A B C D E F G H I J K L M N O P Q R S T U V W X Y Z All
Ahmad, S. M.
- Evolution of Strontium Isotopes in Seawater During the Middle Miocene: New Results from ODP Site 758A
Authors
1 National Geophysical Research Institute, Uppal Road, Hyderabad 500 007, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 55, No 3 (2000), Pagination: 307-316Abstract
Thc 87Sr/86Sr ratio of seawater has increased significantly over the last 40 Ma. This increase was marked by very high rate of increase in 87Sr/86Sr and almost zero growth rate. The middle Miocene is one such period when considerable changes in the slope of 87Sr/86Sr curve have been reported. This study tests and improves on previously published strontium isotopic records for this period.We report 34 high precision strontium isotopic analyses of well preserved planktonic foraminifera from Ocean Drilling Program (ODP) site 758A in the eastern Indian Ocean. This site was selected because of the high quality magnetostratigraphy, suitable sedimentation rate (0.3 to 1.5 cm/ka) and excellent preservation of calcareous microfossils in sediments. Samples studied represent the period from 15.2 Ma to 10.3 Ma with approximately one sample per 100-200 ka.
The results show that the seawater 87Sr/86Sr ratio during this period increased from 0.70881 to 0.70892. The rare of increase in 87Sr/86Sr during 15.2 Ma to 14.3 was -31 x 10-6/m.y., whereas significantly from 14.3 to 12.8 Ma it was about a factor of two higher (-59x10-6/ m.y.). High rate of growth in seawater 87Sr/86Sr during this time interval indicates exceptionally high rates of continental weathering. However, during 12.8 to 10.3 Ma the growth in 87Sr/86Sr of seawater remained almost constant with an average value of 0.708914. This constant value may he due to decrease in weathering rates of continents as a result ofextensive glaciation. It is suggested that the tectonicuplift and denudation of Himalayas coupled with climatic changes have resulted in the variations of seawater strontium isotopes during this period. The inflection points in the curve may be used as event marker for stratigraphic corretation of marine sediments.
Keywords
Marine Geology, Stratigraphy, Strontium Isotopes, Miocene, Planktonic Foraminifera.
- Geology and Geochemistry of Paragneisses from the Javanahalli Schist Belt, Karnataka, India
Authors
1 National Geophysical Research Institute, Hyderabad-500007, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 21, No 12 (1980), Pagination: 577-592Abstract
The lowermost member of the Javanahalli schist belt consists of meta-arkoses/ paragneisses with interbedded epidote-quartzite and calc-silicate bands. These meta-arkoses exhibit relict current-bedding, arkosic texture and grain-to-grain variation in anorthite content within the same sample. Plagioclase is subordinate to K-feldspar and is relatively more weathered and sericitized. Orthoclase predominates over microcline in the total K-feldspar content and the quartz/feldspar ratio is generally very low. The meta-arkoses appear to be first-cycle sediments.
The composition of these paragneisses is unusual for Archaean metasediments. They are enriched in K, Si, Fe, Mg, Th, Rb, Ni, Cr and Co and depleted in Na, AI, Sr and often in Ca. The meta-arkoses of Javanahalli schist belt may be the product of strong and/or rapid chemical disintegration of Archaean tonalitic gneisses and basic and ultrabasic rocks. all of which were prominent components of the continental crust at the time the Javanahalli belt formed. If the meta-arkoses have not undergone major metasomatism, then their high abundance of K and Rb may indicate intense chemical leaching of these tonalitic source areas. Most of the saIt of the primitive oceans might have been derived from this intense leaching of the ancient cratonic tonalite gneisses.
- Anorthositic Enclaves in the Gneisses Around Holenarasipur Schist Belt, Hassan District, Karnataka
Authors
1 National Geophysical Research Institute, Hyderabad 500 007, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 23, No 8 (1982), Pagination: 387-391Abstract
Two occurrences of anorthosite enclaves in the 3.4 b.y. old gneisses surrounding the Holenarasipur schist belt are reported. The Mandagere enclave has An92 whereas, Arkalgud enclave has An83. Both the bodies are metamorphosed to higher amphibolite facies and exhibit granoblastic texture. They are associated with ultramafic enclaves. These occurrences along with the lunar type anorthosites of Holenarasipur Group greenstone belts are significant in early crustal evolution models.- Granite Rock Standard Sample: NGRI-G
Authors
1 National Geophysical Research Institute, Hyderahad 500 007, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 27, No 4 (1986), Pagination: 392-396Abstract
As a part of standard rock sample preparation program, NGRI has prepared a standard rock sample of granite for chemical analysis of granite rock samples using instrumental methods of analysis and to calibrate the instruments. International granite rock standards prepared by United States Geological Survey are no available and, thererore, it was felt necessary to prepare our own standards for different rock types.
