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ABUNDANT APATITE IN GRANULITE 79215: SPOOR OF ANOTHER VOLATILE-RICH LUNAR FLUID

ABUNDANT APATITE IN GRANULITE 79215: SPOOR OF ANOTHER VOLATILE-RICH LUNAR FLUID. A.H. Treiman - Lunar and Planetary Institute J. Gross - American Museum of Natural History. CLSE Team Mtg. Jan 31, 2012. CLSE and Granulites. Granulites, granulitic impactites, are important to CLSE

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ABUNDANT APATITE IN GRANULITE 79215: SPOOR OF ANOTHER VOLATILE-RICH LUNAR FLUID

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  1. ABUNDANT APATITE IN GRANULITE 79215: SPOOR OF ANOTHER VOLATILE-RICH LUNAR FLUID A.H. Treiman - Lunar and Planetary Institute J. Gross - American Museum of Natural History CLSE Team Mtg. Jan 31, 2012

  2. CLSE and Granulites • Granulites, granulitic impactites, are important to CLSE • Records of impact events • Ages (of cooling after impact). • Identity of impactor via highly siderophile element signatures. • Records of early crustal chemistry • Granulites carry several geochemical signatures not seen in ‘pristine’ Apollo or meteorite samples. • E.g., Th/Sm > KREEP

  3. 79215 - Setting

  4. 79215 - The Rock • Massive granulitic breccia, 28% clasts • Metamorphosed at ~3.9 Ga (Ar-Ar) • Zap pits, CR exposure age > 170 Ma

  5. 79215 - Thin Sections ,51

  6. 79215,51 - Thin Section 2

  7. a FeS chr a chr a a a a a a ilm a a a Ag ! a a chr 79215,51 Thin Section 3

  8. Modal Mineralogy • Bickel et al. (1976) optical point counts • Rock is anorthositic olivine-gabbro (IUGS)! • Unusually high proportion of apatite • Variable 0.0 to 3.3% ! • Probably low - optical misses small grains • Bulk P2O5 = 0.4 %

  9. Distribution of Apatite • Abramov/Joy image • red=Fe, green= P, blue=Ca • Apatite=cyan • Abundant 0.7% • Along curved traces, not random

  10. Apatite - EMP Analysis • Method of Goldoff & Webster (submitted) • low V for F, Na • grains untouched by ebeam • 44 analyses, 5 grains • All indistinuishable

  11. Geochemistry • Confirm literature An-En - Mg-suite-ish • REE content is like other A17 Mg-suite rocks (e.g., 76535, 78235)

  12. What about P and apatite? • P abundance far in excess of expected from REE in KREEP !! • > 10 x !

  13. Few Other Geochem Anomalies • High K/Sm vs. KREEP • Lots of cpx • BUT • Hf @ 7xCI • Zr @ 12xCI • Th @ 14xCI • HSE are high, from meteorite component

  14. Inference • Before its consolidation and annealing, 79215 was enriched in P, F, Cl, Ca, and K. But not enriched in REE. • The P enrichment is not directly related to the KREEP component. • The P is represented by subhedral and anhedral grains of fluor-apatite, arranged in curvilinear traces. • The apatite was deposited along fractures and fracture zones, presumably by a fluid or vapor.

  15. What happened? • Not clear how 79215 could become enriched in P but not REE. • Any lunar source rocks rich in P but not REE? • What sort of fluid could mobilize P but not REE? • Fluids • Probably not ionic melt (like chloride or fluoride). • Probably not silicate melt. • Possibly aqeuous, which can probably cause P/REE fractionation. But, no hydrous phases. • Vapor can deposit apatite in lunar breccias, and in chondrites. Some REE poor, some not. • What sort of vapor?

  16. Silicate melt - F/OH basically not defined Huh M. et al. (2011) Experimental determination of hydrogen partitioning in an apatite-haplobasalt andesite melt system at 1200°C and 1 Gpa. AGU abstract.

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