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1. Constraining the timing and conditions of magmatism and metamorphism in the lower crust of the Southern California batholith using U-Pb zircon geochronology and Ti-in-zircon thermometry

   Robles, F. ; Schwartz, J. ; Miranda, E. ; Klepeis, K.A. ; and Mora-Klepeis, G. ( October 2022 , Geological Society of America Abstracts with Programs)

   The Southern California batholith contains a geologic record that can help clarify the timing of events that occurred during the Late Cretaceous (100-65 Ma) along the western margin of the North American Cordillera. The subduction of the oceanic conjugate Shatsky plateau beneath North America is postulated to have ended active magmatism in the arc at 88-70 Ma; however, the timing of this event is poorly constrained in Southern California. We use U-Pb laser ablation zircon petrochronology to document the timing and conditions of magmatism and metamorphism in the lower crust of the Cretaceous arc. We focus on the Cucamonga terrane in a part of the Southern California batholith located northeast of Los Angeles in the southeastern San Gabriel Mountains. These rocks contain exhumed lower crustal (7-9 kbar) rocks predominantly composed of granulite-facies metasedimentary rocks, migmatites, charnockite and dioritic to tonalitic gneiss. We report 20 new zircon dates from 11 samples, including 4 mafic biotite gneisses, 3 mylonitic tonalites, 2 charnockites, a quartzite, and a felsic pegmatite dike crosscutting granulite-facies metasedimentary rocks. New 206Pb/238U ages show that magmatism occurred in the Middle Jurassic (ca. 172-166 Ma), the Early Cretaceous (ca. 120-118 Ma), and the Late Cretaceous (88-86 Ma) at temperatures ranging from 740 to 800 oC. Granulite-facies metamorphism and partial melting of these rocks occurred during the 88-74 Ma interval at temperatures ranging from 730°C to 800oC. Our data indicate that high-temperature arc magmatism and granulite-facies metamorphism continued through the Late Cretaceous and overlapped in timing with postulated subduction of the conjugate Shatsky plateau from previous models. We speculate that termination of arc activity and cooling of the lower crust in response to plateau subduction must postdate ca. 74 Ma. 

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2. CONSTRAINING THE TIMING AND CONDITIONS OF MAGMATISM AND GRANULITE- TO UPPER AMPHIBOLITE-FACIES METAMORPHISM IN THE LOWER CRUST OF THE SOUTHERN CALIFORNIA BATHOLITH USING U-PB ZIRCON GEOCHRONOLOGY AND TI-IN-ZIRCON THERMOMETRY

   https://doi.org/10.1130/abs/2022AM-381996  

   Robles, Francine ; Schwartz, Joshua ; Miranda, Elena ; Klepeis, Keith ; Mora-Klepeis, Gabriela ( January 2022 , Geological Society of America abstracts)
3. CHICXULUB ZIRCON (AND APATITE!)

   Wittmann, Axel ( May 2018 , Annual meeting of the Meteoritical Society)

   Poster presentation of shock metamorphic features of accessory minerals in drill core samples of the Lower Peak Ring section of the Chicxulub impact crater. 

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4. Influence of Fe underlayers on stress evolution of Ti in Ti/Fe multilayers

   https://doi.org/10.1116/1.4962291  

   Wan, Li ; Thompson, Gregory ( November 2016 , Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films)
5. Valences of Ti, Cr, and V in Apollo 17 high‐Ti and very low‐Ti basalts and implications for their formation

   https://doi.org/10.1111/maps.13123  

   Simon, Steven B. ; Sutton, Stephen R. ( June 2018 , Meteoritics & Planetary Science)

   To assess the variability of redox states among mare basalt source regions, investigation of the valence of Ti, Cr, and V and the coordination environment of Ti in pyroxene and olivine in lunar rocks viaXANES(X‐ray absorption near‐edge structure) spectroscopy has been extended to Apollo 17 basalts: two high‐Ti (70017 and 74275) hand samples, and three very low‐Ti (70006,371, 70007,289B, and 70007,296) basalt fragments from the Apollo 17 deep drill core. Valences of Ti in pyroxene of both suites range from 3.6 to 4, or from 40% to 0% Ti³⁺, averaging 15–20% Ti³⁺. Assuming Ti³⁺is more compatible in pyroxene than Ti⁴⁺, then even lower Ti³⁺proportions are indicated for the parental melts. TheVLTpyroxene exhibits a slightly wider range of V valences (2.57–2.96) than the high‐Ti pyroxene (2.65–2.86) and a much wider range of Cr valences (2.32–2.80 versus 2.68–2.86); Cr is generally reduced inVLTpyroxene compared to high‐Ti pyroxene. Valences of Ti and Cr inVLTpyroxene become less reduced with increasing FeO contents, possibly indicating change in oxygen fugacity during crystallization. Olivine in all samples has very low (<20%) proportions of Ti³⁺, with no Ti³⁺and higher proportions of Ti in tetrahedral coordination in theVLTs than in the high‐Ti basalts. Olivine in 74275, including that in a dunite clast, has much higher proportions of Cr²⁺than the pyroxene in that sample, consistent with previous studies indicating that the olivine grains in this sample are xenocrysts and possibly indicating oxidation just prior to pyroxene crystallization. Results for this sample, theVLTs, and previously studied Apollo 14 and 15 basalts all indicate that mare magmas were in reducing environments at depth, as recorded in early crystallization products, and that later, presumably shallower environments, were relatively oxidizing; single, characteristicfO₂s of formation cannot be assigned to these samples. A process likely to account for this feature seen in multiple samples is loss by degassing of a reducing, H‐rich vapor (probably H₂) during ascent and/or eruption, causing oxidation of the residual melt, recorded in relatively late‐crystallized pyroxene.

    

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