Evidence for a Lunar Magma Ocean What is

Evidence for a Lunar Magma Ocean • What is a magma ocean? • Evidence: – Global, highly anorthositic crust – Complementary patters of Eu anomalies – Old ages of the ferroan anorthosite suite – Uniformity of the KREEP component Jeff Taylor Evidence for Magma Ocean 1

What is a Magma Ocean? • Important question because planets might have experienced extensive early partial melting, but not produce an extensive molten layer • A simple definition: – Magma behaves rheologically as a liquid (low yield strength, relatively low viscosity), which implies it contains < 50% crystals – Magma encompasses a substantial fraction of the body, say > 10% (equivalent to 170 km on the Moon), and probably more – Its composition does not necessarily equal the planet’s bulk composition (but probably does at first? ) Jeff Taylor Evidence for Magma Ocean 2

Crust Rich in Plagioclase • Basic concept of the magma ocean envisions plagioclase floating to the top of the system and forming a primary, feldspar-rich crust. • It does not require crust be pure plagioclase because of precipitation of olivine and pyroxene from trapped liquids and floatation of cumulate mafics with plagioclase crystals Jeff Taylor Evidence for Magma Ocean 3

Crust Rich in Plagioclase • Idea was inspired by fragments of anorthosites and other plagioclase-rich rocks in Apollo 11 regolith • And supported by finding large samples of anorthosite at Apollo 15 and 16 Apollo 11 2 -4 mm regolith sample Anorthosite 15415 Jeff Taylor Evidence for Magma Ocean 4

Crust Rich in Plagioclase • Many anorthosites collected, but: – Several groups discernable by their Na/Ca and Mg/Fe, and trace elements – Some with very complicated histories Jeff Taylor Evidence for Magma Ocean 5

Crust Rich in Plagioclase Jeff Taylor Evidence for Magma Ocean 6

Crust Rich in Plagioclase • Large regions of highlands are rich in Al 2 O 3 • Also expected, low contents of incompatible elements • All this in spite of subsequent intrusions of Mg-suite and an unknown amount of ancient (>3. 9 Gy) mare volcanism, and basin-forming impacts distributing more mafic materials Jeff Taylor Evidence for Magma Ocean 7

Crust Rich in Plagioclase • Can infer Al 2 O 3 from Fe. O content of lunar rocks – Not a bad anti-correlation between Al and Fe – Actually a kink at around 11 wt% Fe. O, so maria fall on different trend Jeff Taylor Evidence for Magma Ocean 8

Crust Rich in Plagioclase 4 wt% Fe. O Corresponds to 27 wt% Al 2 O 3, but lots of areas 5 -6 wt%. Is this high Enough to prove magma ocean concept? Low Th consistent with cumulates Jeff Taylor Evidence for Magma Ocean 9

Crust Rich in Plagioclase Where anorthosites? Very low Fe. O (hence high Al 2 O 3) material found in basin rings, implying burial beneath a mafic layer Jeff Taylor From Hawke et al. (2003) Evidence for Magma Ocean 10

Plagioclase-Rich Crust: Anorthosites Ohtake et al (2009) Jeff Taylor Evidence for Magma Ocean 11

Anorthosite Crust Pure anorthosite is distributed widely throughout lunar crust, implying a global layer, but associated with basins and large craters (Yamamoto et al. (2012). Jeff Taylor Evidence for Magma Ocean 12

Plagioclase-Rich Crust Crites and Lucey (2015) Jeff Taylor Evidence for Magma Ocean 13

Plagioclase-Rich Crust Crites et al. (2015) Jeff Taylor Evidence for Magma Ocean 14

Complementary Eu Anomalies • A striking discovery was the presence of positive Eu anomalies in anorthosites and complementary negative anomalies in mare basalts and KREEP (basalts and component in breccias) Jeff Taylor Evidence for Magma Ocean Mare 15

Evidence for Magma Ocean-2: Complementary Eu Anomalies Warren (2004) Jeff Taylor: The Moon Jeff Taylor

REE Partition Coefficients La Ce Nd Olivine. 0001 Sm Eu Gd Dy Er Yb Lu . 0006. 0007. 003. 008. 019. 03 Jeff Taylor Plagioclase Orthopyroxene. 042. 007. 0302. 009. 0236. 014. 017 1. 2. 0105. 0089. 0077. 0065. 0068 Evidence for Magma Ocean . 022. 015. 037. 06. 10. 17. 22 17

Magma Ocean Crystallization Phase diagram for highlands rocks from Walker et al (1972) Silica Molar proportions Opx An Ol Spinel Jeff Taylor Evidence for Magma Ocean 18

Magma Ocean Crystallization Silica Fractional crystallization of bulk Moon composition. Opx An Bulk composition is from Longhi (2006). Ol Spinel Jeff Taylor Evidence for Magma Ocean 19

Magma Ocean Crystallization Silica Opx An Ol Spinel Jeff Taylor Evidence for Magma Ocean 20

Old Ferroan Anorthosites • Magma ocean is the first igneous event on the Moon, so FAS rocks should be oldest, and not too far from 4. 5 Gy. • But… Jeff Taylor Evidence for Magma Ocean 21

But Ages Appear to be Young Only the most reliable ages are shown. FAN not 4. 5 or even 4. 4 Ga. Some Mg-suite rocks are as old or older Jeff Taylor Evidence for Magma Ocean 22

Uniformity of KREEP • Relative abundances of incompatible elements in KREEP are very similar at all sampling locations • Implication: global, common product • Alternative: common regional product (we do not know far side composition) Jeff Taylor Evidence for Magma Ocean Warren and Wasson (1979) 23