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, 30 January 2008, Pages 323–338
Ti-in-zircon thermometry applied to contrasting Archean metamorphic and igneous systems, , , , , a Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australiab Institute of Geology, Chinese Academy of Geological Sciences, 26 Baiwanzhuang Road, Beijing, 100037, ChinaTi-in-zircon thermometry with SHRIMP II multi-collector has been applied to two well-documented Archean igneous and metamorphic samples from southern West Greenland. Zircons from 2.71&Ga partial melt segregation G03/38 formed in a small (& 1&m3), closed system within a mafic rock under high pressure granulite facies conditions. Results of 14 Ti analyses present a mean apparent zircon crystallization temperature of 679 & 11&&C, underestimating independent garnet-clinopyroxene thermometry by 20&50&&C but consistent with reduced aTiO2 in this system. 36 spot analysis on 15 zircons from 3.81&Ga meta-tonalite G97/18, with an estimated magmatic temperature & 1000&&C, yield a low-temperature focused normal distribution with a mean of 683 & 32&&C, further demonstrated by high resolution Ti mapping of two individual grains. This distribution is interpreted to represent the temperature of the residual magma at zircon saturation, late in the crystallization history of the tonalite. Hypothetically, Ti-in-zircon thermometry on Eoarchaean detrital zircons sourced from such a high temperature tonalite would present a low-temperature biased image of the host magma, which could be misconstrued as being a minimum melt granite. Multiple analyses from individual zircons can yield complex Ti distributions and associated apparent temperature patterns, reflecting cooling history and local chemical environments in large magma chambers. In addition to inclusions and crystal imperfections, which can yield apparent high temperature anomalies, zircon surfaces can also record extreme (& 1000&&C) apparent Ti temperatures. In our studies these were traced to 49Ti (or a molecular isobaric interference) contamination derived from the double sided adhesive tape used in sample preparation, and should not be assigned geological significance.KeywordsTi thermometry; Zircon; Archean; Greenland; Isua; Tonalite
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No articles found.This website uses cookies to ensure you get the best experience on our websiteTransmetalation from B to Rh in the course of the catalytic asymmetric 1,4-addition reaction of... - Abstract - Europe PMC
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You-Gui Li
School of Chemistry and Chemical Engineering, Hefei University of Technology, 193 Tunxi Road, Hefei 230009, China. liyg@ ekantchev@ .
Hua-Li Qin
Eric Assen B Kantchev
School of Chemistry and Chemical Engineering, Hefei University of Technology, 193 Tunxi Road, Hefei 230009, China. liyg@ ekantchev@ .
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Transmetalation is a key elementary reaction of many important catalytic reactions. Among these, 1,4-addition of arylboronic acids to organic acceptors such as &,&-unsaturated ketones has emerged as one of the most important methods for asymmetric C-C bond formation. A key intermediate for the B-to-Rh transfer arising from quaternization on a boronic acid by a Rh-bound hydroxide (the active catalyst) has been proposed. Herein, DFT calculations (IEFPCM/PBE0/DGDZVP level of theory) establish the viability of this proposal, and characterize the associated pathways. The delivery of phenylboronic acid in the orientation suited for the B-to-Rh transfer from the very beginning is energetically preferable, and occurs with expulsion of Rh-coordinated water molecules. For the bulkier binap ligand, the barriers are higher (particularly for the phenylboronic acid activation step) due to a less favourable entropy term to the free energy, in accordance with the experimentally observed slower transmetalation rate.
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