8 results on '"Locock, Andrew J."'
Search Results
2. Dense hydrated Mg-silicates in diamond: Implications for transport of H 2 O into the mantle
- Author
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Carvalho, Luísa D. V., primary, Stachel, Thomas, additional, Luth, Robert W., additional, Locock, Andrew J., additional, Pearson, D. Graham, additional, Steele-MacInnis, Matthew, additional, Stern, Richard A., additional, Nestola, Fabrizio, additional, Scholz, Ricardo, additional, Jalowitzki, Tiago, additional, and Fuck, Reinhardt A., additional
- Published
- 2024
- Full Text
- View/download PDF
3. Magnetite-apatite ores record widespread involvement of molten salts
- Author
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Xu, Xinyue, primary, Bain, Wyatt M., additional, Tornos, Fernando, additional, Hanchar, John M., additional, Lamadrid, Hector M., additional, Lehmann, Bernd, additional, Xu, Xiaochun, additional, Steadman, Jeffrey A., additional, Bottrill, Ralph S., additional, Soleymani, Majid, additional, Rajabi, Abdorrahman, additional, Li, Peng, additional, Tan, Xuehai, additional, Xu, Shihong, additional, Locock, Andrew J., additional, and Steele-MacInnis, Matthew, additional
- Published
- 2024
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- View/download PDF
4. Heamanite-(Ce), (K0.5Ce0.5)TiO3, a new perovskite supergroup mineral found in diamond from Gahcho Kué, Canada
- Author
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Anzolini, Chiara, primary, Siva-Jothy, William K., additional, Locock, Andrew J., additional, Nestola, Fabrizio, additional, Balić-Žunić, Tonči, additional, Alvaro, Matteo, additional, Chinn, Ingrid L., additional, Stachel, Thomas, additional, and Pearson, D. Graham, additional
- Published
- 2022
- Full Text
- View/download PDF
5. Incorporating previously neglected excess oxygen associated with ferric iron in matrix corrections of microprobe data from cubic and rhombohedral Fe-Ti oxides.
- Author
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Dungan, Michael A., Donovan, John J., Locock, Andrew J., and Bullock, Emma S.
- Subjects
IRON ,ELECTRON probe microanalysis ,LIGHT elements ,OXYGEN ,OXIDATION states ,OXIDES - Abstract
Estimates of the oxidation states of magmas are important to current investigations of the geochemical characteristics of their source regions and of evolved magmatic series created during differentiation. One means of achieving such estimates is to capitalize on compositions of coexisting cubic and rhombohedral Fe-Ti oxides determined by electron microprobe. A combination of experimental calibration points and thermodynamic modeling provides a basis for translating such compositions into T-ƒ
O2 values. This has been done until recently by estimating Fe3+ /ΣFe on the basis of charge balance and stoichiometry by the method of Droop (1987), after matrix corrections of X-ray intensity data have been performed, as EPMA cannot be used routinely to distinguish different elemental valence states, much less accurately quantify abundances of Fe3+ and Fe2+ . The traditional approach of undertaking post-data-reduction calculations falls short of attaining the best possible quantitative results. The tactical choice of not accounting for light elements that have not been explicitly analyzed prior to matrix corrections of X-ray intensity data leads to systematic errors in reported oxide abundances for measured elements. This article addresses one such issue, the oxygen associated with Fe3+ (hereafter "excess oxygen"), on the basis of coexisting Fe-Ti oxides from Andean lavas. A new software routine in probe for EPMA (PFE) uses an iterative calculation scheme to calculate amounts of excess oxygen that would not be considered if all iron were assumed to be ferrous and then applies this excess oxygen during matrix corrections. The PFE approach reveals that Fe-concentrations have been underestimated, universally, in these minerals because O atoms absorb FeKα radiation: discrepancies increase as total Fe and Fe3+ /Fe2+ , hence excess oxygen, increase. Analyses of the most Fe-rich cubic oxide compositions in this data set have ~6 