Six new K-Ar ages were determined for three of four volcanic systems that constitute the San Pedro-Ceboruco graben of western Mexico, in order to give constraints on stratigraphic relationships and problems of magma genesis in an area characterized by the coexistence of magmas with different petrologic affinities (calc-alkaline and intra-plate type). Previous age determinations are scarce for this region. Samples were dated using the unspiked K-Ar sensitivity method with a mass-fractionation correction procedure. Our new data, along with those in literature, suggest that volcanism started at ∼2.5 Ma in the southwest part of the graben. Between ∼2.3 Ma to ∼0.1 Ma volcanic activity continued in the central part of the graben and starting from ∼0.5 Ma, volcanism also extended to the northern part. This northward shifting of the volcanic activity with time occurred along NW-SE trending lines which coincide with the main regional fault systems, and could be linked to the resumption of convergence after a period of near-cessation. The major phase of the magmatic activity is younger than 0.5 Ma, but the presence of volcanic activity strictly differentiated in magmatic affinities (orogenic and intra-plate), started earlier, around 1.1 Ma and is linked to sub-slab melts flowing in the mantle wedge. The age obtained on the dome complex (San Pedro-Cerro Grande Volcanic Complex) along with literature data suggest that the caldera collapse event occurred before ∼1.1 Ma and was followed by intracaldera and pericaldera activities. After ∼0.6 Ma, subsidence and consequent reactivation of the eastern caldera rim, can have determined the cutting of some pericaldera domes.
An improved method for chlorine isotopic analysis by thermal ionization mass spectrometry of Cs2Cl+ has been investigated for precise measurement of 37Cl/35Cl isotopic ratios for small amount of samples. Chlorine in organic and inorganic samples was recovered as AgCl and converted to CsCl with metallic Mg and Cs-form ion exchange resin. The CsCl (containing 2 μg of Cl) was loaded on a Ta filament together with graphite powder. The 37Cl/35Cl ratio measurement was carried out using Cs2Cl+ ions with multi Faraday cup (static mode) for 80–100 min each single run. Replicate analysis of ocean water and laboratory standard (CsCl reagent) yield the in-run precision of 0.1–0.2‰ (2σm) for 300–400 ratios and external precision of 0.1–0.2‰ (1σSD). This method has been tested on a variety of chemical reagents including metal chlorides, hydrochloric acid and chlorinated organic solvents. It was found that the artificial chlorine compound show large variations of 37Cl/35Cl ratio (total ranges from −5 to +3‰ relative to standard mean ocean chloride) depending upon manufacturing processes. We suggest that the high sensitivity and precision technique established in this work would allow us to detect small isotopic variations of chlorine in small amounts of natural samples such as terrestrial materials (minerals/rocks, sediments, groundwater, etc, as well as environmental materials), meteorites and other planetary materials.
Radiochemical neutron activation was applied for the determination of mercury isotopic ratios (196Hg/202Hg) for standard rock samples and meteorite samples. After neutron activation, Hg was released upon heating from 100°C to 500°C at 100°C steps and the activity ratio of 197Hg/203Hg was measured for each released fraction. After the correction for the interfering γ-rays of 75Se to 203Hg, the ratios obtained from the Dhajala meteorite (bulk and magnetically separated samples) were identical to those of the Hg monitor as well as standard rock samples (JB-1, basalt, and JG-1, granodiorite). An apparently low ratio of 196Hg/202Hg was observed at high temperature (500°C) for the Allende meteorite reference sample. As such a low value was not reproduced in another run, the presence of isotopically anomalous Hg in the Allende reference sample can not be concluded. Hg/Se ratio was observed to be very high in the nonmagnetic fraction of Dhajala compared with that in the magnetic fraction, suggesting that Hg is less chalcophile than Se. Yamato 82050 (CO3) has an anomalously high content of Hg with a normal 196Hg/202Hg ratio, being at least an order of magnitude higher than the expected value. This meteorite must have been contaminated with Hg, presumably on Antarctica.
Rare metals mineralization in Jabal Gattar, N. E. Egypt, occurs at the sheared tectonic contact between a younger granite and Hammamat sediments. The contact is affected by a local reverse fault directed N75°E and dipping 45°SSE. The Hammamat sediments consist substantially of rhythmic siltstones and immature greywackes. The paragenetic history of the mineralized sediments involves many types of alteration, such as dissolution of primary quartz (episyenitization), sericitization, albitization, hematization, pyritization, silicification, kaolinization and bleaching. The calculation of the mass-balance indicates preferential mobilization of REE, Rb, Cs, Be and S from Gattar granite to Hammamat sediments in the alteration zone. The elements; U, Y, W, Nb, Cu, Pb, Sb and Zn are enriched in both granite and sediments, but with higher magnitude for the latter. The hematized sediments are generally better accumulators for rare metals. The Gattar granite is classified as a meta-aluminous leucogranite, containing low Ca and high alkali. The low Ba and Sr and the high REE + Y reflect a highly fractionated (low P) A-type granite. The mineralized granite is affected by several metasomatic alterations. The mineralization causes enrichment in the content of U, Nb, W, Y and the chalcophile elements. The REE content displays portioning towards preferential mobilization of the LREE to the adjacent mineralized Hammamat sediments while higher quotient of the HREE is adopted within the secondary fluorite. The volume loss of granite due to leaching reactions (e.g., episyenitization) causes apparent enrichment of the immobile elements such as Sc, Th and Zr. The mineralization was caused by alkaline and oxidizing hot fluids, with contribution of meteoric water volume heated by convection. The hot fluids flowed up along ENE-WSW and NE-SW faults to cause a rather complicated series of metasomatic reactions. The rare metals were mobilized from the younger granite pluton of Gattar and concentrated in the sheared tectonic contact. The reaction of the mineralizing solutions with the wall rocks and the pseudomorphic oxidation of pyrite caused reduction and fixation of uranium. The study area can be considered as a possible potential deposit for U, REE + Y and other rare metals as well.
Methyl chloride (CH3Cl), methyl bromide (CH3Br), and methyl iodide (CH3I) in marine boundary air were measured over the western Pacific and the southeastern Indian Ocean during the period of December 1996–February 1997. The mean concentrations of CH3Cl, CH3Br, and CH3I were 623, 10.3, and 1.1 pptv, respectively, and their highest concentrations were observed in the tropics. The enhancement of CH3Cl concentration in the tropics, particularly near islands, was consistent with the recently reported finding on its high emissions from tropical coastal lands. The enhancement of CH3Br in the tropics was not related to the closeness of the sampling sites to islands, suggesting that land sources were not so important for CH3Br as for CH3Cl. The atmospheric CH3I concentrations showed a little higher concentration in the tropics and in the southern hemisphere (SH) than in the northern hemisphere (NH). This finding suggests that its higher emission from the tropics and SH in austral summer compensates the higher rate of photolytic decay of this compound in the regions.