Collaborative Research: Revisiting the water-saturated granite solidus

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Earth’s uppermost continental crust is composed on average of granitic material. Granites are igneous rocks, a subset of rocks that form by the cooling and subsequent crystallization of molten materials. The temperature at which a water-bearing granite melts (or, upon cooling of a magma, the temperature at which the last drop of molten material crystallizes) is known as the granitic water-saturated solidus (G-WSS). The G-WSS is one of the most important phase boundaries in all of geology. Its location in pressure-temperature space controls the formation of our continents, the generation of economically important gem and ore deposits (e.g., sapphire, lithium, gold, and copper), the eruption of devastating and explosive volcanic eruptions, and how rapidly our planet has cooled over eons. The position of the G-WSS changes with depth (pressure; P), temperature (T) and bulk composition. The G-WSS is analogous to the freezing point of aqueous fluids, and the compositional effect on magmatic freezing points is analogous to changes to the freezing point depression of water caused by addition of various salts (e.g., NaCl, KCl, CaCl, etc.). Pioneering work performed over 60 years ago remains the basis for our understanding of the G-WSS. However, numerous observations from natural systems suggests igneous rocks crystallize at temperatures ~75–100 degrees C lower than the widely accept¬¬ed G-WSS. These observations combined with advances in experimental and analytical techniques provide the motivation and opportunity to re-investigate the location of the G-WSS. The PI’s preliminary work surprisingly demonstrated that the G-WSS is 100 degrees C lower than previous findings, which will transform long-standing views on granite formation processes, continental crust formation, thermal structure in terrestrial bodies, plate tectonics, innumerable aspects in hard-rock petrology and affect explorations of economically important ores. The PIs will conduct a series of laboratory-based experiments to systematically re-define the G-WSS, and then apply observations to the natural rocks contained in the National Museum of Natural History collections. Beyond providing research opportunities to PhD students and Washington DC high school students from under-served communities, the PIs will also produce a series of educational outreach experiences to teach National Mall visitors how ancient magmatic systems generated building stone rocks that compose many of the National Mall’s most famous monuments and buildings.Granitic and rhyolitic rocks are the end-product of continental crust differentiation. Most magmatic systems evolve towards granitic bulk compositions during crystallization, and the first melts of many rocks are broadly granitic. The granitic water-saturated solidus (G-WSS) is the lowest temperature phase boundary fundamentally separating metamorphic and igneous realms; thus, understanding its location in -pressure-temperature-composition space is critical for interpreting the rock record. The accepted G-WSS was largely determined 60 years ago using experimental and analytical techniques that leave open the possibility that the G-WSS may be inaccurate. In natural systems, various thermobarometric applications to granitic and rhyolitic composition rocks commonly return temperature estimates ~75–100 degrees C lower than the widely accept¬¬ed G-WSS. The availability of modern experimental and analytical approaches and the low temperature estimates for mineral crystallization in granitic rocks raise two overarching questions that will be resolved by performing work outlined in this proposal: (1) What is the P–T position of the G-WSS?, and (2) What are the compositions of melts and crystals that coexist along the G-WSS? The PIs will perform a systematic experimental and analytical program to determine the P–T position of the G-WSS and related compositional variations over conditions that span the continental crust. Experiments will be conducted in cold-seal pressure vessels (P5 kbar) and piston-cylinder devices (P5 kbar). The PIs will use electron probe microanalysis to measure major element compositions of experimental run products. Fourier transform infrared and Raman spectroscopy will be used to measure water concentrations in the melt. A statistically rigorous experimental approach, called a design of experiments, will be employed to determine compositions along the G-WSS over a range of pressures spanning the continental crust. Geochemical analyses and thermobarometry of natural granitic rocks will reveal the extent to which low temperatures are recorded in the rock record. Preliminary results from experiments performed from 0.5 to 10 kbar on granitic composition rocks demonstrate that the G-WSS is significantly lower than unanimously accepted estimates. A more accurate understanding of the position of the G-WSS will help to reconcile interpretations of granite formation and storage conditions within silicic magmatic systems, provide new opportunities to understand the thermal structure of the crust on Earth and other terrestrial bodies, and will influence myriad aspects of hard-rock petrology, geophysics, and mineral/ore exploration that will benefit from an accurate description of the G-WSS. This program also includes research opportunities for graduate students, DC-local high school students from underserved communities, development/implementation of Next Generation Science Standards for 5-8 grade students across the country, and an outreach program called “Magmas on the Mall” aimed at educating the broad public on magmatism and how it created the building stones used across the National Mall.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.