wt% excess oxygen and ~1 wt% more FeO+Fe2 O3 than would be reported without incorporating the impact of excess oxygen in matrix corrections. Minor to negligible differences in other elements are also observed. These effects are not because excess oxygen is directly attributed to these elements, although some may be present in multiple valence states, as matrix corrections are undertaken on the basis of the conventional assumptions that they occur as Cr3+ , V3+ , Mn2+ , Mg2+ , Ca2+ , and Si4+ . Rather, variably small increases in total Fe propagate through the matrix corrections for other elements, and these differences may be recorded as minor increases or decreases in some concentrations, depending on the particular element and the amount of change in Fe-concentration. Fe3+ /ΣFe in analyses produced with the PFE routine are essentially identical to those determined in the traditional mode, as cation proportions calculated on the basis of charge balance and stoichiometry, with the method of Droop (1987), is a necessary step. The new method: (1) provides more accurate concentrations, mainly for Fe and Ti; (2) is applicable to any mineral containing ferric iron (subject to stoichiometric constraints); (3) provides more accurate analytical totals, which can be advantageous for evaluating analytical quality; and (4) does not impact estimates of oxidation state. Oxygen fugacities and temperatures determined with the model of Ghiorso and Evans (2008) are essentially unchanged. [ABSTRACT FROM AUTHOR]- Published
- 2023
- Full Text
- View/download PDF
6. Heamanite-(Ce), (K0.5Ce0.5)TiO3, a new perovskite supergroup mineral found in diamond from Gahcho Kué, Canada.
- Author
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Anzolini, Chiara, Siva-Jothy, William K., Locock, Andrew J., Nestola, Fabrizio, Balić-Žunić, Tonči, Alvaro, Matteo, Chinn, Ingrid L., Stachel, Thomas, and Graham Pearson, D.
- Subjects
MINERALS ,PEROVSKITE ,DIAMONDS ,RADIOACTIVE dating ,ELECTRON probe microanalysis ,CRYSTAL structure - Abstract
Heamanite-(Ce) (IMA 2020-001), ideally (K
0.5 Ce0.5 )TiO3 , is a new perovskite-group mineral found as an inclusion in a diamond from the Gahcho Kué mine in the Northwest Territories, Canada. It occurs as brown, translucent single crystals with an average maximum dimension of ~80 μm, associated with rutile and calcite. The luster is adamantine, and the fracture conchoidal. Heamanite-(Ce) is the K-analog of loparite-(Ce), ideally (NaCe)Ti2 O6 . The Mohs hardness is estimated to be 5½ by comparison to loparite-(Ce), and the calculated density is 4.73(1) g/cm3 . Electron microprobe wavelength-dispersive spectrometric analysis (average of 34 points) yielded: CaO 10.70, K2 O 7.38, Na2 O 0.16, Ce2 O3 13.77, La2 O3 8.22, Pr2 O3 0.84, Nd2 O3 1.59, SrO 6.69, BaO 2.96, ThO2 0.36, PbO 0.15, TiO2 45.77, Cr2 O3 0.32, Al2 O3 0.10, Fe2 O3 0.09, Nb2 O5 0.87, UO3 0.01, total 99.98 wt%. The empirical formula, based on 3 O atoms, is: [(K0.268 Na0.009 )Σ0.277 (Ce0.143 La0.086 Pr0.009 Nd0.016 )Σ0.254 (Ca0.326 Sr0.110 Ba0.033 Pb0.001 )Σ0.470 Th0.002 ]Σ1.003 (Ti0.979 Nb0.011 Cr0.007 Al0.003 Fe0.002 )Σ1.002 O3 . The Goldschmidt tolerance factor for this formula is 1.003. Heamanite-(Ce) is cubic, space group Pm3m, with unit-cell parameter a = 3.9129(9) Å, and volume V = 59.91(4) Å3 (Z = 1). The crystal structure was solved using single-crystal X‑ray diffraction data and refined to R1 (F) = 2.61%. Heamanite-(Ce) has the aristotypic perovskite structure and adopts the same structure as isolueshite and tausonite. The six strongest diffraction lines are [dobs in angstroms (I in percentages) (hkl)]: 2.764 (100) (110), 1.954 (41) (200), 1.596 (36) (211), 1.045 (16) (321), 1.236 (13) (310), and 1.382 (10) (220). The Raman spectrum of heamanite-(Ce) shows two broad bands at 560 and 787 cm−1 , with no bands observed above 1000 cm−1 . Heamanite-(Ce) is named after Larry Heaman, a renowned scientist in the field of radiometric dating applied to diamond-bearing kimberlites, mantle-derived eclogites, and lamprophyre dikes. The dominant REE should appear as a Levinson suffix, hence heamanite-(Ce). [ABSTRACT FROM AUTHOR]- Published
