This document describes an experimental protocol to measure the density of Alpine ophiolite rocks and understand subduction zones functioning.
Extract
[...] Hornblende Metagabbro Hornblende (amphibole) : main mineral constituent, giving the dark color to the rock. Plagioclase : often present in association with hornblende, contributing to the overall mineralogical composition. Métagabbro à Glaucophane : Glaucophane : characteristic mineral of this rock, presenting a blue-green hue and indicating high-pressure and low-temperature conditions associated with subduction. Plagioclase : may be present in smaller quantities. Eclogite : Jadeite one of the main minerals of eclogite, belonging to the pyroxene group. It forms under high pressure and low temperature conditions, typical of subduction. [...]
[...] Note the mass of each sample. Filling the test tube: Fill a graduated test tube with a reference liquid (water) of known density Measurement of the displaced volume: Gently immerse each sample in the reference liquid contained in the test tube. When you immerse each sample, the liquid will be displaced, allowing you to measure the volume displaced by each sample. Note the volume of liquid displaced for each sample. Calculation of the density : For each sample, use the density formula [...]
[...] Here is the cross-section at the level of a subduction zone: Convergence Movement : The oceanic plate plunges under the overriding plate due to the subduction movement. Wadati-Bénioff Zone (distribution of seismic foci) : This plan represents the distribution of earthquakes associated with subduction. Seismic foci are generally inclined downwards towards the plunging plate. Oceanic Trench The subduction zone is often associated with an oceanic trench, a deep topographic depression on the ocean surface, formed by the plunging of the oceanic plate. [...]
[...] These minerals are associated with metamorphic processes, such as those observed in subduction zones. - The eclogite, with its higher density and the presence of minerals such as garnet and jadeite, indicates formation conditions at higher pressures and temperatures, typical of greater depths reached during subduction. Using the pressure/temperature stability diagram of minerals, we can confirm these conclusions. The metamagabbros form at lower depths in the crust, where temperatures and pressures are lower, while the eclogite forms at greater depths where the pressure and temperature conditions are higher. [...]
