RE-OS BEHAVIOR IN MAFIC MAGMAS AND TERRESTRIAL DIAGENETIC MATERIALS: EXTRACTING INFORMATION FROM OPEN SYSTEMS

Re and Os are both chalcophile/siderophile elements that are fixed in sulfide and oxide phases relative to silicate or aqueous phases. Strongly controlled by redox reactions, both are relatively soluble under oxidizing conditions, but precipitated or adsorbed by reducing materials. They are therefore concentrated in sulfide and oxide phases in magmatic systems, and in sulfides and organic material in terrestrial sedimentary rocks. Analyses of organic material and diagenetic pyrite from the Late Jurassic Morrison Formation in NW New Mexico and SW Colorado document open system behavior of the Re-Os system for up to 60 million years after deposition. Similarly, magmatic sulfides and oxides from the Suwalki massif, Poland, show variable initial ¹⁸⁷Os/¹⁸⁸Os reflecting variable crustal contamination in a dynamic magma system. Nevertheless, with appropriate sampling procedures, these materials promise useful geochronologic information.

Samples taken only a few meters apart in a single bed in the Morrison Formation show variable initial ¹⁸⁷Os/¹⁸⁸Os ratios, the result of variable interaction with groundwater over an extended diagenetic history. Splits of pyrite with variable magnetic properties from hand samples no more than 10 cm in diameter, however, yield isochrons with ages 50 to 60 m.y. younger than the depositional age and crustal initial 187Os/188Os ratios (~ 0.8 to 1.0). Related hand samples yield parallel isochrons with similar ages but variable initial ¹⁸⁷Os/¹⁸⁸Os ratios. This reflects variable adsorption of Os over time from groundwater, with the isochron age reflecting final closure to rock/water interaction. While age information is imprecise because the timing of closure is variable, the method may be useful for establishing approximate ages of Precambrian sedimentary rocks where closure periods of 50 to 60 m.y. may not be discernible within analytical error, and uncertainties from other age data are large.

This process is analogous to those in magmatic sulfide deposits, for which enrichment of immiscible sulfide melt in Os by variably contaminated silicate magmas in a dynamic magma system results in spatial variations in initial ¹⁸⁷Os/¹⁸⁸Os ratios. Here, too, parallel isochrons, each based on closely spaced samples, provide information on the age and range of initial ¹⁸⁷Os/¹⁸⁸Os ratios.