Abstract Otoliths, the earbones of teleost (bony) fish, are constructed from alternating layers of aragonite and protein. Laser ablation inductively coupled plasma mass spectrometry and proton-induced X-ray emission are used to obtain spatially well-resolved trace element line-scans that show trace-element concentrations are correlated with the annular structure. Zoned Sr and Zn signatures are common whereas other elements such as Cu, Pb, Li and Cs can be related to the proximity of mineral deposits. Aragonite in otoliths can incorporate a wide range of trace elements at the low-ppm level including alkali- and alkaline-earth elements and base metals; Se has also been detected in proximity to coal mines. These trace elements, in combination with the annular structures, are an important archive for recording information on environments occupied by fish, environmental change and exposure to pollutants.
I am pleased to introduce Frank C. Hawthorne as the recipient of the 2013 Roebling medal. Frank is a colleague and friend of some 31 years, someone with whom I have shared a great deal, and someone from whom I have learned a great deal. Frank was an undergraduate in pure geology at the Royal School of Mines, Imperial College, London, and did his Ph.D. in geology at McMaster University. From there, he went to the University of Manitoba for a post-doctoral fellowship in 1973, became a Research Associate in 1975, a federally funded University Research Fellow in 1980, an Associate Professor in 1984, Distinguished Professor in 1997, and was appointed a Canada Research Chair in Crystallography and Mineralogy in 2001, a position that he still holds. I arrived at the University of Manitoba in 1983 to set up the geochemistry laboratories. Frank was responsible for the MAC 5 electron microprobe and had just been awarded funds to buy a single-crystal diffractometer, the first in a geology department in Canada, so we connected quickly over the temperamental behavior of instrumentation and the problems of laboratory space, issues that still concern/annoy us both today.
Frank did his Ph.D. on the crystal chemistry of the amphiboles using a combination of single-crystal X-ray and neutron crystal-structure refinement and Mossbauer spectroscopy, and he has maintained an interest in this area through his whole career. In 1983, he began a collaboration with the Crystallography Group in Pavia: Roberta Oberti, Luciano Ungaretti, and Giuseppi Rossi. This led to his spending a total of four years in Italy, the publication of numerous papers on …
Perovskite (CaTiO3) is a common early crystallizing accessory phase in a variety of alkaline rocks, and has been shown to contain enough U and Th for U-Pb dating. U and Pb analysis of perovskite has been primarily carried out using the SHRIMP or ID-TIMS techniques, and the resulting U-Pb dates commonly yield the emplacement age of the host rock. To our knowledge, only one U-Pb study of perovskite has been done using the LA-ICP-MS (Cox and Wilton, 2006). Some of the advantages of this method over the SHRIMP and ID-TIMS techniques include greater speed and lower cost of analysis.
We examined the mode of occurrence, pattern of zoning and composition of magnetite and associated spinel-group minerals in three types of calciocarbonatite from the Kerimasi volcano, in Tanzania. In all samples, magnetite is one of the earliest phases to have crystallized, and shows an appreciable compositional variation. The majority of compositions correspond to magnetite with low to moderate proportions of magnesioferrite and ulvospinel components (10–28 and 2–28 mol.%, respectively) and 2 O 4 ). The two trace elements consistently present in appreciable amounts are V (400–2000 ppm) and Zn (700–3300 ppm); the abundances of other trace elements are much lower and very variable (≤15 ppm Cr, 170 ppm Ni, 220 ppm Co, 490 ppm Zr, 14 ppm Hf, 95 ppm Nb, 3 ppm Ta, and 80 ppm Ga). Magnetite is thus a minor host of Zr, Hf, Nb and Ta in carbonatites. The composition of magnetite crystallizing from carbonatitic magma evolves by becoming depleted in Mg and Ti, whereas its Al content inversely correlates with the V content and, thus, is sensitive to variations in f (O 2 ). The compatibility of V is interpreted to decrease, and that of Mn to increase, with increasing f (O 2 ). Covariation between the Mn and Zn contents suggests that the partitioning behavior of Zn is controlled by the coupled substitution Zn 2+ Mn 3+ Fe 2+ −1 Fe 3+ −1 . The Mg–Ti depletion trend is accompanied by a decrease in Zr and Ta contents at constant or decreasing levels of Nb and Hf, which has implications for the partitioning behavior of high-field-strength elements in carbonate melts. In addition to the magmatic evolutionary trend, the Kerimasi magnetite exhibits a previously unrecognized trend arising from a reaction of the magnetite with the carbonatitic magma. This trend involves enrichment of the peripheral parts of magnetite crystals in Mg, Al, Mn, Zr and Nb, and their mantling by Fe-rich spinel. This trend requires that a (Mg 2+ ) and a (Al 3+ ) in the magma increase with evolution, whereas a (SiO 2 ) remains low to impede the precipitation of Mg–Al silicates.
