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Earth's particular style of plate‐tectonics—characterized by localized deformation along dynamic plate boundaries and long‐lived stable plate interiors—appears to be unique among rocky objects in the solar system. However, it is entirely unknown how common plate tectonics and related lithospheric phenomena are among the vast population of exoplanets discovered astronomically or assumed to exist throughout the Universe. In this study, we explore the effect of planetary composition on mylonitization—a set of microphysical processes that is commonly associated with shear localization and plate boundary deformation on Earth. A model for planet compositions, based on stellar spectroscopy, is used to define a plausible range of theoretical mineral abundances in the mantles of rocky Earth‐sized exoplanets. These mineral abundances, along with experimental rock rheology, are used to model microphysical evolution with two‐phase mixing. The model is then used to determine the effect of composition on the time‐scales for shear zone formation. We demonstrate that lithospheres composed of sub‐equal proportions of two mineral phases will form shear zones over relatively short time‐scales, a more favorable condition for forming Earth‐like plate boundaries. In contrast, lithospheres that are nearly monomineralic may require unrealistically long time‐scales to form plate boundary shear zones. Using this approach, we identify specific nearby stars with the optimal range of compositions to be targeted by future astronomical missions, including the Habitable Worlds Observatory. 

Abstract The chemical and isotopic characteristics of a solidified pluton represent the integration of magmatic and sub-solidus processes operating across a range of spatial and temporal scales during pluton construction, crystallization, and cooling. Disentangling these processes and understanding where chemical and isotopic signatures were acquired requires the combination of multiple tools tracing processes at different time and length scales. We combine whole-rock oxygen and Sr-Nd isotopes, zircon oxygen isotopes and trace elements, and mineral compositions with published high-precision U-Pb zircon geochronology to evaluate differentiation within the bimodal Guadalupe Igneous Complex, Sierra Nevada, California (USA). The complex was constructed in ~300 k.y. between 149 and 150 Ma. Felsic magmas crystallized as centimeter- to meter-sized segregations in gabbros in the lower part of the complex and as granites and granophyres structurally above the gabbros. A central mingling zone separates the mafic and felsic units. Pluton-wide δ18O(whole-rock), δ18O(zircon), and Sr-Nd isotopic ranges are too large to be explained by in situ, closed-system differentiation, instead requiring open-system behavior at all scales. Low δ18O(whole-rock) and δ18O(zircon) values indicate assimilation of hydrothermally altered marine host rocks during ascent and/or emplacement. In situ differentiation processes operated on a smaller scale (meters to tens of meters) for at least ~200 k.y. via (1) percolation and segregation of chemically and isotopically diverse silicic interstitial melt from a heterogeneous gabbro mush; (2) crystal accumulation; and (3) sub-solidus, high-temperature, hydrothermal alteration at the shallow roof of the complex to modify the chemical and isotopic characteristics. Whole-rock and mineral chemistry in combination with geochronology allows deciphering open-system differentiation processes at the outcrop to pluton scale from magmatic to sub-solidus temperatures over time scales of hundreds of thousands to millions of years.