ALBERT

Description: Quantitative constraints on the ages of melt-forming impact events on the Moon are based primarily on isotope geochronology of returned samples. However, interpreting the results of such studies can often be difficult because the provenance region of any sample returned from the lunar surface may have experienced multiple impact events over the course of billions of years of bombardment. We illustrate this problem with new laser microprobe 40Ar/39Ar data for two Apollo 17 impact melt breccias. Whereas one sample yields a straightforward result, indicating a single melt-forming event at ca. 3.83 Ga, data from the other sample document multiple impact melt–forming events between ca. 3.81 Ga and at least as young as ca. 3.27 Ga. Notably, published zircon U/Pb data indicate the existence of even older melt products in the same sample. The revelation of multiple impact events through 40Ar/39Ar geochronology is likely not to have been possible using standard incremental heating methods alone, demonstrating the complementarity of the laser microprobe technique. Evidence for 3.83 Ga to 3.81 Ga melt components in these samples reinforces emerging interpretations that Apollo 17 impact breccia samples include a significant component of ejecta from the Imbrium basin impact. Collectively, our results underscore the need to quantitatively resolve the ages of different melt generations from multiple samples to improve our current understanding of the lunar impact record, and to establish the absolute ages of important impact structures encountered during future exploration missions in the inner Solar System.

Description: Several boulders located at the bases of the North and South Massifs were among the primary field targets of the Apollo 17 mission to the Taurus-Littrow Valley on the Moon [1]. Some boulders are polylithologic, including Boulder 1 at Station 2 and the boulders at Stations 6 and 7. These boulders were the subjects of consortium studies [2, 3] that included 40Ar/39Ar geochronology to determine the ages of distinct lithologies within each boulder [e.g., 4-6]. We report new 40Ar/39Ar data for the impact melt breccias 72255, 76315, 77075, and 77135 obtained using the UV laser ablation microprobe (UVLAMP) methods of [7]. For 72255, we obtained a preliminary isochron date ca. 3814 Ma from 22 melt analyses, which is younger than published plateau dates (e.g., 3951-3835 Ma [4, 8]). Fifteen melt analyses of 76315 yield a preliminary isochron date ca. 3850 Ma, younger than the 3900 ± 16 Ma date reported by [8]. Melt analyses of 77075 yield preliminary dates between ca. 3797-3584 Ma, possibly reflecting partial loss of 40Ar. In this case, the oldest date may provide a minimum age for the formation of melt in 77075. Finally, the UVLAMP dates for the 77135 melt range from 3810-3361 Ma and corresponding Ca/K ratios range from ca. 100-6. Electron microprobe analyses of small (ca. 10s of microns wide) pockets of K-rich materials show that both K-rich glass and K-feldspar are present. The UVLAMP dates for 77135 likely reflect spatially variable 40Ar loss, consistent with published step heating results [e.g., 6]. References: [1] Schmitt (1973) Science, 182, 681–690. [2] Ryder (1993). Catalog of Apollo 17 Rocks: Volume 1 – Stations 2 and 3 (South Massif). LPI. [3] Ryder (1993). Catalog of Apollo 17 Rocks: Volume 4 – North Massif. LPI. [4] Leich et al. (1975) The Moon, 14, 407–444. [5] Cadogan & Turner (1976). LPSC, 7, 2267–2285. [6] Stettler et al. (1978). LPSC, 9, 1113–1115. [7] Mercer et al. (2015) Sci. Adv., 1, e1400050. [8] Dalrymple & Ryder (1996). JGR, 101, 26069–26084.

