Granitic melt-mantle reaction experiments: insights into crust-mantle interaction products during the subduction of the continental crust

During the deepest stages of subduction, melts and fluids released from the slab are responsible for crust-mantle interaction and the transfer of elements to the mantle. Primary melt inclusions of crustal origin now trapped in eclogite lenses within peridotite bodies of the Granulitgebirge, Bohemian Massif (Germany), represent witnesses of this process. The scenario proposed to explain the origin of these eclogites was that a granitic crustal melt interacted with a mafic/ultramafic layer already present in the peridotite to produce the eclogites at around 2.2 GPa and 1000°C. However, the nature of the protolith is still unknown because of the lack of mineralogical relics.

To fill this gap and better constrain the role of crustal melt-mantle interaction in generating garnet-bearing rocks, we performed melt-rock reaction experiments at the conditions at which metasomatism took place in the Granulitgebirge. Piston cylinder experiments were performed at 1.5, 2.2 and 2.9 GPa and 1100°C, after an initial stage at 1300°C for 1 hour. As starting materials, we used a homogeneous synthetic glass with the same composition as the granitic melt measured in the Granulitgebirge eclogites and two mantle protoliths: a fertile lherzolite pre-synthesized starting from a gel, and a natural spinel clinopyroxenite (bulk X_Mg = 0.74). Two initial melt:rock weight proportions were chosen: 1:9 and 3:7, which simulate a rock-dominated metasomatic reaction.

The lherzolite-granitic melt reaction generally produces orthopyroxene-rich ± garnet websteritic assemblages. At 2.2 GPa, reaction products are orthopyroxene along garnet and rare phlogopite. At 1.5 GPa, the same reaction does not produce garnet, and pargasitic amphibole is present instead.

Independent of the pressure and melt:rock ratio, the spinel clinopyroxenite-granitic melt reactions completely consumed spinel and produced garnet and new clinopyroxene coexisting with an andesitic residual melt. New clinopyroxene has higher X_Mg (0.84-0.89) and Na at lower Ti contents. At 1.5 GPa, a coarse poikilitic grain (around 1 mm in size) of garnet developed, suggesting a very fast crystallisation rate likely promoted by a high amount of Al-rich reacted melt. The modal abundance of garnet, as well as its grossular content, increases with pressure and decreases with temperature, according to the reaction: spinel + cpx₁ + granitic glass = garnet + cpx₂ + andesitic reacted glass. Interestingly, garnets resulting from this last set of reaction experiments have a composition similar to those in the Granulitgebirge eclogites. On the contrary, clinopyroxenes crystallized by reacted melt are Al-rich diopside with low jadeite contents (Jd < 0.14), much lower than clinopyroxenes in the natural eclogites.

Our study shows that the best fit between nature (Granulitgebirge case study) and experiments is visible when the reacting rock is a spinel clinopyroxenite rather than a peridotite. However, the differences between nature and experiments still need to be evaluated.