Basalts

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Basalts

• Why study basalts?
• How are they classified?
• What are the significant differences chemical
  between terrestrial and extraterrestrial basalts?
• Summary models for origin of terrestrial and
  lunar basalts and basaltic achondrite meteorites.

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Trace Element Fractionation During Partial
Melting
From http//www.geo.cornell.edu/geology/classes/g
eo302
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Basalt Types-Major Element Variation
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Alkaline and Subalkaline Rock Suites
15,164 samples
Irregular solid line defines the boundary between
Ne-norm rocks
Le Bas et al., 1992 Le Roex et al., 1990 Cole,
1982 Hildreth Moorbath, 1988
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Tholeiitic vs. Calc-alkaline Trends
Terms emerged from tangled history spanning
many decades. CA label proposed by Peacock
in 1931. Tholeiite originated in
mid-1800s from Tholey, western Germany.
Rocks show stronger Fe/Mg
enrichment than CA trend.
Tholeiites are commonly found
island arcs, while CA rocks
are more commonly found in
continental arcs.
Cole, 1982
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K2O content of subalkaline rocks
K2O content may broadly correlate with crustal
thickness. Low-K 12 km Med-K 35 km High-K 45 km
Ewart, 1982
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Classification of Basalts

• Three basalt types recognized based on their
  degree of silica saturation
• Quartz-hypersthene normative (Q Hy)
• quartz tholeiite
• Olivine-hypersthene normative (Ol Hy)
• olivine tholeiite
• Nepheline normative (Ne)
• alkaline basalt
• Tholeiitic basalts make up the oceanic crust,
  continental flood basalt provinces, and some
  large intrusions.
• Alkaline basalts are found in oceanic islands and
  some continental rift environments.

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Yoder Tilley Basalt Tetrahedron
Yoder Tilley, 1962 Le Maitre
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Basalt Types - Trace Elements
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Partition Coefficients for REEs
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Partition Coefficients for REE in Melts
Dbulk X1D1 X2D2 X3D3 XnDn
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Chondrite Normalized REE patterns

• By normalizing (dividing by abundances in
  chondrites), the sawtooth pattern can be
  removed.

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Differentiation of the Earth

• Melts extracted from the mantle rise to the
  crust, carrying with them their enrichment in
  incompatible elements
• Continental crust becomes incompatible element
  enriched
• Mantle becomes incompatible element depleted

From http//www.geo.cornell.edu/geology/classes/g
eo302
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Sr Isotope Evolution on Earth
87Sr/86Sr)0
Time before present (Ga)
87Sr/86Sr)0
Time before present (Ga)
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Sr and Nd Isotope CorrelationsThe Mantle Array
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Terrestrial Basalt Generation Summary

• MORBs are derived from the partial melting of a
  previously depleted upper mantle under largely
  anhydrous conditions at relatively shallow
  depths.
• True primary mantle melts are rare, although the
  most primitive alkali basalts are thought to
  represent the best samples of direct mantle
  melts.
• The trace element and isotopic ratio differences
  among N-MORB (normal), E-MORB (enriched), IAB,
  and OIB indicate that the Earths upper mantle
  has long-lived and physically distinct source
  regions.
• Ancient komatiites (gt2.5 Ga) indicate that the
  Earths upper mantle was hotter in the Archean,
  but already depleted of continental crustal
  components.

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Lunar Surface
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Apollo 15 Basalt Sample
Vesicles - Probably derived from CO degassing
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Lunar Olivine Basalt Thinsection
Fe-Ti oxides
Plagioclase
Olivine aligned MIs
Pyroxenes
Plane Polarized Light
Sample collected from the SE end of Mare
Procellarum by the Apollo 12 mission. Interpreted
as a Lava Lake basalt.
Cross Polarized Light
From http//www.union.edu/PUBLIC/GEODEPT/COURSES/
petrology/moon_rocks/12005.htm
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Lunar Anorthosite Thinsection
Pyroxenes
Fractured Plagioclase Feldspar
Rock is 98 fsp, An95 to An97
Plane Polarized Light
Highly brecciated lunar anorthosite was collected
by the Apollo 16 mission to the lunar highlands
SW of Mare Tranquillitatis. It has been dated at
4.44 Ga.
Cross Polarized Light
From http//www.union.edu/PUBLIC/GEODEPT/COURSES/
petrology/moon_rocks/12005.htm
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Earth Mars-sized Impact Model for Lunar Origin
Impact 0.5 hr
Impact 5hr
From Kipp Melosh, 1986 (above) and W. Hartmann
paintings of Cameron, Benz, Melosh models
(right)
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Features of the Giant Impact Hypothesis

