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
Breccias
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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
Breccias
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Classification
Jeff Taylor
Breccias
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Fragmental Breccias
• Angular rock and
mineral clasts
• Big size range
• Porous, frequently
sintered, and friable
0.25 mm
Jeff Taylor
Breccias
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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
Breccias
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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: 15455
0.5 mm
Jeff Taylor
Breccias
Clast-poor: A-16 coarse fine
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Dimict Breccias
• Composed of
impact melt and
plutonic or
metamorphic rock
• Frequently show
mutually intrusive
relationships
Jeff Taylor
Dimict breccia: 64475
Breccias
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Granulitic Breccias
• Rock and mineral clasts in
granoblastic matrix
• Mineral phases homogeneous
• Lots of 120o triple junctions
• Two-pyx equilibrium T up to
1100oC
• 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
72559
50 m
50 m
79215
Jeff Taylor
Breccias
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Regolith Breccias
• Fragmental breccia
made by shocklithification of regolith
• Contain soil
components (glass,
solar wind gases)
Jeff Taylor
Breccias
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Crater Settings
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Breccias
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Impact Melt Breccias and Processes
• Impact melts tend to be uniform in composition
for a given impact:
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Breccias
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Impact Melt Breccias and Processes
• Uniformity has been used to associate compositional
clusters of impact melts:
McKinley et al. (1984)
Apollo 16 Impact Melt Groups
Jeff Taylor
Breccias
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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
Breccias
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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
Breccias
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Clasts in Impact Melts: Mistastin Lake
Example
Impact melt rocks
Breccias
Mistastin Lake Crater:
Labrador, Canada
28 km in diameter
38 My old
Jeff Taylor
Much of impact melt breccia
eroded away, but occurs on
western side of structure
Target: Anorthosites and granitic
rocks
Breccias
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Clasts in Impact Melts: Mistastin Lake
Example
0.25 mm
Although melt composition is uniform
from place to place, clast population
varies substantially.
Jeff Taylor
Breccias
Pie diagrams: Black
shows % of anorthosite in
clast population
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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
Breccias
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Impact Melt Breccias and Processes:
Cooling
Onorato et al. (1976)
Jeff Taylor
Breccias
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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
Breccias
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Impact Melt Breccias and Processes:
Synthesis
Onorato et al. (1976)
Jeff Taylor
Breccias
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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
Breccias
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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
Breccias
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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
Breccias
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Why call them basaltic impact melts?
Lunar Impact
Melts (LKFM)
Terrestrial
basalts
Lunar Mare
Basalts
SiO2
45-51
46-52
38-48
Al2O3
16-24
9-19
7-15
FeO
5-11
8-13
17-22
MgO
6-15
4-21
6-20
CaO
9-13
10-13
6-12
0.6-0.7
0.7-0.9
0.8-1.3
CaO/Al2O3
Jeff Taylor
Breccias
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Why call them basaltic impact melts?
• LKFM melts plot
along cotectic,
implying an
igneous, basaltic
composition
(though they are
not igneous)
Jeff Taylor
Main LKFM
Group
Breccias
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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
Breccias
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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
Breccias
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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
– P2O5 varies directly with REE
• K2O:
– 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
• Al2O3:
LKFM
– Varies from 16 to 24 wt%
– May be slightly bimodal (16-20; 21-24)
Jeff Taylor
Breccias
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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
Breccias
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Geographic Distribution of Basaltic Impact Melts
SPA  80
• 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)
Jeff Taylor
Breccias
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Geographic Distribution
Jeff Taylor
Breccias
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Jeff Taylor
Breccias
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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
Jeff Taylor
Breccias
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