Barren Island (Andaman Sea) is the northernmost active volcano of the Indonesian Arc. To construct the eruptive history of
this little studied volcano, we measured 14C dates of inorganic carbon in sediment beds, and Sr and Nd isotopic ratios of seven discrete ash layers, in a marine sediment
core collected from 32 km southeast of the volcano. The study reveals that the volcano had seven major ash eruptions at ~70,
69, 61, 24, 19, 15, and 10 ka. The ash layers erupted from 70 ka through 19 ka have highly uniform Nd isotopic composition,
and since the ~15 ka eruption to the present the isotopic composition has been highly variable. Between ~24 ka and ~10 ka,
the volcano had large ash eruptions spaced at 4,500 year intervals. Isotopically correlating the precaldera lavas and ash
exposed on the volcano to the uppermost ash layer in the core, we infer that the caldera of Barren Island volcano is younger
than 10 ka. 相似文献
We report the result of simultaneous multi-wavelength observations of the TeV blazar Mrk 421 during February - March 2003. We observed Mrk 421 using the Pachmarhi Array of Cerenkov Telescopes (PACT) of Tata Institute of Fundamental Research at Pachmarhi, India. Other simultaneous data were taken from the literature and public data archives. We have analyzed the high quality X-ray (2-20keV) observations from the NASA Rossi X-Ray Timing Explorer (RXTE). We obtained a possible correlated variability between X-ray and J band (1.25 μ) near infrared (NIR) wavelength. This is the first case of X-ray and NIR correlated variability in Mrk 421 or any high energy peaked (HBL) blazar. The correlated variability reported here indicates a similar origin for the NIR and X-ray emissions. The emission is not affected much by the environment of the surrounding medium of the central engine of Mrk 421. The observations are consistent with the shock-in-jet model for the emissions. 相似文献
We report the first results of quasi-simultaneous two passband optical monitoring of six quasi-stellar objects to search for micro-variability. We carried out photometric monitoring of these sources in an alternating sequence of R and V passbands, for five radio-quiet quasi-stellar objects (RQQSOs), 0748 + 291, 0824 + 098, 0832 + 251, 1101 + 319, 1225 + 317 and one radio-loud quasi-stellar object (RLQSO), 1410 + 429. No micro-variability was detected in any of the RQQSOs, but convincing micro-variability was detected in the RLQSO on two successive nights it was observed. Using the compiled data of optical micro-variability of RQQSOs till date, we got the duty cycle for micro-variability in RQQSOs is ~10%. The present investigation indicates that micro-variability is not a persistent property of RQQSOs but an occasional incident. 相似文献
Many tholeiitic dyke-sill intrusions of the Late Cretaceous Deccan Traps continental flood basalt province are exposed in
the Satpura Gondwana Basin around Pachmarhi, central India. We present field, petrographic, major and trace element, and Sr–Nd–Pb
isotope data on these intrusions and identify individual dykes and sills that chemically closely match several stratigraphically
defined formations in the southwestern Deccan (Western Ghats). Some of these formations have also been identified more recently
in the northern and northeastern Deccan. However, the Pachmarhi intrusions are significantly more evolved (lower Mg numbers
and higher TiO2 contents) than many Deccan basalts, with isotopic signatures generally different from those of the chemically similar lava
formations, indicating that most are not feeders to previously characterized flows. They appear to be products of mixing between
Deccan basalt magmas and partial melts of Precambrian Indian amphibolites, as proposed previously for several Deccan basalt
lavas of the lower Western Ghats stratigraphy. Broad chemical and isotopic similarities of several Pachmarhi intrusions to
the northern and northeastern Deccan lavas indicate petrogenetic relationships. Distances these lava flows would have had
to cover, if they originated in the Pachmarhi area, range from 150 to 350 km. The Pachmarhi data enlarge the hitherto known
chemical and isotopic range of the Deccan flood basalt magmas. This study highlights the problems and ambiguities in dyke-sill-flow
correlations even with extensive geochemical fingerprinting. 相似文献
Experimental studies have been performed on an olivine tholeiite and tholeiitic picrite at pressure and temperature ranges of 20–40 kb and 1200–1300°C. The lower and upper limits of basalt-eclogite transition zone for tholeiitic picrite are 23 kb and 31·67 kb at 1200°C, and 24·67 kb and 33·67 kb at 1300°C, whereas for olivine tholeiite, these are 27 kb and 32·33 kb at 1200°C, and 28·70 kb and 33·70 kb at 1300°C. While the assemblages for both samples below the transition region are Pl+Px+Mt, they are Pl+Gt+Px+Mt within it. The eclogite field has Gt+Px+Mt. The ratio of garnet to plagioclase increases from the transition zone to the eclogite field and with the disappearance of plagioclase, the percentage of garnet increases to 30 in the eclogite field. Comparison of our results with previous studies on basalt-eclogite transition shows that the transition zone found by us occurs at higher pressure-temperature conditions. Seismic studies of the region below the Deccan Traps show an increase in velocity (1–4%) at depth. It is suggested that after partial melting, during ascent of the basaltic liquid, a significant portion of it crystallizes within the upper mantle as pockets of eclogite. As eclogite is more dense than peridotite, their presence should cause a similar increase in the seismic velocity below the Deccan area. 相似文献
This paper describes a hybrid fuzzy weights-of-evidence (WofE) model for mineral potential mapping that generates fuzzy predictor
patterns based on (a) knowledge-based fuzzy membership values and (b) data-based conditional probabilities. The fuzzy membership
values are calculated using a knowledge-driven logistic membership function, which provides a framework for treating systemic
uncertainty and also facilitates the use of multiclass predictor maps in the modeling procedure. The fuzzy predictor patterns
are combined using Bayes’ rule in a log-linear form (under an assumption of conditional independence) to update the prior
probability of target deposit-type occurrence in every unique combination of predictor patterns. The hybrid fuzzy WofE model
is applied to a regional-scale mapping of base-metal deposit potential in the south-central part of the Aravalli metallogenic
province (western India). The output map of fuzzy posterior probabilities of base-metal deposit occurrence is classified subsequently
to delineate zones with high-favorability, moderate favorability, and low-favorability for occurrence of base-metal deposits.
An analysis of the favorability map indicates (a) significant improvement of probability of base-metal deposit occurrence
in the high-favorability and moderate-favorability zones and (b) significant deterioration of probability of base-metal deposit
occurrence in the low-favorability zones. The results demonstrate usefulness of the hybrid fuzzy WofE model in representation
and in integration of evidential features to map relative potential for mineral deposit occurrence. 相似文献
We conducted a series of melting experiments in the join forsterite–diopside–leucite under 0.1 and 2.3 GPa and in the join forsterite–leucite–åkermanite under 2.3 GPa to understand paragenetic relationships amongst different types of lamproitic and lamprophyric magmas with K-rich mafic and ultramafic volcanic (kamafugitic) rocks. Both the joins were studied in the presence of excess water. The experimental results of the join forsterite–diopside–leucite at 0.1 GPa show that the five-phase point of forsterite (Fo)ss + diopside (Di)ss + leucite (Lc)ss + liquid (Liq) + vapour (V) (equivalent to ugandite lava) occurs at Fo2Di50Lc48 at 880 ± 5 °C. Phlogopite appears as the last phase at 830 ± 15 °C. The final crystalline assemblage of forsteritess + diopsidess + leucitess + phlogopite is similar to the phenocryst assemblage of missourite lava. Present study suggests that an olivine leucitite (ugandite) can be derived from an olivine italite, a slightly potassic peridotite and a leucitite magma.
A study of the join Fo–Di–Lc [P(H2O) = P(Total)] at 2.3 GPa shows that liquid compositions penetrate the primary phase volumes of forsteritess, phlogopitess, kalsilitess, K-feldsparss and diopsidess. It has the following three five-phase points: 1) one occurring at Fo9Di49Lc42 and 1005 ± 5 °C, where liquid and vapour coexists with forsteritess, phlogopitess and diopsidess (phlogopite-bearing madupite), 2) the second one at Fo4Di50Lc46 and 990 ± 10 °C, where diopsidess, K-feldsparss and phlogopitess coexist with liquid and vapour (pyroxene-bearing minette), and 3) the third one at Fo3Di21Lc76 and 775 ± 5 °C, where phlogopitess, kalsilitess and K-feldsparss are in equilibrium with liquid plus vapour (kalsilite-bearing minette).
The experimental results of the join Fo–Lc–åkermanite (Ak) show that the join 40 penetrates the primary phase volumes of forsteritess, phlogopitess, kalsilite, K-feldsparss, diopsidess and merwinitess. The data indicate the presence of four five-phase points: 1) one occurring at Fo7Lc42Ak51 and 1165 ± 5 °C, where phlogopitess, forsteritess, diopsidess coexists with liquid and vapour (olivine-bearing madupite), 2) the second one at Fo3Lc49Ak48 and 1140 ± 10 °C, where a liquid is in equilibrium with phlogopitess, K-feldsparss, diopsidess and vapour (pyroxene-bearing minette), 3) the third one at Fo18Lc21Ak61 and 1255 ± 10 °C, where merwinitess, forsteritess and diopsidess are in equilibrium with liquid and vapour (merwinite-bearing wherlite), and 4) the fourth one at Fo5Lc73.5Ak21.5 and 770 ± 5 °C, where kalsilitess, phlogopitess and K-feldspar coexist with liquid and vapour (kalsilite-bearing minette). The present data suggest that high pressure heteromorphic equivalent of a katungite magma is represented by a kalsilite-bearing minette, a pyroxene-bearing minette, or an olivine-bearing madupite. 相似文献
A centrographic method for analysing mineral deposit clusters is illustrated using the komatiite-hosted Kambalda nickel sulphide deposit cluster, Yilgarn craton, Western Australia. In this method, the standard distance circle divides the cluster into a more endowed inner part and a less endowed peripheral part. The standard deviational ellipse, another centrographic object, depicts the preferred northwest–southeast trend of nickel orebodies at Kambalda. Weighted centrography shows that nickel endowment is greater in the eastern than western part of the cluster. The spatio-geometric interaction of the circle and ellipse splits the cluster into several partitions. The relative concentration of nickel orebodies or endowment within a partition in relation to their concentration within the entire cluster is termed ‘capture efficiency’. Komatiite areal trace exhibits higher nickel orebody capture efficiency than spatio-geometric partitions; however, some spatio-geometric partitions exhibit nickel endowment capture efficiencies comparable to that of komatiite. Furthermore, nickel orebody and endowment capture efficiencies of komatiite are elevated only within the standard distance circle. These results suggest that at Kambalda, (i) the standard distance circle is a prime window for understanding the komatiite-hosted nickel system, and (ii) spatio-geometric partitions are plausible locales for spatial analysis of nickel orebodies and endowment. The proposed centrographic method is potentially useful in mineral resource estimations and mineral exploration targeting. 相似文献