Lunar Highland Igneous Rocks Pristinity Concept Early bombardment

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 Pristine rocks 1

Why Knowing you have a certifiable igneous rock is important 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 Pristine rocks 2

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

Stillwater Igneous Complex Jeff Taylor Pristine rocks 4

A Breccia: Not Pristine but has a little pristine rock fragment Arrow points to a pristine rock fragment. Scale is in cm. Jeff Taylor Pristine rocks 5

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… We can see through all that…we’re petrologists Jeff Taylor Pristine rocks 0. 5 mm 6

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 rocks 7

Lunar Highland Igneous Rocks: Pristinity Concept • Criteria: 1. Low content of siderophile elements (Ir, Ni, Au, Ag, Os, Pt, Pd…) 2. Cumulate plutonic character (or volcanic texture in some cases). 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 rocks 8

Lunar Highland Igneous Rocks: Pristinity Concept 1. Low content of siderophile elements (< 3 x 10 -4 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. Jeff Taylor Pristine rocks Warren (2004) 9

Lunar Highland Igneous Rocks: Pristinity Concept • Official criterion for bulk analyses: < 3 x 10 -4 CI chondrite abundance Co • Basically the same approach as used to identify an impact at the K -T boundary on Earth • Metal grains: Ni – Present in many rocks; pristine rocks have nonmeteoritic Co and Ni Metal grains important because not all pristine rocks large enough to measure siderophile elements. Jeff Taylor Pristine rocks 10

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, >~3 mm Orthocumulate Adcumulate Jeff Taylor Pristine rocks 11

Cumulates Orthocumulate Mesocumulate Adcumulate Decreasing amount of trapped liquid Jeff Taylor Pristine rocks 12

Cumulate Textures 0. 5 mm 72503, 7085 Ferroan Anorthosite Orthocumulate Jeff Taylor 76535 Troctolite Adcumulate Pristine rocks 13

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 Jeff Taylor Pristine rocks Aluminous mare basalt Apollo 16 impact melt Both fields of view 2. 5 mm across. Impact melt 14

Lunar Highland Igneous Rocks: Pristinity Concept 3. Phase homogeneity • • • Jeff Taylor Suggests slow cooling, hence a plutonic environment But extrusive pristine rocks will have chemicallyzoned mineral grains Caution: polymict breccias can be metamorphosed to produce homogeneous minerals…though they end up with metamorphic textures Pristine rocks 15

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 (< 0. 6992) - Reasoning: most impacts are < 4. 0 Ga (maybe) 5. Low concentrations of incompatible elements • • Jeff Taylor < 5 x 10 -3 KREEP levels Problem: there are pristine KREEP-rich rocks Pristine rocks 16