Download Terrestrial and Extraterrestrial Basalts

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.
Trace Element Fractionation
During Partial Melting
La
Lu
Ni
La
Rb
Sr
Region of
Partial Melting
Nd
Sm
M elting Residue
Co
La
Lu
From: http://www.geo.cornell.edu/geology/classes/geo302
Basalt Types-Major Element Variation
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
Tholeiitic vs. Calc-alkaline Trends
Terms emerged from tangled history
spanning many decades. CA label
proposed by Peacock in 1931.
Tholeiite originated in mid-1800’s
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
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
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.
Yoder & Tilley Basalt Tetrahedron
Yoder & Tilley, 1962; Le Maitre
Basalt Types - Trace Elements
Partition Coefficients for REEs
Partition Coefficients for REE in Melts
Dbulk = X1D1 + X2D2 + X3D3 + … + XnDn
Chondrite Normalized REE patterns
• By “normalizing” (dividing by abundances in chondrites), the
“sawtooth” pattern can be removed.
Differentiation of the Earth
Rb>Sr
Nd>Sm
La>Lu
Continental Crust
La
Lu
Rb>Sr
Nd>Sm
La>Lu
Mantle
(After partial
melt extraction)
Rb<Sr
Nd<Sm
La<Lu
La
Lu
• 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/geo302
Sr Isotope Evolution on Earth
87Sr/86Sr)
0
Time before present (Ga)
87Sr/86Sr)
0
Time before present (Ga)
Sr and Nd Isotope Correlations:
The Mantle Array
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 NMORB (normal), E-MORB (enriched), IAB, and OIB
indicate that the Earth’s upper mantle has long-lived and
physically distinct source regions.
• Ancient komatiites (>2.5 Ga) indicate that the Earth’s upper
mantle was hotter in the Archean, but already depleted of
continental crustal components.
Lunar Surface
Apollo 15 Basalt Sample
Vesicles Probably
derived from
CO degassing
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
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
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)
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 Earth’s core formation event
because of the small lunar Fe core and difference in bulk
density (rMoon = 3.3 g/cc << 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. EarthMoon 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.
Lunar Interior Composition
From: BVSP, 1986 and Taylor, 1987
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.
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.
Asteroid Vesta - Eucrite Parent Body?
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
Spectroscopy of surface
from HST indicates
two types of basalts.
Large crater found near
South Pole! Source of
achondrites?
From: B. Zellner and NASA
Viking 2 Lander Site Mars - Basalt Flow Field?
Image Source: NASA
Shergotty Meteorite - Martian Basalt Sample?
Photo credit: Robby Score, JSC
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.