Transcript Planet Formation in the Inner Solar System

Planet Formation and Evolution in the
Inner Solar System:
Origin and Nature of Impacting Projectiles
Gerhard Schmidt
Condensation sequence of metals and minerals from a
cooling gas of solar composition
Refractory Metal Alloys
Re, Os, W, Ir, Mo, Ru, Pt, Rh
• refractory metals are the first
condensates from a cooling
nebular gas
e.g. Berg et al. 2009 LPSC
• complex particles (tenths of μm)
contain the most refractory metals
Re, Os, W, Ir, Mo, Ru, Pt, Rh
• refractory metal nuggets occur
embedded in Ca,Al-rich
inclusions (CAIs)
• refractory metal nuggets
represent the most pristine solar
system material found so far
Nebular Condensation of Sub-micron Refractory Metal Alloys
RMN from
CM chondrite
Murchison
Fig. from Berg, Marosits, Maul, Schönhense, Hoppe, Ott, Palme (2009) LPSC
RMN with a composition incompatible with geochemical processes
(by weight %: W 5.8, Os, 13.0, Ir 24.0, Mo 14.2, Ru 31.1, Pt 9.6, Ni 1.9, Fe 0.4)
and polyhedral surface characteristics indicating a monocrystalline cubic structure
Planet Formation (Earth, 4.567 billion years ago)
After D. L. Anderson (1990) in „The New Solar System“
Cambridge University Press
1.
in a first step metals
condense first out of a hot
nebular gas and then
silicates → dust and small
particles accumulate and
transformed into the protoplanet Earth
2.
impacts dominate earliest
history of Earth formation
3.
in a next step nickel-iron
separated from silicates and
stripped the proto-Earth’s
mantle of all the highly
siderophile elements
(Os, Ir, Ru, Pt, Rh, Pd)
→ core formation was
finished about 30 Ma
after Earth formation
Exogenic Contribution to Earth and Moon
4. collision with a Mars-sized
object
→ Re, Os, Ir, Ru, Pt, Rh, Pd
and Au were added to
accreting Earth by a late
bombardment after
core formation (30-50 Ma)
© Fahad Sulehria
5.
crust formation, erosion
and sediment recycling as
early as 4.35 Ga
6.
Late Heavy Bombardment
on Moon and Earth
(4.5 to 3.8 billion years
ago)
Highly Siderophile Elements in Earth Mantle
and Apollo 17 Melt Samples
Apollo 17
Ru/Ir=<1.7
3.9 Ga signature
EH or LL chondrite
Ru/Ir=1.8
Norman et al. 2002
Earth
Ru/Ir=2
• HSE in 3.9 Ga old Apollo 17
melt rocks from impactor
that formed Serenitatis basin
• Moon significantly lower
Ru/Ir and Pd/Ir in
comparison to Earth mantle
 siderophile enrichment in
Apollo 17 impact melts
different from Earth
mantle’s exogenic
contribution (“late veneer”)
 HSE of Earth and lunar
breccias from different
projectiles
Volatility-Related Abundances of
Highly Siderophile Elements (HSE) in Earth
•
relative abundances of HSE are
not CI-chondritic
•
systematic enrichment correlated with
the volatility of the elements
→ HSE decreases with increasing
condensation temperatures from Pd
to Os
•
the more volatile element Pd is more
abundant by about a factor of 2
•
excess of non-refractory element Pd
and depletions in refractory elements
Ir and Os consistent
→ high-temperature gas condensation
fractionation process
Chondrite Formation Model
• carbonaceous, ordinary, and enstatite chondrites exhibit successively
greater degrees of reduction and successively lower contents of
refractory elements
• features most likely associated with formation at successively smaller
heliocentric distances in hotter portions of the solar nebula
Baedecker and Wasson 1975; Hertogen et al. 1983
• EH chondrites → more refractory HSE (Re, Os, Ir, Ru, Pt) fractionated
from moderately volatile siderophile elements
(Pd, Au, As, Ga)
Earth mantle abundances normalized to
enstatite chondrites (EH)
Schmidt 2004
HSE systematics of Earth mantle‘s exogenic contribution resembles materials more
fractionated than enstatite chondrites → an indication that some inner solar system
materials were more highly fractionated than enstatite chondrites
Where does the Material come from?
