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Nearly two decades since the first oxygen isotope (δ18O) studies of zircon in the Sierra Nevada Batholith, California, USA, a far more extensive picture of spatial and temporal patterns of magmatic δ18O has emerged in parallel with a tenfold increase in geochronologic coverage, and many new radiogenic isotope (Sr, Nd, Hf) analyses. Over this time, models of Cordilleran-type arc systems have sought to elucidate flare-ups of magmatism as cyclic, with radiogenic isotope “excursions” tracing variable input of crust and mantle into arc magmas [e.g., 1]. Such models haven't incorporated oxygen isotopes to full advantage because of apparent complexity in the signals they record [2]. New, single zircon δ18O analyses—of plutonic, volcanic, and detrital zircon—from the Sierra amplifiing the findings of previous studies [e.g., 3], that δ18O records are well-suited for detecting relatively fast (<10 million year) recycling of subducted supracrustal rock and accreted terranes in forearc settings. Such recycling is not resolved by radiogenic isotope systems. A wealth of new volcanic δ18O zircon data from the Sierra, along with δ18O of hydrothermal minerals like skarn garnet, also records periods of significant δ18O “pull-downs” as lower-δ18O hydrothermal waters alter surface rocks whose assimilation subsequently embeds these surface signals in silicic volcanic systems. Such re-melting and volcanic episodes are often brief (< 5 million years) and small volume, so have often been overlooked, however such, δ18O values may be key to detecting plutonic from volcanic zircon in detrital records when used in conjunction with trace elements. Low-δ18O domains are becoming recognized in other arcs and to be useful to detect episodic resampling of crustal domains [4]. Morover, discovery of fossil low-δ18O systems in screens of wallrock in mid-crustal levels [e.g., 5] documents wholesale rapid burial of these domains in arcs, during transitions to episodes of shortening or transpression. All together, zircon δ18O uniquely traces surface- to-source transport and recycling in Cordilleran arcs as it relates to changing arc stress regime, at periods that may fail to be recorded in excursions of radiogenic isotopes, such as relaxation of stress regimes in upper plate domains. [1] DeCelles, P. G. et al. Nature Geoscience 2, 251-257 (2009) ; [2] Chapman, J. B. et al. Lithos 398- 299, (2021); [3] Lackey, J. S., et. al. J. Petrology 49, 1397–1426 (2008); [4] Turnbull, R. E. et al. Gondwana Res. 121, 436-471; [5] Ryan-Davis, J. et al. Contributions to Min. and Pet 174, 19 (2019) 

Garnet U‐Pb dating by laser ablation‐inductively coupled plasma‐mass spectrometry requires the development of matrix‐matched reference materials of variable chemistry and U mass fraction for accurate analysis. Additional calibration of existing primary reference materials is also justified based on the relatively poor calibration of some of the widely available primary reference materials that are currently utilised by the geoscience community. We present a micro sampling workflow combined with a refined ID‐TIMS methodology for the generation of high precision (~ 0.1%) U‐Pb dates from domains within garnet single crystals. Using this workflow, we calibrated two new natural andradite reference materials, the Jumbo andradite (And₉₉; 110.34 ± 0.03 (0.04) [0.13] Ma,n= 7, MSWD = 1.21) and the Tiptop andradite (And₈₇; 209.57 ± 0.11 (0.13) [0.26] Ma,n= 6, MSWD = 1.39). We also present additional calibration of the widely utilised Willsboro‐Lewis andradite primary reference material (And₉₀; 1024.7 ± 9.5 (9.6) [9.6] Ma (2s; overdispersed),n= 6). Wafers of the Jumbo and Tiptop andradite reference materials are available from the authors upon request.