50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132) 2214.pdf

RECENT SUCCESSES AND FUTURE PROMISE OF ABLATION MICROPROBE NOBLE GAS GEOCHRONOLOGY OF PLANETARY MATERIALS K.V. Hodges1, A.E. Brunner1, J. A. Cartwright2, C.S. McDonald1, C.M., Mercer1, M.C van Soest1. 1School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, USA ([email protected]); 2Department of Geological Sciences, University of Alabama, Tusca- loosa, AL 35487, USA.

Introduction: Most published 40Ar/39Ar analyses of recent and on-going UVLAMP 40Ar/39Ar studies of im- planetary materials have been obtained through incre- pact melt samples from the as well as howardite mental heating experiments on relatively large, whole- , presumably sourced from the regolith of as- rock samples or mechanically separated minerals or teroid 4 Vesta. glasses. Typically, such experiments consume tens to Lunar Materials: UVLAMP 40Ar/39Ar results are hundreds of milligrams of material, and even the most available for a relatively small number of lunar samples conservative procedures require sample masses of tens thus far, but they provide a wealth of information about of micrograms [e.g., 1]. An alternative – and highly the often complex and sometimes diachronous history complementary – approach to 40Ar/39Ar dating of plan- of melt formation in individual impact melt breccia etary materials relies on an ultraviolet mi- samples. Some samples – such as Apollo 17 sample croprobe (UVLAMP). Laser technologies have been ex- 77115 – show evidence of only a single impact melt- ploited in 40Ar/39Ar dating studies of meteorites and lu- forming event [4]; for 77115, the UVLAMP data indi- nar materials since the days of the Apollo program, but cate that this event occurred at 3.834 ± 0.020 Ga (2σ). of those studies have employed visible or infrared However, UVLAMP results for Apollo 17 sample for gas extraction. These lasers can only heat or melt 73217 indicate that the rock was affected by at least samples, which limits their usefulness for high-spatial- three impact events between 3.81 Ga and ≤3.27 Ga, an resolution analyses because they extract significant (but interval of more than 500 Ma [4]. Furthermore, SIMS unquantifiable) amounts of Ar from the region sur- U/Pb studies of zircons and phosphates in 73217 have rounding the target footprint of the laser beam through been interpreted as evidence for even older impact thermally-activated diffusion. In contrast, high-fluence, events at ca. 4.34 and 3.93 Ga [5], implying that this short-pulse-duration ultraviolet lasers ablate the target, single, hand-sized sample records over a billion years of vaporizing material beneath the laser footprint. The re- impact history. Such findings underscore the im- sulting rapidly escapes before it can cause per- portance of multi-chronometer studies of lunar impact ceptible “collateral damage” of the surrounding region. breccias, and caution against inferring the ages of spe- Consequently, UVLAMPs enable precise targeting and cific, basin-forming impact basins on the Moon from dating of distinctive domains within petrographic sec- geochronologic studies of one or even several impact tions of lunar and meteoritic samples, with a quantifia- melt breccia samples from a single collection site. ble spatial resolution of 100 µm or less. Typical ana- The capacity of a UVLAMP to target specific do- lyzed masses are 1 to 6 µg – far less that the sample mains in petrographic context and to use the results to masses required for incremental heating analysis – yet better interpret incremental-heating datasets is illus- UVLAMP dates for such small samples still typically trated by a study of Apollo 17 sample 77135 [6]. Previ- have 2σ uncertainties of less than 1% when state-of-the- ously published incremental heating 40Ar/39Ar data for art mass spectrometers are used for the isotopic meas- Apollo 17 sample 77135 [e.g., 7] have been interpreted urements. Although careful incremental-heating exper- as evidence that the whole-rock sample experienced iments on separated crystals provide additional infor- post-impact radiogenic 40Ar (40Ar*) loss, but the details mation regarding the intracrystalline distribution of 40Ar of how this occurred were unclear. UVLAMP data indi- in a crystal, incremental-heating data for polymineralic cate that 40Ar* loss was most pronounced in fine- fragments – the targets of most published incremental- grained potassium feldspar and K-rich glass, and less heating studies of planetary materials – are much more pronounced in pyroxene and plagioclase. Variable de- difficult to interpret unambiguously, and doing so re- grees of 40Ar* loss in different petrographic domains of quires the use of sophisticated multi-phase and multi- the sample confound attempts to establish a high-preci- domain diffusion models that require a variety of as- sion age for impact melt formation in this rock, but the sumptions [2,3]. UVLAMP 40Ar/39Ar studies can guide correlation between K concentration and post-impact more robust geologic interpretations of the results of 40Ar* loss implies that a robust minimum estimate of the such incremental-heating experiments, and – in many melt age is ca. 3.78 to 3.81 Ga. cases – can yield unique insights regarding samples Howardite Meteorites: Brecciated materials are with complex histories. Here, we review the results of also found in abundance within the asteroid belt, and the 50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132) 2214.pdf

