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Earliest known modern humans from Kibish, Ethiopia
Goldschmidt Conference Abstracts 2006
Crust–mantle and core–mantle recycling
Earliest known modern humans from Kibish, Ethiopia
W.F. MCDONOUGH, R.D. AREVALO Department of Geology, University of Maryland, College Park, MD 20742, USA ([email protected]) The formation, evolution and composition of the depleted mantle, the OIB source region and other mantle domains (present-day and ancient) are evaluated and constrained by using a variety of ‘‘constant’’ element ratios (e.g., Ce/Pb, Nb/U). Such element ratios typically involve pairs that have similar bulk partition coefficients during mantle melting, but have contrasting behavior during crust formation and/or during core separation, the two most significant differentiation events in the Earth. It is also highly advantageous to consider element ratio pairs across a spectrum of incompatibility (highly to moderately incompatible to compatible). Examples of these ratios include: Ba/W, Ba/Cs, Th/W, Mo/Ce, P/Nd, Re/Yb, Fe/Mn, Ge/Si, Mg/Ni. Moreover, in harmony with the insights discussed in Sun and McDonough (1989), it is necessary to link these chemical variations with radiogenic isotopic compositional variations that are consistent with the sense recorded in parent–daughter fractionation. By quantifying the variations in both the concentrations of the elements and the absolute values of the ratios we can more narrowly define the limits on the composition of the mantle and its domains, as well as the composition of the Earth’s core. These data are, in turn, used to constrain crust to mantle recycling as well as core–mantle exchange. We have recently acquired data (laser ablation ICP-MS) for such element ratios on glassy basalt chips from various tectonic settings in the oceanic environment with our target precision on the element ratios being <±5%. We find limited evidence for crustal contributions to OIB source regions, with an upper limit of <2%. In addition, these data reveal that contributions from the core to plume magmatism, which may be ascending from core–mantle boundary depth, are limited to <0.5%. This abstract is dedicated to the memory of Shen-su Sun, a great mentor, colleague and friend. doi:10.1016/j.gca.2006.06.824
A409
IAN MCDOUGALL1, F.H. BROWN2, J.G. FLEAGLE3 1
Research School of Earth Sciences, The Australian National University, Australia ([email protected]) 2 Geology and Geophysics, University of Utah, Salt Lake City, Utah, USA ([email protected]) 3 Anatomical Sciences, Stony Brook University, Stony Brook, New York, USA (jfl[email protected]) Two hominin skulls (calvaria), Omo I and Omo II, recovered from the Kibish Formation in southern Ethiopia, have been assigned to Homo sapiens. These specimens are regarded as representative of early anatomically modern humans. However, the provenance and age of the fossils have been much debated. We confirm that Omo I and the somewhat more primitive-looking Omo II calvaria are from similar stratigraphic levels in Member I of the Kibish Formation. Based on 40Ar/39Ar age measurements on alkali feldspar crystals from pumice clasts within a tuffaceous horizon in Member I just below the hominin levels we place an older limit of 198 ± 14 ka (weighted mean age = 196 ± 2 ka) for the hominins. A younger limit of 104 ± 7 ka (weighted mean age = 104 ± 1 ka) is provided by feldspars separated from pumice clasts in the Aliyo Tuff in Member III. Geological evidence indicates rapid deposition of each member of the Kibish Formation, concurrent with deposition of sapropels in the Mediterranean Sea. The 40Ar/39Ar age measurements, together with correlations with sapropels, indicate that the hominin fossils are close in age to the older limit. Our preferred estimate of the age of the hominins is 195 ± 5 ka, making them the earliest well-dated anatomically modern humans yet described. The four members comprising the Kibish Formation are separated by disconformities and have a cumulative thickness of about 100 m. Each member represents an interval of mainly deltaic deposition associated with the Omo River where it entered a much expanded Lake Turkana, about 100 km north of the present lake. In addition to the 40Ar/39Ar ages on alkali feldspars from Members I and III, 14C ages on Member IV show deposition occurred between about 10 and 3 ka ago. The numerical ages for the threedated members of the Kibish Formation are remarkably similar to ages derived via the astronomical time scale for several sapropels in the eastern Mediterranean Sea, 195 ka for sapropel S7, 102 ka for S4 and 8 ka for S1. The link between the two widely separated regions is paleoclimatic. The Omo River and the Nile River systems share a drainage divide and both the formation of the sapropels and the deposition of the members of the Kibish Formation reflect greatly increased precipitation in the Ethiopian highlands and high runoff during periods of intensification of the African monsoon when precession was at a minimum and solar insolation was at a maximum. doi:10.1016/j.gca.2006.06.825
