary chondritic spinels

    Get PDF
    Abstract During the late Eocene there was an enigmatic enhancement in the flux of extraterrestrial material to Earth. Evidence comes from sedimentary 3He records indicating an increased flux of interplanetary dust during ca. 2 Myr, as well as two very large impact structures, Popigai (100 km diameter) and Chesapeake Bay (40–85 km), that formed within 10–20 kyr at the peak of the 3He delivery. The Massignano section in Italy has one of the best sedimentary records of these events, including a well-defined 3He record, an Ir-rich ejecta bed related to the Popigai impact event, and two smaller Ir anomalies. Recently we showed that the Popigai ejecta is associated with a significant enrichment of chromite grains (>63 μm) with an H-chondritic elemental composition (17 grains in 100 kg of rock). Most likely these grains are unmelted fragments from the impactor. Slightly higher up (ca. 20 cm) in the section, where a small Ir anomaly possibly related to the Chesapeake Bay impact has been measured, we found a weak enrichment in L-chondritic grains (8 grains in 208 kg of rock). Here we report an extended data set increasing the total amount of sediment dissolved in acid and searched for extraterrestrial chromite grains from 658 to 1168 kg. In altogether 760 kg of background sediment from 17 levels over 14 m of strata outside the interval corresponding to the Popigai and Chesapeake Bay impacts, we only found 2 extraterrestrial chromite grains. Both grains have L-chondritic compositions and were found in a 100 kg sample from the ca. 10.25 m level in the section where the second of the smaller Ir anomalies has been reported. A correlation appears to exist between Ir, 3He and chromite from ordinary chondrites. We also report oxygen three-isotope measurements of the extraterrestrial chromite grains associated with the Popigai ejecta and confirm an H-chondritic composition. The new results strengthen our scenario that the upper Eocene 3He and Ir enrichments originate from the asteroid belt rather than the Oort cloud as originally proposed when the 3He anomaly was discovered. The generally low background concentrations of extraterrestrial chromite through the section speak against any major single asteroid breakup event such as in the mid-Ordovician after the break-up of the L-chondrite parent body. Instead the data reconcile with a small, possibly a factor of 2–3, increase in the flux of extraterrestrial material to Earth, but of both H- and L-chondritic composition. We also report the composition of all the 2310 terrestrial chrome spinel grains recovered, and show that their chemical composition indicates a dominantly regional ophiolitic source. Four anomalous chrome spinel grains with high Ti and V concentrations were found in the Popigai ejecta. These grains originate from Siberian Traps basalts in the Popigai crater at the time of impact

    In search of the Earth-forming reservoir: Mineralogical, chemical, and isotopic characterizations of the ungrouped achondrite NWA 5363/NWA 5400 and selected chondrites

    Get PDF
    High-precision isotope data of meteorites show that the long-standing notion of a “chondritic uniform reservoir” is not always applicable for describing the isotopic composition of the bulk Earth and other planetary bodies. To mitigate the effects of this “isotopic crisis” and to better understand the genetic relations of meteorites and the Earth-forming reservoir, we performed a comprehensive petrographic, elemental, and multi-isotopic (O, Ca, Ti, Cr, Ni, Mo, Ru, and W) study of the ungrouped achondrites NWA 5363 and NWA 5400, for both of which terrestrial O isotope signatures were previously reported. Also, we obtained isotope data for the chondrites Pillistfer (EL6), Allegan (H6), and Allende (CV3), and compiled available anomaly data for undifferentiated and differentiated meteorites. The chemical compositions of NWA 5363 and NWA 5400 are strikingly similar, except for fluid mobile elements tracing desert weathering. We show that NWA 5363 and NWA 5400 are paired samples from a primitive achondrite parent-body and interpret these rocks as restite assemblages after silicate melt extraction and siderophile element addition. Hafnium-tungsten chronology yields a model age of 2.2 ± 0.8 Myr after CAI, which probably dates both of these events within uncertainty. We confirm the terrestrial O isotope signature of NWA 5363/NWA 5400; however, the discovery of nucleosynthetic anomalies in Ca, Ti, Cr, Mo, and Ru reveals that the NWA5363/NWA 5400 parent-body is not the “missing link” that could explain the composition of the Earth by the mixing of known meteorites. Until this “missing link” or a direct sample of the terrestrial reservoir is identified, guidelines are provided of how to use chondrites for estimating the isotopic composition of the bulk Earth

    New records of Conidae (Mollusca, Gastropoda) from Andhra Pradesh, east coast of India

    Get PDF
    641-647A benthic investigation from the coastal corridor of Andhra Pradesh revealed 17 species of cone snails. Of these, seven species, namely Conasprella aculeiformis Reeve, C. coromandelica (E. A. Smith), Conus amadis (Gmelin), C. caracteristicus (Fischer von Waldheim), C. coronatus (Gmelin), C. monile Hwass in Bruguiere, and C. sulcatus Hwass in Bruguière are recorded for the first time from the coastal waters of Andhra Pradesh. The present findings of Conus from Andhra Pradesh coast are indicative of the species extended distribution to the northeast coast along the Bay of Bengal. Additionally, the live specimens collected could be of resource value for species description and historical analyses

    New records of Conidae (Mollusca, Gastropoda) from Andhra Pradesh, east coast of India

    Get PDF
    A benthic investigation from the coastal corridor of Andhra Pradesh revealed 17 species of cone snails. Of these, seven species, namely Conasprella aculeiformis Reeve, C. coromandelica (E. A. Smith), Conus amadis (Gmelin), C. caracteristicus (Fischer von Waldheim), C. coronatus (Gmelin), C. monile Hwass in Bruguiere, and C. sulcatus Hwass in Bruguière are recorded for the first time from the coastal waters of Andhra Pradesh. The present findings of Conus from Andhra Pradesh coast are indicative of the species extended distribution to the northeast coast along the Bay of Bengal. Additionally, the live specimens collected could be of resource value for species description and historical analyses

    First report of Lioconcha polita (Röding, 1798) from East peninsular India, Andhra Pradesh

    Get PDF
    Incidence of a bivalve Lioconcha polita (Röding, 1798) in the benthic collections dredged from a depth of 21 m, off Andhra Pradesh, on the east coast of India, is the first report from coastal waters of the mainland. The present findings indicate a westward range expansion of the species from the Andaman and Nicobar Islands

    First record of Veneridae clam Protapes ziczac (Linnaeus, 1758) from east coast of India, Andhra Pradesh

    Get PDF
    250-252A venerid bivalve Protapes ziczac (Linnaeus, 1758) from benthic collections is reported for the first time from the coastal waters of Andhra Pradesh, east coast of India. The findings of this study indicate the extended distributional range of the species, until now documented from the west coast of India

    First report of Lioconcha polita (Röding, 1798) from East peninsular India, Andhra Pradesh

    Get PDF
    666-669Incidence of a bivalve Lioconcha polita (Röding, 1798) in the benthic collections dredged from a depth of 21 m, off Andhra Pradesh, on the east coast of India, is the first report from coastal waters of the mainland. The present findings indicate a westward range expansion of the species from the Andaman and Nicobar Islands
    corecore