Interpretation of the provenance of small‐scale heterogeneity as documented in a single eruptive unit from Mt. Jefferson, Central Oregon Cascades

Abstract

Mt. Jefferson is a large composite volcano located in the central Oregon Cascades that has erupted a diverse compositional suite of lavas from basalt to rhyodacite (50–72 wt. % SiO2). Individual eruptive units contain multiple populations of plagioclase, and a variety of mafic textural/mineralogical components often preserved as large centimeter to millimeter‐sized enclaves. Understanding the processes active in any volcanic center requires that we document the products of those processes. In this contribution, we documented the small‐scale compositional diversity within a single eruptive unit at Mt. Jefferson, the Whitewater Creek andesite, in order to answer three questions: (1) what are the characteristics and scale of diversity in a single eruptive unit, (2) what is the provenance of the observed components, and (3) how does that observed small‐scale diversity relate to the larger‐scale diversity observed between other flows at Mt. Jefferson. Our analyses are based on major, trace element concentrations for phenocrysts, and melt inclusions from a single eruptive unit, the Whitewater Creek andesite which is one of the most heterogeneous units erupted at Mt. Jefferson. We have identified at least four distinct components present at the centimeter scale. These components, identified on the basis of their mineralogy and composition, include three mafic (two pyroxene + plagioclase; plagioclase‐hornblende, and olivine‐orthopyroxene) and one silicic component (dacitic groundmass). To understand the relationship between the observed textural components and the magma types erupted at Mt Jefferson, equilibrium liquid compositions were calculated from the phenocryst compositions. The range exhibited by those calculated liquids covers much of the entire range represented by the lavas at Mt. Jefferson. However, it is difficult to directly connect them to specific magma types observed at Mt. Jefferson. We attribute our inability to directly link the textural components to the known magma types to post mixing diffusive reequilibration during storage and transport. These results suggest that great care should be taken in interpretation of whole rock data.

Geophysical and Geochemical Constraints on Magma Storage Depths Along the Cascade Arc: Knowns and Unknowns

“…Only N = 3 of these Opx-Cpx pairs are in high-T K D equilibrium following Putirka ( 2008). • N = 25 Amp analyses from the ∼10 ka Whitewater Creek andesite reported by Ustunisik et al (2016).…”

“…The only MI measurements from Mount Jefferson to our knowledge are from Ustunisik et al. (2016). However, they do not measure MI H 2 O or CO 2 , so no barometric constraints can be obtained from these samples.…”

“…A transition from widespread mafic volcanism to more focused intermediate and silicic volcanism and the development of the modern edifice began at ∼300 ka (Conrey, 1991; DiGuilio, 2015). Erupted rocks are characterized by very complex, heterogenous crystal cargoes (Conrey, 1991; Ustunisik et al., 2016).…”

Trace Element and Isotopic Characteristics of Plagioclase Megacrysts in Plagioclase Ultraphyric Basalts (PUB)

“…The chemical zonation in plagioclase phenocrysts has already been widely employed to better understand magma dynamics within the crust in arc systems (Bindeman & Bailey, 1999;Davidson et al, 2005;Ginibre et al, 2002;Lange et al, 2013;Nielsen, 2011;Singer et al, 1995;Ustunisik et al, 2014Ustunisik et al, , 2016. Plagioclase crystallization is also limited to maximum pressures of~1 GPa in dry MORB type magmas (Fram & Longhi, 1992;Kohut & Nielsen, 2003;Tepley et al, 2000;Ustunisik et al, 2018), which restricts plagioclase occurrence to the crust and upper mantle.…”

MGP‐1 Plagioclase: A Potential Element and Sr Isotope Reference Material for In Situ Microanalysis

“…Owing to a slow CaAl-NaSi diffusion exchange, compositional and textural zoning patterns are often preserved in plagioclase during primary growth (Grove et al 1984), preserving records of the chemical and physical evolution of magma chambers (e.g., Blundy andShimizu 1991, Singer et al 1995). For example, trace elements such as Sr, Ba, Ti and rare earth elements (REEs) can resolve temporal records of compositional evolution and convective regimes and cooling histories, with wide implications for understanding petrogenesis (mafic and felsic magma) and metallogenesis (mafic magma-derived ore-forming components) (Bindeman and Bailey 1999, Ginibre et al 2002, Nielsen 2011, Lange et al 2013, Ustunisik et al 2016, Cao et al 2019, Nielsen et al 2020, M€ uller et al 2022. In addition, Sr isotopic composition is an important geochemical tracer in solid Earth sciences (Davidson et al 2001, Jackson and Hart 2006, Zhang et al 2018; for example, the radiogenic 87 Sr/ 86 Sr ratio of plagioclase is particularly useful in studies of processes such as magma mixing and contamination (Davidson and Tepley 1997, Knesel et al 1999, Browne et al 2006, Gao et al 2015.…”