Textures and compositions of olivine and plagioclase in primitive basalts of the Poison Lake chain in the Lassen Region of the southern Cascades

The Poison Lake chain (PLC) is located 30 km east of the Lassen Volcanic Center, at the eastern boundary of the southern Cascades and the western margin of the Basin and Range. PLC basalts erupted from monogenetic cinder cones and are subdivided into 9 geochemical groups, ranging in age from 100±10 ka (1). One group, "the basalts of old railroad grade" (bg), is split into four units that have phenocrysts of euhedral olivine and fine grained plagioclase. Point counts indicate samples contain 5-10% olivine phenocrysts (cores with Fo: 84-88%) and 7-20% plagioclase phenocrysts (cores with An: 84-87%, except bg1 with cores ≥ An72%) that make three Textural Types. Texture Type I includes the most primitive bg units (bg3 and bg4) and features embayed olivine with homogeneous cores (Fo: 85-88%) with slightly lower forsterite rims (Fo: 80-84%) and normal zoned plagioclase (cores: An 85-87%; rims: An 70-75%). Unit bg1 represents Texture Type II with glomerocrysts of embayed, resorbed olivine with homogeneous cores (Fo: 84-86%; rims Fo: 81-82%) and plagioclase that are embayed, resorbed, swallow-tailed and normal zoned (cores: An 72-88%; rims: An 55-70%), some of which are also oscillatory zoned. Texture Type III includes bg2 samples with embayed, resorbed, and normal zoned olivine (cores Fo: 84-88%; rims: Fo: 78-80%) and embayed, resorbed, and normal zoned plagioclase (cores: An 79-85%; rims: An 76-84%), with some oscillatory zoned crystals. Texture Types are the result of compositional or temperature variations in the magma. Cooling experiments can be used to test temperatures of crystallization and development of textures. MELTS (2) models predict olivine crystallization started at 1252C, which can be used as a starting point for crystallization experiments, followed by heating and cooling experiments to replicate development of embayed textures. (1) Muffler et al., 2011 (2) Ghiorso and Sack, 1995