Abstract Although gypsum has been predicted to precipitate in sea ice, it has never been observed. Here we provide the first report on gypsum precipitation in both experimental and natural sea ice. Crystals were identified by X‐ray diffraction analysis. Based on their apparent distinguishing characteristics, the gypsum crystals were identified as being authigenic. The FREeZing CHEMistry (FREZCHEM) model results support our observations of both gypsum and ikaite precipitation at typical in situ sea ice temperatures and confirms the “Gitterman pathway” where gypsum is predicted to precipitate. The occurrence of authigenic gypsum in sea ice during its formation represents a new observation of precipitate formation and potential marine deposition in polar seas.
Abstract We report measurements of pH, total alkalinity, air‐ice CO 2 fluxes (chamber method), and CaCO 3 content of frost flowers (FF) and thin landfast sea ice. As the temperature decreases, concentration of solutes in the brine skim increases. Along this gradual concentration process, some salts reach their solubility threshold and start precipitating. The precipitation of ikaite (CaCO 3 .6H 2 O) was confirmed in the FF and throughout the ice by Raman spectroscopy and X‐ray analysis. The amount of ikaite precipitated was estimated to be 25 µmol kg −1 melted FF, in the FF and is shown to decrease from 19 to 15 µmol kg −1 melted ice in the upper part and at the bottom of the ice, respectively. CO 2 release due to precipitation of CaCO 3 is estimated to be 50 µmol kg −1 melted samples. The dissolved inorganic carbon (DIC) normalized to a salinity of 10 exhibits significant depletion in the upper layer of the ice and in the FF. This DIC loss is estimated to be 2069 µmol kg −1 melted sample and corresponds to a CO 2 release from the ice to the atmosphere ranging from 20 to 40 mmol m −2 d −1 . This estimate is consistent with flux measurements of air‐ice CO 2 exchange. Our measurements confirm previous laboratory findings that growing young sea ice acts as a source of CO 2 to the atmosphere. CaCO 3 precipitation during early ice growth appears to promote the release of CO 2 to the atmosphere; however, its contribution to the overall release by newly formed ice is most likely minor.
Abstract. A major issue of Arctic marine science is to understand whether the Arctic Ocean is, or will be, a source or sink for air-sea CO2 exchange. This has been complicated by the recent discoveries of ikaite (CaCO3·6H2O) in Arctic and Antarctic sea ice, which indicate that multiple chemical transformations occur in sea ice with a possible effect on CO2 and pH conditions in surface waters. Here we report on biogeochemical conditions, microscopic examinations and x-ray diffraction analysis of single crystals from an actively melting 1.7 km2 (0.5–1 m thick) drifting ice floe in the Fram Strait during summer. Our findings show that ikaite crystals are present throughout the sea ice but with larger crystals appearing in the upper ice layers. Ikaite crystals placed at elevated temperatures gradually disintegrated into smaller crystallites and dissolved. During our field campaign in late June, melt reduced the ice flow thickness by ca. 0.2 m per week and resulted in an estimated 1.6 ppm decrease of pCO2 in the ocean surface mixed layer. This corresponds to an air-sea CO2 uptake of 11 mmol m−2 sea ice d−1 or to 3.5 ton km−2 ice floe week−1.
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