ORIGIN OF MANTLED (RAPAKIVI) FELDSPARS - EXPERIMENTAL-EVIDENCE OF A DISSOLUTION-CONTROLLED AND DIFFUSION-CONTROLLED MECHANISM

Experiments designed to simulate the dissolution of alkali feldspar during magma mixing produced plagioclase mantles that are texturally and compositionally similar to those in some hybrid volcanic rocks. In hydrous dacite melt (69% SiO2) at 0.8 GPa, 850-degrees-C, orthoclase (Or93) and sanidine (Or30) partially dissolved and were mantled by sodic plagioclase (approximately An25-30). Although plagioclase nucleated epitaxially as a thin shell on the alkali feldspar surface near the time of initial resorption, plagioclase subsequently grew inward - mostly in the form of parallel blades - toward the receding dissolution surface. Orthoclase dissolved at a rate approximately proportional to the square root of run duration, indicating diffusional control. Plagioclase grew inward within a static "boundary zone" of melt that formed between the original crystal-dacite interface and the dissolution surface. During orthoclase dissolution, this boundary zone rapidly and simultaneously gained Na (by diffusion from dacite) and lost K (by diffusion into dacite); Ca diffused more slowly into this zone, from which non-feldspar species were mostly excluded. Plagioclase was stable where sufficient Ca had diffused in that the boundary zone melt intersected the plagioclase-saturation liquidus. Plagioclase subsequently grew toward the receding dissolution surface as thc Ca compositional gradient (and hence the site of plagioclase saturation) stepped inward. Crystallization of plagioclase in the form of parallel blades allowed continued diffusive exchange of melt components between the dissolution surface and the host melt. Bladed growth also served to maintain (at blade tips) proximity of plagioclase to the dissolution surface, thereby apparently preserving (locally) a thin zone of low-variance melt. In natural systems, mantling of alkali feldspar by plagioclase will occur in a similar manner when (a) P, T, or X are changed to induce alkali feldspar dissolution, (b) sufficient Ca is available in the host melt to drive (by diffusion) boundary zone melt compositions to plagioclase saturation, and (c) temperatures are low enough to stabilize sodic plagioclase and to maintain a coherent boundary zone. These requirements are satisfied in volcanic systems when alkali feldspar is juxtaposed during mixing with hybrid melts of approximately dacitic composition. Mantled feldspars in some intrusive systems (i.e., "rapakivi granites") may form by a similar dissolution- and diffusion-controlled mechanism. Textural evidence of a similar origin may be obscurred in intrusive rocks, however, by products of late-stage magmatic and subsolidus processes.