Petrogenesis and chronology of lunar meteorite Northwest Africa 4472: A KREEPy regolith breccia from the Moon

113

106

Abstract-New petrography and40 Ar ages have been obtained for 1-3 mm sized rock fragments from Apollo 16 Station 13 soil 63503 (North Ray crater ejecta) and chips from three rocks collected by Apollo 16 and Apollo 17 missions. Selection of these samples was aimed at the old 40 Ar-39 Ar ages to understand the early history of the lunar magnetic field and impact flux. Fifteen samples were studied including crustal material, polymict feldspathic fragmental breccias, and impact melts. The impact ages obtained range between approximately 3.3 and 4.3 billion years (Ga). Polymict fragmental breccia 63503,1 exhibits the lowest signs of recrystallization observed and a probable old relic age of 4.547 AE 0.027. The plateau age of 4.293 AE 0.044 Ga obtained for impact melt rock 63503,13 represents the oldest known age for such a lithology. Possibly, this age represents the minimum age for the South Pole-Aitken (SPA) Basin. In agreement with literature data, these results show that impact ages >3.9 Ga are found in lunar rocks, especially within soil 63503. Impact exhumation of deep-seated warm crustal material onto the lunar surface is considered to explain the common 4.2 Ga ages obtained for weakly shocked samples from soil 63503 and Apollo 17. This would directly imply that one or more basin-forming events occurred at that time. Some rock fragments showing none to limited petrologic features indicate thermal annealing. These rocks may have lost Ar while resident within the hot-ejecta of a large basin. Concurrent with previous studies, these results lead us to advocate for a complex impact flux in the inner solar system during the initial approximately 1.3 Ga.

Section: Discussionmentioning

confidence: 97%

The bombardment history of the Moon as recorded by ⁴⁰Ar‐³⁹Ar chronology

Fernandes

Fritz

Weiss

et al. 2013

113

106

“…We modified the method of Joy et al. () and Mercer et al. () choosing to use the low spatial resolution (5° per pixel) Clementine Ti and Fe ultraviolet‐visible‐infrared (UVVIS) data sets.…”

Section: Discussionmentioning

confidence: 99%

High crustal diversity preserved in the lunar meteorite Mount DeWitt 12007 (Victoria Land, Antarctica)

Collareta

D’Orazio

Gemelli

et al. 2016

The meteorite Mount DeWitt (DEW) 12007 is a polymict regolith breccia mainly consisting of glassy impact-melt breccia particles, gabbroic clasts, feldspathic clasts, impact and volcanic glass beads, basaltic clasts and mingled breccia clasts embedded in a matrix dominated by finegrained crystals; vesicular glassy veins and rare agglutinates are also present. Main minerals are plagioclase (typically An>85) and clinopyroxene (pigeonites and augites, sometimes interspersed). The presence of tranquillityite, coupled with the petrophysical data, the O-isotope data (D17O = -0.075) and the FeOtot/MnO ratios in olivine (91), pyroxene (65) and bulk rock (77) indicate a lunar origin for DEW 12007. Impactites consist of Al-rich impact melt splashes and plagioclase-rich meta-melt clasts. The volcanic products belong to the Very Low Titanium (VLT) or Low Titanium (LT) suites; an unusual subophitic fragment could be cryptomare-related. Gabbroic clasts could represent part of a shallow intrusion within a volcanic complex with prevailing VLT affinity. DEW 12007 has a mingled bulk composition with relatively high incompatible element abundances and shows a high crustal diversity, comprising clasts from the Moon's major terranes and rare lithologies. First order petrographic and chemical features suggest that DEW 12007 could be launch-paired with other meteorites including Y 793274/981031, QUE 94281, EET 87521/96008, and NWA 4884

“…Following the method of Joy et al. (, ), we searched the Lunar Prospector gamma‐ray spectrometer datasets using bulk composition parameters of 1.73 ± 1 ppm Th and 9.75 ± 1 wt% FeO (Korotev et al. ).…”

Section: Discussionmentioning

confidence: 99%

New lunar meteorite Northwest Africa 2996: A window into farside lithologies and petrogenesis

Mercer

Treiman

Joy

2013

Abstract-The Northwest Africa (NWA) 2996 meteorite is a lunar regolith breccia with a "mingled" bulk composition and slightly elevated incompatible element content. NWA 2996 is dominated by clasts of coarse-grained noritic and troctolitic anorthosite containing calcic plagioclase (An#~98) and magnesian mafic minerals (Mg#~75), distinguishing it from Apollo ferroan anorthosites and magnesian-suite rocks. This meteorite lacks basalt, and owes its mingled composition to a significant proportion of coarse-grained mafic clasts. One group of mafic clasts has pyroxenes similar to anorthosites, but contains more sodic plagioclase (An#~94) distinguishing it as a separate lithology. Another group contains Mgrich, very low-titanium pyroxenes, and could represent an intrusion parental to regional basalts. Other clasts include granophyric K-feldspar, disaggregated phosphate-bearing quartz monzodiorites, and alkali-suite fragments (An#~65). These evolved lithics are a minor component, but contain minerals rich in incompatible elements. Several anorthosite clasts contain clusters of apatite, suggesting that the anorthosites either assimilated evolved rocks or were metasomatized by a liquid rich in incompatible elements. We used Lunar Prospector gamma-ray spectrometer remote sensing data to show that NWA 2996 is most similar to regoliths in and around the South Pole Aitken (SPA) basin, peripheral regions of eastern mare, Nectaris, Crisium, and southern areas of Mare Humorum. However, the mineralogy of NWA 2996 is distinctive compared with Apollo and Luna mission samples, and is likely consistent with an origin near the SPA basin: anorthosite clasts could represent local crustal material, mafic clasts could represent intrusions beneath basalt flows, and apatite-bearing rocks could carry the SPA KREEP signature.