• Original idea paper by Hartmann Davis, 1975
  additional geochemical research by Michael Drake
  and computer models by Jay Melosh and colleagues.
• Impact occurs soon after Earths core formation
  event because of the small lunar Fe core and
  difference in bulk density (rMoon 3.3 g/cc ltlt
  rEarth 5.5 g/cc).
• Impact event must occur before formation of the
  lunar highlands at 4.4 Ga, which formed as a
  result of the crystallization of the lunar magma
  ocean. Lunar differentiation continues w/ basalt
  genesis (3.95 to 3.15 Ga).
• Oxygen isotope compositions of lunar and
  terrestrial rocks are similar, but different from
  Mars and meteorites. Earth-Moon must be made of
  the same stuff.
• Volatiles are depleted in the proto-moon during
  impact event. This is consistent with
  geochemistry and petrology of lunar samples.

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Lunar Interior Composition
From BVSP, 1986 and Taylor, 1987
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Lunar Basalt Generation Summary

• All lunar basalts are ancient in comparison with
  MORBs (100 Ma average age). Lunar basalt ages
  range between 3.95 to 3.15 Ga.
• Mare regions resemble continental flood basalt
  provinces and ocean plateaus in areal extent.
• Several distinctly different compositions (e.g.
  KREEP, Hi-Ti, Low-Ti), which likely reflect
  different source regions that developed during
  post magma-ocean crystallization.
• Strong positive Eu anomalies in highlands
  Anorthosites is complemented by Eu depletion in
  all lunar basalts.
• Younger basalts are more primitive and may be
  derived from deeper sources. This could reflect
  increased internal heating from radioactive decay.

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Other Extraterrestrial Basalts - I

• Basaltic achondrite meteorites have compositions,
  textures, and mineralogies that are broadly
  similar to terrestrial basalts.
• Eucrites and Howardites all have ancient
  crystallization ages of 4.6 Ga. Again very
  different from Earth and the Moon.
• Oxygen isotopic ratios are distinct from
  terrestrial rocks, thus they are derived from a
  different region in the solar system.
• Achondrites are derived from Eucrite Parent Body,
  which must have had a mantle dominated by olivine
  and pyroxene depleted in alkalis and volatiles
  and a high Fe/Mg. Melting occurred in the
  presence of plagioclase, so the body must be
  small to have low P at high T!
• Asteroid Vesta (540 km diameter) is a candidate
  for the EPB as its surface is covered in basalt,
  but this is just speculation.

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Asteroid Vesta - Eucrite Parent Body?
Spectroscopy of surface from HST indicates two
types of basalts. Large crater found near South
Pole! Source of achondrites?
From B. Zellner and NASA
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Viking 2 Lander Site Mars - Basalt Flow Field?
Image Source NASA
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Shergotty Meteorite - Martian Basalt Sample?
Photo credit Robby Score, JSC
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SNC and Martian Basalt Summary

• SNC (Shergotty, Nakhla, Chassigny) meteorites are
  thought to be derived from Mars. Shergottites are
  most similar to terrestrial basalts while
  nakhilites are cumulate peridotites. All SNC show
  shock metamorphism.
• Oxygen isotopic signatures different from Earth
  and Moon.
• Noble gas ratios are similar to modern Mars
  atmosphere and very different from Earth. Fe/Mg
  ratio higher than Earth. Mars may not have a
  Fe-rich core.
• 1.3 Ga crystallization ages are much young than
  other basaltic meteorites. Corresponds to period
  of active basaltic resurfacing on Mars based on
  crater density.
• Mechanism for ejection from Mars surface is still
  problematical.