T266 Early Career Investigators in Mineralogy and Crystallography

Wednesday, 18 October 2023
1:30 PM - 5:30 PM
334

MULTIPLE FLUID INTERACTIONS RECORDED IN TOURMALINE FROM THE DOROTHY CHINA CLAY PIT, CORNWALL, UK

Paper number: T266-12
Program: Technical Sessions
Time: 4:15 PM – 4:30 PM

Authors

Abstract

Tourmaline (tur) can record geochemical signatures of its host rock and environment of formation, including multiple generations of fluid phase interactions. Fluid-mineral interactions in tur physically manifest as intricate chemical-zoning patterns, dissolution features, and recrystallization textures. These processes are highlighted in a tur sample from the Dorothy China Clay Pit (CCP), St. Austell, Cornwall, U.K. The sample is composed of elaborately zoned tur (85%), with crystals ranging in size from 1mm – 2 cm, within a quartz matrix (15%). Chemical and textural characterization of tur grains was carried out primarily through EPMA and BSE analyses.

A large euhedral crystal oriented along the c axis displays zoning from the base to the tip, which is replaced by feathery zoning and fibrous tur, and surrounded by an oscillatory zoned rim. A late homogeneous tur overgrows these crystals and appears to also partially replace the matrix quartz. A lengthwise traverse of 275 analytical points reveals a wide variety of tur species within the single grain, reflective of the fine scale compositional heterogeneity. The core is composed of Li-bearing species, rossmanite, elbaite, and fluor-elbaite which are partially replaced by Fe-richer species fluor-schorl, oxy-schorl, schorl and foitite. The outer rim predominantly consists of Fe-bearing oxy-dravite. Such zoning and replacement textures suggest at least four distinct generations (gen) of tur growth. Gen 1 Li-rich tur, Gen 2 schorl, Gen 3 oxy-dravite with Gen 4 overgrowing the previous tur generations and quartz. The extreme compositional variation and fine-scale zoning result from multiple stages of fluid interactions in an open system. When new fluids infiltrated, tur dissolved and reprecipitated to attain equilibrium. Likely the original elbaite crystallized in a hydrothermal environment. Gen 2 tur formed by infiltrating fluids richer in Fe, B, Na, and F which removed Li and Al. Gen 3 fluids had lower Na and higher Mg. The final fluids, richer in Fe, B, Na, Al, and F with low Si, resulted in Gen 4 Fe-rich schorl overprinting the original matrix phase, retaining its habit. This study provides a detailed chemical characterization of tur from the Dorothy CCP deposit, underscoring its utility as a geochemical recorder of hydrothermal activity reflected by multi-generational self-cannibalization of tur grains.

Geological Society of America Abstracts with Programs. Vol. 55, No. 6

doi: 10.1130/abs/2023AM-391665

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