Squeezing Water From a Stone: H₂O In Nominally Anhydrous Minerals From Granulite Xenoliths and Deep, Hydrous Fractional Crystallization

Abstract

Water is key to plate tectonics on Earth, which, in turn, is vital to the production of continental crust. Although arc lavas erupt in a volatile-rich state and calc-alkaline arc plutons are distinguished by the presence of hydrous minerals such as hornblende and biotite, the water content of arc magmas earlier in their evolution-in the deep crust-remains poorly constrained. Here, we report H 2 O contents in nominally anhydrous minerals measured in situ on petrographic thin sections by secondary ion mass spectrometry of Proterozoic deep crustal xenoliths from Colorado, USA. Clinopyroxene, orthopyroxene, and garnet contain average H 2 O contents ranging from 75-760, 233-410, and 42-139 ppm, respectively. Reconstructed bulk rock H 2 O contents range from~60 to~650 ppm. Intermineral H 2 O ratios overlap experimental mineral/melt D values and are used to calculate H 2 O of melts last in equilibrium with the xenoliths. We propose that these xenoliths represent cumulates fractionated from a primitive, hydrous (≥1 wt.% H 2 O) melt at high (~1 GPa) pressures, similar to conditions in modern subduction zones and potentially associated with widespread arc accretion that formed the core of North America in the Precambrian.

Fluorine-rich mafic lower crust in the southern Rocky Mountains: The role of pre-enrichment in generating fluorine-rich silicic magmas and porphyry Mo deposits

“…Altogether, the observations of granulite textures in the mafic lower crustal xenoliths (Bradley 1985;Farmer et al 2005;Chin et al 2020; Fig. S1), as well as amphibole compositions and Ti in calcium amphibole data presented here, strongly suggest that portions of the mafic lower crust of northern Colorado were subjected to high or UHT (>900 °C) peak metamorphic conditions.…”

“…Fluorine enrichment must have occurred prior to Laramide hydration due to the age of the xenoliths (e.g., Humphreys et al 2003), and it could be an ancient feature of the mafic lower crust that extends throughout the southern Rocky Mountain region (Snelson et al 2005). Our results suggest that ancient enrichment of F in the deep mafic lower crust, either by UHT granulite grade metamorphism or deep crustal magma processes (e.g., Chin et al 2020), could be a necessary pre-condition for generating F-rich silicic magmas at a later stage. In light of xenolith compositions reported here, we suggest that F-rich leucogranites and rhyolites in the Colorado Mineral Belt and Never Summer complex inherited their initially high halogen abundances mainly from their lower crustal source.…”

“…S1), as well as trace to minor quartz, alkali feldspar, biotite, rutile, ilmenite, zircon, and apatite (Bradley 1985). The variable mineralogy of the xenoliths has been interpreted to reflect equilibration of compositionally similar rocks over a range of mid-to lower-crustal temperature and pressure conditions (Bradley and McCallum (Farmer et al 2005;Chin et al 2020). Zircon U/Pb geochronology indicates primary igneous crystallization occurred at approximately 1720 Ma, but a subset of approximately 1360 Ma zircons showed sector zoning in cathodoluminescence images were interpreted as metamorphic overprint (Farmer et al 2005).…”

“…Furthermore, the presence or absence of For submission to American Mineralogist garnet in the State Line mafic lower crustal xenoliths has been interpreted to be a magmatic feature related to crystallization of basaltic melts at varying crustal depths (Bradley and McCallum 1984;Bradley 1985). Chin et al (2020) suggested that the garnet-bearing mafic lower crust in northern Colorado represent cumulates that fractionated from primitive hydrous melts during the Proterozoic. Hence, the higher T recorded in shallower rocks garnet-absent mafic granulite xenoliths could be due to a different UHT metamorphism event that did not affect the lowermost, garnet-bearing mafic lower crust.…”

“…6). The protolith for mafic granulite lower crust xenoliths is interpreted to be hydrous (>1 wt% H 2 O) arc-related basaltic melts (Farmer et al 2005;Chin et al, 2020), and therefore the Sr/Y ratio of silicic melts derived from deep mafic crustal anatexis is likely For submission to American Mineralogist controlled by the stability of amphibole (± garnet) in the source (Jacob et al, 2015). The low Sr/Y, high Nb and Y trace element signature is also observed in F-rich leucogranites from Mount Antero, as well as high silica rocks related to Mo mineralization at Climax and Henderson (Fig.…”