Crust–mantle and core–mantle recycling
Earliest known modern humans from Kibish, Ethiopia
W.F. MCDONOUGH, R.D. AREVALO Department of Geology, University of Maryland, College Park, MD 20742, USA ([email protected]) The formation, evolution and composition of the depleted mantle, the OIB source region and other mantle domains (present-day and ancient) are evaluated and constrained by using a variety of ‘‘constant’’ element ratios (e.g., Ce/Pb, Nb/U). Such element ratios typically involve pairs that have similar bulk partition coefficients during mantle melting, but have contrasting behavior during crust formation and/or during core separation, the two most significant differentiation events in the Earth. It is also highly advantageous to consider element ratio pairs across a spectrum of incompatibility (highly to moderately incompatible to compatible). Examples of these ratios include: Ba/W, Ba/Cs, Th/W, Mo/Ce, P/Nd, Re/Yb, Fe/Mn, Ge/Si, Mg/Ni. Moreover, in harmony with the insights discussed in Sun and McDonough (1989), it is necessary to link these chemical variations with radiogenic isotopic compositional variations that are consistent with the sense recorded in parent–daughter fractionation. By quantifying the variations in both the concentrations of the elements and the absolute values of the ratios we can more narrowly define the limits on the composition of the mantle and its domains, as well as the composition of the Earth’s core. These data are, in turn, used to constrain crust to mantle recycling as well as core–mantle exchange. We have recently acquired data (laser ablation ICP-MS) for such element ratios on glassy basalt chips from various tectonic settings in the oceanic environment with our target precision on the element ratios being <±5%. We find limited evidence for crustal contributions to OIB source regions, with an upper limit of <2%. In addition, these data reveal that contributions from the core to plume magmatism, which may be ascending from core–mantle boundary depth, are limited to <0.5%. This abstract is dedicated to the memory of Shen-su Sun, a great mentor, colleague and friend. doi:10.1016/j.gca.2006.06.824
A409
IAN MCDOUGALL1, F.H. BROWN2, J.G. FLEAGLE3 1
Research School of Earth Sciences, The Australian National University, Australia ([email protected]) 2 Geology and Geophysics, University of Utah, Salt Lake City, Utah, USA ([email protected]) 3 Anatomical Sciences, Stony Brook University, Stony Brook, New York, USA (jfl[email protected]) Two hominin skulls (calvaria), Omo I and Omo II, recovered from the Kibish Formation in southern Ethiopia, have been assigned to Homo sapiens. These specimens are regarded as representative of early anatomically modern humans. However, the provenance and age of the fossils have been much debated. We confirm that Omo I and the somewhat more primitive-looking Omo II calvaria are from similar stratigraphic levels in Member I of the Kibish Formation. Based on 40Ar/39Ar age measurements on alkali feldspar crystals from pumice clasts within a tuffaceous horizon in Member I just below the hominin levels we place an older limit of 198 ± 14 ka (weighted mean age = 196 ± 2 ka) for the hominins. A younger limit of 104 ± 7 ka (weighted mean age = 104 ± 1 ka) is provided by feldspars separated from pumice clasts in the Aliyo Tuff in Member III. Geological evidence indicates rapid deposition of each member of the Kibish Formation, concurrent with deposition of sapropels in the Mediterranean Sea. The 40Ar/39Ar age measurements, together with correlations with sapropels, indicate that the hominin fossils are close in age to the older limit. Our preferred estimate of the age of the hominins is 195 ± 5 ka, making them the earliest well-dated anatomically modern humans yet described. The four members comprising the Kibish Formation are separated by disconformities and have a cumulative thickness of about 100 m. Each member represents an interval of mainly deltaic deposition associated with the Omo River where it entered a much expanded Lake Turkana, about 100 km north of the present lake. In addition to the 40Ar/39Ar ages on alkali feldspars from Members I and III, 14C ages on Member IV show deposition occurred between about 10 and 3 ka ago. The numerical ages for the threedated members of the Kibish Formation are remarkably similar to ages derived via the astronomical time scale for several sapropels in the eastern Mediterranean Sea, 195 ka for sapropel S7, 102 ka for S4 and 8 ka for S1. The link between the two widely separated regions is paleoclimatic. The Omo River and the Nile River systems share a drainage divide and both the formation of the sapropels and the deposition of the members of the Kibish Formation reflect greatly increased precipitation in the Ethiopian highlands and high runoff during periods of intensification of the African monsoon when precession was at a minimum and solar insolation was at a maximum. doi:10.1016/j.gca.2006.06.825