- 2022
- Full Text
- View/download PDF
7. Magnetite-apatite ores record widespread involvement of molten salts.
- Author
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Xinyue Xu, Bain, Wyatt M., Tornos, Fernando, Hanchar, John M., Lamadrid, Hector M., Lehmann, Bernd, Xiaochun Xu, Steadman, Jeffrey A., Bottrill, Ralph S., Soleymani, Majid, Rajabi, Abdorrahman, Peng Li, Xuehai Tan, Shihong Xu, Locock, Andrew J., and Steele-MacInnis, Matthew
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APATITE , *FUSED salts , *TITANIUM oxides , *ORES , *MAGNETITE , *PRECAMBRIAN , *MINERALS - Abstract
The origins of magnetite-apatite deposits are controversial, and the crux of the debate is what types of fluids form these rocks. We present evidence from 20 magnetite-apatite deposits worldwide showing ubiquitous involvement of molten salts. The studied deposits are distributed globally, from various tectonic settings, and from Precambrian to Quaternary in age. In every case, water-poor polycrystalline melt inclusions in ore-stage minerals are dominated by sulfate, chloride, and carbonate components plus variable proportions of calc-silicates, phosphates, and iron ± titanium oxides that re-melt between 285 °C and 1100 °C. These fluids are very different from what is generally expected in most geologic settings, but their ubiquitous presence in magnetite-apatite rocks indicates that molten salts are widespread and essential to the formation of these deposits. [ABSTRACT FROM AUTHOR]
- Published
- 2024
- Full Text
- View/download PDF
8. Dense hydrated in Mg-silicates diamond: Implications for transport of H2O into the mantle.
- Author
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Carvalho, Luísa D. V., Stachel, Thomas, Luth, Robert W., Locock, Andrew J., Pearson, D. Graham, Steele-MacInnis, Matthew, Stern, Richard A., Nestola, Fabrizio, Scholz, Ricardo, Jalowitzki, Tiago, and Fuck, Reinhardt A.
- Subjects
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MAGNESIUM isotopes , *DIAMONDS , *EARTH'S mantle , *FLUID inclusions , *OXYGEN isotopes , *MINERALS in water , *MINERAL waters - Abstract
Water in Earth's upper mantle is a minor and yet critically important component that dictates mantle properties such as strength and melting behavior. Minerals with stoichiometric water, such as those of the humite group, are important yet poorly characterized potential reservoirs for volatiles in the upper mantle. Here, we report observation of hydroxyl members of the humite group as inclusions in mantle-derived diamond. Hydroxylchondrodite and hydroxylclinohumite were found coexisting with olivine, magnesiochromite, Mg-bearing calcite, dolomite, quartz, mica, and a djerfisherite-group mineral in a diamond from Brazil. The olivine is highly forsteritic (Mg# 97), with non-mantle-like oxygen isotope composition (18O +6.2%), and is associated with fluid inclusions and hydrous minerals--features that could be inherited from a serpentinite protolith. Our results constitute direct evidence for the presence of deserpentinized peridotitic protoliths in subcratonic mantle keels, placing important constraints on the stability of hydrous phases in the mantle and the origin of diamond-forming fluids. [ABSTRACT FROM AUTHOR]
- Published
- 2024
- Full Text
- View/download PDF
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