Fig. 2: Evidence for synmetamorphic reduction.
From: Slab-derived devolatilization fluids oxidized by subducted metasedimentary rocks
a–c, Plane-polarized light photomicrographs. a, Quartzite (jagti134N). Magnified inset shows haematite inclusions in a garnet (Grt) that is enclosed by magnetite. The rock initially contained only haematite, which was incorporated into growing garnets, but no magnetite. Subsequently, magnetite grew in the matrix and around garnets as a result of haematite reduction. Some haematite remains in the matrix. Garnet cores with haematite inclusions have high Fe3+/(Fe3+ + Fe2+) values of ~0.35, whereas the rims have much lower values (~0.05). b, Albitite (jagti75A). Anastomosing ‘veinlets’ (light) containing magnetite formed by the reduction of fine-grained haematite are shown (haematite occurs in cloudy domains, which may also contain fine-grained albite, quartz, phengite, epidote, sodic clinopyroxene, and Na and Na–Ca amphiboles (Amp)). These anastomosing features are interpreted as infiltration channels for reducing fluids. c, Garnet containing inclusions of Na–Ca amphibole (green, right) that have ~25% greater Fe3+/(Fe3+ + Fe2+) values than matrix Na amphibole (blue, left) (jagti154F-1). The garnet core also contains small (tens of micrometres) inclusions of jadeite–aegerine clinopyroxene with ~0.4–0.6 Fe3+/(Fe3+ + Al) (not visible). d, Reflected light photomicrograph showing haematite and rutile inclusions in garnet (main image and inset) and matrix ilmenite (Ilm) (jagti90B). Note that matrix ilmenite surrounds and postdates the haematite-bearing garnets. e, Cut rock slab containing the alteration selvage (tan ‘bleached’ appearance) adjacent to a quartz vein cutting purple-red haematite-bearing phyllite. The haematite has been destroyed in the selvage by reducing fluids that infiltrated along the vein (jagcr00A). Siderite–magnesite in veins is the inferred sink for the reduced iron.
