T87 Non-Traditional Stable Isotope Fractionation at Extreme Conditions: In Honor of Anat Shahar, 2016 MSA Awardee

Monday, 26 September 2016
8:00 AM - 9:45 AM
Room 404

THE BORON ISOTOPIC COMPOSITION OF TOURMALINE FROM PEGMATITES AT THE LITTLE THREE MINE, RAMONA, CALIFORNIA

Paper number: T87-6
Program: Technical Sessions
Time: 9:25 AM – 9:40 AM

Authors

Abstract

Tourmaline, mica, hambergite, danburite, and axinite, from the Main, Swamp, and Spessartine pegmatite dikes at the Little Three mine near Ramona, California, have been analyzed by secondary ion mass spectrometry (SIMS) for their boron isotopic compositions (δ11B). Tourmaline is the primary host of B in the footwall and hanging wall sections of the dikes. In the thinner Spessartine and Swamp dikes, the pocket mineralogy includes foitite, spessartine, muscovite, axinite, danburite, and rare beryl. The thicker Main dike culminates in cavities containing topaz, elbaite, lepidolite, boromuscovite, hambergite, and rare stibiotantalite. The values of δ11BTur are strikingly heavy (-0.6 to +9.7‰) compared to most other pegmatitic tourmaline, which may reflect inheritance from altered oceanic crust with or without a marine sediment component. Across the footwall aplite of the Swamp dike, values of δ11BTur are constant up to the lower intermediate zone, at which point the values abruptly decrease (~2‰). The footwall aplite of the Main dike, which can be divided into massive and layered sections, displays small, 2-4‰, fluctuations in δ11BTur. There is a symmetrical decrease in δ11BTur from the upper and lower intermediate zones into miarolitic cavities of the Main and Swamp dikes. The difference in δ11B between tourmaline from the intermediate zones and miarolitic cavities in the Main and Spessartine dikes is nil. The δ11B of tourmaline increases sharply from intermediate zone to miarolitic cavity where danburite and axinite (both with 4-coordinate B) co-crystallized.

Uncertainties in the boron isotopic fractionation factors for melt-aqueous fluid and tourmaline-granitic melt hamper interpretations of magmatic values of δ11BTur. However, considering the growth rate of tourmaline should have been fast relative to the diffusivity of B through granitic melt, an equilibrium distribution of 11B and 10B between tourmaline, bulk melt, and an aqueous fluid (i.e. Rayleigh fractionation process) is unlikely. The boron isotopic composition of aqueous fluid present during the formation of miarolitic cavities and the tourmalinized host tonalite can be reconciled to equilibria using boron isotopic fractionation factors between tourmaline and aqueous fluid insofar as these are known.

Geological Society of America Abstracts with Programs. Vol. 48, No. 7

doi: 10.1130/abs/2016AM-287540

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