HR: 16:15h
AN: V22H-02    [PDF]
TI: Thermochemical Erosion of Mantle Lithosphere by Melt/Rock reaction in the Lanzo peridotites (Italy), and Implications for the Transition from Rifting to Ultra-Slow Seafloor Spreading
AU: * M\"{u}ntener, O
EM: othmar.muentener@unine.ch
AF: Geology Institute, University of Neuchatel, Rue Emile Argand, 11, Neuchatel, 2007 Switzerland
AU: Piccardo, G B
EM: piccardo@dipteris.unige.it
AF: Dipteris, University of Genova, Corso Europa, 26, Genova, 16132 Italy
AU: Pettke, T
EM: pettke@erdw.ethz.ch
AF: Isotope geochemisty and mineral resources, ETH Zurich, Sonneggstrasse, 6, Zurich, 8092 Switzerland
AU: Zanetti, A
EM: zanetti@crystal.unipv.it
AF: CNR-Istituto di Geoscienze e Georisorse, University of Pavia, via Ferrata, 1, Pavia, 27100 Italy
AB: A review of existing data together with recent field, petrographic, petrologic and geochemical data from the Lanzo peridotites in northern Italy provides new ideas on the scales and causes of chemical heterogeneitiy in mantle rocks evolving from rifting to ultraslow seafloor spreading. We propose that large masses of plagioclase peridotite with a fertile composition in Lanzo and elsewhere are hybrid rocks created by melt infiltration and melt/rock reaction superposed on an 'old' lithospheric history. The collected dataset allows us to address the igneous history of the Lanzo peridotite after their accretion to the conductive lithosphere and prior to their seafloor exposure in mid Jurassic time. Our data show that diffuse porous flow of ascending liquids produced by the underlying hot asthenosphere dissolved clinopyroxene (cpx)+spinel and precipitated orthopyroxene (opx)+plagioclase (plg)+olivine, forming opx+plg-rich peridotite. Migrating liquids became progressively saturated in cpx, and then precipitated microgranular aggregates of cpx-bearing gabbronorite. Melt/rock reaction in the stability field of plagioclase produced variably depleted light rare earth element (REE) patterns in cpx (La$_{N}$/Sm$_{N}$ and La$_{N}$/Yb$_{N}$ in the range of 0.006-0.25 and 0.006-0.36, respectively) with high absolute middle REE contents (Gd$_{N}$: 11 to 29 chondrite), Gd$_{N}$/Yb$_{N}$ $>$ 1 and negligible to significant negative Eu anomalies. Sr$_{N}$ always shows a marked negative anomaly. Migrating liquids, which infiltrated peridotite and formed gabbroic rocks span a wide range of compositions from silica-rich single melt fractions to T- and N-MOR basalts, characteristic of the melting column beneath mid-ocean-ridges. Later, diffuse porous melt flow was replaced by focused porous flow, producing a system of discordant dunite bodies. Upon cooling, liquids migrating in dunite channels became progressively saturated in cpx and plg, forming interstitial cpx at olivine triple points followed by cpx+plg megacrysts and gabbro veinlets within the dunite, and gabbro dykelets within plagioclase peridotites. Calculated liquids in equilibrium with cpx have REE slopes and concentrations similar to MORB crystallised from low percentage aggregate liquids (less than 5%). Subsequent cooling was accompanied by intrusion of km-scale gabbroic dikes evolving from troctolite to Mg-Al and Fe-Ti gabbros. Explanations for the progressive evolution of the melt migration mechanism from diffuse to focused porous flow of melt and finally diking include the competing effects of heating of the thermal lithosphere by ascending magmas from the hot asthenosphere below and conductive cooling by exhumation from above. The change in the melt migration mechanism suggest that the rheology of the mantle was modified from 'lithospheric' to 'asthenospheric' and back again. It is possible that the softening of the thermal lithosphere by melt impregnation could have played a fundamental role in the mechanisms of extension and the establishment of an ultra-slow spreading ridge.
DE: 3035 Midocean ridge processes
DE: 3640 Igneous petrology
DE: 3670 Minor and trace element composition
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8434 Magma migration
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting