Alexander V. Ignatiev

Oxygen isotope and trace element zoning in hydrothermal garnets: Windows into large-scale fluid-flow behavior

Single garnet crystals from a large, multicyclic hydrothermal system in Far East Russia are strongly growth zoned. Microscale ion microprobe analysis reveals consistent intracrystalline patterns in oxygen isotope composition and boron and iron contents, reflecting evolution of the hydrothermal system from magmatic- to meteoric-dominated conditions, and repetitive influx of magmatic fluids. Large intracrystalline variations in oxygen isotope composition (>12‰ δ 18 O) delimit temporal evolution of magmatic and meteoric fluid input within different portions of the hydrothermal system, and suggest that fluid-flow regimes differed between central and distal portions of the system. These results provide compelling evidence that microanalytical data can be extrapolated from intracrystalline scales to study macroscale processes in complex, large-scale fluid-rock systems.

Relationship between δ13C and δ18O values of the RecentNautilus and brachiopod shells in the wild and the problem of reconstruction of fossil cephalopod habitat

Ontogenetic change in δ18O values of the investigatedNautilus pompilius Linne shell in the Philippines (Tagnan, Panglao Islands, Bohol Island area) confirms data, in whichNautilus in the wild shows a marked increase in oxygen isotopic composition between embryonic and post-embryonic stages. The increase in δ18O in post-embryonic septa reflects a migration into colder, deeper (about 300 m) waters. Judging from the isotopic composition of aboral and adoral parts of the last septum, the amplitude of the short term vertical migration for the investigated sample seems to be about 70 m. Relative fluctuation in δ13C values for the livingNautilus, as well as living brachiopods, might be connected with annual cycles of the phytoplankton development. The negative δ13C excurse (−2.5‰) fixed in the septa 9 of the investigatedNautilus pompilius shell seems to be connected with a weakening of phytoplankton bioproductivity related to low solar activity. It is suggested that ammonoids, like livingNautilus spent the most part of their lives near the sea-floor but unlikeNautilus and late Cretaceous belemnites they did not experience, apparently, significant short-term vertical migration.

Carbon and oxygen isotope microanalysis of carbonate

Technical modification of the conventional method for the delta(13)C and delta(18)O analysis of 10-30 microg carbonate samples is described. The CO(2) extraction is carried out in vacuum using 105% phosphoric acid at 95 degrees C, and the isotopic composition of CO(2) is measured in a helium flow by gas chromatography/isotope ratio mass spectrometry (GC/IRMS). The feed-motion of samples to the reaction vessel provides sequential dropping of only the samples (without the sample holder) into the acid, preventing the contamination of acid and allowing us to use the same acid to carry out very large numbers of analyses. The high accuracy and high reproducibility of the delta(13)C and delta(18)O analyses were demonstrated by measurements of international standards and comparison of results obtained by our method and by the conventional method. Our method allows us to analyze 10 microg of the carbonate with a standard deviation of +/-0.05 per thousand for delta(13)C and delta(18)O. The method has been used successfully for the analyses of the oxygen and carbon isotopic composition of the planktonic and benthic foraminifera in detailed palaeotemperature reconstructions of the Okhotsk Sea.

Sulfur, carbon, and nitrogen stable isotope ratios in soft tissues and trophic relationships of fish from the near-shore waters of the peter the great bay in the Sea of Japan

Stable isotope ratios of sulfur (34S/32S), carbon (13C/12C), and nitrogen (15N/14N) were analyzed in the soft tissues of 12 common species of fish from the near-shore waters of the Peter the Great Bay in the Sea of Japan. The average δ13C values of individual species varied from −20.7‰ for planktivorous fish to −16.8‰ for benthivorous fish, reflecting the growing relative contribution of benthic primary producers to fish nutrition. The majority of the various species representatives studied can be assigned to one trophic level, as indicated by their narrow range of δ15N values (9.9 to 12.6‰). Large interspecific variations were found in the sulfur stable isotope ratios of fish (the mean δ34S values ranged from 11.2 to 19.5‰). This is the result of the different contributions to fish nutrition of infaunal invertebrates that are depleted in 34S due to the microbial food chain of the bottom sediments.

Peculiarities of the oxygen isotope ratio in precious opals

This paper presents the results of the δ18O study of the precious opals from Primor’e (Raduzhnoe deposit), Australia, and Ethiopia and the modern opals from the hydrotherms of the Mendeleev Volcano (Kunashir Island, Kuril Islands). It is established that the oxygen isotope ratio in opals may serve as a criterion for the estimation of their formation temperature. The low-temperature sedimentary opals are relatively enriched in the heavy oxygen isotope independently of the sedimentary or volcanic host rocks. Examples are the Australian and Slovakian opals of the A-type. The hydrothermal opals are enriched in the light oxygen isotope, which depends on the precipitation temperature. The higher the temperature, the lighter the oxygen isotope ratio of the precipitating opal is and the closer it is to that of the hydrothermal fluid.

A continuous flow mass spectrometry technique of argon isotope measurement for K/Ar geochronology

A new method for the measurement of argon isotope composition in a continuous flow of helium for potassium/argon geochronology is described. Extraction of argon from geological samples in multiple-sample holders was carried out in a chamber by heating with a continuous Nd-YAG laser. The extracted and pre-concentrated argon is passed through a chromatographic capillary column in a flow of helium. Argon is separated from possible contaminants in the column and is injected through an open split into the ion source of an isotope ratio mass spectrometer. Measurement of the (36)Ar, (38)Ar and (40)Ar isotopes was carried out in dynamic mode, using a triple-collector ion detector. These experiments have shown that continuous flow mass spectrometry can be used for the analysis of radiogenic argon in picogram quantities with an accuracy that is satisfactory for the solution of many geochronological problems. The method of argon isotope measurement in a continuous flow of helium is an alternative to the measurement of argon isotopes in the static mode. The sensitivity and accuracy of argon measurement by this method are comparable with those provided by the classical static method. The measurement of argon isotopes in a continuous flow of helium is simpler and more reliable than measurement in the static mode.