Jeff Taylor Breccias1 Lunar Highlands Breccias Types of breccias and their crater settings –Monomict, polymict –Fragmental –Glassy melt breccia and impact glass –Impact melt breccias Crystalline melt breccias Clast-poor impact melt breccias –Granulitic breccias –Dimict Breccias –Regolith breccias Impact melts and processes LKFM (Low-K Fra Mauro Basalt)

Jeff Taylor Breccias2 Monomict and Polymict Monomict: composed on one lithology, although rock is fragmental –Many pristine highland rocks are monomict breccias –Related term: genomict—breccias composed of group of related rocks Polymict: composed of more than one lithology; often several lithologies

Jeff Taylor Breccias3 Classification

Jeff Taylor Breccias4 Fragmental Breccias Angular rock and mineral clasts Big size range Porous, frequently sintered, and friable 0.25 mm

Jeff Taylor Breccias5 Glassy Melt Breccia and Impact Glass Glass or devitrified glass matrix May or may not contain mineral clasts Small objects, in general Can be isolated objects, clasts in breccias, or coatings on rocks Adapted from Zellner et al. (2002)

Jeff Taylor Breccias6 Impact Melt Breccias Igneous-textured matrix Contain rock and mineral clasts Range widely in abundance of clasts Clast size range typically a power law Grain size of matrix inversely proportional to clast abundance 0.5 mm Clast-rich: mm Clast-poor: A-16 coarse fine

Jeff Taylor Breccias7 Dimict Breccias Composed of impact melt and plutonic or metamorphic rock Frequently show mutually intrusive relationships Dimict breccia: 64475

Jeff Taylor Breccias8 Granulitic Breccias Rock and mineral clasts in granoblastic matrix Mineral phases homogeneous Lots of 120 o triple junctions Two-pyx equilibrium T up to 1100 o C Probably fragmental rocks to begin with, but some might have started as impact melt breccias Source of heat for metamorphism is impact melt nearby Seem to have average crustal composition: noritic anorthosite, mix of FAN and Mg-suite, but may represent separate magma type  m

Jeff Taylor Breccias9 Regolith Breccias Fragmental breccia made by shock- lithification of regolith Contain soil components (glass, solar wind gases)

Jeff Taylor Breccias10 Crater Settings

Jeff Taylor Breccias11 Impact Melt Breccias and Processes Impact melts tend to be uniform in composition for a given impact:

Jeff Taylor Breccias12 Impact Melt Breccias and Processes Uniformity has been used to associate compositional clusters of impact melts: Apollo 16 Impact Melt Groups McKinley et al. (1984)

Jeff Taylor Breccias13 Impact Melt Breccias and Processes Uniformity in composition plays important role in trying to date specific impact basins on the Moon. The trick is to know what basin to associate with each melt group We are not positive that big basins will have perfectly uniform compositions in their impact melt sheets From Spudis (1993), which assembled data from several sources.

Jeff Taylor Breccias14 Impact Melt Breccias and Processes In principle, we can even characterize the impactor that created a basin This would be very useful for understanding accretion Problems: –Fractionation of the elements because of the impact –More than one impact contributing to a given impact cluster

Jeff Taylor Breccias15 Clasts in Impact Melts: Mistastin Lake Example Mistastin Lake Crater: Labrador, Canada 28 km in diameter 38 My old Impact melt rocks Breccias Much of impact melt breccia eroded away, but occurs on western side of structure Target: Anorthosites and granitic rocks

Jeff Taylor Breccias16 Clasts in Impact Melts: Mistastin Lake Example 0.25 mm Pie diagrams: Black shows % of anorthosite in clast population Although melt composition is uniform from place to place, clast population varies substantially.

Jeff Taylor Breccias17 Impact Melt Breccias and Processes: Cooling Impact melts are composed of: –A superheated melt phase –Variable amounts of cold clasts This assemblage cools, but not simply by conduction of heat away from the melt sheet. Clasts act as heat sinks inside the melt, making cooling very fast.

Jeff Taylor Breccias18 Impact Melt Breccias and Processes: Cooling Onorato et al. (1976)

Jeff Taylor Breccias19 Impact Melt Breccias and Processes: Cooling Cooling rate depends on clast abundance and mean size of clasts Once melt sheet has reached equilibrium temperature, it cools by loss of heat from the top and bottom and can be modeled by conventional approaches.

