In situ concretions are important sources of fossil mammals in the Sucker Creek Formation (middle Miocene) in southeastern Oregon. Three discrete mammal-bearing concretion horizons occur within a 20-m volcanogenic paleosol sequence in the Devil's Gate area. Approximately 35% of collected concretions contain visually discernible bone. Concretions may also contain root casts or inorganic nuclei (e.g., pumice clasts, volcanic clays); or may be internally homogenous. The concretions themselves are mineralogically distinct from marine-derived concretion types (i.e., siderite and calcium carbonate) that more commonly preserve vertebrate remains, such as fish and sharks. Energy dispersive spectrometry (EDS), X-ray diffraction, and Microprobe techniques were used to infer geochemical interactions that occurred between bone and enclosing volcaniclastics during early concretion diagenesis. Both concentrations and distributions of chemical phases and minerals have been used to suggest that the following events occurred during their development: (1) partial dissolution of bone hydroxyapatite by groundwater and diffusion into adjacent volcaniclastic material; (2) filling of bone voids by calcite, quartz, hematite, and zeolites; (3) development of a porous fabric around the bone by the precipitation of dissolved bone hydroxyapatite, seeded by inorganically derived hydroxyapatite and calcic zeolites derived from volcanic ash; (4) deposition of a secondary zeolite fabric into voids of the dominant fabric; and (5) development of bentonitic clays around concretions during pedogenesis. Although some superficial bone was consumed during concretion diagenesis, relatively fast concretion development in this volcanic environment reduced the chance of prolonged chemical and physical destruction during later soil development. Consequently, bone-volcaniclastic interactions played an important role in the preservation of large mammal skeletal remains and rare skeletal elements of small mammals.