The sample is homogeneous and gives reproducible values for all the major and minor oxides and ten trace elements. Replicate analysis at NGRI using XRF, AAS, Spectrophotometer and at other laboratories in USA and West Germany have given confident results. Proposed values for nine oxides and seven trace elements are given in this paper and the sample is offered to be used as a geochemical reference standard for granite samples.
- Physico-Chemical and Mineralogical Studies on Some of the Clay Mineral Deposits of Karnataka, India
Authors
1 National Geophysical Research Institute, Hyderabad 500 007, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 27, No 3 (1986), Pagination: 298-302Abstract
Physico-chemical properties such as EC and CEC along with X-ray diffraction studies, have been carried out on some of the clay mineral deposits of Karnataka. The results indicate that most of these deposits contain kaolinite as predominant clay mineral except Kurvi clay. Attapulgite is found to be the dominant mineral in the deposits from Kurvi.Full Text
- Spinifex Textured Peridotitic Komatiite from Nuggihalli and R.N. Pur Schist Belts, Karnataka
Authors
1 National Geophysical Research Institute, Hyderabad-500007, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 49, No 1 (1997), Pagination: 33-38Abstract
Spinifex textured ultramafic rocks occur at Bhaktarahalli in Nuggihalli schist belt and as an enclave within the Peninsular Gneiss near Dodda Gudda, Holenarasipur belt in Hassan District. They have Komatiitic chemistry pointing to ultramafic volcanism in the Dharwar Craton during the Archaean Era.Keywords
Petrology, Geochemistry, Spinifex Texture, Komatiite, Karnataka.- Changes in Seawater 87Sr/86Sr Curve during Early Miocene to Early Late Miocene: Implications of ODP Site 758A Record
Authors
1 National Geophysical Research institute, Uppal Road, Hyderabad - 500 007, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 65, No 2 (2005), Pagination: 147-157Abstract
A seawater 87Sr/86Sr record is generated for early Miocene to early late Miocene based on measurements of 114 samples of cleaned planktonic foraminifera from Ocean Drilling Program site 758 (Hole 758A) in the eastern Indian Ocean. Age model for this Site is based on nannofossil events and interpolated numerical ages are compared with Sr Isotopically-Derived ages, There is a fairly good agreement between the two stratigraphic tools (<±1.5 Ma). 87Sr/86Sr ratio during ∼23.7 Ma to ∼8.1 Ma increased from 0.70828 to 0.70895, with variations in the rate of 87Sr/86Sr increase. Seventh-Order polynomial fit curve shows the 87Sr/86Sr increase rates of 44 ×10-6m .y.-1 for -23.7 to ∼19.2 Ma, 41 × 10-6 m.y.-1 for -18.2 to - 10.8 Ma, and 25 × 10-6 m.y.-1 for ∼10.8 to ∼8.1 Ma. These rates are broadly consistent with the growth rates of 87Sr/86Sr reported from other DSDP and ODP sites with some minor differences.A composite Sr isotope curve constructed for the 24-8 Ma is broken into three linear segments with a different linear regression equation. The overall rate of 87Sr/86Sr increase for ∼24-8 Ma at site 758A (40 x 10-6 m.y.-1) is in agreement with the rate of 87Sr/86increase of the composite curve (42 x 10-6 m.y.-1).
Keywords
Sr Isotopes, Miocene, Stratigraphy, Planktonic Foraminifera, Eastern Indian Ocean.- Seawater Neodymium Isotopic Composition in the Northeastern Indian Ocean during the LGM to Holocene: Response to Glacial and Monsoonal Weathering in Himalaya-Tibet
Authors
1 National Geophysical Research Institute, Uppal Road, Hyderabad - 500 007, IN
2 National Geophysical Research Institute, Uppal Road, Hyderabad - 500 007