High Spatial Resolution 40Ar/39Ar Geochronology of Impact Melt Breccias from Apollo 17 Boulders at Stations 2, 6, and 7.

Mercer, C., Hodges, K., Jolliff, B., van Soest, M., Wartho, J.-A. and Weirich, J. (2015) High Spatial Resolution 40Ar/39Ar Geochronology of Impact Melt Breccias from Apollo 17 Boulders at Stations 2, 6, and 7. [Poster] In: AGU Fall Meeting 2015. , 14.-18.12.2015, San Francisco, USA .

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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.

Document Type: Conference or Workshop Item (Poster)
Additional Information: Abstract # P33C-2136
Research affiliation: OceanRep > GEOMAR > FB4 Dynamics of the Ocean Floor > FB4-MUHS Magmatic and Hydrothermal Systems
Date Deposited: 16 Nov 2015 13:34
Last Modified: 16 Nov 2015 13:35
URI: http://oceanrep.geomar.de/id/eprint/30203

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