There is a general consensus that melts erupted on the seafloor are not in equilibrium with the residual mantle under low-pressures (Stolper, 1980). To preserve the original geochemical signature, the primitive melts must rise without reequilibrating with surrounding peridotite. This may be accomplished by replacive mantle dunites (Kelemen et al., 1995). These rocks represent the main pathways for the extraction of the primitive mantle-derived melts and are expected to preserve the chemical characteristics of the parental melts acquired upon mantle melting. This contribution discusses the chemistry of olivine from MORB-type replacive mantle dunites exposed at the Lanzo South Massif. Three different analytical techniques were used to obtain a complete picture of the chemistry of olivine from these rocks, by coupling Laser Ablation and Secondary Ion Mass Spectrometry with Fourier Transform Infrared Spectroscopy. Minor (Ni, Mn and Co) and trace (Sc, V, Ti, Zr, Y and HREE) element compositions of the dunite olivines are consistent with formation by interaction between shallow peridotites and MORB-type melts (Piccardo et al., 2007). Chemical differences between the different dunite bodies led us to propose that the melts extracted through these high permeability conduits were melt batches not fully aggregated after their formation in the asthenospheric source. Positive correlations among the concentrations of Ni, Mn, Co, Sc and V indicate equilibration with melts produced under different pressure conditions (Sanfilippo et al., 2014). New determinations of H, Li and B in the dunite olivines are consistent with this idea and suggest the involvement of a garnet-bearing component in the source of MORB. The chemistry of olivine from replacive mantle conduits is thus a powerful tool to explore the mode of partial melting of the MORB mantle, preserving a chemical heterogeneity not entirely documented in erupted melts. Kelemen P.B., Shimizu N. & Salters V. 1995. Extraction of mid-ocean ridge basalt from the upwelling mantle by focussed flow of melt in dunite channels. Nature, 375, 747-753. Piccardo G.B., Zanetti A., Poggi E., Spagnolo G. & Muntener O. 2007. Melt/peridotite interaction in the Southern Lanzo peridotite: field, textural and geochemical evidence. Lithos, 94, 181-209. Sanfilippo A., Tribuzio R. & Tiepolo M. 2014. Mantle-crust interaction in the oceanic lithosphere: constraints from minor and trace elements in olivine. Geoch. Cosmoch. Acta, 141, 423-439. Stolper E. 1980. A phase diagram for mid-ocean ridge basalts: Preliminary results and implications for petrogenesis. Contrib. Mineral. Petrol., 74, 13-27.
Olivine from replacive mantle bodies reveals modes of partial melting of the MORB mantle source
Ottolini L;
2015
Abstract
There is a general consensus that melts erupted on the seafloor are not in equilibrium with the residual mantle under low-pressures (Stolper, 1980). To preserve the original geochemical signature, the primitive melts must rise without reequilibrating with surrounding peridotite. This may be accomplished by replacive mantle dunites (Kelemen et al., 1995). These rocks represent the main pathways for the extraction of the primitive mantle-derived melts and are expected to preserve the chemical characteristics of the parental melts acquired upon mantle melting. This contribution discusses the chemistry of olivine from MORB-type replacive mantle dunites exposed at the Lanzo South Massif. Three different analytical techniques were used to obtain a complete picture of the chemistry of olivine from these rocks, by coupling Laser Ablation and Secondary Ion Mass Spectrometry with Fourier Transform Infrared Spectroscopy. Minor (Ni, Mn and Co) and trace (Sc, V, Ti, Zr, Y and HREE) element compositions of the dunite olivines are consistent with formation by interaction between shallow peridotites and MORB-type melts (Piccardo et al., 2007). Chemical differences between the different dunite bodies led us to propose that the melts extracted through these high permeability conduits were melt batches not fully aggregated after their formation in the asthenospheric source. Positive correlations among the concentrations of Ni, Mn, Co, Sc and V indicate equilibration with melts produced under different pressure conditions (Sanfilippo et al., 2014). New determinations of H, Li and B in the dunite olivines are consistent with this idea and suggest the involvement of a garnet-bearing component in the source of MORB. The chemistry of olivine from replacive mantle conduits is thus a powerful tool to explore the mode of partial melting of the MORB mantle, preserving a chemical heterogeneity not entirely documented in erupted melts. Kelemen P.B., Shimizu N. & Salters V. 1995. Extraction of mid-ocean ridge basalt from the upwelling mantle by focussed flow of melt in dunite channels. Nature, 375, 747-753. Piccardo G.B., Zanetti A., Poggi E., Spagnolo G. & Muntener O. 2007. Melt/peridotite interaction in the Southern Lanzo peridotite: field, textural and geochemical evidence. Lithos, 94, 181-209. Sanfilippo A., Tribuzio R. & Tiepolo M. 2014. Mantle-crust interaction in the oceanic lithosphere: constraints from minor and trace elements in olivine. Geoch. Cosmoch. Acta, 141, 423-439. Stolper E. 1980. A phase diagram for mid-ocean ridge basalts: Preliminary results and implications for petrogenesis. Contrib. Mineral. Petrol., 74, 13-27.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
