Millimeter– to decimeter–scale compositional mapping using a scanning X–ray analytical microscope and its application to a reaction zone in high–grade metamorphic rock

Abstract

A new method for quantitatively joining compositional maps measured by a scanning X-ray analytical microscope (SXAM) to visualize a larger scale element distribution (i.e., a joined element map) is proposed, and applied to the analysis of a 25-cm-long sample across a reaction zone from high-grade metamorphic rock. The method involves the in situ measurement of a standard material during a sample scan, which enables correction of the different sensitivities of multiple maps. The appropriate background intensity correction, spectrum processing, and X-ray intensity correction proposed in this study enable the production of a semiquantitative element map at a decimeter scale with relatively high resolution (~0.1 mm). The one-dimensional quantitative transect across the reaction zone has high resolution as well as high precision (e.g., relative standard deviation of <2% for Fe). The transect shows both a sharp boundary controlled by phase stability (as well as a millimeterscale gradual reaction boundary) and a decimeter-scale gradual compositional gradient simultaneously, and these features are difficult to identify using conventional methods (i.e., electron probe microanalyzer, X-ray fluorescence analysis, or SXAM with prior data processing). These compositional gradients, which range from submillimeter to decimeters in length, provide a key to understanding the formation mechanisms of rock and/or mineral reaction zones.

Fluid Infiltration Through Oceanic Lower Crust in Response to Reaction‐Induced Fracturing: Insights From Serpentinized Troctolite and Numerical Models

“…Thin‐section‐scale elemental maps were obtained by scanning X‐ray analytical microscopy (SXAM; Horiba Scientific XGT‐7000V at Tohoku University, Japan) on an instrument equipped with an energy dispersive spectrometer (EDS). The measurement conditions were the same as descried by Uno et al (2014). The SXAM measurements were conducted at an accelerating voltage of 50 kV and tube diameter of 10 μm.…”

Fluid Pocket Generation in Response to Heterogeneous Reactivity of a Rock Fracture Under Hydrothermal Conditions

“…After the experiments and CT analyses, the fracture surface was analyzed using a scanning X‐ray analytical microscope (X‐ray guide tube (XGT) 7000V, Horiba Scientific; Uno et al, ) at Tohoku University to obtain X‐ray images of Al, K, Ca, Si, and Fe. The element maps were used to create a mineral distribution map.…”

Macro-scale ore-controlling faults revealed by micro-geochemical anomalies

“…In this SXAM system, a sample is mounted on a PC-controllable motor-driven X–Y stage placed in an open space outside the vacuum chamber. Therefore, among the elements that SXAM can analyze 46 (i.e., sodium to uranium), it is difficult to obtain compositional images for the lightest elements (e.g., Al), especially if these are present in very low concentrations, because low-energy fluorescence X-rays are absorbed by air and the film of the window. However, compared to other elemental imaging methods like electron probe micro-analyser or scanning electron microscopy with energy dispersive X-ray analyser, the SXAM can measure two-dimensional distribution of chemical elements over a significantly wider range of scales (e.g., it can acquire elemental images for a sample set on a standard size 24 × 46 mm petrographic thin section) because the scanning area can be set from 2.56 × 2.56 mm with a resolution of 10 mm, to roughly 200 × 400 mm with a resolution of 0.78 mm.…”