Ages of Pristine Highlands Rocks Ages of lunar

Ages of Pristine Highlands Rocks • Ages of lunar rocks informative about: – – Timing of magma ocean crystallization Timescales of planetary accretion Time scale of lunar differentiation Models of planetary formation and evolution of the solar system Jeff Taylor Ages of Highland Rocks 1

Ages of Pristine Highlands Rocks • • Overview of ages Ferroan anorthosite suite Mg-suite and Alkali suite rocks Duration of magma ocean crystallization Jeff Taylor Ages of Highland Rocks 2

Highlands Age Summary Elkins-Tanton et al. (2011) FAN Rocks f: 67075 g: mafic frac in FAN h: 60025 (Sm-Nd; Carlson and Lugmair, 1988) m: 60025 (Pb-Pb and Sm-Nd; Borg et al. , 2011) p: 62236 (Rb-Sr, Sm-Nd; Borg et al. , 1999) Jeff Taylor Ages of Highland Rocks 3

Ferroan Anorthosites • Four have been dated by Rb-Sr and Sm-Nd. • Rb-Sr system is disturbed. • Sm-Nd gives range of apparent ages, 4. 29 ± x Gy to 4. 53 ± 0. 12 Gy. – Magma ocean hypothesis predicts all > 4. 4 Gy • Nd for the four rocks are > 0 (up to +3) – Magma ocean hypothesis predicts formation of FAS and related rocks early in lunar history from magma with chondritic relative abundances of REE, so Nd should be close to 0. • Norman et al (2003) suggest Sm-Nd slightly disturbed and that all rocks are old and Nd ~0 Jeff Taylor Ages of Highland Rocks 4

Ferroan Noritic Anorthosite 67215 c Norman et al. (2003) No valid isochron—System is disturbed Jeff Taylor Ages of Highland Rocks 5

Ferroan Noritic Anorthosite 67215 c Norman et al. (2003) Reasonable isochron—System is not disturbed Jeff Taylor Ages of Highland Rocks 6

Ferroan Anorthosite 62236 Young Ar-Ar age consistent with disturbed Rb-Sr All plots from Borg et al. (1999) Jeff Taylor Ages of Highland Rocks 7

Ferroan Anorthosite 62236 Borg et al. (1999) • Large Nd implies formation from a depleted source—not consistent with magma ocean. • Norman et al. (2003) point out that there is no trace element evidence for such a depleted source. Jeff Taylor Ages of Highland Rocks 8

Age of Ferroan Anorthosites Individual FAS ages (Sm-Nd only): 60025: 4. 44 ± 0. 02 67016 c 1: 4. 65 ± 0. 1 62236: 4. 29 ± 0. 058 67215 c: 4. 44 ± 0. 11 Jeff Taylor Ages of Highland Rocks 9

More on 60025 4. 367 Ga 4. 359 Ga 4. 318 Ga Sm-Nd: For FAN 60025, plagioclase, pyroxene and whole rocks define a 147 Sm– 143 Nd age of 4, 3676± 11 Myr (a) and a 146 Sm– 142 Nd age of 4, 318(+30, 38 Myr (b) Pb-Pb: 4, 359± 2. 4 Myr Borg et al. (2011) Jeff Taylor Ages of Highland Rocks 10

Rules to Determine Reliable Ages Read this for an overview: http: //www. psrd. hawaii. edu/Oct 15/age-rules. html Read this to be baffled: Borg, L. E. , Gaffney, A. M. , and Shearer, C. K. (2015) A Review of Lunar Chronology Revealing a Preponderance of 4. 34– 4. 37 Ga Ages, Meteoritics & Planetary Science, v. 50, p. 715 -732, doi: 10. 1111/maps. 12373. Rule #1: Use multiple clocks Jeff Taylor Ages of Highland Rocks 11

Highland Igneous Ages Jeff Taylor Ages of Highland Rocks 12

So how old are ferroan anorthosites? • We have not dated a pure anorthosite (>98% plagioclase) – Need to have enough minerals for isochrons, so need enough mafic minerals – Maybe those somewhat mafic anorthosites really are younger • In addition, a big norite clast in impact melt 15445 gives excellent isochrons for two analyses (Sm-Nd) in the same lab but with ages of 4. 28± 0. 03 Ga and 4. 46 ± 0. 07 Ga. • We do not know the ages as well as we’d like! Jeff Taylor Ages of Highland Rocks 13