howardite group, in particular, represents a References: [1] Lindsay F.N. et al. (2014) GCA, source of regolith material from the second-largest as- 129, 96–110. [2] Cassata, W. S. and Renne, P. R. (2014) teroid Vesta. Thus far, we have employed the UVLAMP GCA, 112, 251-287. [3] Boehnke, P. et al. (2016) EPSL, technique to determine 40Ar/39Ar dates of melt clasts 453, 267-275. [4] Mercer C.M et al. (2015) Sci. Adv., 1, within two howardites (NWA 1929 and Dho 485) to as- e1400050. [5] Grange M.L. et al. (2009) GCA, 73, sess whether a similar impact history is recorded in 3093–3107. [6] Mercer C.M. et al. (in press) Meteoritics these materials compared to that recorded in lunar melt & . Sci. [7] Dalrymple, G. B. and Ryder, G. samples. Previous studies [8-10] were limited to <20 (1996) JGR, 101, 26069-26084. [8] Bogard, D.D. melt clasts extracted using dental tools from a few select (2011) Chem. Erde Geochem., 71, 207-226. [9] Ken- howardites and subsequently dated by incremental- nedy, T. et al. (2013) GCA 115,162-182. [10] Cohen, heating 40Ar/39Ar. While these data yielded a range of B.A. (2013) Meteoritics & Planet. Sci. 48, 771- apparent ages, the authors inferred the existence of a 785. [11] Cartwright J. A. et al. (2016) LPSC (abs# peak in apparent ages at ca. 3.9 Ga, consistent with the 2865). [12] Boyce et al. (2006) GCA, 70, 3031-3039. Late Heavy Bombardment model. However, our [13] Horne et al. (2015) GCA, 178, 106-123. [14] Horne UVLAMP data (62 analyses of individual melt clasts) et al. (2019) Chem. Geol. 506, 40-50. [15] Young et al. included a broad range of 40Ar/39Ar dates between ca. (2013) GRL, 40, 3836-3840. 4.4 and ca. 3.2 Ga, suggesting an extended period of bombardment on the vestan surface [11]. This range, obtained from only two samples, highlights the im- portance of performing a comprehensive study of avail- able regolithic meteorites to constrain details of the im- pact history of Vesta. Prospects: Current research at ASU also includes efforts to capitalize on the high-spatial-resolution of UVLAMPs to map intracrystalline 40Ar diffusion gradi- ents in large feldspar xenocrysts in impact breccias, and to use these data to model melt cooling histories as a way to constrain whether or not specific breccia samples represent large melt sheets associated with basin-form- ing impacts or smaller, localized impacts. Such infor- mation is essential for understanding the larger signifi- cance of melt ages for those samples with respect to lu- nar impact history. A significant number of lunar and Martian meteorite samples are polylithologic breccias, and careful UVLAMP 40Ar/39Ar dating of individual clasts in petrographic context is providing new con- straints on their varied age provenances. UVLAMP methods also have been developed for U/Pb and (U- Th)/He “double” dating of individual zircon and phos- phate crystals [12-14]. The application of these tech- niques – in conjunction with UVLAMP 40Ar/39Ar dating of feldspars – could lead to an unprecedented under- standing of the thermal histories of individual samples [15]. Opportunities for UVLAMP applications to plan- etary materials are likely to expand substantially in light of on-going efforts to improve the design of UVLAMP analytical systems to achieve even greater resolution and analytical precision, just in time to take maximum advantage of small samples that will be returned from the Moon, Mars, and asteroids by exploration missions planned for the near future.