Compositional models of the Earth depend on the chemical and petrologic models of peridotite-basalt melting relationship. Thus, the investigation of the upper mantle is fundamental to understand the differentiation processes that gave rise to the crust, starting from the primitive mantle (PM). Li and B importance for such purpose has long been recognised, but their use remained scarce, due to analytical difficulties. Particularly, the concentration of B in PM is still a matter of debate. That is, it seems that only mantle rocks with up to 0.1 ppm B [1, 2], far lower the CI content (0.27-1.20 ppm [3]), are consistent with non-arc basalt sources. One the other hand, upper mantle data, as obtained from peridotites investigations [2, 4] exhibit higher contents, in the chondritic range. This discrepancy probably results from the use of peridotite whose non-metasomatized character has not been established for Li and B. The lack of a comprehensive model describing the behaviour of both elements through various metasomatic processes is the first cause of this unsuitable mantle sampling. The in-situ analysis by ion microprobe (SIMS) permits to measure Li and B in minerals at the 10 ppb level, thus allowing the direct investigation of Li and B in mantle rocks [5]. On the basis of new SIMS data [6], we propose discriminating diagrams for Li and B metasomatic effects in the sub-continental depleted mantle. After identifying non-metasomatised samples, we provide for them the correlative evolution of Li(m) and B(m) vs. Mg#(ol) in mantle minerals (m=opx, cpx, sp, ol), and ascribe the obtained lines to the partial melting evolution. The same samples were subsequently used to derive the PM Li and B contents by reverse modelling of the fusion process. Our data assess that the primitive mantle has a chondritic content for Li (1.6-1.8 ppm), whereas B concentration results to be much lower (0.09-0.11 ppm) than that of CI, as suspected from preliminary data [5]. These new data are consistent with melting models of fertile peridotites, and represent today the most direct confirmation of that was previously an assumption. 1 Leeman and Sisson, 1996 in: Reviews in Mineralogy 33: Boron, Grew E and Anovitz L Eds, p. 645 2 Chaussidon M and Marty B, 1995 Science 269:383 3 McDonough W and Sun S-s, 1995 Chem. Geol. 120:223 4 Higgins M and Shaw D, 1984 Nature 308:172 5 Ottolini L and McDonough W, 1996 J. Conf. Abstracts 1:446 6 Ottolini L et al. submitted to Earth. Planet. Sci. Lett.
SIMS investigation of upper mantle peridotites confirm that the Bulk Earth is chondritic for lithium, not for boron
OTTOLINI L;
2004
Abstract
Compositional models of the Earth depend on the chemical and petrologic models of peridotite-basalt melting relationship. Thus, the investigation of the upper mantle is fundamental to understand the differentiation processes that gave rise to the crust, starting from the primitive mantle (PM). Li and B importance for such purpose has long been recognised, but their use remained scarce, due to analytical difficulties. Particularly, the concentration of B in PM is still a matter of debate. That is, it seems that only mantle rocks with up to 0.1 ppm B [1, 2], far lower the CI content (0.27-1.20 ppm [3]), are consistent with non-arc basalt sources. One the other hand, upper mantle data, as obtained from peridotites investigations [2, 4] exhibit higher contents, in the chondritic range. This discrepancy probably results from the use of peridotite whose non-metasomatized character has not been established for Li and B. The lack of a comprehensive model describing the behaviour of both elements through various metasomatic processes is the first cause of this unsuitable mantle sampling. The in-situ analysis by ion microprobe (SIMS) permits to measure Li and B in minerals at the 10 ppb level, thus allowing the direct investigation of Li and B in mantle rocks [5]. On the basis of new SIMS data [6], we propose discriminating diagrams for Li and B metasomatic effects in the sub-continental depleted mantle. After identifying non-metasomatised samples, we provide for them the correlative evolution of Li(m) and B(m) vs. Mg#(ol) in mantle minerals (m=opx, cpx, sp, ol), and ascribe the obtained lines to the partial melting evolution. The same samples were subsequently used to derive the PM Li and B contents by reverse modelling of the fusion process. Our data assess that the primitive mantle has a chondritic content for Li (1.6-1.8 ppm), whereas B concentration results to be much lower (0.09-0.11 ppm) than that of CI, as suspected from preliminary data [5]. These new data are consistent with melting models of fertile peridotites, and represent today the most direct confirmation of that was previously an assumption. 1 Leeman and Sisson, 1996 in: Reviews in Mineralogy 33: Boron, Grew E and Anovitz L Eds, p. 645 2 Chaussidon M and Marty B, 1995 Science 269:383 3 McDonough W and Sun S-s, 1995 Chem. Geol. 120:223 4 Higgins M and Shaw D, 1984 Nature 308:172 5 Ottolini L and McDonough W, 1996 J. Conf. Abstracts 1:446 6 Ottolini L et al. submitted to Earth. Planet. Sci. Lett.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
