EXPERIMENTAL SIMULATIONS OF PEGMATITIC TEXTURES AND ZONATION

There is strong evidence that the internal zonation and textures of pegmatites scale in proportion to the size of the magma body. Hence, experimental simulations of pegmatites can represent meaningful proxies to the real things. We have continued our program of crystallization with the Macusani obsidian (Peru), an ideal analogue to the compositions of Li-rich rare-element pegmatites. Cores of obsidian were hydrated to 3-8 wt% H₂O at 800°C, 200 MPa for one week, quenched, and reloaded either with or without powdered amphibolite + 2 mg H₂O at one end of the capsule. Capsules were run to 750°C, 200 MPa, then cooled quickly and isobarically to temperatures of 450°, 500°, or 550°C (ΔT = 300°, 250°, and 200°C, respectively) for durations up to three months. Amphibolite (Hbl + Pl) was added both to simulate a silicate host rock in contact with a pegmatite-forming melt, and to assess its chemical interaction with the melt. In all cases, crystallization commenced with a very fine-grained border zone, followed by a unidirectional wall zone that completely surrounds the melt body. The wall zone contains substantially less normative quartz than the bulk composition; consequently, the latest interior zones are quartz-rich, and pure quartz cores have formed in some experiments. Feldspar compositions are zoned around the wall zone, with comparatively An-rich Pl adjacent to the amphibolites versus Kfs-rich feldspar that is prevalent at the opposite end of the melt body. Experiments that start with high H₂O contents and those that crystallize extensively develop miarolitic textures, but most remnant melt pools do not contain miarolitic cavities. Residual glass compositions are extremely fractionated from the initial composition of the obsidian. Starting with 580 ppm Cs, residual glass contains > 5700 ppm Cs at ~ 80% crystallization (an effective bulk D_Cs < 0.001), and pollucite is the last phase to crystallize in some experiments. Despite the high degree of chemical fractionation attained, and the formation of miarolitic cavities in some experiments, there is no evident reaction with the amphibolite. That is, exsolved vapor remained within the pegmatite body, and none escaped to the surrounding host rock.