Chemical variation in garnets from aplites and pegmatites, peninsular Thailand

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Abstract
Electron microprobe analyses of almandine-spessartine garnets from pegmatites and aplites from the Hub Kapong batholith and Phuket Island in peninsular Thailand show three types of zoning. Garnets from pegmatites show extreme zoning, with Mn-rich cores (⋍ 80 % spessartine) and Mn-poor rims (⩾ 40 % spessartine), whereas those from aplites are either unzoned (35–40% spessartine) within the limits of detection, or have Mn-enriched rims (cores 45 %, rims 55 % spessartine). The origin of the three types of zoning is considered to reflect different crystal growth histories in each case. The pegmatitic garnets grew under conditions favourable for the development and preservation of marked concentration gradients—low nucleation density, rapid growth rate and slow cation diffusion rates for the crystal and rapid diffusion rates for the pegmatitic liquid. The aplitic garnets had a more complex growth history, with slower growth rates and faster diffusion rates for the crystal, coupled with slower diffusion rates within the aplite magma, tending to prevent the formation and preservation of concentration gradients. Subsequent resorption of aplitic garnets at lower temperatures gave rise to Mn-rich rims because of the preferential retention of Mn by the garnet; lower cation diffusion rates within the crystal permitted these marginal concentration gradients to be preserved.