Description: As part of an interdisciplinary study of the polylithologic Station 7 boulder at the base of the North Massif in the Taurus Littrow Valley on the Moon [1], Stettler et al. [2-4] reported 40Ar/39Ar spectra for whole-rock fractions from sample 77135, generally interpreted to be the youngest impact melt breccia present in the boulder. The spectra were highly disturbed, with young apparent ages (ca. 2.17-1.70 Ga) for the low-temperature (T) steps at the beginning of the experiment rising to older apparent ages (ca. 3.87-3.79 Ga) for the high-T steps. The corresponding Ca/K ratios for these release spectra are low (ca. 4-5) for the low-T steps, and higher (ca. 48-119) for the high-T steps. The simplest interpretation of such spectra is that the low-T steps reflect partial Ar loss due to one or more reheating events or prolonged cooling, whereas the higher-T steps more closely approximate the formation age of the melt. One potential implication of this interpretation is that any total-gas or whole rock fusion dates, such as those obtained by laser ablation techniques, would be systematically younger than the formation age of the melt. We have revisited 77135 with the UV laser ablation microprobe (UVLAMP) 40Ar/39Ar method and have found that this is not necessarily the case. BSE and X-ray element maps were made for 77135,209 using the electron microprobe at Washington University in St. Louis, and UVLAMP analyses were conducted at Arizona State University following the methods of [5]. The maps show that K is highly concentrated in discrete interstitial pockets (typically 〈100 µm in diameter) scattered throughout the melt matrix. We specifically targeted these small K-rich domains with cylindrical laser spots (ca. 110-180 µm in diameter with typical ablated depths of 40-60 µm) and obtained 40Ar/39Ar dates between ca. 3.56-3.36 Ga and corresponding Ca/K ratios of 26.5-7.9. When we tried to avoid the K-rich domains (using similar spot sizes), we obtained a narrow range of dates between ca. 3.77-3.66 Ga with corresponding Ca/K ratios between 91.2-69.6. Owing to the small sizes of the K-rich domains compared to the laser pit sizes required for each analysis, we were not able to ablate a sample volume containing only the K-rich material. However, mixtures of the K-rich domains and surrounding materials have low apparent ages. In contrast, our analyses with higher Ca/K ratios tend towards the formation age of the melt, implying that UVLAMP techniques permit us to avoid the analysis of material that has a tendency to experience preferential post-crystallization Ar loss.

Description: In the fall of both 1999 and 2000, unexpected “rapid tides” occurred along the coast of the Avalon Peninsula of Newfoundland. These rapid tides have been linked to the passing of Tropical Storm Jose (1999) and Tropical Storm Helene (2000) over the Grand Banks. Here we examine the dynamic ocean response to Tropical Storm Helene (2000) using a barotropic shallow water ocean model forced by atmospheric pressure and surface winds derived from a simulation of Helene using a dynamical model of the atmosphere. The ocean model is able to capture the main features of the observed response at the coast of Newfoundland as seen in the available tide gauge data. Results show that the simulated sea level response at the coast is driven by a combination of wind stress and atmospheric pressure forcing, the former generally dominating. An exception is Conception Bay, Newfoundland, where the response is captured mainly by atmospheric pressure forcing. Offshore near the edge of the Grand Banks, atmospheric pressure and wind stress forcing are equally important. The wind‐forced response depends on the divergence of the surface wind stress and hence on the structure of the storm in the atmospheric model simulation. Sensitivity studies show the importance of having a small time interval (on the order of minutes) at which the atmospheric forcing is supplied to the ocean model and show the importance of the location of the storm track.

Description: 40Ar/39Ar incremental heating experiments on whole‐rock lunar samples commonly provide evidence of varying degrees of radiogenic 40Ar (40Ar*) loss. However, these experiments provide limited information about whether or not 40Ar* is preferentially lost from specific glasses, minerals, or polyphase domains. Ultraviolet laser ablation microprobe (UVLAMP) 40Ar/39Ar dating and electron probe microanalysis of mineral clasts and polyphase melt assemblages in Apollo 17 poikilitic impact melt rock 77135 show evidence of geochemical controls on 40Ar/39Ar dates. Potassium‐rich glass and K‐feldspar in the mesostasis are the dominant sources for Ar released during low‐temperature steps of published 40Ar/39Ar release spectra for this rock, while pyroxene oikocrysts with enclosed plagioclase chadacrysts contribute Ar predominantly to intermediate‐ to high‐temperature steps. Additionally, UVLAMP analysis of a mm‐scale plagioclase clast demonstrates the potential to use stranded 40Ar* diffusive loss profiles to constrain the thermal evolution of lunar impact melt deposits and indicates that the melt component of 77135 cooled quickly. While some submillimeter clasts of plagioclase are distinctly older than the melt, other small clasts yield dates younger than the oldest melt components in 77135, plausibly due to subgrain fast diffusion pathways and/or 40Ar* loss during brief episodes of reheating at high temperatures. Our data suggest that integrated petrologic and microanalytical geochronologic studies are necessary complements to bulk sample geochronologic studies in order to fully evaluate competing models for the impactor flux during the first billion years of the Moon's evolution.