Noble gas isotope sites and mobility in mafic rocks and olivine

Diffusion of helium in olivine at 1 atm and 2.7 GPa

“…The He diffusivities measured in this work are generally consistent with results from the study of Futagami et al (1993), who measured He diffusion in natural olivine by outgassing 4 He implanted samples, and with the diffusivities measured by bulk-release of He by , but are about 2 orders of magnitude slower than the recent findings of Tolstikhin et al (2010) and Blard et al (2008). An up-temperature extrapolation of our data also show reasonable agreement with the higher-temperature measurements of Hart (1984); the coincident diffusivities from these studies under the high-temperature conditions used by Hart et al (2008) in their He diffusion/production modeling supports the applicability of their modeling results.…”

“…T ( o C) (Futagami et al, 1993) Hart (1984 He -Olivine Hart (1984), Futagami et al (1993), Trull et al (1991), Trull and Kurz (1993), , Blard et al (2008) and Tolstikhin et al (2010). Diffusivities from the present study are similar to those measured by Futagami et al (1993) and , but about two orders of magnitude lower than those from the studies of Trull et al (1991), Blard et al (2008) and Tolstikhin et al (2010). study, find high 3 He releases in the initial and low-temperature heating steps, resulting in anomalously high diffusivities compared with those determined from later release.…”

“…Such distributions of He may behave differently during step-heating studies than He atoms migrating singly through the lattice, since the latter would not have to exit a bubble or inclusion to escape; these different conditions may be the cause of observed double peaks in release patterns, and measurements of "apparent" diffusivities that differ markedly from others determined for volume diffusion (e.g., Trull and Kurz, 1993;Tolstikhin et al, 2010). Tolstikhin et al (2010) have developed a model to account for effects on He release patterns observed during heating, considering the presence of various amounts of He in the lattice and in inclusions and the partitioning of He between them, finding good agreement with various laboratory measurements.…”

“…Much of the present knowledge of diffusion behavior in olivine has been obtained from outgassing studies, where samples are heated, either in a stepwise manner or isothermally, and the He released measured by mass spectrometry (e.g., Hart, 1984;Trull et al, 1991;Trull and Kurz, 1993;Futagami et al, 1993;Blard et al, 2008;Tolstikhin et al, 2010). In these studies, the release of either naturally-present He (primordial or cosmogenic 3 He or 4 He produced through radioactive decay) or He introduced by irradiation (e.g., ion implantation or spallation reactions induced by a proton beam) are measured.…”

“…Helium may be nonuniformly distributed, depending on the distribution of U and Th in mineral grains (either the grains themselves, or adjacent or included grains that may implant He into the olivine during decay sequences), alpha ejection from mineral grains during radioactive decay, and differential He losses due to past thermal histories. In addition to the aforementioned potential effects on apparent diffusivities of the presence of bubbles or inclusions, microstructures within mineral grains may create diffusion fast paths or other anomalous behavior (e.g., Trull and Kurz, 1993;Blard et al, 2008;Tolstikhin et al, 2010), which cannot be readily distinguished from lattice diffusion because He concentration profiles are not measured directly in these analyses. In some of these studies, the potential complication of nonuniform distributions of helium have been circumvented by producing homogeneous distributions of 3 He by irradiation with high-energy protons (e.g., Blard et al, 2008) or by introducing He through ion implantation (e.g., Futagami et al, 1993), where implanted He ions will be deposited at a known depth and distribution in the sample depending on the incident He energy and the characteristics of the sample material; nonetheless, other limitations of bulk-release studies remain.…”