Tracing Oxygen Fugacity in Asteroids and Meteorites through Olivine Composition

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Tracing Oxygen Fugacity in Asteroids and
Meteorites through Olivine Composition

• Jessica SunshineScience Applications
  International Corporation (SAIC)
• Tim McCoy and Cari CorriganSmithsonian
  Institution
• Bobby BusInstitute for Astronomy
• Tom BurbineMt. Holyoke College

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Does Olivine-Rich ? Mantle ?

• Asteroid Conventional Wisdom
• Olivine-rich rocks are restricted to the
  mantles of differentiated bodies

• Meteorite Reality
• Olivine-dominated chondrites exist
  (R-chondrites)
• gt 90 olivine
• Primitive achondrites exist (Brachinites)
• Produced in localized magma systems?
• difficult to produce a hemispherically averaged
  signature

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Does Mantle Rock ? Only Olivine ?

• Asteroid CW
• Silicates in mantles of asteroids are pure
  olivine

• Meteorite Reality
• Pyroxene-rich pallasites exists
• Pure olivine residuals require high
  temperatures and extensive melting
• May not be possible in a convecting mantle

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OLV and OPX Abundance During Melting
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Does Differentiation ? MgO-Rich ?

• Asteroid CW
• All differentiated olv-rich meteorites are
  MgO-rich

• Meteorite Reality
• Brachinites are differentiated, but FeO-rich (Fa
  40)due to oxidization

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Does No Pyroxene ? Mantle ?

• Asteroid CW
• Olivine-rich asteroids that lack pyroxene must
  be mantles

• Reality
• R-Chondrites have so little pyroxene (lt10)
  that they could be olivine only

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Petrologic Origin of Asteroidal Olivines

• Not Necessarily Mantles
• Olivine-Rich, Pyroxene, FeO Poor
• Likely mantle sample
• MgO-rich, olv-rich chondrites don't exist
• oxidation also increases FeO
• Olivine-Rich, Pyroxene, FeO Rich
• Achondritic (oxidized but some differentiation)
• e.g. Brachinites
• Chondritic (oxidized region of nebula)
• e.g. R-Chondrites
• At present, can't distinguish from spectra alone

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Olivine Composition from Spectra

• Determine composition of olivine in lab spectra
• Prove with meteorites
• Brachinites and R-Chondrites (Fa 40) vs.
  Pallasites (Fa 11)
• Olivines from SNC's (Fa 70, Fa 32, Fa 25)
• Asteroid spectra
• Requires full 1 µm feature with high SNR
• Now have with SpeX
• At different temperature than lab (160-180K vs.
  300K)
• Understand temperature and compositional
  variability
• Space weathering
• Red slope
• Methodology must work in the presence of pyroxene
• Want to extend to more pyroxene-rich asteroids

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Olivine Variations with Composition
Sunshine and Pieters, 1998
King and Ridley, 1987
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Olivine Variations with Composition
MgO
FeO
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Olivine Dominated Meteorites
Pallasite Thiel Mountains R-Chondrite
Rumuruti Brachinite EET 99402 R-Chondrite NWA 753
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Composition of Olivine in SNC's
ALHA 77005 Chem Fa 25 MGM Fa 20
Chassigny Chem Fa 32 MGM Fa 40
Nakhla Chem Fa 70 MGM Fa 70
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Olivine Temperature Spectra
Sunshine, Hinrichs, and Lucey, LPSC 31
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Relative Band Strength
Fe-rich
MgO
Mainbelt
Mg-rich

• Temperature is known i.e., Mainbelt, NEA, or Lab
  
• Then constrain models to determine composition

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354 Eleonora

• No obvious pyroxene
• Within MgO-rich range
• Consistent with 170 K
• MGM results Fa 10

180K
Mantle Origin

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289 Nenetta

• Outside MgO-rich range
• MGM results Fa 30
• Hemispheric signature

180K
Oxidized Chondrite
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Olivine-Rich Asteroids

• 354 Eleonora MgO-Rich
• 289 Nenetta FeO-Rich
• Others246 Asporina,446 Aeternitas,863
  Benkoela,984 Gretia
• Include 5 Pyx
• Appear to be MgO-Rich

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Petrologic Origin of Olivine-Rich Asteroids

• Not necessarily mantles
• Can distinguish spectrally from olivine
  composition
• Olivine-dominated, FeO-Poor
• 354 Eleonora (and preliminarily other
  olivine-rich asteroids)
• Likely mantle sample
• Olivine-dominated, FeO-Rich
• 289 Nenetta
• Chondritic oxidized region of nebula
  R-Chondrites
• Achondritic oxidized, some differentiation
  Brachinites
• Can't distinguish from spectra alone
• - hemispheric scale suggest chondritic

Olivine Composition Oxidized Chondrite
vs. Mantle