The potential of crinoids as (U+Th+Sm)/He thermochronometers

- Crinoids form with sometimes substantial amounts of common He in their columnals.
- Common He is due to either a vital effect or interstellar dust particles.
- Crinoids can be used as a low-T thermochronometer if common He is removed by burial.
- The conditions needed to remove He are similar to the early part of the oil window.

(U+Th+Sm)/He dating of crinoid fossils from Late Paleozoic rocks from a variety of sites in North America suggest that this material can be used as a useful low-T thermochronometer under certain conditions. Available data suggest that common He is present in crinoids and at the low α-production rates typical of calcite, this common He will be sufficient to produce spurious ages in crinoid samples of any age. Heating of samples to temperatures in excess of the opening temperature will expel all He (common and radiogenic), with all subsequent He accumulation being exclusively radiogenic; such samples have potential for being robust thermochronometers sensitive to temperatures in the range of ∼60-110 °C. Without common He, analysts can take advantage of the large size of crinoid columnals and produce reliable ages even when young (Miocene) or with low α-production rates ([eU] = 0.1 ppm). Samples not subjected to post-depositional heating above the opening temperature (burial to depths of 2 to 4 km, depending on geothermal gradient, sedimentation rate, and residence time at peak T) will likely retain common He and be unsuitable for thermochronologic studies.