Transcript GG 673C: Petrologic Evolution of the Moon and Mars

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
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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.
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Pristine rocks
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Lava flows—clearly igneous rocks
~3 m
Photo by Scott Rowland
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Pristine rocks
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Wave-cut cliff section, Makapu‘u, O‘ahu
Slide Courtesy of Scott Rowland
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Pristine rocks
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Stillwater Igneous Complex
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Pristine rocks
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A Breccia: Not Pristine
but has a little pristine rock fragment
Arrow points to
a pristine rock
fragment. Scale
is in cm.
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Pristine rocks
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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
0.5 mm
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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
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Pristine rocks
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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 rocks
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Lunar Highland Igneous Rocks:
Pristinity Concept
1. Low content of siderophile
elements (< 3 x 10-4 times
CI chondrites)
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•
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)
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Lunar Highland Igneous Rocks:
Pristinity Concept
• Official criterion for bulk
analyses:
Co
10-4
<3x
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 nonmeteoritic Co and Ni
Jeff Taylor
Ni
Metal grains important because not all
pristine rocks large enough to measure
siderophile elements.
Pristine rocks
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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
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Cumulates
Orthocumulate
Mesocumulate
Adcumulate
Decreasing amount of trapped liquid
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Cumulate Textures
0.5 mm
72503,7085
Ferroan Anorthosite
Orthocumulate
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76535
Troctolite
Adcumulate
Pristine rocks
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Lunar Highland Igneous Rocks:
Pristinity Concept
Aluminous mare basalt
• 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
Impact melt
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Lunar Highland Igneous Rocks:
Pristinity Concept
3. Phase homogeneity
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•
•
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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
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Lunar Highland Igneous Rocks:
Pristinity Concept
4. Old age
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> 4.2 Ga
-
•
Very vague, as some pristine rocks are 4 Ga
Problem: some granulitic breccias may have ages > 4.2 Ga
Low 87Sr/86Sr initial ratio (< 0.6992)
- Reasoning: most impacts are < 4.0 Ga (maybe)
5. Low concentrations of incompatible elements
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•
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< 5 x 10-3 KREEP levels
Problem: there are pristine KREEP-rich rocks
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