Abstract
We report here the results of an investigation of W and Nd isotopes in the SNC (Shergottite-Nakhlite-Chassignite (martian)) meteorites. We have determined that ε182W values in the nakhlites are uniform within analytical uncertainties and have an average value of ∼3. Also, while ε182W values in the shergottites have a limited range (from 0.3-0.7), their ε142Nd values vary considerably (from -0.2-0.9). There appears to be no correlation between ε182W and ε142Nd in the nakhlites and shergottites. These results shed new light on early differentiation processes on Mars, particularly on the timing and nature of fractionation in silicate reservoirs. Assuming a two-stage model, the metallic core is estimated to have formed at ∼12 Myr after the beginning of the solar system. Major silicate differentiation established the nakhlite source reservoir before ∼4542 Ma and the shergottite source reservoirs at 4525 [+19 -21] Ma. These ages imply that, within the uncertainties afforded by the 182Hf-182W and 146Sm-142Nd chronometers, the silicate differentiation events that established the source reservoirs of the nakhlites and shergottites may have occurred contemporaneously, possibly during crystallization of a global magma ocean. The distinct 182W-142Nd isotope systematics in the nakhlites and the shergottites imply the presence of at least three isotopically distinct silicate reservoirs on Mars, two of which are depleted in incompatible lithophile elements relative to chondrites, and the third is enriched. The two depleted silicate reservoirs most likely reside in the Martian mantle, while the enriched reservoir could be either in the crust or the mantle. Therefore, the 182W-142Nd isotope systematics indicate that the nakhlites and the shergottites originated from distinct source reservoirs and cannot be petrogenetically related. A further implication is that the source reservoirs of the nakhlites and shergottites on Mars have been isolated since their establishment before ∼4.5 Ga. Therefore, there has been no giant impact or efficient global mantle convection to thoroughly homogenize the Martian mantle following the establishment of the SNC source reservoirs.
| Original language | English (US) |
|---|---|
| Pages (from-to) | 4557-4571 |
| Number of pages | 15 |
| Journal | Geochimica et Cosmochimica Acta |
| Volume | 69 |
| Issue number | 18 |
| DOIs | |
| State | Published - Sep 15 2005 |
| Externally published | Yes |
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ASJC Scopus subject areas
- Geochemistry and Petrology
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The early differentiation history of Mars from 182W-142Nd isotope systematics in the SNC meteorites. / Foley, C. Nicole; Wadhwa, Meenakshi; Borg, L. E.; Janney, P. E.; Hines, R.; Grove, T. L.
In: Geochimica et Cosmochimica Acta, Vol. 69, No. 18, 15.09.2005, p. 4557-4571.Research output: Contribution to journal › Article
}
TY - JOUR
T1 - The early differentiation history of Mars from 182W-142Nd isotope systematics in the SNC meteorites
AU - Foley, C. Nicole
AU - Wadhwa, Meenakshi
AU - Borg, L. E.
AU - Janney, P. E.
AU - Hines, R.
AU - Grove, T. L.
PY - 2005/9/15
Y1 - 2005/9/15
N2 - We report here the results of an investigation of W and Nd isotopes in the SNC (Shergottite-Nakhlite-Chassignite (martian)) meteorites. We have determined that ε182W values in the nakhlites are uniform within analytical uncertainties and have an average value of ∼3. Also, while ε182W values in the shergottites have a limited range (from 0.3-0.7), their ε142Nd values vary considerably (from -0.2-0.9). There appears to be no correlation between ε182W and ε142Nd in the nakhlites and shergottites. These results shed new light on early differentiation processes on Mars, particularly on the timing and nature of fractionation in silicate reservoirs. Assuming a two-stage model, the metallic core is estimated to have formed at ∼12 Myr after the beginning of the solar system. Major silicate differentiation established the nakhlite source reservoir before ∼4542 Ma and the shergottite source reservoirs at 4525 [+19 -21] Ma. These ages imply that, within the uncertainties afforded by the 182Hf-182W and 146Sm-142Nd chronometers, the silicate differentiation events that established the source reservoirs of the nakhlites and shergottites may have occurred contemporaneously, possibly during crystallization of a global magma ocean. The distinct 182W-142Nd isotope systematics in the nakhlites and the shergottites imply the presence of at least three isotopically distinct silicate reservoirs on Mars, two of which are depleted in incompatible lithophile elements relative to chondrites, and the third is enriched. The two depleted silicate reservoirs most likely reside in the Martian mantle, while the enriched reservoir could be either in the crust or the mantle. Therefore, the 182W-142Nd isotope systematics indicate that the nakhlites and the shergottites originated from distinct source reservoirs and cannot be petrogenetically related. A further implication is that the source reservoirs of the nakhlites and shergottites on Mars have been isolated since their establishment before ∼4.5 Ga. Therefore, there has been no giant impact or efficient global mantle convection to thoroughly homogenize the Martian mantle following the establishment of the SNC source reservoirs.
AB - We report here the results of an investigation of W and Nd isotopes in the SNC (Shergottite-Nakhlite-Chassignite (martian)) meteorites. We have determined that ε182W values in the nakhlites are uniform within analytical uncertainties and have an average value of ∼3. Also, while ε182W values in the shergottites have a limited range (from 0.3-0.7), their ε142Nd values vary considerably (from -0.2-0.9). There appears to be no correlation between ε182W and ε142Nd in the nakhlites and shergottites. These results shed new light on early differentiation processes on Mars, particularly on the timing and nature of fractionation in silicate reservoirs. Assuming a two-stage model, the metallic core is estimated to have formed at ∼12 Myr after the beginning of the solar system. Major silicate differentiation established the nakhlite source reservoir before ∼4542 Ma and the shergottite source reservoirs at 4525 [+19 -21] Ma. These ages imply that, within the uncertainties afforded by the 182Hf-182W and 146Sm-142Nd chronometers, the silicate differentiation events that established the source reservoirs of the nakhlites and shergottites may have occurred contemporaneously, possibly during crystallization of a global magma ocean. The distinct 182W-142Nd isotope systematics in the nakhlites and the shergottites imply the presence of at least three isotopically distinct silicate reservoirs on Mars, two of which are depleted in incompatible lithophile elements relative to chondrites, and the third is enriched. The two depleted silicate reservoirs most likely reside in the Martian mantle, while the enriched reservoir could be either in the crust or the mantle. Therefore, the 182W-142Nd isotope systematics indicate that the nakhlites and the shergottites originated from distinct source reservoirs and cannot be petrogenetically related. A further implication is that the source reservoirs of the nakhlites and shergottites on Mars have been isolated since their establishment before ∼4.5 Ga. Therefore, there has been no giant impact or efficient global mantle convection to thoroughly homogenize the Martian mantle following the establishment of the SNC source reservoirs.
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UR - http://www.scopus.com/inward/citedby.url?scp=26444434837&partnerID=8YFLogxK
U2 - 10.1016/j.gca.2005.05.009
DO - 10.1016/j.gca.2005.05.009
M3 - Article
VL - 69
SP - 4557
EP - 4571
JO - Geochmica et Cosmochimica Acta
JF - Geochmica et Cosmochimica Acta
SN - 0016-7037
IS - 18
ER -