The role of melt/olivine ratio in dissolution and reactive crystallization: an experimental study at 0.5 GPa

Microstructural and geochemical evidence have emphasized that melt-rock reactions play an important role in the origin of gabbroic rocks in the lower oceanic crust. It is widely accepted that olivine-rich rocks might form through dunite infiltration followed by reactive crystallization of interstitial melts. Experiments on the origin of olivine-rich troctolites through basalt-dunite interactions suggested that the melt/olivine ratio plays an important role (Borghini et al., 2018). In this study, we aim to experimentally quantify the effect of the melt/olivine ratio on dissolution and reactive crystallization processes. We performed piston cylinder experiments at 0.5 GPa on San Carlos olivine (Fo90) variably mixed to MORB-type glass (10, 25 and 50 wt%). We performed both isothermal runs at 1300°C, lasted 24 hours, and step-cooled runs carried out by lowering the temperature from the isothermal dwell at 1300°C down to 1100°C at 1°C/min. Run products of isothermal experiments consist of olivine and glass. Texturally we distinguish two generations of olivine: i) large subhedral crystals (~ 50 µm) with both straight and lobate rims and ii) smaller (5-20 µm) rounded grains. At increasing initial melt amount (from 25 to 50 wt%), olivine grain size increases (up to ~ 80 µm) and the amount of smaller rounded grain decreases. Relatively high initial melt/olivine ratio enhanced the reaction promoting olivine crystal growth and dissolution resulting in higher olivine tortuosity. Olivine XMg increases from 0.91 to 0.92 at initial melt amount increasing (from 25 to 50 wt%). NiO content decreases via reaction with initial melt as a function of the melt/olivine ratio. Reacted glass records higher XMg and NiO contents and lower Na2O, CaO and Al2O3 abundances. Mass-balance calculations support an increase of olivine dissolution at increasing initial melt fraction. When a cooling path is followed and the temperature is lowered down to 1100°C, run products, in starting mixture with 25 wt% of melt, consist of olivine, glass and interstitial clinopyroxene. Crystallization of reacted melt leads to the early appearance of clinopyroxene as compared with plagioclase. This results from the decrease of melt MgO (7.6 ± 0.4 wt%) via olivine crystallization promoting early clinopyroxene crystallization. Raman spectroscopy has revealed H2O dissolution in experimental glass, suggesting that experimental conditions are not perfectly dry, but nominally anhydrous conditions. This could further inhibit the crystallization of plagioclase (Husen et al., 2016). After cooling, olivine XMg is similar (XMg = 0.91) to its analogue isothermal experiment. Clinopyroxene crystallizing from the reacted melt shows high XMg (0.91) and Al2O3 content (7.5 ± 1.5 wt%). Experimental results provide new insights on the origin of primitive gabbros at the base of the oceanic crust. References: Borghini et al., (2018) Lithos, 323, 44-57. Husen et al., (2016) J. Petrol. 57, 309-344.