Jeff Taylor Breccias20 Impact Melt Breccias and Processes: Synthesis Onorato et al. (1976)

Jeff Taylor Breccias21 Low-K Fra Mauro Basalt (LKFM) What are basaltic impact melts (a.k.a LKFM)? General characteristics of target lithologies Diversity of basaltic impact melts Geographic distribution Origin of target rocks for basaltic impact melts

Jeff Taylor Breccias22 Synonyms LKFM (Low-K Fra Mauro basalt) MKFM (Medium-K Fra Mauro basalt) HKFM (High-K Fra Mauro basalt) Norite KREEP norite High-Al basalt Very high alumina basalt (VHA) Alkali high alumina basalt Range of names reflects range in chemical compositions

Jeff Taylor Breccias23 Why call them basaltic impact melts? They have textures of typical impact melts: –Angular rock and mineral clasts in igneous matrix All are enriched in siderophile elements –Metallic spherules with meteoritic Co/Ni –Enrichments in Ir, Au, etc. –Implies meteoritic contamiation

Jeff Taylor Breccias24 Why call them basaltic impact melts? Lunar Impact Melts (LKFM) Terrestrial basalts Lunar Mare Basalts SiO Al 2 O FeO MgO CaO CaO/Al 2 O

Jeff Taylor Breccias25 Why call them basaltic impact melts? LKFM melts plot along cotectic, implying an igneous, basaltic composition (though they are not igneous) Main LKFM Group

Jeff Taylor Breccias26 General Characteristics of Target Lithologies Generally basaltic Entrained clasts cannot be mixed in any proportion to produce melt compositions Missing component(s) is(are) rich in K, REE, P, U, and transition metals such as Cr, Ti, and Sc –Resembles gabboric rock or mare basalts In contrast, terrestrial impact melts: –Can be modeled as mixtures of clasts and target rocks –But proportion of target lithologies varies widely within clast population

Jeff Taylor Breccias27 General Characteristics of Target Lithologies Korotev (2000) argues that you can make LKFM by mixing KREEP basalt, forsteritic olivine (dunite), and feldspathic crust: –Dunite with Fo >90 –Average KREEP basalt –Feldspathic crust from highlands meteorites –Used 33 elements in mixing model –Note large variation in composition "Mathematical success of a mass-balance model such as that described here supports but does not and cannot prove the hypothesis that the breccias are actual mixtures of the lithologies represented by the components of the model. Models involving other components may provide equivalently good or better fits..." Korotev (2000)

Jeff Taylor Breccias28 Diversity of Basaltic Impact Melts Rare earth elements: –Patterns usually the same: La/Lu = 2 – 2.4 (chondrite normalized) –Abundances vary: Sm = 3 to 40 ppm –P 2 O 5 varies directly with REE K 2 O: –Varies from 0.05 to 0.8 wt% –Usually, but not always, correlated with REE Molar Mg/(Mg+Fe): –Varies from 0.5 to 0.8 Al 2 O 3 : –Varies from 16 to 24 wt% –May be slightly bimodal (16-20; 21-24) LKFM

Jeff Taylor Breccias29 Compositional Relation of Basaltic Impact Melts to Other Lunar Rock Types Compared to other highland rocks, basaltic impact melts are: –Elevated in Ti, Cr, Sc –Similar FeO to Mg-suite –Similar Mg# to Mg-suite –Elevated REE, K, P, Th, U Lucey et al. (1998)

Jeff Taylor Breccias30 Geographic Distribution of Basaltic Impact Melts Appear to be associated with basins If associated with basin ejecta, then LKFM might represent composition of lower crust But might be restricted to Procellarum KREEP terrain SPA falls way off the trend Spudis and Davis (1986) SPA  80

Jeff Taylor Breccias31 Geographic Distribution

Jeff Taylor Breccias32

Jeff Taylor Breccias33 Origin of Basaltic Impact Melts Definitely formed by impact melting, implying that they are mixtures of lithologies Likely represent lower crust in Procellarum KREEP terrain Maybe represent lower crust in SPA region General cotectic nature suggests igneous processes involved somehow, yet they seem to be mixtures of KREEP, feldspathic crust, and dunite Probably related to final stages of magma ocean crystallization Igneous fractionation suggested by range in Mg# Some extreme fractionation to decouple K from P and REE in some samples