Cosmogenic 3He in terrestrial rocks: A review

Abstract

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Low-volume magmatism linked to flank deformation on Isla Santa Cruz, Galápagos Archipelago, using cosmogenic 3He exposure and 40Ar/39Ar dating of fault scarps and lavas

“…The cosmogenic component of the 3 He can be calculated from the two measurements, assuming that the inherited 3 He/ 4 He ratio is negligibly affected by cosmogenic 4 He (Kurz 1986b). Radiogenic 4 He ( 4 He*) can also affect apparent cosmogenic age (Blard 2021), but for the relatively young samples in this study, 4 He* is estimated to be less than the error on the 4 He melt measurement (Online Resource 2) and is negligible. Atmospheric helium adsorption, occurring after crushing and prior to melting, may lead to overcorrection of the magmatic 3 He component by several percent (Protin et al 2016) to an extent that cannot be quantified, but was minimized by venting to nitrogen following the crushing step.…”

“…Cosmogenic 3 He can be used to quantify the duration of near-surface exposure to cosmic rays, which serves as a mechanism to date volcanic eruptions (e.g., Kurz 1986a;Blard 2021). This approach can be adapted to the direct dating of deformation.…”

“…To do this, lava samples are collected from fault scarps, where their initial exposure is associated with deformation (Mackey and Quigley 2014;Blahůt et al 2020). In this scenario, the exposure age of a sample reflects the age of a scarp-forming event assuming no erosion and negligible non-cosmogenic 3 He production (Blard 2021). We collected samples for 3 He exposure dating from basaltic lava flows, typically within vertical fissures formed by normal faulting in the subsurface (Martel and Langley 2006).…”

Cosmogenic 10Be in pyroxene: laboratory progress, production rate systematics, and application of the 10Be-3He nuclide pair in the Antarctic Dry Valleys

“…For quality control, internal comparability, and interlaboratory calibration, we also measured helium isotopes in the CRONUS-P pyroxene reference material, which is derived from the NXP 93*52 surface sample (Blard et al, 2015;; Table 2). For all samples, we normalize the measured 3 He concentrations to the accepted CRONUS-P value [(5.02 ± 0.05) x 10 9 ; Tables 1 and 2; Blard, 2021;Blard et al, 2015]. At all laboratories, relative measurement uncertainties for the Labyrinth core samples increased with depth owing to the decreasing 3 He concentrations, ranging from 2% in the uppermost sample to 4% in the lowermost sample.…”

“…At CRPG, 34 measurements were made on 15 CRPG-prepared Labyrinth core samples and 5 measurements were made on 5 BGC-prepared Labyrinth core samples. Sample extraction was realized by complete fusion at 1500°C in our home-design extraction furnace (Blard, 2021;Zimmermann et al, 2018). Typical blanks were (2.1 ± 1.4) x 10 -20 mol of 3 He and (1.9 ± 0.1) x 10 -15 mol of 4 He.…”