Mineral replacement during scapolitization

The coronitic gabbros of the Bamble sector of South Norway are variably scapolitized and albitized. Scapolitization of the Ødegården and Langøy metagabbros, transforms the magmatic mineralogy (plagioclase, olivine, clinopyroxene and ilmenite) to an assemblage dominated by Cl-scapolite, phlogopite, amphibole and rutile. In addition, apatite and tourmaline are abundant in the scapolitized parts. The preservation of a three-dimensional framework of small zircons originally positioned on ilmenite grain boundaries allows us to trace the replacement reactions and assess the element transport. The following replacement textures have been observed: (i) ilmenite to phlogopite, (ii) phlogopite to talc, (iii) clinopyroxene to amphibole, (iv) plagioclase to Cl-scapolite, and (iv) fluorapatite - chlorapatite - hydroxyapatite. Transformation of plagioclase starts as a sericitisation, followed by the replacement to scapolite. The reaction progress to form scapolite and phlogopite is associated with both an intermittent growth of prehnite, as well as sapphirine occurring as inclusions within scapolite. The scapolitization process is driven by fluid infiltration. The observed mineral chemical changes need a fluid rich in Cl, K, P, and cause an enrichment of Mg and depletion of Fe. The observed multistage reactions are interpreted to reflect the fluid evolution and propagation of metasomatic fronts through the gabbro, in combination with changing conditions for mineral stability during consumption and evolution in fluid composition. Scanning and transmission electron microscopy of chlorapatite partially replaced by hydroxyapatite illustrate the typical replacement mechanism during metasomatism. The mechanism is characterized by a sharp reaction front which moves through the parent phase, while preserving the morphology and crystallographic orientation. The product hydroxyapatite is porous, allowing mass transport between the fluid reservoir and the reaction front. These observations are entirely consistent with an interface-coupled dissolution-reprecipitation mechanism.