P33C-2136
High Spatial Resolution 40Ar/39Ar Geochronology of Impact Melt Breccias from Apollo 17 Boulders at Stations 2, 6, and 7
Wednesday, 16 December 2015
Poster Hall (Moscone South)
Cameron Mark Mercer1, Kip V Hodges1, Bradley L Jolliff2, Matthijs C Van Soest3, Jo-Anne Wartho4 and John R Weirich5, (1)Arizona State University, Tempe, AZ, United States, (2)Washington University, Saint Louis, MO, United States, (3)Arizona State University, School for Earth and Space Exploration, Tempe, AZ, United States, (4)GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany, (5)Planetary Science Institute Tucson, Tucson, AZ, United States
Abstract:
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.
