FLUID ALTERATION OF THE FAMENNIAN HUSHOOT SHIVEETIIN GOL SECTION (BARUUNHUURAI TERRANE, MONGOLIA) PRESERVED IN TITANIA MINERALS: HOW DOES THIS AFFECT GEOCHEMICAL FINGERPRINTING OF SHALLOW WATERS?

The Hushoot Shiveetiin gol section is a 167-meter-thick stratigraphic section that spans the Famennian to early Carboniferous within the Samnuuruul Formation in the Gobi-Altai region of southwestern Mongolia. This section represents a shallow marine to lagoonal sequence deposited on the flanks of the East Junggar volcanic island arc system. The section was expected to preserve the Hangenberg ocean anoxia event and the associated extinction at the Devonian-Carboniferous (D-C) boundary, but small unconformities are likely in parts of the section. This complicates the search for the D-C boundary extinction using field-based evidence alone, and instead requires the use of geochemical and mineralogical signatures of ocean anoxia to help constrain the location of the Hangenberg ocean anoxia event.

Micron-scale mineral textures and chemistry within layers of ash deposits and pyroclastic flows were analyzed using SEM-EDS, with particular attention paid to the presence of titania minerals, which indicates subsurface groundwater flow (either as syndepositional submarine groundwater discharge or post-depositional fluid alteration) and results in element mobility. Of the ash deposits analyzed, those from shallow marine facies showed heavy titania alteration, whereas those from lagoonal facies were associated with only minor Ti alteration (if any). Ti alteration is commonly correlated with apatite, kaolinite, and illite, indicating that Ti mobility was due to submarine groundwater discharge and early post-depositional diagenesis rather than late diagenesis from deep basinal fluids. These interactions between shallow marine waters with submarine meteoric groundwater will alter the results of any geochemical and stable isotope proxies commonly used to detect anoxia and paleotemperatures in marine environments. We therefore recommend that shallow-water, near-shore sequences be tested for Ti alteration prior to carbon, oxygen, or strontium stable isotope analysis.