Impact of three common post-depositional environmental settings on siliceous sinter diagenesis: An eight year experiment

• Mineralogical and morphological changes of opal-A sinter tracked over eight years
• Overprinting of newly-formed opal-A sinter by steam, organics and meteoric water
• After eight years sinter in all three settings remained opal-A.
• Morphological changes varied in each setting with time.

Samples of newly-formed, opal-A, filamentous siliceous sinter were placed inside three contrasting environments to assess their effect on sinter diagenesis: (1) an actively discharging fumarole; (2) buried in steaming vegetation; (3) buried in soil in a non-thermal area. Each setting was chosen to represent common, sinter post-depositional environments. The sinter remained in these settings for eight years, during which time, samples were taken from the sinter and examined using X-ray powder diffraction (XRPD) and scanning electron microscopy (SEM). These techniques enabled changes in the silica phase mineralogy and morphology to be tracked over time. While all three sinters remained as opal-A during the eight years, minor but significant silica phase changes did take place and were reflected by the increase in maximum intensity value and shift in apex position on the XRPD trace. Silica phase transitions were greatest in the sinter buried in the steaming vegetation, followed by the sinter in the fumarole. The sinter buried in the non-thermal garden, revealed almost no change. Throughout the duration of the experiment, in all three settings, the filamentous nature of the sinter was preserved. Botryoidal clusters and new generations of opal-A spheres were observed in samples, from all three sites. At various time periods, in all environments, silicified filaments formed smooth, non-porous ridges separated by a more open arrangement of silicified filaments.Major, minor and no dissolution textures were observed in the sinter buried in the steaming vegetation, non-thermal soil and the fumarole settings respectively. Rows on aligned, opal-A spheres were only present in the fumarole sinter at 33 weeks. Tracking sinter diagenesis in three commonly found, post-sinter formation environments, contributes to our understanding of the preservation potential of siliceous sinters and the influence steam, meteoric water and organics have on sinter diagenesis.