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1.
Chris Yakymchuk Adrian Rehm Zhuoheng Liao John M. Cottle 《Journal of Metamorphic Geology》2019,37(6):839-862
Sapphirine–quartz granulites from the Cocachacra region of the Arequipa Massif in southern Peru record early Neoproterozoic ultrahigh‐temperature metamorphism. Phase equilibrium modelling and zircon petrochronology are used to quantify timing and pressure–temperature (P–T) conditions of metamorphism. Modelling of three magnetite‐bearing sapphirine–quartz samples indicates peak temperatures of >950°C at ~0.7 GPa and a clockwise P–T evolution. Elevated concentrations of Al in orthopyroxene are also consistent with ultrahigh‐temperature conditions. Neoblastic zircon records ages of c. 1.0–0.9 Ga that are interpreted to record protracted ultrahigh‐temperature metamorphism. Th/U ratios of zircon of up to 100 reflect U‐depleted whole‐rock compositions. Concentrations of heavy rare earth elements in zircon do not show systematic trends with U–Pb age but do correlate with variable whole‐rock compositions. Very large positive Ce anomalies in zircon from two samples probably relate to strongly oxidizing conditions during neoblastic zircon crystallization. Low concentrations of Ti‐in‐zircon (<10 ppm) are interpreted to result from reduced titania activities due to the strongly oxidized nature of the granulites and the sequestration of titanium‐rich minerals away from the reaction volume. Whole‐rock compositions and oxidation state have a strong influence on the trace element composition of metamorphic zircon, which has implications for interpreting the geological significance of ages retrieved from zircon in oxidized metamorphic rocks. 相似文献
2.
In the coastal and estuarine waters of Goa, particulate organic carbon (POC) varied from 0.52 to 2.51 mg l?1 and from 0.28 to 5.24 mg l?1 and particulate phosphorus (PP) varied from 0.71 to 5.18 μg l?1 and from 0.78 to 20.34 μg l?1, respectively. The mean values of chlorophyll and primary productivity were 1.94 mg m?3 and 938.1 mg C m?2 day?1 in the coastal waters and 4.3 mg m?3 and 636.5 mg C m?1 day?1 in the estuarine waters, respectively. ratios were low in June and October even when POC values were quite high. The POC in surface waters was linearly correlated with the chlorophyll content. Also PP increased when chlorophyll and primary productivity remained high. The results suggest that the phytoplankton was sharply increasing and contributed to POC and PP content. The percentage of detritus calculated from the intercept values of chlorophyll on POC varied from 46 to 76% depending on season. Results indicate that the major portion of POC and PP during postmonsoon (October–January) is derived from phytoplankton production while the allochthonous matter predominate during monsoon (June–September). 相似文献
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Vertical measurements of , and N2O concentrations, and uptake, and oxidation rates were measured at 5 sites in western Cook Strait, New Zealand, between 31 March and 3 April 1983. Nitrate increased with depth at all stations reaching a maximum of 10.5 μg-atom at the most strongly stratified station whereas was relatively constant with depth at all stations (~0.1 μg-atom ). The highest rates of oxidation generally occurred in the near surface waters and decreased with depth. N2O levels were near saturation with respect to the air above the sea surface and showed no obvious changes during 24 h incubation. oxidation by nitrifying bacteria may account for about 30% of the total utilization (i.e. bacterial+agal) and, assuming oxidation through to , may supply about 40% of the algal requirements of in the study area. These results suggest that bacterial nitrification is of potential importance to the nitrogen dynamics of the western Cook Strait, particularly with respect to the nitrogen demands of the phytoplankton. 相似文献
6.
Age estimates of coastal terraces in the Andaman and Nicobar Islands and their tectonic implications
Kusala Rajendran C.P. Rajendran Anil Earnest G.V. Ravi Prasad K. Dutta D.K. Ray R. Anu 《Tectonophysics》2008,455(1-4):53-60
The great Indian Ocean earthquake of December 26, 2004 caused significant vertical changes in its rupture zone. About 800 km of the rupture is along the Andaman and Nicobar Islands, which forms the outer arc ridge of the subduction zone. Coseismic deformation along the exposed land could be observed as uplift/subsidence. Here we analyze the morphological features along the coast of the Andaman and Nicobar Islands, in an effort to reconstruct the past tectonics, taking cues from the coseismic effects. We obtained radiocarbon dates from coastal terraces of the island belt and used them to compute uplift rates, which vary from 1.33 mm yr− 1 in the Little Andaman to 2.80 mm yr− 1 in South Andaman and 2.45 mm yr− 1 in the North Andaman. Our radiocarbon dates converge on 600 yr and 1000 yr old coastal uplifts, which we attribute to the level changes due to two major previous subduction earthquakes in the region. 相似文献
7.
Manoj K. Pandit Helga de Wall Narendra K. Chauhan 《Journal of Earth System Science》2008,117(3):201-209
A number of fine-grained sericite bearing pelitic, schistose lithologies occur along the Archean (Banded Gneiss Complex)-Proterozoic
(Aravalli Supergroup) contact (APC) in the Udaipur valley in NW Indian craton. These Al-rich lithologies (subsequently metamorphosed)
have been described as ‘paleosols’, developed over a 3.3 Ga old Archean gneissic basement and are overlain by Paleoproterozoic
Aravalli quartzite. The paleosol was developed between 2.5 and 2.1, coincident with the globally recognized Great Oxidation
Event (GOE). In previous studies these paleosol sections were interpreted to have developed under reducing environment, however,
the finding of a ‘ferricrete’ zone in the upper part of Tulsi Namla section (east of Udaipur) during the present study (in
addition to earlier reported lithologies) has led to an alternative suggestion of oxygen-rich conditions during paleosol development.
