Nano- and micro-geochronology in Hadean and Archean zircons by atom-probe tomography and SIMS: New tools for old minerals

Abstract

Atom-probe tomography (APT) and secondary ion mass spectrometry (SIMS) provide complementary in situ element and isotope data in minerals such as zircon. SIMS measures isotope ratios and trace elements from 1-20 μm spots with excellent accuracy and precision. APT identifies mass/ charge and three-dimensional position of individual atoms (±0.3 nm) in 100 nm-scale samples, volumes up to one million times smaller than SIMS. APT data provide unique information for understanding element and isotope distribution; crystallization and thermal history; and mechanisms of mineral reaction and exchange. This atomistic view enables evaluation of the fidelity of geochemical data for zircon because it provides new understanding of radiation damage, and can test for intracrystalline element mobility. Nano-geochronology is one application of APT in which Pb isotope ratios from sub-micrometer domains of zircon provide model ages of crystallization and identify later magmatic and metamorphic reheating.Based on SEM imaging and SIMS analysis, 11 needle-shaped specimens ~100 nm in diameter were sampled from one Archean and two Hadean zircons by focused ion-beam milling and analyzed with APT. The three-dimensional distribution of Pb and nominally incompatible elements (Y, REEs) differs at the atomic scale in each zircon. Zircon JH4.0 (4.007 Ga, Jack Hills, Western Australia) is homogeneous in Pb, Y, and REEs. In contrast, Pb and Y and REEs are clustered in sub-equant ~10 nm diameter domains, spaced 10-40 nm apart in zircons ARG2.5 (2.542 Ga, Grouse Creek Mountains, Utah) and JH4.4 (4.374 Ga, Jack Hills). Most clusters are flattened parallel to (100) or (010). U and Th are not collocated with Pb in clusters and appear to be homogeneously distributed in all three zircons. The analyzed domains experienced 4 to 8 × 10 15 α-decay events/mg due to U and Th decay and yet all zircons yield U-Pb ages by SIMS that are better than 97% concordant, consistent with annealing of most radiation damage. The 207 Pb/ 206 Pb ratios for the 100 nm-scale specimens measured by APT average 0.17 for ARG2.5, 0.42 for the JH4.0, and 0.52 for JH4.4. These ratios are less precise (±10-18% 2σ) due to the ultra-small sample size, but in excellent agreement with values measured by SIMS (0.1684, 0.4269, and 0.5472, respectively) and the crystallization ages of the zircons. Thus Pb in these clusters is radiogenic, but unsupported, meaning that the Pb is not spatially associated with its parent isotopes of U and Th. For the domain outside of clusters in JH4.4, the 207 Pb/ 206 Pb ratio is 0.3, consistent with the SIMS value of 0.2867 for the zircon overgrowth rim and an age of 3.4 Ga. In ARG2.5, all Pb is concentrated in clusters and there is no detectable Pb remaining outside of the clusters. The Pb-Y-REE-rich clusters and lack of correlation with U in ARG2.5 and JH4.4 are best explained by diffusion of Pb and other elements into ~10 nm amorphous domains formed by α-recoil. Diffusion distances of ~20 nm for these elements in crystalline zircon are consistent with heatin...

Metallic Pb nanospheres in ultra-high temperature metamorphosed zircon from southern India

“…Investigations of the location of Pb in zircon on the atomic scale using atom probe tomography (APT) have been reported by several groups (Valley et al 2014(Valley et al , 2015Peterman et al 2016;Piazolo et al 2016). All of these studies documented clustering of Pb atoms in zircon but not the formation of metallic Pb spheres similar to those observed in UHT zircon by TEM (this study; Kusiak et al 2015Kusiak et al , 2017.…”

Advances and Opportunities in Ore Mineralogy

“…APT couples chemical sensitivity and three-dimensional capabilities with a spatial resolution near the atomic scale to provide unique information about nanoscale structures. The technique has been demonstrated as a valuable tool to complement SIMS and allow for smallest-scale geochronology (e.g., [103]), to understand nanoscale diffusion within zircon [104,105], and for the examination of homogeneity within geochronological reference materials [106].…”

Magma reservoir dynamics at Toba caldera, Indonesia, recorded by oxygen isotope zoning in quartz

“…1). These tools permit determination of isotope ratios on a sub-crystal scale, allowing the analysis of small sample volumes and thus production of high spatial resolution data within and between individual crystal growth zones (e.g., refs 2, 3, 4). …”

“…Crystal-scale oxygen isotope analysis is now routinely carried out using the SIMS technique2345. For example, the oxygen isotopic composition of zircon is a robust source of information on the crystallisation environment346 and, as zircon also lends itself to U-Pb dating, it can help to identify and place constraints on the timing of geological events (e.g., refs 6 and 7).…”

“…For example, the oxygen isotopic composition of zircon is a robust source of information on the crystallisation environment346 and, as zircon also lends itself to U-Pb dating, it can help to identify and place constraints on the timing of geological events (e.g., refs 6 and 7). Quartz is a common mineral phase in high-silica metamorphic and magmatic rocks and, like zircon, is largely resistant to alteration and retains its magmatic δ 18 O values post-eruption8.…”