Formation Location of Exogenic Mantle Material
Enstatite formation location:
•
in the innermost solar system (perhaps less
than 1 AU from the Sun, possibly in the Mercury-Venus region)
Baedecker and Wasson (1975), Clayton et al. (1976), Wasson and Wetherill (1979)
Arguments:
1)
oxygen isotope composition is close to that of Earth and Moon
(single fractionation line)
2)
lower FeO and greater depletion of refractories than other meteorites and
Earth
Earth mantle‘s exogenic metals formation location:
•
HSE more highly fractionated than EH chondrites → indication of formation
at smaller heliocentric distances in hotter portions of the solar nebula
Condensation along with Isolation of Metal
Grains by Gravitational Settling
•
Larimer and Anders (1970) proposed that high-temperature materials were
removed by gravitational settling (as particles) to the nebular median plane
•
continuous loss of refractory nebular condensates (refractory metal alloys)
from the chondrite formation region during condensation could thus
account for the depletion and fractionation of refractory elements in the
Earth late accreted component (Earth mantle‘s exogenic contribution)
•
gas-solid fractionation during condensation from a solar gas could be one
of the most important process leading to fractionations in primitive objects
of our solar system (Wasson and Chou 1974)
Impact Cratering
A major process in the origin and evolution of
Planets
→ key influence on geochemical evolution of solid bodies
(Moon, Mars, Earth)
• Moon provide insight into first few hundred million years of solar
system existence
→ insight into early development of Earth and other planetary
bodies
• Earth record of early evolution is destroyed by geological
activity (erosion and tectonics)
Earth
• ~175 impact craters up to ~300 km in diameter and
up to ~2 Ga in age
→ ~ 20 craters (Ø = 0.015 - 23 km) → iron projectiles
(e.g. Schmidt et al. 1997, Geochimica et Cosmochimica Acta 61, 2977)
→ ~ 20 craters (Ø = 1 - 300 km) → chondrites as bolide types
• highly siderophile elements (HSE: Os, Ir, Ru, Pt, Rh, Pd)
and Ni
• HSE composition → key issue for understanding origin and
influence of impactors on chemical composition of planets
 nature of materials that bombarded early Earth, Moon and Mars
Barringer crater, Arizona
Locations of Impact Craters
~ 175 identified impact craters up to ~ 300 km in diameter and up to ~ 2 Ga in age
~ 20 craters (Ø = 0.015 to 23 km) → iron projectiles
~ 20 craters (Ø = 1 to 300 km) → chondrites as impactors
http://www.lpl.arizona.edu
Barringer crater, Arizona
J. A. Wood
Barringer crater, Arizona
Ø = 1.2 km; depth = 200 m; age ~ 50.000 years;
projectile = IAB iron; projectile diameter ~ 33 m; impact velocity ~ 15 km/sec
Impact craters with no detectable
meteoritic contaminations of Iridium
Nördlinger Ries (Germany)
From G. Schmidt (1993)
Manicougan (Canada)
~ 214 Ma, 100 km diameter
• ~ 15.1 Ma
• 24 - 26 km diameter crater
• excavated from carbonate-bearing
sedimentary sequences and underlying
crystalline silicate basement materials
Abundance (atom fraction) of the chemical elements in
Earth’s upper continental crust as a function of
atomic number
US Geological Survey
Earth's upper continental crust
→ very low Ir and Os contents of ~15 ppt (Nördlinger Ries)
Clearwater East impact structure (Canada)
D = 22 km; age = 287 Mio. J.
PGE ratios and Cr-Isotopie
ordinary chondrite
(Schmidt 1997; Shukolyukov and Lugmair 2000)
→ bolide typ
LL chondrite?
L chondrite?
