THE ROLE OF MELT/OLIVINE RATIO IN DISSOLUTION AND REACTIVE CRYSTALLIZATION: AN EXPERIMENTAL AND MICROSTRUCTURAL STUDY (ELECTRON BACK-SCATTERED DIFFRACTION - EBSD) AT 0.5 GPA.

The role of melt/olivine ratio in dissolution and reactive crystallization: an experimental and microstructural study (Electron Back-Scattered Diffraction - EBSD) at 0.5 GPa. ABSTRACT Microstructural and geochemical studies have emphasized that melt-rock reactions play a paramount role in the origin of gabbroic rocks in the lower oceanic crust. It is widely accepted that olivine-rich troctolites might form through dunite infiltrated followed by reactive crystallization of interstitial melts. Furthermore, experiments on the origin of olivine-rich troctolites have suggested that melt/olivine ratio is a fundamental factor in basalt-dunite interactions (Borghini et al., 2018). However, the effect of melt/rock ratio on the chemistry and the development of microstructures induced by reactive dissolution and crystallization processes have not been well defined yet. This work aims to experimentally evaluate and quantify the effect of melt/olivine ratio on mineral chemistry, melt evolution and microstructural development of olivine-rich troctolites formed by reactive processes. Electron Back-Scattered Diffraction (EBSD) analysis gives the opportunity to parametrize microstructures as a function of different melt/olivine ratio. Ten experiments were performed in piston cylinder apparatus at 0.5 GPa using San Carlos olivine (Fo90) and a MORB-type glass (AH6) as starting materials, mixed together in different proportions (i.e., the Initial Melt Amount, IMA, was equal to 10, 25 or 50wt%). Out of ten experiments, seven were isothermal runs at 1200, 1250 and1300°C, lasted 24 hours, and three were step-cooled runs carried out by lowering the temperature, with a fix rate of 1°C/min, from an isothermal dwell (at 1300°C) down to 1100°C. Run products of isothermal experiments at 1200°C consist of olivine, plagioclase, clinopyroxene and traces of glass, whereas at 1300°C consist only of olivine and glass. Texturally we distinguish two types of olivine: i) large subhedral crystals (up to100 µm) with both straight and lobate rims and ii) smaller (5-20 µm) rounded grains. Olivine grain size increases with the IMA; on contrary, the amount of smaller rounded grains is inversely correlated. At 1300°C, high IMA enhances the melt-olivine reaction, promoting olivine dissolution and crystal growth that leads to an increase in the olivine grain boundaries tortuosity. At 1200°C, mineral chemistry of olivine suggests that XMg values are not correlated with different IMA. At 1300°C, XMg in olivine increases from 0.89 ± 0.01 to 0.91 ± 0.01 with the increase of the IMA. The NiO content in olivine is strongly temperature dependent. At 1300°C, the NiO content in olivine decreases with the IMA. Clinopyroxene presents XMg = 0.89 ± 0.00IMA10wt%., 0.89 ± 0.02IMA25wt% and 0.84 ± 0.02IMA50wt%. Plagioclase has XAn = 0.68 ± 0.03IMA10wt%, 0.61 ± 0.06IMA25wt%. and 0.66 ± 0.02IMA50wt%. Reacted glasses at 1200°C have higher SiO2 and lower XMg than glasses at 1300°C, thus suggesting a strong control of olivine and interstitial phases on the residual glass composition. EBSD analysis suggests that the interaction between olivine matrix and basalt at 50wt%IMA and 1300°C strongly affects the pre-existing dunite matrix recovering its plastic deformation (i.e., deformed olivines grains underwent an annealing re-crystallization), thus generating an “undeformed” olivine-rich matrix. The average intragranular deformation parameter defined by the Grain Orientation Spread (GOS), indicates that no differences occur between the two different olivine morphologies. The smaller and rounded olivine has GOS = ~ 0.49 and the larger one with straight and lobate rims has GOS value = ~ 0.41. Hence, in terms of deformation grade, the assessment of a pre-existing olivine grain (Olivine1) and a new crystallized olivine (Olivine2) is not straightforward. Step-cooled experiments result in dunite (10wt%IMA), olivine-rich troctolite (25wt%IMA) and olivine-gabbro (50wt%IMA). The olivine texture is formed by elongated crystals with high tortuosity of grain boundaries independently of the grain size and IMA. In contrast to a normal fractional crystallization path, the reactive crystallization of melt within an olivine matrix leads to early clinopyroxene crystallization rather than plagioclase. This is likely related to a combined effect of: i) the MgO enrichment of reacted melt due to the high extent of olivine dissolution and ii) small amount of H2O in reacted melts that can suppress the plagioclase crystallization. EBSD analysis suggests that the reactive dissolution and crystallization processes efficiently promote the “recovery” of olivine plastic deformation independently of the grain size (and shape) and the different IMA. Experimental results of this work show several textural and chemical analogies with observations made on natural olivine-rich troctolite from oceanic and ophiolitic massifs. The results achieved in this thesis provide new and useful insights into melt-rock reactions between a dunite and an impregnating MORB-type melt