75-16 Comparison of Phase Assemblages and Microstructures of Holocene and Miocene Basaltic Lavas at Dotsero Crater, Colorado

Session: Mineralogy, Geochemistry, Petrology, and Volcanology Student Session (Posters)

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Nearby Dotsero Crater, Colorado are two basaltic lava flows that are spatially proximal, but temporally distinct. The younger flow extruded during the Holocene at ~4 ka (amidst the sequence of volcanism that produced Dotsero Crater) about a mile southwest of the older lava flow erupted during the Miocene (~ 20 Ma). Here, we present preliminary mineralogical and textural data from samples of both lava flows to test whether xenocrysts from any crystallized remnants of the Miocene system were incorporated into the Holocene magmatic system during its existence. In this initial investigation, we prepared thin sections for petrographical analysis using a polarized light microscope, as well as for semi-quantitative mineral compositional analysis using energy dispersive spectroscopy on a scanning electron microscope. Both Holocene and Miocene lava samples have similar phase assemblages and compositions but differences in their mineral modes and textures. The Miocene flow is holocrystalline with a fine-grained matrix consisting of sub-ophitic plagioclase, clinopyroxene, and Fe-Ti oxides, with slightly coarser phenocrysts of plagioclase, clinopyroxene, altered olivine (iddingsite), and orthopyroxene in order of decreasing abundance. In the Miocene flow, some of the rare orthopyroxene grains are rimmed by clinopyroxene and clinopyroxene grains are occasionally rimmed by unaltered olivine. Interestingly, many of the altered olivine cores are also rimmed by unaltered olivine in the Miocene flow. In contrast, our sample of the Holocene flow is hypocrystalline with ~20% phenocrysts of plagioclase, clinopyroxene, unaltered olivine, and Fe-Ti oxides. Plagioclase in the Holocene flow makes up a smaller proportion of the mineral assemblage than in the Miocene flow.  Across both lavas, we observed compositionally similar plagioclase (labradorite) and clinopyroxene (mostly diopsidic augites).  Olivine from the Holocene lava ranges in composition from Fo₇₅ - Fo_82, consistent with the results of Rowe et al. (2011). The olivine grains from the Miocene flow are more Fe-rich (Fo₄₆ - Fo₆₆), likely as a result of Mg-loss during their alteration, and therefore are not reflective of equilibrated olivine compositions in the Miocene system. The similarity of plagioclase and clinopyroxene compositions in the two flows precludes a semi-quantitative assessment from yielding a conclusive interpretation about the extent of Miocene xenocryst incorporation into the Holocene flow. Further examination of major/minor elements and trace element concentrations using more precise quantitative methods could provide additional clarity.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

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