Oxygen isotope ratios in authigenic magnetites from the Belden Formation, Colorado

Abstract

Authigenic magnetites from the Belden Formation at Bassam Park, Colorado, were analyzed for their 18 O/ 16 O composition using secondary ion mass spectrometry. The δ 18 O values of magnetite‐rimmed pyrite (MagPy) grains averaged −8.5‰ (versus VSMOW). Small (25–50 μm) iron oxide grains had a range of δ 18 O values from −9.7‰ to −5.5‰, consistent with a variably hematized magnetite population having a δ 18 O value of around −9‰ for magnetite. The δ 18 O results suggest that the magnetite rims and grains formed in contact with a water having δ 18 O of near 0‰ or less, implying a meteoric or connate source rather than a highly evolved orogenic or basinal fluid. This supports paleomagnetic results that suggest the formation of authigenic magnetites under Mesozoic burial conditions at some Belden Formation localities, rather than in response to orogenic fluid migration driven by Early Tertiary tectonic uplift. The δ 18 O results also support the observation that chemical remanent magnetizations related to authigenic magnetite formation cannot be universally applied to the dating of fluid migration events associated with orogenic deformation.