QUANTITATIVE 3D PETROGRAPHY USING X-RAY TOMOGRAPHY: DOCUMENTING ACCESSORY PHASES WITH DIFFERENTIAL ABSORPTION TOMOGRAPHY

Accessory minerals preserve important records of the evolution of magmatic systems, but study of their textures and contact relations is hindered by the lack of suitable methods for characterization. Differential absorption x-ray tomography (DAT) can be used to qualitatively and quantitatively document textures of zircon and REE-minerals in situ and in 3D.

The method takes advantage of large jumps in x-ray attenuation near absorption edges to generate 3D elemental maps for selected elements. A key limitation is the strong dependence of the maximum sample diameter on the absorption edge energy. For most major elements in silicate rocks, only sub-mm samples can be imaged. Many elements with higher atomic number (e.g. Sr) appear dispersed in major minerals and are unsuited for DAT. Zr and REE are major components of certain accessory minerals and have absorption edges at energies suitable for imaging samples up to ~1 cm in diameter. This is particularly fortunate, given that zircon and REE-minerals preserve rich records of magmatic evolution.

We apply DAT to pumice from Peach Spring Tuff (PST) and Mount St. Helens (MSH) and present a few illustrative examples of the kinds of data that can be extracted using a combination of DAT and conventional tomography. Several factors make DAT particularly interesting for studies of accessory minerals:

(1) Textures and contact relations between accessory minerals can be documented in 3D, as exemplified by the discovery of numerous zircon crystals included in titanite and in contact with allanite and magnetite in PST pumice;

(2) Different populations of accessory minerals can be recognized and studied in textural context, as demonstrated by the identification of two populations of zircon crystals in MSH pumice: a population of zircon crystals included in other minerals, and a population of zircon crystals fully embedded in glass;

(3) Crystal size distributions can be readily obtained, and provide information on the crystal nucleation and growth histories, including inferences on the timescales of magmatic crystallization.

Particularly when combined with compositional and age data, the ability to visualize and document accessory minerals in 3D and in textural context opens exciting new possibilities for the study of accessory minerals and of the rocks in which they are contained.