Marine biogeochemistries of Be and Al: A study based on
cosmogenic 10 Be, Be and Al in marine calcite, aragonite, and opalWeiquan Dong
, Devendra Lal, Barbara Ransom, Wolfgang Berger and Marc W Caffee 1Scripps Institution of Oceanography, Geosciences Research Division, La Jolla, CA 92093.
1
Lawrence Livermore National Laboratory, Center for Accelerator Mass Spectrometry,Livermore, CA 94551.
Present address: University of Nevada, Geosciences Department, 4505 Maryland Parkway,Box 454010, Las Vegas, NV 89154-4010.
The geochemical behaviors of Be and Al in ocean waters have been successfully studied in recent years using natural, cosmogenic, radioactive 10 Be and 26 Al as tracers. The present day dissolved concentrations and distribution of the stable and radioactive isotopes of Be and Al in ocean waters have revealed their short residence times and appreciable effects of exchange uxes at the coastal and ocean-sediment interfaces. It follows that concentrations of these particle-active elements must have varied in the past with temporal changes in climate, biological productivity and aeolian ux of continental detritus to the oceans. We therefore investigated the feasibility of extending the measurements of Be and Al isotope concentrations in marine systems to the 10 3 {10 6 y BP time scale. We report here the discovery of significant amounts of intrinsic Be and Al in marine foraminiferal calcite and coral aragonite, and of Al in opal (radiolarians) and aragonite (coral), which makes it possible to determine 10 Be/Be and 26 Al/Al in oceans in the past. We also report measured 10 Be/ 9 Be in foraminiferal calcite in Pacific Ocean cores, which reveal that the concentrations and ratios of the stable and cosmogenic isotopes of Be and Al have varied signicantly in the past 30 ky. The implications of these results are discussed. Download PDF filesHigh Iridium concentration of alkaline rocks of Deccan
and implications to K/T boundaryP N Shukla, N Bhandari
, Anirban Das, A D Shukla and J S RayPhysical Research Laboratory, Ahmedabad 380 009, India.
e-mail: bhandari@prl.ernet.in
We report here an unusually high concentration of iridium in some alkali basalts and alkaline rocks of Deccan region having an age of about 65Ma, similar to the age of the Cretaceous-Tertiary boundary. The alkali basalts of Anjar, in the western periphery of Deccan province, have irid-ium concentration as high as 178pg/g whereas the alkaline rocks and basalts associated with the Amba Dongar carbonatite complex have concentrations ranging between 8 and 80 pg/g. Some of these values are more than an order of magnitude higher than the concentration in the tholeiiticbasalts of Deccan, indicating the signicance of alkaline magmatism in the iridium inventory at the Cretaceous-Tertiary boundary. Despite higher concentration, their contribution to the global inventory of iridium in the Cretaceous-Tertiary boundary clays remains small. The concentration of iridium in uorites from Amba Dongar was found to be < 30 pg/g indicating that iridium is not incorporated during their formation in hydrothermal activity. Download PDF fileGeochemistry and magnetostratigraphy of Deccan ows at Anjar, Kutch
A D Shukla, N Bhandari
, S h e e l a Kusumgar, P N Shukla, Z G Ghevariya, K Gopalany and V BalaramyPhysical Research Laboratory, Navrangpura, Ahmedabad, 380 009, India
Geological Survey of India, Gandhinagar 382 043, Indiay
National Geophysical Research Institute, New Uppal Road, Hyderabad 500 007, India e-mail: bhandari@prl.ernet.inChemical analysis of nine Deccan ow basalts at Anjar, Kutch, western India, indicates that all,except the uppermost ow F-9, are alkaline. In their major and trace element composition, the alkali basalts resemble Ocean island basalts (OIB). Similarities of many diagnostic trace element ratios (e.g. Sm/Nd, Ba/Nb,Y/Nb and Zr/Nb) are similar to those found in the Reunion Island basalts. The uppermost basalt is tholeiitic and chemically resembles the least contaminated Deccan basalt (Ambenali type). The Anjar basalts have iridium concentration ranging between 2 and 178 pg/g. Some of these values are higher by about an order of magnitude compared to the Ir concentration in other basalts of the Deccan. A synthesis of chemical, palaeomagnetic and geochronologic data enables us to construct a chemical and magnetic stratigraphy for these ows.
The three ows below the iridium enriched intertrappean bed (IT III) show normal magnetic polarity whereas all except one of the upper basalts show reversed magnetic polarity. The sequence seems to have started in polarity zones 31N and probably continued up to 28R or 27R. The results presented here support the view that Deccan volcanism in Kutch occurred on a time span of a few million years.
No K/T boundary at Anjar, Gujarat, India: Evidence from magnetic susceptibility and carbon isotopes
H J Hansen
, D M Mohabey and P Toft Geological Institute, Oster Voldgade 10, DK-1350, Denmark Geological Survey of India, Seminary Hills, Nagpur 440 006, IndiaThe paper describes the variation pattern of magnetic susceptibility of Lameta sediments and isotopic variation of organic 13 C from Chui Hill, Bergi, Kholdoda, Pisdura and Girad. The susceptibility pattern and a negative carbon isotopic anomaly allows xation of the K/T boundary at these localities and they dier in these aspects from the inter-trappean sediments at Anjar.
