Download Presentation #1

Magma Oceans in the Inner Solar System
Linda T. Elkins-Tanton
Magma Oceans
Planetecimals accreted first 1.5M.A. of Solar
System
1694- Gottfried Wilhelm Leibniz :
Suggested Earth began as a uniform liquid and differentiated as it cooled
1748- Georges-Louis Leclrec, Comte de
Buffon:
Planets began in the molten state after fragmented from the sun
1955- Harold Urey:
Silicate and Metal Materials in terrestrial planets melted several times
before reaching
their final solid state
1992- Taylor & Norman:
Define Magma oceans by 2 criteria
a) magma behaves as liquid with small crystal fraction
b) magma consists of a substantial fraction of parent body
Magma Oceans
Model:
Core formation Early and crustal formation by 4.4Ga as evidenced by:
• Some compositions and mineralogy suggest early widespread
melting and fractional crystallization
• Compressed crystallization age (Zircons)
•
182W
and 142Nd contents of crustal and mantle rocks suggest early
core formation
• Siderophile content of the mantle is thought to be due to core
formation and accretion
The Moon
Apollo Missions:
• Provoked magma ocean in current form due to discovery of
anorthosites suggesting floatation on magma ocean of early moon
• KREEP ( Potassium, Rare Earth Elements, Phosphorus) basalts
and picritic glasses enriched in incompatible elements consistant
with fractional solidification of a Magma ocean
• Magma ocean was originally ~100s km deep as evidenced by
Europium deficits on Mare basalts and enrichment in Anorthitic
crust
Mars and Earth
Mars:
•
Early core formation suggests accretionary heat concentrated in brief time period giving
greater melting potential
•
core formation enhanced by presence of magma oceans
•
Preservation of Rb-Sr isochrons and W & Nd anomalies suggest little to no crustal recycling
Earth:
• Siderophile content of mantle settled during core segregation at 27GPa and
2000C
• 142Nd/144Nd ratio of chondrites differs from earth’s mantle
• Possible untapped mantle source to balance to chondritic composition
•
Original Crust recycled due to plate tectonics
Vesta
•
•
•
•
HEDs ( Howardite, Eucrites, Diogenites) inferred to originate from vesta
Isotopes indicate igneous rocks in first 10 Ma of SS
Ages of iron meteorites of destroyed planitecimals
• Suggest planitecimal formation and differentiation
• Sources accreted quickly and melted internally
Internal Magma Ocean suggested
http://www.nasa.gov/mission_pages/dawn/multimedia/pia15678.html
Primary Crust formation
Conductive lids on magma oceans formed one of the following ways:
•
Internal magma oceans- outer portion of planetecimal may remain unmelted and conductive (eg. Vesta)
•
Buoyant phases may form in the magma ocean and float to the
surface ( eg. Plagioclase on moon)
•
Mafic silicate magma may quench to form solid crust
Atmosphere initially insulate magma ocean above liquidus and solidus
during the solidification
Magma oceans with 100ppm water retain free liquid magma ocean
surface to prevent quenching
Dense quenched material sinks
Jeff Plescia 2008
Cooling of Magma Ocean
Crystal settling and Entrainment

Oxides ( SiO2, Al2O3, MgO, FeO, and CaO) make 97 wt% of total silicates in terrestrial planets

SiO2 ~41-46%, MgO ~30-40%, FeO ~8-18% others 3-6%

Mars is more iron rich

Mafic phases (Olivine and Pyroxine) are denser and sink
Crystal settling and Entrainment

Oxides ( SiO2, Al2O3, MgO, FeO, and CaO) make 97 wt% of total silicates in terrestrial planets

SiO2 ~41-46%, MgO ~30-40%, FeO ~8-18% others 3-6%

Mars is more iron rich

Mafic phases (Olivine and Pyroxine) are denser and sink
With added volatiles
Effect of
volatiles on
Magma
Ocean
Volatile behavior
Abundance and distribution of Carbon and Hydroxyl determine
mantle viscosity and melting temperature
Eucrites, such as ones in Vesta, lower in volatiles than chondrites
Volatiles thought to have been released to space upon eucritic
volcanic eruptions
Un-melted crusts retain near original water content
Could obtain higher water contents via fluid fluxes from interior
Crystallization of magma ocean facilitates concentration of
volatiles in the melt