CRYSTAL SIZE DISTRIBUTION ANALYSIS OF FORSTERITE IN THE ALTA AUREOLE, UTAH

The Tertiary Alta stock has contact metamorphosed Paleozoic sedimentary rocks in Little Cottonwood Canyon, near Salt Lake City, Utah.  In the southern Alta Aureole, siliceous dolomites show the prograde isograd sequence of talc, tremolite, forsterite and periclase.  Samples were collected from a series of outcrops across the forsterite zone (starting ~700 m from and toward the igneous contact) and the crystal size distribution (CSD) of Fo was analyzed following the procedure described by Higgins (1994, 2000, 2002) and using the program CSDCorrections (Higgins, 2002). 

Upgrade, the average measured 2D Fo crystal size decreases from more than 1 mm to approximately 0.2 mm and the calculated number of Fo crystals per mole of Fo increases from ~2x10⁵ to ~3x10⁷.  The increase in Fo crystals per mole Fo suggests a progressive increase in the rate of crystal nucleation relative to the rate of crystal growth toward the contact. The CSD analysis supports this pattern. 

Overall, CSD patterns for Fo change upgrade from curved, concave-up to near linear plots. Linear CSDs indicate continuous nucleation and growth of Fo while concave-up CSDs indicate a change in nucleation or growth rates, textural coarsening, or a combination of the two.  (Cashman & Ferry, 1989; Higgins, 2003)  The intercepts of the CSDs also increase upgrade.  The slopes of the linear CSD plots are steeper than the slopes of the right-hand leg (toward increasing crystal size) of the concave-up plots.  The slope and intercept of the right-hand leg of the concave-up plot is interpreted as representing initial nucleation and growth rates, if it is assumed that the largest crystals are the oldest.  These upgrade increases in the CSD slopes and intercepts, both linear and concave, support the upgrade increase in nucleation rates suggested by the trends in number and average size of the Fo crystals. These changes are likely a function of increasing temperature and/or increased heating rates toward the intrusion.

Various tests of the spatial disposition of Fo in 2D indicate mostly non-random distributions, suggesting that Fo growth and nucleation is diffusion-controlled.  Apparently, diffusion-controlled growth of Fo is occurring within carbonate rocks that also record evidence (i.e., petrologic reaction progress, ¹⁸O/¹⁶ O depletion) for fluid-infiltration accompanying Fo reaction.