EMBRACE-EAR-Seed: Magmatic Evolution and Timing of the Independence Dike Swarm

Large dike swarms represent the plumbing system of areally vast and volumetrically large magmatic systems and are found both on Earth and other planetary bodies. These dike swarms are the feeders for volcanic eruptions in a range of geologic settings, and thus are the connection between magmas that are generated deep in Earth’s mantle and those that travel through the crust to be erupted at the surface. Such dike swarms exist from the deep geologic past, in areas where the volcanic cover has long since been removed, thus they can provide important evidence to help reconstruct the magmatic history of these regions. The Independence Dike Swarm (IDS) extends over 600 km of eastern California, from east of the Sierra Nevada Mountains and southward into the Mojave Desert. These dikes have a broad range of chemical compositions, suggesting many magmatic processes acted upon the magmas prior to their eruption; however, previous work has largely focused on age determinations and the physical emplacement of the dikes, and thus limited information on the detailed chemical compositions exists. Assessing the chemical compositions of such dikes allows for understanding the melting processes that generated the magmas, as well as later processes that changed the magmas before they erupted. The IDS is generally thought to have an age of 148 mega-annum (Ma), but dikes from some studied sections of the swarm have revealed older ages. This study will focus on age determinations and obtaining chemical compositions of dikes from across the swarm, which can help to determine whether the IDS was emplaced at 148 Ma, or if the magmatism was pulsed and episodic over a longer time span. This study will add to the current understanding of how Sierran Nevada magmatism changed over time and paves the way for a deeper understanding of other large dike swarms throughout the world. In addition to advancing knowledge of large dike swarms, this study will provide research and networking opportunities for many Earth Science undergraduate students, expand the on-site research capabilities of an undergraduate institution, provide support for an early career geoscientist and visiting assistant professor in the development of a long-term research program, and seek to expand the bridge between Indigenous and western scientific knowledge. The Independence Dike Swarm (IDS) in eastern California contains the full range of magmatic compositions, from mafic to felsic endmembers and in between, and thus represents a vast record of the processes that generated and differentiated the magmas from the mantle source of melting to subsequent ascent through the crust. The range of compositions present and the parallel orientation of the dikes over the length of the swarm make the IDS distinctive from many other global and planetary large dike swarms. There has been much previous work on structural emplacement of the dikes, but compositional studies are largely limited to major elements. Geochronological work on the mafic compositions has been hampered by a paucity of zircons and in some cases, metamorphic overprinting of mineral phases. Due to a general lack of mafic dike ages, many segments of the IDS are poorly correlated with the swarm overall and are presently considered “tentatively” associated, mainly through structural data. Additionally, in some segments of the IDS that do have abundant age data, some ages are much older than the accepted 148 Ma age of the swarm, suggesting the dikes may have been emplaced via multiple and earlier pulses of magmatism. To reconcile these issues, this research will include: 1) field work to sample and characterize segments of the IDS that are only tentatively correlated with the swarm, where dates are limited by lack of zircons or metamorphism in mafic compositions, and where conflicting ages have been reported; 2) method development for whole-rock Rb-Sr isochron dating on the Pomona College LA-ICP-MS, a technique that can be utilized on mafic dikes to help see through metamorphic events that may have reset mineral phases therein; 3) whole-rock compositional analysis to assess mantle sources and the crustal differentiation history that contributed to generating the magmas. The data collected will be combined with previous age, major element, and tectonic constraints to generate a comprehensive model of the magmatic evolution of the IDS, distinguishing whether different regions of the swarm were emplaced simultaneously or episodically, and if they are genetically related through melting and crustal differentiation mechanisms. The focus on sampling dikes that are tentatively correlated with the IDS will help to further establish the true extent of the swarm. An interpretive model for the magmatic evolution and emplacement timing of the IDS will further elucidate how Sierran arc magmatism changed over time and paves the way for comparative assessment of the emplacement timing and evolution of other large dike swarms in tectonic regimes worldwide. Additionally, this work will expand the method for Rb-Sr dating via LA-ICP-MS to whole rock glasses. Broader impacts of this work beyond petrologic and geochemical knowledge of large dike swarms include: 1) providing research and networking opportunities for ~10 Earth Science undergraduate students; 2) expanding the analytical capabilities of in-house research at a primarily undergraduate institution (PUI); 3) supporting an early career visiting assistant professor focused on developing a robust research program at a PUI; 4) expansion of the principal investigator’s Indigenous geologic knowledge through field collaboration with an archeologist and research into tribal oral histories of the region, which will inform future teaching and interdisciplinary research. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.