MELT-ROCK INTERACTION AT THE MANTLE-CRUST TRANSITION ZONE IN THE OCEANIC SPREADING LITHOSPHERE: AN EXPERIMENTAL STUDY

Studies on oceanic lithosphere suggest that melt-rock reactions play a key role in the origin of olivine-rich troctolites. We performed reactive dissolution and crystallization experiments in a piston-cylinder in order to provides experimental constraints on the formation of olivine-rich troctolites at the mantle-crust transition zone through melt-rock reaction. Experimental charges consist of three-layers samples made by: (1) basalt glass powder, (2) powder of San Carlos olivine (Fo90) mixed with 9% of basalt, and (3) melt trap. We used 3 synthetic MORB-type glasses (XMg = Mg/(Mg+Fe) = 0.74, 0.62 and 0.58). Experiments have been conducted at 0.5 and 0.7 GPa, following an isobaric step-cooled temperature path from 1300 to 1150 °C, to induce reactive dissolution of olivine and in-situ crystallization of interstitial phases from the reacted melt. Time-solved isothermal time-solved experiments have also been performed at 0.5 and 0.7 GPa and 1200 to 1300 °C for various durations from 20 minutes to 60 hrs. The effect of pressure, melt composition and time on melt-rock reaction have been evaluated by textural observations, phase abundance and mineral chemistry. Moreover, dunite infiltration experiments with a dissolution couple made of sintered dunite rod and basalt powder at 0.7 GPa and 1150 to 1250 °C have been also performed, with the aim of evaluating melt transport properties within dunite matrix. Step-cooled experiments show a layered lithological sequence from olivine-gabbro, to olivine-rich troctolite or dunite. In troctolite and dunite, plagioclase and clinopyroxene poikiloblasts include small rounded olivines but also euhedral to subhedral embayed olivines: evidences of disequilibrium and dissolution. Vermicular glass remains as interstitial phases within the troctolite. Isothermal experiments consist of olivine and glass with olivine habits similar to those observed in step-cooled runs. A textural homogenization is observed as function of run duration, driven by a more regular repartition of grain size and shape testified by recrystallized “smooth” and large subhedral olivine with curvilinear to euhedral grain boundaries. Mineral chemistry significantly varies within the olivine-rich layer along the experimental charge. Olivine ranges from Fo82 at the olivine-gabbro/troctolite interface to Fo90 in the troctolite layer. Clinopyroxene XMg increases from 0.63 to 0.90. Plagioclase composition varies from An50 to An75 , and it tends to increase going far from the interface. As expected, large anorthite variations are coupled with rather constant forsterite values, deviating from fractional crystallization paths. In isothermal experiments, olivine ranges from Fo86 to Fo90 along the olivine-rich layer. Melt compositions vary as function of starting material and run duration. Run duration favors the extent of melt-rock reaction, leading to a chemical homogenization of reacted melts and olivine compositions. Only slow diffuser elements, such as Al and Ti still remain highly heterogeneous in the reacted melt. In the troctolite, higher abundances of interstitial phases are observed at lower pressure suggesting higher extent of olivine dissolution. At higher pressure, early crystallization of interstitial reacted melts, due to stability of clinopyroxene and plagioclase at higher temperature, inhibits the reaction. TEM and EPMA investigations on dunite infiltrations highlighted that melt percolation occurs by the dissolution of olivine at grain boundaries. Melt percolates along planar crystalline surfaces with straight contacts such as “layers”, dissolves olivine creating lobate rims. Olivine dissolution and recrystallization by the reactive melt is localized at the dunite-melt reservoir boundary where olivine varies from Fo75 to Fo90. Experiments are texturally and chemically comparable with olivine-rich troctolites from natural study cases, supporting their origin by melt-olivine reaction.