Geochemical constraints on the origin of mafic and silicic magmas at Cordón El Guadal, Tatara-San Pedro Complex, central Chile

The aim of this study is to quantify the crustal differentiation processes and sources responsible for the origin of basaltic to dacitic volcanic rocks present on Cordón El Guadal in the Tatara-San Pedro Complex (TSPC). This suite is important for understanding the origin of evolved magmas in the southern Andes because it exhibits the widest compositional range of any unconformity-bound sequence of lavas in the TSPC. Major element, trace element, and Sr-isotopic data for the Guadal volcanic rocks provide evidence for complex crustal magmatic histories involving up to six differentiation mechanisms. The petrogenetic processes for andesitic and dacitic lavas containing undercooled inclusions of basaltic andesitic and andesitic magma include: (1) assimilation of garnet-bearing, possibly mafic lower continental crust by primary mantle-derived basaltic magmas; (2) fractionation of olivine + clinopyroxene + Ca-rich plagioclase + Fe-oxides in present non-modal proportions from basaltic magmas at ∼4–8 kbar to produce high-Al basalt and basaltic andesitic magmas; (3) vapor-undersaturated (i.e., P _H2O<P _TOTAL) partial melting of gabbroic crustal rocks at ∼3–7 kbar to produce dacitic magmas; (4) crystallization of plagioclase-rich phenocryst assemblages from dacitic magmas in shallow reservoirs; (5) intrusion of basaltic andesitic magmas into shallow reservoirs containing crystal-rich dacitic magmas and subsequent mixing to produce hybrid basaltic andesitic and andesitic magmas; and (6)␣formation and disaggregation of undercooled basaltic andesitic and andesitic inclusions during eruption from shallow chambers to form commingled, mafic inclusion-bearing andesitic and dacitic lavas flows. Collectively, the geochemical and petrographic features of the Guadal volcanic rocks are interpreted to reflect the development of shallow silicic reservoirs within a region characterized by high crustal temperatures due to focused basaltic activity and high magma supply rates. On the periphery of the silicic system where magma supply rates and crustal temperatures were lower, cooling and crystallization were more important than bulk crustal melting or assimilation. Received: 2 July 1997 / Accepted: 25 November 1997     

Reduction of inorganic compounds in autoclaves by using suspended cellulose

Autoclaves are closed vessels in which liquids can be heated above their ‘normal' boiling points. As a consequence the pressure will rise. This equipment has permitted a variety of processes like the syntheses of organics and the leaching of metal ores. Pressure technologies can be used for accelerating reactions, for specific syntheses or they offer an environmental or economical alternative for existing processes. Wet pressure oxidation of sewage sludge, an organic waste product, has become a proven technology. If less oxygen is added than stoichiometrically required for a complete oxidation, cellulose will break down by a different reaction mechanism, releasing electrons. Under these circumstances reduction of inorganic compounds is possible.     

Salinity reflux and dolomitization of southern Australian slope sediments: the importance of low carbonate saturation levels

Anomalously saline waters in Ocean Drilling Program Holes 1127, 1129, 1130, 1131 and 1132, which penetrate southern Australian slope sediments, and isotopic analyses of large benthic foraminifera from southern Australian continental shelf sediments, indicate that Pleistocene–Holocene meso‐haline salinity reflux is occurring along the southern Australian margin. Ongoing dolomite formation is observed in slope sediments associated with marine waters commonly exceeding 50‰ salinity. A well‐flushed zone at the top of all holes contains pore waters with normal marine trace element contents, alkalinities and pH values. Dolomite precipitation occurs directly below the well‐flushed zone in two phases. Phase 1 is a nucleation stage associated with waters of relatively low pH (ca 7) caused by oxidation of H₂S diffusing upward from below. This dolomite precipitates in sediments < 80 m below the sea floor and has δ¹³C values consistent with having formed from normal sea water (? 1‰ to + 1‰ Vienna Pee Dee Belemnite). The Sr content of Phase 1 dolomite indicates that precipitation can occur prior to substantial metastable carbonate dissolution (< 300 ppm in Holes 1129 and 1127). Dolomite nucleation is interpreted to occur because the system is undersaturated with respect to the less stable minerals aragonite and Mg‐calcite, which form more readily in normal ocean water. Phase 2 is a growth stage associated with the dissolution of metastable carbonate in the acidified sea water. Analysis of large dolomite rhombs demonstrates that at depths > 80 m below the sea floor, Phase 2 dolomite grows on dolomite cores precipitated during Phase 1. Phase 2 dolomite has δ¹³C values similar to those of the surrounding bulk carbonate and high Sr values relative to Phase 1 dolomite, consistent with having formed in waters affected by aragonite and calcite dissolution. The nucleation stage in this model (Phase 1) challenges the more commonly accepted paradigm that inhibition of dolomitization by sea water is overcome by effectively increasing the saturation state of dolomite in sea water.     

