Jeff Taylor Pristine rocks1 Lunar Highland Igneous Rocks: Pristinity Concept Early bombardment messed up igneous rocks in the highlands Lots of impact breccias Impact melting Fragmentation of original rocks

Jeff Taylor Why Knowing you have a certifiable igneous rock is important Pristine rocks2 These data are from a set of volcanic deposits identified by field observations and petrography. The data show a diverse set of rocks that could be related by fractional crystallization in a magma chamber.

Jeff Taylor Lava flows—clearly igneous rocks Pristine rocks3 ~3 m Photo by Scott Rowland

Jeff Taylor Wave-cut cliff section, Makapu‘u, O‘ahu Pristine rocks4 Slide Courtesy of Scott Rowland

Jeff Taylor Stillwater Igneous Complex Pristine rocks5

Jeff Taylor A Breccia: Not Pristine but has a little pristine rock fragment Pristine rocks6 Arrow points to a pristine rock fragment. Scale is in cm.

Jeff Taylor Pristine rocks7 Lunar Highland Igneous Rocks: Pristinity Concept Highland rocks were bashed, busted, and mixed. Their textures have been obscured by shock. But all is not lost… 0.5 mm We can see through all that…we’re petrologists

Jeff Taylor Pristine rocks8 Lunar Highland Igneous Rocks: Pristinity Concept Pristine rocks: –Rocks produced by endogenous igneous processes –Not impact melts or breccias composed of more than one lithology (i.e., they are not polymict breccias) –But they can be crushed, granulated, sheared, and generally messed up –And they can be found in breccias –Synonyms in the literature: meteorite-free, endogenous, indigenous, primitive, plutonic All mare basalts from visible maria are pristine

Jeff Taylor Pristine rocks9 Lunar Highland Igneous Rocks: Pristinity Concept Criteria: 1.Low content of siderophile elements (Ir, Ni, Au, Ag, Os, Pt, Pd…) 2.Cumulate plutonic character 3.Phase homogeneity 4.Old age 5.Low concentrations of incompatible elements These criteria must be used carefully: None is an absolute rule. The first two are most important (actually, the others might be wrong)

Jeff Taylor Pristine rocks10 Lunar Highland Igneous Rocks: Pristinity Concept 1.Low content of siderophile elements (< 3 x times CI chondrites) Works because the lunar interior seems to be depleted in siderophiles; e.g., mare basalts Certified impact products such as the regolith and overtly mixed up breccias, have higher contents of Ir etc. Warren (2004)

Jeff Taylor Pristine rocks11 Lunar Highland Igneous Rocks: Pristinity Concept Official criterion for bulk analyses: < 3 x CI chondrite abundance Basically the same approach as used to identify an impact at the K-T boundary on Earth Metal grains: –Present in many rocks; pristine rocks have non- meteoritic Co and Ni Co Ni Metal grains important because not all pristine rocks large enough to measure siderophile elements.

Jeff Taylor Pristine rocks12 Lunar Highland Igneous Rocks: Pristinity Concept 2.Cumulate or plutonic character Igneous texture, even if only relicts of it Enrichments in one mineral—abundance too high for normal partial melt. Minerals are not present in cotectic proportions Coarse grains, >~3mm Orthocumulate Adcumulate

Jeff Taylor Pristine rocks13 Cumulates OrthocumulateMesocumulateAdcumulate Decreasing amount of trapped liquid

Jeff Taylor Pristine rocks14 Cumulate Textures 72503,7085 Ferroan Anorthosite Orthocumulate Troctolite Adcumulate 0.5 mm

Jeff Taylor Pristine rocks15 Lunar Highland Igneous Rocks: Pristinity Concept What about volcanic rocks? –Tricky because impact melts have fine-grained igneous textures –Impact melts have angular rock and mineral clasts (so do some volcanic rocks) –Non-cotectic proportions of minerals –Siderophiles help, of course Aluminous mare basalt Impact melt

Jeff Taylor Pristine rocks16 Lunar Highland Igneous Rocks: Pristinity Concept 3.Phase homogeneity Suggests slow cooling, hence a plutonic environment But extrusive pristine rocks will have chemically- zoned mineral grains Caution: polymict breccias can be metamorphosed to produce homogeneous minerals…though they end up with metamorphic textures

Jeff Taylor Pristine rocks17 Lunar Highland Igneous Rocks: Pristinity Concept 4.Old age > 4.2 Ga -Very vague, as some pristine rocks are 4 Ga -Problem: some granulitic breccias may have ages > 4.2 Ga Low 87 Sr/ 86 Sr initial ratio (< ) - Reasoning: most impacts are < 4.0 Ga (maybe) 5.Low concentrations of incompatible elements < 5 x KREEP levels Problem: there are pristine KREEP-rich rocks