Schmidt (1997)
concentration of target
rocks corrected
Ru 1.27 ng/g
Rh 1.12 ng/g
Pd 3.03 ng/g
Fig. after McDonald (2002) using data from Schmidt (1997)
Sääksjärvi impact structure (Finland)
Schmidt et al. (1997)
concentration of target
rocks corrected
→ bolide type
iron meteorite
(Schmidt et al. 1997)
Ru 1.6 ng/g
Rh 0.6 ng/g
Pd 3.5 ng/g
impact structure
D = 6 km; age = 560 Ma
Figs. after McDonald (2002) using data from Schmidt et al. (1997)
Magmatic Iron Meteorite as Projectile of
Sääksjärvi Impact Crater
Fig. from Schmidt, Palme, Kratz (1997) GCA
HSE and Ni in Earth crust and
IIIAB iron meteorite Grant
• IIIAB iron meteorites → wide range of Ni/Ir
ratios
•
Ir, Os, Pt, Rh, Ru enriched in early Ni-poor
solid → negatively correlated with Ni
•
Pd, Au, Sb, As, P, Co, Mo concentrate in later
solid → positive correlations with Ni
→ siderophile element fractionations by
fractional crystallisation of a liquid iron core
in parent meteorite bodies (Scott 1972)
→ similar chemical fractionation of highly
siderophile elements and Ni in iron
meteorites and continental upper crust
→ genetic link?
Grant: Os, Ir, Ru, Rh, Pd, and Ni; Mullane et al. (2004), Pt from Pernicka and Wasson (1987)
Crustal data derived from Ries crater; Schmidt and Pernicka (1994) and
Sääksjaärvi, Mien and Dellen craters; Schmidt et al. (1997)
Crustal data are multiplied by a factor of 2.5 x 103 for comparison.
Crustal abundances of HSE and Ni
in Earth and Mars
Earth
•
strongly fractionated element pattern
•
Os and Ir decoupled from Ru, Pt, Rh, Pd, Ni
•
strongly fractionated Ru/Ir unparalleled in
terrestrial magmatic systems
Mars
•
SNC meteorites Nakhla, Shergotty, Zagami
similar HSE and Ni chemistry than
Earth crust
IIIAB irons
•
SNC data from Jones et al. (2003)
Crustal data derived from Ries crater (Schmidt and Pernicka 1994) and
from Sääksjaärvi, Mien and Dellen craters (Schmidt et al. 1997)
similar fractionated element patterns
not distinguishable Earth and Mars
→ HSE in crust of Mars and Earth by impacts?
Mantle abundances of HSE and Ni in
Earth and Mars (SNC meteorites) and IVA Iron Charlotte
•
chemical fractionation of HSE and Ni in
SNC meteorites (ALH 77005, EET
79001A, Dar al Gani 476)
Field photo of Dar al Gani 476 by Zipfel. Sample size is ~ 15 by 10 cm.
•
SNC data from Jones et al. (2003)
IVA Iron data from Walker et al. (2005)
Mantle data from Schmidt (2004)
Iron meteorite Charlotte and Earth
mantle similar PGE pattern
→ genetic link?
Meteorite crater on Mars
Impactcrater Endurance: 132 m diameter, 15 m depth
J. A. Wood
NASA
January 2005 Mars-Rover Opportunity found this iron meteorite – the first one ever discovered
on another planet
Mass of Iron Meteorites in the Earth
Upper Continental Crust (UCC)
•
IIIAB Iron is a common type of iron meteorite
•
about 20 impact craters on Earth produced by iron meteorites
•
Estimation of meteoritic mass (92.4 mg/g Ni Grant, 8 g cm-3 for
magmatic iron meteorites)
Assumtion:
if bulk Ni and Ir abundances in UCC is extraterrestrial in origin
 corresponds to a volume of 6.55 x 1020 cm3 and about
160 spherical impacting asteroids (M-type objects?)
with diameters of 20 km (upper limit)
Main belt M-type asteroids 16 Psyche
and 216 Kleopatra
Psyche
•
D > 200 km
largest of metallic M-type asteroids
•
contain 0.6 percent of the mass of the
entire asteroid belt (Vitaeau 2000)
•
pure iron-nickel composition
Artist image of asteroid Kleopatra
Kleopatra
http://solarsystem.nasa.gov
•
217 × 94 × 81 km
•
consistent with metallic composition
(Mueller, Harris, Delbó, MIRSI-team 2005)
Conclusions
(1) refractory metal systematics of Earth mantle‘s exogenic
component resembles materials more fractionated than
enstatite chondrites (EH)
→ formation location of refractary metals of the Earth
mantle in the innermost solar system, perhaps even less
than 1 AU from the Sun
(2) Martian crust and Earth crust preserves an old imprint of
fractionation processes which occurred in asteroids
→ parent bodies of magmatic irons or pallasites
(M-Type asteroids?)