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Melt/biotite 11B1 10OB isotopic fractionation and the boron local environment in the structure of volcanic glasses. (Articolo in rivista)

Type

Prodotto della ricerca (Classe)
Articolo in rivista (Classe)

Label

Melt/biotite 11B1 10OB isotopic fractionation and the boron local environment in the structure of volcanic glasses. (Articolo in rivista) (literal)

Anno

2003-01-01T00:00:00+01:00 (literal)

Alternative label

TONARINI S., FORTE C., PETRINI R., FERRARA G. (2003)
Melt/biotite 11B1 10OB isotopic fractionation and the boron local environment in the structure of volcanic glasses.
in Geochimica et cosmochimica acta
(literal)

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TONARINI S., FORTE C., PETRINI R., FERRARA G. (literal)

Pagina inizio

1863 (literal)

Pagina fine

1873 (literal)

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This paper addressed an interesting and complicated issue of melt/mineral B isotope fractionation at magmatic temperature in relation to the structural role of B in silicate melts. This is a virtually unexplored field and the possibility to have an important isotopic fractionation during melt/mineral equilibrium is especially important for the interpretation of boron isotope data in geological and environmental (literal)

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http://scienceserver.cilea.it/cgi-bin/sciserv.pl?collection=journals&journal=00167037 (literal)

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26 (literal)

Rivista

Geochimica et cosmochimica acta (Rivista)

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Impact factor: 3,465 (literal)

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This paper is discussing the biotite/melt Boron fractionations, both relative to the concentrations and the isotopic compositions. The data are coming from measurements done on natural rocks that are acidic volcanics from Italy (Roccastrada, Mt. Amiata and Mt Cimini, belonging to the Neogene-Quaternary magmatism of Central Italy). The B concentrations and isotopic composition are measured by TIMS. The link between fractionation and structural environment of Boron is obtained by NMR measurements. The 11B/10B isotopic fractionation between biotite and melt/glass was observed to be large even at magmatic temperatures and was found to be 1.0066 (Roccastrada sample), 1.00535 (Mt. Amiata sample), and 1.00279 (Mt. Cimini sample). Fractionation is mostly related to the relative amount of trigonal and tetrahedral boron sites in the glass network rather than to other processes, including the speciation of hydrous species in the glass structure. The measured values are significantly higher than the calculated ones obtained using the Reduced Partition Function Ratios (RPFR) for B(OH)3 and B(OH)4- as reported in literature. (literal)

Note

ISI Web of Science (WOS) (literal)

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Tonarini S. and Ferrara G.: Istituto di Istituto di Geoscienze e Georisorse, CNR, Via Moruzzi 1, I-56127 Pisa Forte C.: Istituto per i Processi Chimico-Fisici, CNR, Via Moruzzi 1, I-56127 Pisa Petrini R.: Dipartimento di Scienze della Terra, Università di Trieste, via Weiss 8, I-34100 Trieste (literal)

Titolo

Melt/biotite 11B1 10OB isotopic fractionation and the boron local environment in the structure of volcanic glasses. (literal)

Abstract

This paper is focused on the role of boron coordination in determining the 11B/10B isotopic fractionation between melt/glass and biotite at magmatic temperatures. For this purpose three evolved volcanic rocks from Roccastrada, Mt. Amiata and Mt Cimini, belonging to the Neogene-Quaternary magmatism of Central Italy, were studied. In these samples the measured boron biotite-glass partition coefficient ranges between 0.004 and 0.011 indicating that boron behaves as an incompatible element during biotite crystallization. The 11B MAS NMR spectra reveal the presence of trigonal BO3/2 units, tetrahedral BO4/2- sites and three-coordinated BO2/2O- species containing one nonbridging oxygen. The relative contributions of these different boron sites were estimated by spectral deconvolution, and it was observed that the fraction of trigonally coordinated boron decreases with increasing K2O concentration in the glass. The 11B/10B isotopic fractionation between biotite and melt/glass was observed to be large even at magmatic temperatures and was found to be 1.0066 (Roccastrada sample), 1.00535 (Mt. Amiata sample), and 1.00279 (Mt. Cimini sample). Fractionation is mostly related to the relative amount of trigonal and tetrahedral boron sites in the glass network rather than to other processes, including the speciation of hydrous species in the glass structure. The measured ? values are significantly higher than the calculated ones obtained using the Reduced Partition Function Ratios (RPFR) for B(OH)3 and B(OH)4- as reported by Kakihana et al. (1977) and the abundance of trigonal and tetrahedral boron obtained by 11B NMR spectra. Furthermore, a non-linear relationship is observed between the %BO4 in the glass structure and the measured 1000ln?, suggesting that the approximation of monomeric B(OH)3 and B(OH)4- species contribution through ideal mixing in calculating the RPFR in polyanions (Oi et al., 1989) probably does not apply to silicate glasses. The large B isotopic fractionation measured between glass and biotite and its dependence on the boron coordination in the glass, are a limitation to the use of ?11B in the mineral to characterize magmas. Nonetheless, the high incompatible behavior of boron in the most common magmatic minerals rule out that fractional crystallization significantly modified the B isotopic composition of the melt. (literal)

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