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In situ δ7Li, Li/Ca, and Mg/Ca analyses of synthetic aragonites (2011)

Gabitov, R. I. ; Schmitt, A. K. ; Rosner, Martin ; [et al.]

American Geophysical Union

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Publication Date: 2022-05-26

Description: Author Posting. © American Geophysical Union, 2011. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 12 (2011): Q03001, doi:10.1029/2010GC003322.

Description: In situ secondary ion mass spectrometry (SIMS) analyses of δ7Li, Li/Ca, and Mg/Ca were performed on five synthetic aragonite samples precipitated from seawater at 25°C at different rates. The compositions of δ7Li in bulk aragonites and experimental fluids were measured by multicollector inductively coupled plasma–mass spectrometry (MC-ICP-MS). Both techniques yielded similar δ7Li in aragonite when SIMS analyses were corrected to calcium carbonate reference materials. Fractionation factors α7Li/6Li range from 0.9895 to 0.9923, which translates to a fractionation between aragonite and fluid from −10.5‰ to −7.7‰. The within-sample δ7Li range determined by SIMS is up to 27‰, exceeding the difference between bulk δ7Li analyses of different aragonite precipitates. Moreover, the centers of aragonite hemispherical bundles (spherulites) are enriched in Li/Ca and Mg/Ca relative to spherulite fibers by up to factors of 2 and 8, respectively. The Li/Ca and Mg/Ca ratios of spherulite fibers increase with aragonite precipitation rate. These results suggest that precipitation rate is a potentially important consideration when using Li isotopes and elemental ratios in natural carbonates as a proxy for seawater composition and temperature.

Description: SIMS analyses were supported by U.S. NSF, EAR, Instrumentation and Facilities Program. The development of the method for bulk d7Li analysis and the MC‐ICP‐MS measurements were covered by NSF grant EAR/IF‐0318137. Precipitation experiments were supported by NSF through grants OCE‐0402728, OCE‐0527350, and OCE‐0823527 to Glenn Gaetani and Anne Cohen and through grant EAR‐0337481 to Bruce Watson.

Keywords: Isotope ; Aragonite ; Rate ; SIMS ; Magnesium ; Lithium

Repository Name: Woods Hole Open Access Server

Type: Article

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Diffusion of phosphorus in olivine and molten basalt (2015)

Watson, E. B., Cherniak, D. J., Holycross, M. E.

Mineralogical Society of America

In: American Mineralogist

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Publication Date: 2015-10-02

Description: The diffusivity of phosphorus in San Carlos olivine (SCO) was measured at near-atmospheric pressure and 650–850 °C by in-diffusion of P from a surface powder source consisting of pre-reacted SCO and AlPO 4 . The experiments were conducted in evacuated silica-glass ampoules at oxygen fugacities fixed by solid-state buffers, generally Ni-NiO but also including two experiments buffered at wüstite-magnetite. Phosphorus uptake profiles were characterized by Rutherford backscattering spectroscopy (RBS) and nuclear reaction analysis (NRA). The temperature dependence of P diffusion in SCO conforms to the expected Arrhenius relation D = D 0 exp(– E a /R T ), where the constants are as follows: log( D 0 , m 2 /s) = –10.06 ± 0.80 and E a = 229 ± 16 kJ/mol. These values characterize P as a relatively slow diffuser in olivine—slower by about an order of magnitude than Cr and Ca at basalt near-liquidus temperatures—but substantially faster than Si. With a view toward modeling P uptake during rapid growth of natural olivines, P diffusion was also characterized in dry MORB basalt melt over the temperature range 1250–1500 °C at 1 GPa, using traditional diffusion couples contained in graphite. Phosphorus diffusion profiles in the quenched and depressurized samples were quantified by laser-ablation ICP/MS. Phosphorus diffusion in basaltic melt is similar to that of Si, with log( D 0 , m 2 /s) = –6.30 ± 0.7 and E a = 147 ± 22 kJ/mol. The new data for P diffusion in olivine and basalt melt can be used to explore the acquisition of fine-scale zoning in natural olivine phenocrysts through kinetic models, as well as the survival of P zoning in olivine with time spent at elevated temperature. Models of growth entrapment of a P-enriched near-surface layer in the olivine lattice indicate that crystal growth at plausible sustained rates is indeed likely to result in regions of anomalously high P content in the resulting crystal. Phosphorus concentrations above the equilibrium partitioning value can also result from development of a diffusive boundary layer in the melt against a rapidly growing crystal, but this mechanism is ineffective at typical sustained olivine growth rates, requiring dendrite-forming growth speeds. Preservation of P zoning on the scale of a few micrometers apparently requires cooling within a few months of formation of the zoning.

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Electronic ISSN: 1945-3027

Topics: Geosciences

Published by Mineralogical Society of America

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Diffusion of helium in natural monazite, and preliminary results on He diffusion in synthetic light rare earth phosphates (2013)

Cherniak, D. J., Watson, E. B.

