Use of Smectite as a Geothermometer to Understand Emplacement Temperatures of Mount Calavera Dome, Carlsbad, California

Caleb Adkins

Caleb Adkins
BS Candidate
Advisor: Heather Webb and Dr. Jillian Maloney

May 4th, 2017 – CSL 422, 9 am
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Clay minerals have been used as geothermometers to determine source rock maturation and basin analysis in the oil and gas industry, as well as to understand heat generation along faults during rupture. In both of these applications the proportion of discrete illite (I) in mixed-layer illite/smectite (I/S) is correlated to heat. However, in our study we will evaluate if the presence of smectite can be used to limit the temperature regime of emplacement of an adakitic dome, located in Carlsbad, CA. Until recently, Mount Calavera was thought to be the solidified neck of an ancient volcano, however recent research provided by Mohammad El-Najjar suggests that the feature is extrusive in nature, most likely a dome with adakitic composition. Understanding the temperatures at which the dome was emplaced may help us further understand its genesis. The Miocene adakitic dome is in contact with the sediments of the middle Eocene Santiago Fm. on most sides. However, a bake zone is only exposed along the NW portion of the western flank of the dome. During this study seven samples were collected, four from the baked sediments, two from the unbaked sediments, and one from the adakite. All seven samples were analyzed for clay mineralogic composition by x-ray diffraction and petrologic descriptions. The < 4 μm fraction of samples CA 1, CA 2, CA 3, CA 4 (baked Santiago Fm), CA 5, and CA 6 (unbaked Santiago Fm.), include discrete smectite, discrete illite, and lesser amounts quartz. The < 4 μm fraction of CA 7 (adakite) was dominated by smectite with lesser amounts of quartz and feldspars. We view the strong presence of smectite in all the samples as an indication that temperatures during emplacement were < 200 °C, the point at which smecitite collapses to an illite structure. The determination of a low heat signature corroborates with the idea that Mount Calavera is a surface feature, rather than a volcanic plug.