Jeff Taylor Ages of Highland Rocks1 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 Rocks2 Ages of Pristine Highlands Rocks Overview of ages Ferroan anorthosite suite Mg-suite and Alkali suite rocks Duration of magma ocean crystallization

Jeff Taylor Highlands Age Summary Ages of Highland Rocks3 Elkins-Tanton et al. (2011) FAN Rocks f: g: mafic frac in FAN h: (Sm-Nd; Carlson and Lugmair, 1988) m: (Pb-Pb and Sm-Nd; Borg et al., 2011) p: (Rb-Sr, Sm-Nd; Borg et al., 1999)

Jeff Taylor Ages of Highland Rocks4 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 Rocks5 Ferroan Noritic Anorthosite 67215c No valid isochron—System is disturbed Norman et al. (2003)

Jeff Taylor Ages of Highland Rocks6 Ferroan Noritic Anorthosite 67215c Norman et al. (2003) Reasonable isochron—System is not disturbed

Jeff Taylor Ages of Highland Rocks7 Ferroan Anorthosite Young Ar-Ar age consistent with disturbed Rb-Sr All plots from Borg et al. (1999)

Jeff Taylor Ages of Highland Rocks8 Ferroan Anorthosite 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. Borg et al. (1999)

Jeff Taylor Ages of Highland Rocks9 Age of Ferroan Anorthosites Individual FAS ages (Sm-Nd only): 60025: 4.44 ± c1: 4.65 ± : 4.29 ± c: 4.44 ± 0.11

Jeff Taylor More on Ages of Highland Rocks10 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) Ga Ga Ga

Jeff Taylor Rules to Determine Reliable Ages Ages of Highland Rocks11 Read this for an overview: 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 , doi: /maps Rule #1: Use multiple clocks

Jeff Taylor Highland Igneous Ages Ages of Highland Rocks12

Jeff Taylor 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 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! Ages of Highland Rocks13