Collaborative Research: Reconstructing melt compositions across Cordilleran arc segments to evaluate petrologic proxies of crustal thickness and transcrustal differentiation

The thickness of the Earth’s crust, from the surface to the mantle, varies by location. Geophysical tools can tell us about today's crustal thickness. However, they cannot determine the crustal thickness in the geologic past or how it has changed over time.

Knowing the crustal thickness above ancient subduction zones helps us understand several processes, including the formation of mountains, ore deposits, sediments, and the continental crust, where people live. Researchers commonly use compositional markers of rocks to estimate crustal thickness. However, these compositions may be averages, resulting from many processes that rocks experience during their formation.

As a result, there are large uncertainties in determining paleo crustal thickness. The research team will study the melts that formed the final rock composition, using minerals that crystallized early in the magma's history. The calculated melt compositions will be used to test whether the magmas formed in thick or thin crust, and how this changed over space and time.

The research team will collect data from four ancient arc segments in the western US and will create field research videos to showcase their findings and raise awareness about geoscience research. Collaboration between institutions will create new pathways in field and laboratory science for 10 students engaged in the research project.

Petrologic proxies of arc crustal thickness are often applied to plutonic datasets in ancient, exhumed arc systems to generate an estimate of paleo-arc thickness. However, studies have recognized that bulk-rock trace element ratios in plutonic rocks are composite signals of pressure-dependent crystallization and subsequent magmatic processing. Crystal accumulation is of particular concern, since bulk-rock trace element ratios in samples with accumulated crystals do not represent melt values and thus may not retain information about the source depths from which melts were derived.

The research aims to quantify the extent of this processing and to investigate the underlying petrologic mechanisms that define the composition-crustal thickness relationship, utilizing mineral-melt equilibrium relationships in high-temperature minerals. The research team will use high temperature minerals to 1) identify and quantify crystal accumulation in plutonic samples; 2) reconstruct trace element melt compositions and evaluate the preservation of pressure-dependent signals; 3) assess melt evolution across four arc segments that preserve different instances of arc crustal thickness: across space, through time, and vertically through the crustal column.

Investigation of the underlying petrologic processes that define compositional-crustal thickness relationships is critical to our understanding of the effects of melt fractionation and crystal accumulation on trace element signatures in rocks and minerals. Findings will enable testing of existing models, and potentially generate new models, for magma processing and connectivity in the arc crustal column, and contributes to the larger, first-order question of where most magmatic differentiation processes occur in the crust.

The research team will create field research videos to showcase their findings and raise awareness about geoscience research. Collaboration between institutions will create new pathways in field and laboratory science for 10 students engaged in the research project.

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.