Fluid-rock interaction: quantifying changes in whole rock-chemistry at variable scales

Fluid-rock interaction causes gradational changes in rock composition from the millimeter scale to kilometer scale, for example in vein selvages and pervasive alteration in porphyries. These changes are a function of how the physicochemical properties of both the fluid and the rock evolve as they interact with each other.

Analytical results of whole-rock chemistry are commonly used to quantify the equilibrium mineral assemblages expected at different physicochemical conditions. However, the average chemistry of rock samples with strong chemical variability at small spatial scales is challenging to quantify and limits our ability to validate geochemical models of fluid-rock interaction in natural systems.

This study presents a comparison of various analytical techniques used to quantifying changes in whole rock chemistry from the hundred-micron scale to the decimeter scale. The techniques evaluated include raster and spot analyses with laser ablation inductively coupled mass spectroscopy (LAICPMS) of rock slabs and powder pills, X-ray fluorescence (XRF) of powder pills, and calibrated microXRF of rock slabs. The results are compared and evaluated in terms of their precision and accuracy, and the logistic advantages and disadvantages of using each of them.