Paleomagnetic measurements of the Anjar sediment and the overlying basalt ow demonstrate reversed polarity. The Lameta sediments with dinosaur nests at Kheda and the overlying intertrappean sediments are of normal polarity. The clay layers at Anjar, associated closely with Ir-enrichments, are strongly leached, rhyolitic bentonites containing low-quartz paramorphs after high-quartz with glass inclusions. It is concluded, that the inter-trappean lake deposits at Anjar were deposited in the early part of magnetochron 29R and are unrelated to the K/T boundary.
T S G i r i t h a r a n
and V RajamaniSchool of Environmental Sciences, Jawaharlal Nehru University, New Delhi 110067
Presently at Department of Earth Sciences, Pondicherry University, Pondicherry 605 014.e-mail: tsgiridharan@yahoo.com
The eastern Dharwar Craton of southern India includes at least three
2700Ma supracrustal belts (schist belts) which have mesothermal, quartz-carbonate vein gold mineralization emplaced within the sheared metabasalts. In the Hutti and the Kolar schist belts, the host rocks are amphibolites and the ore veins have been anked by only a thin zone of biotitic alteration; in the Ramagiri belt, however, the host rocks to the veins have been a ected by more extensive but lower temperature alteration by uids. The rare earth element (REE) geochemistry of the host metabasalts, alteration zones, ore veins and the bulk sulfides separated from the ore veins and the alteration zones suggest that the REE chemistry of the immediate host rocks has been modified by uids which added LREE, the REE abundance of the ore veins vary with the amount of host rock fragments included in the veins, the sulfides formed during mineralization have significant REE concentration with patternsnearly identical to the ore veins and alteration zones and
therefore the ore uids involved in gold mineralization here could be LREE enriched. Because alteration and mineralization involved addition of REE, more LREE compared to HREE, the uids could be of higher temperature origin. The initial Nd isotope ratios in the host rocks ("Nd calculated at 2700 Ma) showed a large variation (+8 to -4) and a deep crustal source for the uid REE seems likely. A crustal source for Pb and Os in the ore samples of Kolar belt has previously been suggested (Krogstad et al 1995; Walker et al 1989). Such a source for ore uids is consistent with a late Archean (2500Ma) accretionary origin for the terrains of the eastern Dharwar Craton.D i l i p K Mukhopadhyay
Department of Earth Sciences, University of Roorkee, Roorkee 247667, Uttaranchal, India
e-mail: dilipfes@rurkiu.ernet.in
The small leucogranite plutons occurring in linear belts in the Higher Himalayas have formed due to post-collision partial melting within the Himalayan crust. Several studies have documented that the Sr isotopic ratios in the granite bodies show chaotic variation and meaningful Rb-Sr isochron ages are dicult, if not impossible, to obtain. In tectonically overthickened crust, the depth-temperature prole (geotherm) remains strongly transient for the rst tens of millions of years. It is proposed here that the intersecting relations between the transient geotherms and activity-dependent solidus/melting curves may generate small pods of magma at di erent depths and at di erent times. Each of these pods will have its unique Sr isotopic ratios. Coalescence of these small pods of magma without any e ective homogenization due to deformation-induced fast segregation, ascent and emplacement may lead to pluton-wide extreme heterogeneity in Sr isotopic ratios.
Estimation of source parameters of Chamoli Earthquake, India
Y Pandey, R Dharmaraju
and P K S ChauhanCentral Building Research Institute, Roorkee 247667, India
e-mail: silcbri@nde.vsnl.net.in
The devastating earthquake (mb = 6.6) at Chamoli, Garhwal Himalaya, which occurred in the morning hours on 29th March 1999, was recorded on Delhi Strong Motion Accelerograph (DSMA) Network operated by the Central Building Research Institute, Roorkee. In this paper the source parameters of this event calculated from the Strong Motion Data are presented. The seismic moment for this event has been found to be of the order of 10 25 dyne.cm and the moment mag- nitude has been calculated in the range of 6.53 { 6.69 at different stations. The stress drop and source radius for the earthquake are also calculated.
Computing energy budget within a crop canopy from Penmann's formulae
Mahendra Mohan
and K K S r i v a s t a v a Radio and Atmospheric Science Division, National Physical Laboratory, New Delhi 110012, India Department of Chemical Engineering, Institute of Technology, Banaras Hindu University, Varanasi 221005, IndiaThe Lhomme's model (1988a), that extended Penmann's formulae to a multi-layer model, is rede-fined as a function of micrometeorological and physiological profiles of crop canopy. The sources and sinks of sensible and latent heat uxes are assumed to lie on a fictitious plane called zero-displacement plane. Algorithms are given to compute sensible and latent heat ux densities. Per-formance of the algorithms is compared with that of earlier algorithms.