Mineralogical Society of America

In: American Mineralogist

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Publication Date: 2013-08-20

Description: Diffusion of helium has been characterized in natural monazite and synthetic LREE phosphates. Polished slabs of natural monazite and flat growth faces of synthetic phosphates were implanted with 100 keV 3 He at a dose of 5 x 10 15 3 He/cm 2 and annealed in 1 atm furnaces. 3 He distributions in monazites following experiments were measured with nuclear reaction analysis using the reaction 3 He(d,p) 4 He. For diffusion in monazite we obtain the following Arrhenius relation for diffusion normal to (100) \[ \hbox{ D }=1.60\times {10}^{-7}\hbox{ exp }(-150\pm 8\hspace{0.17em}\hbox{ kJ }/\hbox{ mol }/\hbox{ RT })\hspace{0.17em}{\hbox{ m }}^{2}/\hbox{ s }. \] Diffusion in natural monazite exhibits little diffusional anisotropy, as diffusion normal to (001), (100), and (010) appear similar, as do diffusivities of He in natural monazites from two different localities. Over the investigated temperature range, these diffusivities are similar to those of Farley and Stockli (2002) , and values obtained for some of the monazite grains analyzed by Boyce et al. (2005) , obtained through bulk-release of He by step heating, and are bracketed by diffusivities obtained by Farley (2007) for monazite-structure synthetic REE phosphates. Helium diffusion measured in synthetic REE phosphates (LaPO 4 , NdPO 4 , SmPO 4 , and EuPO 4 ) with the monazite structure appears to behave systematically, with diffusion in the lighter REE phosphates slightly faster than in the heavier REE phosphates. This trend is in broad agreement with the findings of Farley (2007) , but the difference in diffusivities among the REE phosphates is smaller than that reported by Farley (2007) . Activation energies for He diffusion in the synthetic LREE phosphates are similar to that for natural monazite measured in this study, but He diffusivities in synthetic LaPO 4 are about an order of magnitude faster than in the natural monazites. The differences in He diffusivities among the natural monazites and synthetic REE phosphates may be a consequence of the density and distribution of interstitial apertures in the crystal structure, which may result in differences in He migration rates. The similarities in He diffusivities for natural monazites containing varying mixtures of rare-earth elements, as well as differences in Th contents, however, indicate that the above Arrhenius relation is most relevant for describing He diffusion in monazite in nature. We use this Arrhenius relation to evaluate He retentivity in monazite experiencing thermal events, using both the closure temperature formulation of Dodson (1973) and recently developed expressions ( Watson and Cherniak 2013 ) for prograde thermal events and heating and cooling trajectories. Calculations indicate that monazite is relatively retentive of He compared with accessory mineral phases.

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Kinetic Fractionation of Non-Traditional Stable Isotopes by Diffusion and Crystal Growth Reactions (2017)

Watkins, J. M., De; Paolo, D. J., Watson, E. B.

Mineralogical Society of America

In: Reviews in Mineralogy and Geochemistry

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Publication Date: 2017-02-01

Print ISSN: 1529-6466

Electronic ISSN: 1943-2666

Topics: Geosciences

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Experimental investigation into the substitution mechanisms and solubility of Ti in garnet (2017)

Ackerson, M. R., Watson, E. B., Tailby, N. D., Spear, F. S.

Mineralogical Society of America

In: American Mineralogist

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Publication Date: 2017-01-04

Description: Garnet is a common and important mineral in metamorphic systems, but the mechanisms by which it incorporates Ti—one of the major elements in the crust—are not well constrained. This study draws upon garnets synthesized at a range of temperatures and pressures to understand Ti solubility and the substitution mechanisms that govern its incorporation into garnet at eclogite and granulite facies conditions. Garnets from these synthesis experiments can incorporate up to several wt% TiO 2 . Comparison of Ti content with deficits in Al and Si in garnet indicates that Ti is incorporated by at least two substitution mechanisms ( VI Ti 4+ + VI M 2+ 2 VI Al 3+ , and VI Ti 4+ + IV Al 3+ VI Al 3+ + IV Si 4+ ). Increasing Ti solubility is correlated with increasing Ca and Fe/Mg ratios in garnet, clinopyroxene and melt. The complexity of the substitution mechanisms involved in Ti solubility in garnet makes practical Ti-in-garnet thermobarometry infeasible at present. However, a model fit to Ti partitioning between garnet and melt can be used to predict melt compositions in high-grade metamorphic systems. Additionally, the solubility and substitution mechanisms described here can help explain the presence of crystallographically aligned rutile needles in high-grade metamorphic systems.

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XAFS spectroscopic study of Ti coordination in garnet (2017)

Ackerson, M. R., Tailby, N. D., Watson, E. B.

Mineralogical Society of America

In: American Mineralogist

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Publication Date: 2017-01-04

Description: Titanium can be incorporated either tetrahedrally ( IV Ti) or octahedrally ( VI Ti) in most silicate minerals. Ti K -edge X-ray absorption fine structure (XAFS) spectroscopy enables observation of Ti coordination in minerals and melts. In this study, XAFS is used to determine the coordination of Ti in synthetic and natural garnets. Garnets grown synthetically at eclogite- and granulite-facies conditions can contain several wt% TiO 2 , most of which is incorporated as VI Ti. This observation aligns with major element trends in these garnets. In natural garnets grown at lower temperatures and pressures, on the other hand, Ti is observed to occupy both the octahedral and tetrahedral sites in garnet—in some cases Ti is almost entirely fourfold coordinated. Combined with previous research (see Ackerson et al. 2017, this issue) on substitution mechanisms for VI Ti, the results of this study demonstrate that Ti is incorporated on two crystallographic sites in garnet by at least three primary substitution mechanisms. In both natural and synthetic garnets, there is a discernible increase in VI Ti content in garnet with increasing temperature and pressure, suggesting a significant role for these two parameters in determining Ti solubility. However, a continuous increase in VI Ti with increasing grossular content also suggests that the Ca content of the garnet plays a critical role.

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Electronic ISSN: 1945-3027

Topics: Geosciences

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