Giorgio Ferrara

Melt/Biotite 11B/10B isotopic fractionation and the boron local environment in the structure of volcanic glasses.

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 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 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 α 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 11B NMR spectra. Furthermore, a nonlinear relationship is observed between the percentage of BO4 in the glass structure and the measured 1000lnα, 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. 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 δ11B in the mineral to characterize magmas. Nonetheless, the high incompatible behavior of boron in the most common magmatic minerals rules out that fractional crystallization significantly modified the B isotopic composition of the melt.

Flood basalts of the Gedo Region (Southern Somalia): Geology, petrology and isotope geochemistry

Basaltic flows and sills occur in the Gedo region and in the Shebelle valley (Buulobarde area) of the western-central part of Somalia. These basalts have a subalkaline-transitional tholeiitic character but, on the basis of geochemical and isotope data, three different types can be evidenced. The basalts of Gedo region along the Juba river, from Luuq to Baardheere, are different from those of the Laaf Maakada plateau due to their slightly higher SiO2 and lower FeOt and TiO2 contents, alkalis and Mg values being almost equivalent. The Webe Shebelle basalts, as indicated by their lower Mg values, can be classified as more evolved rocks; they also exhibit slightly higher TiO2 content. The different groups cannot have been derived by simple fractionation processes, as indicated by the87Sr/86Sr isotope composition which ranges from 0.70388 to 0.70530. These variations are correlated with Rb and K2O values, suggesting a crustal contamination process. The Laaf Maakada basalts, characterized by the lowest SiO2, Rb, K2O contents and minimum87Sr/86Sr value, may represent the less contaminated products.RiassuntoNella regione di Gedo e nella valle del Webe Shebelle (Area Buulobarde) affiorano colate e «sills» di basalti subalcalini a carattere transizionale-tholeiitico. I dati geochimici ed isotopici permettono di distinguere tre diversi tipi di basalti. I basalti della regione di Gedo, lungo il fiume Giuba, da Luuq a Baardheere, differiscono da quelli del plateau di Laaf Maakada a causa dei valori leggermente più aid in SiO2 e più bassi in FeOt e TiO2, mentre molto simili sono i valori degli alcali e di MgO. I basalti di Webe Shebelle corrispondono a termini più evoluti, come indicato dai bassi «Mg values»; essi hanno anche un contenuto in TiO2 leggermente più alto. Questi diversi gruppi non possono essere derivati l’uno dall’altro mediante proccssi di semplice frazionamento dato ehe la composizione isotopica 87Sr/86Sr varia ira 0.70388 a 0.70530. Tali variazioni sono correlate con i valori di Rb e K2O e suggeriscono processi di contaminazione crostale. I basalti di Laaf Maakada, caratterizzati dai più bassi contenuti in SiO2, Rb, K2O e dai più bassi rapporti87Sr/86Sr dovrebbero corrispondere ai prodotti meno contaminati.