TY - JOUR
T1 - Hf-W evidence for rapid differentiation of iron meteorite parent bodies
AU - Scherstén, Anders
AU - Elliott, Tim
AU - Hawkesworth, Chris
AU - Russell, Sara
AU - Masarik, Josef
N1 - Funding Information:
We thank Ian Parkinson who shared a split of his UMD Os-standard. Alex Halliday, Tibor Dunai Gareth Davies, Pieter Vroon and Wim Van Westrenen provided valuable comments and discussions. Nicole Foley and Carsten Münker are thanked for constructive journal reviews. An EU Marie Curie post-doctoral fellowship (AS) and a Philip Leverhulme Prize (TE) supported this work, which are gratefully acknowledged.
PY - 2006/1/31
Y1 - 2006/1/31
N2 - New, high-precision W isotope data on iron meteorites are presented that provide important constraints on the timing of silicate-metal segregation in planetesimals. Magmatic iron meteorites all have ε182W within error or less radiogenic than initial ε182W estimated by studies of chondritic meteorites. At face value this implies that iron meteorites are as old and older than refractory calcium-aluminium rich inclusions (CAI), which are widely thought to be the oldest solar system objects. Moreover, different meteorites from the same magmatic groups, believed to be derived from the same planetissimal core, display a range of ε182W. We suggest that the paradoxical ε182W values more negative than initial Solar System Initial (SSI) are most readily explained as a result of secondary, spallation reactions with cosmic rays during transit between parent body and the earth. This is supported by the most negative ε182W being found in meteorites with the oldest exposure ages and the magnitude of the effect is shown to be consistent with known nuclear reactions. On the other hand, it is also striking that none of the magmatic iron group meteorites have ε182W analyses, outside error, more radiogenic than the estimated solar system initial ratio. This suggests that core formation in parent bodies of magmatic iron meteorites occurred ≤ 1.5 Myr after the formation age of CAI [Y. Amelin, A.N. Krot, I.D. Hutcheon, and A.A. Ulyanov, Lead isotopic ages of chondrules and calcium-aluminum inclusions, Science 297, 1678-1683, 2002]. This extremely early metal-silicate differentiation is coeval with the first chondrules [M. Bizzarro, J.A. Baker, and H. Haack, Mg isotope evidence for contemporaneous formation of chondrules and refractory inclusions, Nature 431, 275-278, 2004, A.N. Krot, Y. Amelin, P. Cassen, and A. Meibom, Young chondrules in CB chondrites from a giant impact in the early Solar System, Nature 436, 989-992, 2005]. Formation of later chondrules, and hence the parent bodies of some chondritic meteorites, must therefore have occurred in the presence of planetesimals large enough to possess iron cores. We conclude that early planetary accretion and differentiation was sufficiently fast for 26Al-decay to be an important heat source. Non-magmatic iron meteorites, however, display more radiogenic and varied W isotope signatures. This is in keeping with them being generated later, by impact melting during which the metal (partially) re-equilibrated with the then more radiogenic silicate fraction.
AB - New, high-precision W isotope data on iron meteorites are presented that provide important constraints on the timing of silicate-metal segregation in planetesimals. Magmatic iron meteorites all have ε182W within error or less radiogenic than initial ε182W estimated by studies of chondritic meteorites. At face value this implies that iron meteorites are as old and older than refractory calcium-aluminium rich inclusions (CAI), which are widely thought to be the oldest solar system objects. Moreover, different meteorites from the same magmatic groups, believed to be derived from the same planetissimal core, display a range of ε182W. We suggest that the paradoxical ε182W values more negative than initial Solar System Initial (SSI) are most readily explained as a result of secondary, spallation reactions with cosmic rays during transit between parent body and the earth. This is supported by the most negative ε182W being found in meteorites with the oldest exposure ages and the magnitude of the effect is shown to be consistent with known nuclear reactions. On the other hand, it is also striking that none of the magmatic iron group meteorites have ε182W analyses, outside error, more radiogenic than the estimated solar system initial ratio. This suggests that core formation in parent bodies of magmatic iron meteorites occurred ≤ 1.5 Myr after the formation age of CAI [Y. Amelin, A.N. Krot, I.D. Hutcheon, and A.A. Ulyanov, Lead isotopic ages of chondrules and calcium-aluminum inclusions, Science 297, 1678-1683, 2002]. This extremely early metal-silicate differentiation is coeval with the first chondrules [M. Bizzarro, J.A. Baker, and H. Haack, Mg isotope evidence for contemporaneous formation of chondrules and refractory inclusions, Nature 431, 275-278, 2004, A.N. Krot, Y. Amelin, P. Cassen, and A. Meibom, Young chondrules in CB chondrites from a giant impact in the early Solar System, Nature 436, 989-992, 2005]. Formation of later chondrules, and hence the parent bodies of some chondritic meteorites, must therefore have occurred in the presence of planetesimals large enough to possess iron cores. We conclude that early planetary accretion and differentiation was sufficiently fast for 26Al-decay to be an important heat source. Non-magmatic iron meteorites, however, display more radiogenic and varied W isotope signatures. This is in keeping with them being generated later, by impact melting during which the metal (partially) re-equilibrated with the then more radiogenic silicate fraction.
KW - Asteroid differentiation
KW - Core formation
KW - Iron meteorite
KW - Planetary accretion
KW - Tungsten isotopes
UR - http://www.scopus.com/inward/record.url?scp=30744471701&partnerID=8YFLogxK
U2 - 10.1016/j.epsl.2005.11.025
DO - 10.1016/j.epsl.2005.11.025
M3 - Article
VL - 241
SP - 530
EP - 542
JO - Earth and Planetary Science Letters
JF - Earth and Planetary Science Letters
SN - 0012-821X
IS - 3-4
ER -