Age of the Ust’-Gilyui sequence in the Stanovoi Complex of the Selenga-Stanovoi Superterrain, Central Asian fold belt

According to the results of U-Pb geochronological investigations, the age of the amphibolite protoliths (metabasalts) in the Ust??-Gilyui sequence within the Stanovoi Complex of the Amazar-Gilyui structural and formational zone in the Selenga-Stanovoi Superterrain of the Central Asian fold belt can be estimated at 193 ± 1 Ma. The Nd model age of the Ust??-Gilyui metasedimentary rocks is in the interval of t _Nd(DM) = 1.1?C3.1 Ga. This information along with the previously obtained geochronological data are indicative of the fact that the Ust??-Gilyui sequence consists of metasedimentary and metavolcanic rocks of various ages: (1) volcanic rocks with the age of 193 ± 1 Ma; (2) metasedimentary and metavolcanic rocks broken through by the Paleozoic granitoids dated to 370 Ma and characterized by minimum estimations of t _Nd(DM) = 1.1 Ga, i.e., rocks with an age of 1.1?C0.4 Ga. In addition, it is quite possible that this sequence also includes more ancient rocks. The SSS Amazar-Gilyui structural and formational zone is likely to be a tectonic mélange composed of the metasedimentary and metavolcanic rocks of the Mesozoic and, probably, Paleozoic and Early Precambrian ages. The studied zone was formed in the Mesozoic, most likely, in the course of the collision processes initiated by the closing up of the Mongol-Okhotsk Ocean.     

Biosorptive behaviour of mango leaf powder and rice husk for arsenic(III) from aqueous solutions

The present study deals with the biosorption of As(III) from aqueous solution using mango leaves powder (MLP) and rice husk (RH) in a batch operation. Scanning electron microscopy and Fourier transformation infrared spectrometry analysis shows the surface texture of biosorbents and metal binding of functional groups of before and after biosorption of As(III). The optimum pH was obtained at 7 and 6 with 7 and 6?g/l of dosage of MLP and RH, respectively. The adsorption of As(III) onto MLP and RH was favourably influenced by an increase in temperature. Equilibrium data were well represented by the Freundlich isotherm model. Nitric acid and ethylenediaminetetra acetic acid was found to be a better eluant for the desorption followed by hydrochloric acid and sodium hydroxide of As(III) with a maximum desorption efficiency of 69.5, 48.5 and 79.4, 86.3?%, respectively. The pseudo-second-order kinetic model was found to best fitted of the experimental data over the equilibrium time at 32?h. The positive values of heat of adsorption (23.89?kJ/mol for MLP and 52.26?kJ/mol for RH) indicate the endothermic nature of the adsorption process. The thermodynamic study showed the spontaneous nature of the sorption of As(III) onto MLP and RH.     

Chemical classification of iron meteorites—VIII. Groups IC. IIE,IIIF and 97 other irons

Concentrations of Ni, Ga, Ge and Ir in 106 iron meteorites are reported. Three new groups are defined: IC, IIE and IIIF containing 10, 12 and 5 members, respectively, raising the number of independent groups to 12. Group IC is a cohenite-rich group distantly related to IA. Group IIE consists of those irons previously designated Weekeroo Station type and five others having similar compositions though diverse structures. The IIE irons are compositionally similar to the mesosiderites and pallasites, and the three groups probably formed at similar heliocentric distances. The mixing of the globular IIE silicates with the metal probably occurred during shock events. Group IIIF is a well-defined group of low-Ni and low-Ge irons. The compositions of these groups are summarized as follows:

     

1000.

Precipitation of arsenic under sulfate reducing conditions and subsequent leaching under aerobic conditions     

T.C. Onstott  E. ChanM.L. Polizzotto  J. LanzonM.F. DeFlaun 《Applied Geochemistry》2011,26(3):269-285

     

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Group	Ni (%)	Ga (ppm)	Ge (ppm)	Ir (ppm)
IC	6.1–6.8	42–54	85–250	0.07–10
IIE	7.5–9.7	21–28	62–75	0.5–8
IIIF	6.8–7.8	6.3–7.2	0.7–1.1	1.3–7.9