The results, which relied on the direct Secondary Ion Mass Spectrometry (SIMS) analysis of Li and B in mantle minerals (cpx, opx, ol and sp) at CNR-IGG (Pavia), indicated the following: - The major host mineral phase for Li is, in general, ol, whereas for B it is cpx. Sp is confirmed to be very poor in B. The main carrier for both Li and B is ol owing to its high modal abundance. - The combination of our graph (Ce/B)cpx vs. (Li/Yb)cpx with that of Licpx (ppm) vs. Liol (ppm) is able to distinguish the metasomatized samples, taking as an un-metasomatized reference the domain where the sub-continental depleted mantle (SCDM) samples plot. In particular Fig. 4a of the paper allows for a discrimination between fluid and magma influences, while Fig. 4c can be used to identify the effect of carbonatitic magmas from that of silicate ones, as represented in this study by sample Z7. - The calculations based on samples most representative of the SCDM, indicate that the average composition of Li in the pyrolitic mantle is chondritic, whereas the lower B content (0.07-0.09 ppm), is below the chondritic values. - Li distribution between major mineral phases in mantle rocks is independent of rock chemical composition, which agrees with literature data. - Two-mineral partition coefficients for B confirm that cpx is the main host phase for this element.
The importance of Li and B geochemistry has long been recognised, owing specifically to their characteristic behaviours during processes involving fluid phases. However, the lack of a set of validated reference data for the "normal" Earth mantle has hampered the development of models for Li and B metasomatic effects on mantle rocks. In particular, the concentration of B in the mantle is still a matter of debate. An estimate of 0.1 ppm B seems to be consistent as a source for non-arc basalts, but to date such data have not been directly confirmed. Li and B literature contents for peridotites are derived from samples whose nonmetasomatized character has not been established for both elements, due to the aforementioned lack of a comprehensive metasomatism model for Li and B. We have looked at two groups of mantle rocks, with and without clear compositional evidence that they are metasomatized. The latter rocks provide the best constraints on "normal" mantle B and Li contents. We propose a diagnostic diagram based on (Ce/B) vs. (Li/Yb) as measured by SIMS in peridotite clinopyroxenes, which is useful in identifying metasomatized samples. After discovering samples with no metasomatic alteration, which are considered representative of the "normal" mantle, we derived for each mantle mineral phase (ol, opx, cpx and sp) the Mg#, Li and B partial-melting evolution trends. Additionally, considering that the "normal" mantle rocks have evolved through partial melting only, we assessed Li and B contents in the parental mantle of our samples, and assumed that the calculated values (1.6-1.8 ppm Li and 0.07-0.10 ppm B) are representative of the contents in MORB mantle sources. These new data are consistent with current melting models of fertile peridotites.
Direct assessment of mantle boron and lithium contents and distribution by SIMS analyses of peridotite minerals.
Ottolini L;
2004
Abstract
The importance of Li and B geochemistry has long been recognised, owing specifically to their characteristic behaviours during processes involving fluid phases. However, the lack of a set of validated reference data for the "normal" Earth mantle has hampered the development of models for Li and B metasomatic effects on mantle rocks. In particular, the concentration of B in the mantle is still a matter of debate. An estimate of 0.1 ppm B seems to be consistent as a source for non-arc basalts, but to date such data have not been directly confirmed. Li and B literature contents for peridotites are derived from samples whose nonmetasomatized character has not been established for both elements, due to the aforementioned lack of a comprehensive metasomatism model for Li and B. We have looked at two groups of mantle rocks, with and without clear compositional evidence that they are metasomatized. The latter rocks provide the best constraints on "normal" mantle B and Li contents. We propose a diagnostic diagram based on (Ce/B) vs. (Li/Yb) as measured by SIMS in peridotite clinopyroxenes, which is useful in identifying metasomatized samples. After discovering samples with no metasomatic alteration, which are considered representative of the "normal" mantle, we derived for each mantle mineral phase (ol, opx, cpx and sp) the Mg#, Li and B partial-melting evolution trends. Additionally, considering that the "normal" mantle rocks have evolved through partial melting only, we assessed Li and B contents in the parental mantle of our samples, and assumed that the calculated values (1.6-1.8 ppm Li and 0.07-0.10 ppm B) are representative of the contents in MORB mantle sources. These new data are consistent with current melting models of fertile peridotites.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
