Unravelling titanium’s place in garnet
Si, Al, Ca, Mg, Fe, and O are the major elements that, in most cases, constitute the bulk of most garnet crystals formed in Earth’s crust. Other elements, such as Ti, Cr, and Mn, may also be present in minor amounts. Of these, Ti has the most uncertainty regarding its substitution mechanisms in the garnet structure. Garnet Ti content is positively correlated with higher pressures and temperatures of metamorphism and represents a promising opportunity to develop thermobarometric tools for assessing rock pressure and temperature histories, but a quantitative calibration remains elusive because of how complex garnet Ti substitutions are.
In this paper, we use the chemical compositions of natural and experimentally-produced garnets to constrain the dominant mechanisms that control Ti substitution in volatile-poor garnets formed at the conditions of Earth’s crust.
Image: Garnet crystal in pegmatite matrix, Ishikawa District, Fukushima Prefecture, Japan
Major slab melting events during the Proterozoic eon
Massif-type anorthosites are giant igneous intrusions that formed only during Earth’s middle age and characterize Proterozoic convergent margins worldwide. They hold important ore deposits of critical minerals, and their restricted formation in time has represented a fundamental mystery in geology for many decades. Why were giant plagioclase formed and intruded into the crust during the Proterozoic? What can these rocks tell us about tectonics and magmatism on the early Earth?
Using B, O, Nd, and Sr isotope data and petrogenetic modelling, we showed that melting of subducted oceanic crust is required to explain the geochemistry of the Marcy and Morin anorthosites in the Grenville orogen. We present a slab melting hypothesis that is relevant to all massif-type anorthosites and postulate that these rocks formed during slab melting events on the early Earth. Their disappearance from the rock record reflects the cooling upper mantle and the rarity of volumetrically major slab melting events since the Cambrian.
Image: mm-scale labradorescent plagioclase mineral separate viewed through a binocular microscope. Plagioclase mineral separates were used to collect O, Nd, and Sr isotope data for our study.
Tracing the fate of magmatic copper with isotope geochemistry
What factors determine the locations of ore deposits? In this study we used osmium and oxygen isotopic tracers to study the processes that produce hydrous sulfide-bearing cumulates in arc magmas. Results suggest that assimilation of metasedimentary country rocks by mafic magmas during their residence in the lower crust encourages sulfide saturation and deposits Cu in the arc root.
Image: Hydrous ultramafic rocks exposed in the exhumed lower crust of the Acadian Orogen, CT, USA.
See Tassara et al. 2022, Tassara et al. 2021, Keller and Ague 2018, and Keller and Ague 2020 for more information on the locality.
Considering the effects of subducted iron formations on the Earth system
Earth’s Archean and Proterozoic oceans produced enormous amounts of iron formations, chemical sediments rich in iron. What effects upon the deep Earth would these rocks have had once introduced into the mantle (e.g., by subduction or foundering)? We explore the possibility that due to the behavior of Fe-oxides at lower mantle conditions, dense subducted iron formations might form highly conductive regions of the lower mantle that facilitate mantle plume upwellings, ultimately producing large igneous provinces (LIPs).
Featured on the cover of Nature Geoscience, vol. 16, issue 6 (June 2023)
Image: Polished banded iron formation from Western Australia. Photo by L. Welzenbach, Rice University. FOV approx. 2 cm.
Predicting and explaining exsolution behavior
Why do exsolved crystalline phases adopt the alignments relative to their hosts that we observe? We examined the crystallography of exsolved rutile, ilmenite, apatite, corundum, and quartz precipitates in garnet and applied a concept from metallurgy called “edge-to-edge matching” that can explain much of the observed behavior. Read about how this concept works, the some promising ways in which it can be applied to retrieve information about a rock’s history, and exciting frontiers for testing it in other mineral systems.
Recognizing sample contamination
Coesite and diamond are the gold-standard indicator minerals for ultrahigh-pressure metamorphism. We document several scenarios from our sample preparation and analysis that run the risk of being misinterpreted as genuine metamorphic coesite and diamond, including contamination from diamond and SiC polishing grit.
The full paper is available to subscribers on the NJMA website, linked by the button below. A preprint is available at this link to the preprint repository here on my site.
Ultrahigh-pressure metamorphism during Appalachian orogenesis left fingerprints in garnet
Exsolution lamellae of silicates, oxides, and phosphates in garnet reveal metasediment burial to ultrahigh-pressures >5 GPa during the formation of the Appalachian mountains. Return of these extraordinary rocks to the base of the crust shows geochemical recycling of subducted material. Findings have implications for recognizing ultrahigh-pressure rocks using garnet chemistry and for understanding the dynamics of orogenesis.
Image: Thin section photomicrograph of quartz, ilmenite, apatite, and rutile exsolution lamellae in garnet, Brimfield Schist, Connecticut, U.S.A. Vertically integrated focus.
Identifying exsolved phases in garnet using crystallography
Common crystallographic traits connect precipitates in garnet from multiple localities and shed light on the factors influencing exsolution in garnet. Similarities between rutile exsolution from garnet and exsolution textures in meteorites open avenues for comparative study.
Image: Exsolution textures in Sacramento Mountains iron meteorite, YPM MIN.101193. Courtesy of the Division of Mineralogy & Meteoritics; Peabody Museum of Natural History, Yale University; peabody.yale.edu. Photography by Duncan Keller.
Exhumed Appalachian mountain roots in Connecticut
Garnet, spinel, and corundum, combined with temperature estimates from ternary feldspars, show that rocks from the New England Appalachians formed at the base of over-thickened crust.
Image: Thin section photomicrograph of perthite and antiperthite with other feldspar in thin section from high-pressure granulite, Brimfield Schist, Connecticut, U.S.A. Field of view is ~5.5 mm.