The case for a melt matrix in plagioclase‐POIK mesosiderites

The plagioclase‐POIK mesosiderites, Bondoc, Budulan, and Mincy, have poikilitic matrix textures exactly like igneous rocks including the olivine zone of the Palisades sill and Apollo 17 mare basalts. They contain olivine grains with embayed margins characteristic of resorption in liquid rather than coronas formed during solid state reactions. There is a nonrandom (i.e., nonmetamorphic) arrangement of pyroxene phases in the matrix: orthopyroxene is enclosed poikilitically by plagioclase, but inverted pigeonite is interstitial and locally subophitic to plagioclase, as in an igneous crystallization sequence. These mesosiderites are therefore reclassified from subgroup 3B (highly recrystallized matrix) to 4B (igneous matrix). The 4A mesosiderites have fine‐grained intergranular textures because they are richer in plagioclase than subgroup 4B, so that plagioclase crystallized earlier than in 4B, but also because they contained abundant nuclei associated with relict plagioclase clasts. The only 4A melt‐rock mesosiderite, Pinnaroo, with an intergranular‐poikilitic matrix texture transitional between 4A and 4B mesosiderites, contains very few plagioclase clasts. Over half the mesosiderites contain melt matrix or melt‐rock clasts. The heterogeneity within mesosiderites as a whole and within specific mesosiderites (e.g., Estherville) may be due to variations in the proportions of hot melt and cold clasts incorporated.