Three-dimensional microstructures and mineralogy of mud volcano from Salton Sea, California, USA

Abstract

Mud volcano is a unique geological formation that constantly ejects clay-rich fluids and gases to the surface. These fluids is generally formed due to the mixing and heating process between rock or sediment layers and subsurface water. Over time clay-rich fluids can become overpressured and be forced upward through fractures in the overlying rocks when experience extremely high pressure from tectonic activities in the area. Despite the abundance of sedimentary rocks formed by mud volcano activities worldwide, their physical and chemical properties are still poorly understood. This is mainly due to the extremely fine grain size of clay minerals, which cannot be effectively characterized by classical methods. This study thus aims to determine mineral compositions, crystallographic preferred orientation (CPO), and microstructures of 2 sedimentary samples obtained from mud volcano near the Salton Sea in California, USA by using synchrotron X-ray techniques. Results from synchrotron X-ray diffraction show that both samples have high clay content approximately 69 – 75wt.%, including illite-mica, smectite, and kaolinite as major phases. A small amount of calcite and dolomite are only observed in Sample B, indicating a compositional variation of fluids ejected from different periods of the same mud volcano. Despite a slight compositional difference, these samples have a noticeable variation of microstructures. Sample A has very weak degrees of clay preferred orientation (1-2 m.r.d.) while Sample B has moderate degrees of preferred orientation (~3 m.r.d.). These results are consistent with three-dimensional morphology and distribution of clay minerals obtained from synchrotron x-ray microtomography measurement as most of clay particles in sample B are very fine and preferentially align parallel to sub-parallel to the horizontal direction. On the contrary, clay particles in sample A are mostly randomly orientated. Due to the grain size effect. Sand- or silt-size particles in the matrix play a major role in controlling the preferred orientation of sheet-like clay particles during sedimentation.