Amphibole lamellae formation in the upper mantle due to interaction of fluid inclusions and host minerals: a case study from Persani Mountains Volcanic Field, Transylvania

Amphibole is one of the most abundant ’water’-bearing minerals in the Earth’s upper mantle. Amphiboles occur as interstitial grains, lamellae within pyroxenes or as daughter minerals within fluid inclusions.  Most commonly amphibole formation is related to mantle metasomatism, where the agent has a subducted slab (e.g. Manning 2004) or an asthenospheric origin (e.g. Berkesi et al. 2019).  After the formation of fluid inclusions, a subsolidus interaction can take place where the H₂O content of fluid inclusions may crystallize pargasite (e.g. Plank et al. 2016).

Here we present amphibole lamellae formation in mantle xenoliths from the Persani Mountains Volcanic Field that is interrelated to a reaction between fluid inclusions and host clinopyroxene.  Newly formed amphibole lamellae occur only in the surroundings of the fluid inclusions and grow within the host clinopyroxene in a preferred crystallographic direction.  Studied lamellae do not reach the rim of the host mineral implying that components needed for formation of amphibole lamellae in clinopyroxene could have only originated from the fluid inclusion itself.  We measured the major element composition of amphibole lamellae and host clinopyroxene (1) and used Raman spectroscopy and FIB-SEM on fluid inclusion study situated next to the lamellae (2).  Results support the hypothesis that chemical components (dominantly H⁺) migrated sub-solidus from the fluid inclusion into the host mineral after fluid entrapment via subsolidus interaction.  Beyond the clinopyroxene-hosted fluid inclusions, fluid inclusions in orthopyroxenes were also studied as a reference.  Our study shows that post-entrapment diffusion from a fluid inclusion into the host mineral changes the solid/fluid ratio of the mantle  which could modify the rheology of the lithospheric mantle.

Berkesi, M. et al. 2019. Chemical Geology, 508, 182-196.

Kovács et al. (2017) Acta Geodaetica et Geophysica, 52(2), 183-204.

Manning C. E. 2004. Earth and Planetary Science Letters, 223, 1-16.

Plank, T. A. et al. 2016. In AGU Fall Meeting Abstracts.