We measured the thickness of each sample to the nearest millimeter (±1 mm) using calipers before crushing and milling the sample to a grain size of 210 to 750 μm. We then put the 210- to750-μm grain-size fraction through a series of o-phosphoric acid, sodium hydroxide, and dilute hydrofluoric-nitric acid leaches to etch quartz and dissolve other minerals. Once we obtained pure quartz, we spiked the samples and a process blank with a known mass of 9Be carrier and digested them in hydrofluoric acid. We used a 9Be carrier developed from a deeply buried beryl crystal. For carrier concentrations, see table S1. We then used a modified version of the procedures described in (49) to isolate beryllium from each sample and the process blank. 10Be/9Be ratios of samples were measured at the Center for Accelerator Mass Spectrometry at Lawrence Livermore National Laboratory (LLNL) and normalized to the 07KNST3110 standard (table S1) (50). Once obtained, we subtracted the 10Be/9Be process blank ratio from the 10Be/9Be ratios of measured samples and used the blank-corrected sample ratios to determine the concentration of 10Be in each sample per gram of quartz. For process blank ratios, see table S1.

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