The Tulsi Namla paleosol section shows all the features characteristic of a complete paleosol section described from other
Archean cratons. The paleosol includes sericite schist with kyanite as the prevalent Al-silicate in the lower part of profile
while chloritoid and Fe-oxides typify the Fe-rich upper part. Alumina has remained immobile during the weathering process
while Fe and Mn show a decrease in the lower part of the section and an abrupt rise in the upper part, in the ferricrete zone.
The field and geochemical data indicate that the Tulsi Namla section is an in situ weathering profile and at least the upper part shows evidence of oxidizing conditions. 相似文献
8.
Correlations between heavy metals and organic carbon extracted by dry oxidation procedure in urban roadside soils 总被引:1,自引:0,他引:1
Xue Song Wang 《Environmental Geology》2008,54(2):269-273
The organic fraction in soils has a significant influence on heavy metal transport. In this study, the organic carbon content
was measured by dry oxidation procedure from 21 Xuzhou urban roadside soils to assess the relationships between the concentrations
of heavy metals (Pb, Cu, Zn, and Cr) and the amount of organic carbon. The anthropogenic heavy metals (e.g. Pb, Cu, Zn) were
strongly correlated with organic carbon (denoted by Corg−c) extracted by dry oxidation while natural heavy metal (e.g. Cr) showed no correlation to the Corg−c. The anthropogenic heavy metals were also strongly correlated with the amount of the total carbon. These results show that
the anthropogenic heavy metals are mainly enriched in the organic matter in the Xuzhou urban roadside soils. 相似文献
9.
R. K. Prasad N. C. Mondal Pallavi Banerjee M. V. Nandakumar V. S. Singh 《Environmental Geology》2008,55(3):467-475
Remote Sensing and Geographic Information System has become one of the leading tools in the field of hydrogeological science,
which helps in assessing, monitoring and conserving groundwater resources. It allows manipulation and analysis of individual
layer of spatial data. It is used for analysing and modelling the interrelationship between the layers. This paper mainly
deals with the integrated approach of Remote Sensing and geographical information system (GIS) to delineate groundwater potential
zones in hard rock terrain. The remotely sensed data at the scale of 1:50,000 and topographical information from available
maps, have been used for the preparation of ground water prospective map by integrating geology, geomorphology, slope, drainage-density
and lineaments map of the study area. Further, the data on yield of aquifer, as observed from existing bore wells in the area,
has been used to validate the groundwater potential map. The final result depicts the favourable prospective zones in the
study area and can be helpful in better planning and management of groundwater resources especially in hard rock terrains. 相似文献
10.
Pressures of Crystallization of Icelandic Magmas 总被引:1,自引:0,他引:1
Iceland lies astride the Mid-Atlantic Ridge and was createdby seafloor spreading that began about 55 Ma. The crust is anomalouslythick (20–40 km), indicating higher melt productivityin the underlying mantle compared with normal ridge segmentsas a result of the presence of a mantle plume or upwelling centeredbeneath the northwestern edge of the Vatnajökull ice sheet.Seismic and volcanic activity is concentrated in 50 km wideneovolcanic or rift zones, which mark the subaerial Mid-AtlanticRidge, and in three flank zones. Geodetic and geophysical studiesprovide evidence for magma chambers located over a range ofdepths (1·5–21 km) in the crust, with shallow magmachambers beneath some volcanic centers (Katla, Grimsvötn,Eyjafjallajökull), and both shallow and deep chambers beneathothers (e.g. Krafla and Askja). We have compiled analyses ofbasalt glass with geochemical characteristics indicating crystallizationof ol–plag–cpx from 28 volcanic centers in the Western,Northern and Eastern rift zones as well as from the SouthernFlank Zone. Pressures of crystallization were calculated forthese glasses, and confirm that Icelandic magmas crystallizeover a wide range of pressures (0·001 to 1 GPa), equivalentto depths of 0–35 km. This range partly reflects crystallizationof melts en route to the surface, probably in dikes and conduits,after they leave intracrustal chambers. We find no evidencefor a shallow chamber beneath Katla, which probably indicatesthat the shallow chamber identified in other studies containssilica-rich magma rather than basalt. There is reasonably goodcorrelation between the depths of deep chambers (> 17 km)and geophysical estimates of Moho depth, indicating that magmaponds at the crust–mantle boundary. Shallow chambers (<7·1 km) are located in the upper crust, and probablyform at a level of neutral buoyancy. There are also discretechambers at intermediate depths (11 km beneath the rift zones),and there is strong evidence for cooling and crystallizing magmabodies or pockets throughout the middle and lower crust thatmight resemble a crystal mush. The results suggest that themiddle and lower crust is relatively hot and porous. It is suggestedthat crustal accretion occurs over a range of depths similarto those in recent models for accretionary processes at mid-oceanridges. The presence of multiple stacked chambers and hot, porouscrust suggests that magma evolution is complex and involvespolybaric crystallization, magma mixing, and assimilation. KEY WORDS: Iceland rift zones; cotectic crystallization; pressure; depth; magma chamber; volcanic glass 相似文献