This paper is focused on the role of boron coordination in determining the B-11/B-10 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 B-11 magic-angle spinning nuclear magnetic resonance (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 B-11/B-10 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 a values are significantly higher than the calculated ones obtained using the reduced partition function ratios (RPFRs) 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 B-11 NMR spectra. Furthermore, a nonlinear relationship is observed between the percentage of BO, in the glass structure and the measured 1000lnalpha, suggesting that the approximation of monomeric B(OH)(3) and B(OH)(4)(-) species contributions through ideal mixing in calculating the RPFRs in polyanions (Oi et al., 1989) probably does not apply to silicate glasses.
Melt/biotite B-11/B-10 isotopic fractionation and the boron local environment in the structure of volcanic glasses
Forte C;
2003
Abstract
This paper is focused on the role of boron coordination in determining the B-11/B-10 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 B-11 magic-angle spinning nuclear magnetic resonance (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 B-11/B-10 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 a values are significantly higher than the calculated ones obtained using the reduced partition function ratios (RPFRs) 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 B-11 NMR spectra. Furthermore, a nonlinear relationship is observed between the percentage of BO, in the glass structure and the measured 1000lnalpha, suggesting that the approximation of monomeric B(OH)(3) and B(OH)(4)(-) species contributions through ideal mixing in calculating the RPFRs in polyanions (Oi et al., 1989) probably does not apply to silicate glasses.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
