UNDERSTANDING THE EVOLUTION OF COMPOSITIONAL ZONING IN SAN JUAN VOLCANIC FIELD IGNIMBRITES USING IRON ISOTOPE RATIOS

The San Juan Volcanic Field of southwestern Colorado represents a large Tertiary aged calc-alkaline ignimbrite flare up. Work by Lipman and coworkers have led to detailed characterization of the volcanic stratigraphy and temporal evolution of many large calderas. Ignimbrites include both compositionally zoned rhyolite to dacite eruptions as well as monotonous intermediates, yet the petrogenetic origin of the field and the mechanisms of magma compositional evolution within eruptions remain largely unknown.

Here we present new iron (Fe) isotope ratio data aimed at better understanding the origin of zoned ignimbrites. We sampled in detail the zoned 33.2 Ma Bonanza Tuff and the 27.55 Ma Carpenter Ridge Tuff (Lipman and McIntosh, 2008) and collected representative samples from the 26.9 Ma Nelson Mountain, 26.9 Ma Rat Creek, and 28.02 Ma Fish Canyon tuffs (Lipman and McIntosh, 2008) as well as two intrusive units from the Bonanza Caldera. Detailed X-ray mapping and compositional analysis were performed; compositions range from andesite to rhyolite, and granodiorite to aplite. Fe was purified from dissolved whole rock samples and analyzed by high resolution MC-ICPMS.

Preliminary Fe isotopic ratio data display relatively constant δ⁵⁶Fe hovering around the mean mafic earth value of 0.09 over much of the compositional range but importantly, increasing to significantly higher δ⁵⁶Fe at the highest silica contents (74% and higher). This behavior mimics the trend in δ⁵⁶Fe observed in many igneous differentiation suites including both zoned ignimbrites and granitoid plutons. The origin of isotopic variations in Fe within high temperature samples remains unidentified; however temperature gradient driven diffusion has been shown to produce large isotopic fractionations in experiments. Based on this, Lundstrom (2009) has proposed that the Fe isotopic behavior with silica content in granodiorite plutons reflects a top down magma differentiation process. The same signature in ignimbrites may mean that their magma chambers form similarly. We will present further data and models of this and traditional differentiation processes to assess the origin of ignimbrite compositional zoning.