Abundance and distribution of ultramafic microbreccia in moses rock dike: Quantitative application of mapping spectroscopy

Abstract

Moses Rock dike is a Tertiary diatreme containing serpentinized ultramafic microbreccia (SUM) located on the Colorado Plateau in Utah. Field evidence indicates that SUM was emplaced first followed by breccias derived from the Permian strata exposed in the walls of the diatreme and finally by complex breccias containing basement and mantle‐derived rocks. SUM is primarily found dispersed throughout the matrix of the diatreme. We examined the nature of SUM abundance and spatial distribution using data collected remotely by the Airborne Imaging Spectometer (AIS). The minerals serpentine, gypsum, and illite as well as desert varnish and the lithologies SUM and various sandstones were identified from the AIS data. Six end‐members (SUM, two types of sandstone, gypsiferous soil, clay‐rich soil, desert varnish) were chosen to represent the dominant lithologies of the surface in Moses Rock dike region. Spectra of these end‐members were used in an intimate mixing model to deconvolve the AIS spectral data into surface abundance coefficients for each component using a nonnegative least squares inverse algorithm. The results of this calculation of surface composition are consistent with field observations and investigations. SUM distribution and abundance in the matrix of the diatreme were examined in detail, and two distinct styles of SUM dispersion were observed. One style is characterized by high to moderate SUM abundance surrounded by halos of lesser SUM abundance. The dispersion halos grade steeply into regions of the matrix with little to no SUM. The second style is characterized by moderate SUM abundances and broad dispersion halos with SUM distributed across the entire width of the diatreme. These dispersion styles are consistent with emplacement of the dike as a fluidized solid‐volatile system where SUM, which is emplaced early, becomes reincorporated into the turbulent flow of the diatreme by abrasion and comminution by paniculate matter carried along in the eruption. This eroded material is then dispersed throughout the matrix by eddy diffusion. The first style represents an early arrested phase of the eruption sequence, while the second style represents a more mature phase. Distributions of the second style probably indicate the location of channels where flow was concentrated during later stages of eruption. Since both styles are observed at the same level of erosion, this indicates that the duration of eruption varied along strike at Moses Rock dike.