[BPT05-P01] Geochemistry and Re-Os and 146Sm-142Nd isotope systematics of mafic rocks in the Acasta Gneiss Complex: Discovery of the oldest terrestrial rock and implications for the Earth-forming materials
キーワード：冥王代、初期分化
The first billion years’ history of the earth is still poorly understood because terrestrial rocks with the ages are scarcely preserved. The Acasta Gneiss Complex (AGC) is one of the rare Eoarchean geologic bodies, located in the western margin of the Slave Craton, Canada, and is composed of felsic to intermediate gneisses with subordinate amounts of mafic rocks. This study aims to obtain physico-chemical constraints on the Hadean mantle and propose a new picture of early mantle evolution based on geological, petrological and geochemical investigations of the oldest mafic rocks in AGC.
The mafic rocks mainly consist of fine to coarse-grained hornblende and plagioclase with small amount of quartz, chlorite, garnet and ilmenite. They commonly suffered from amphibolite to upper amphibolite facies metamorphism. The mafic rocks are subdivided into three groups based on the rare earth element (REE) patterns: highly variable light REE-enriched pattern, flat REE pattern and slightly light REE-enriched pattern with positive Eu anomaly, respectively. The samples, which have flat REE patterns and whose high field strength element (HFSE) and REE contents are well correlated with immobile Zr contents, were selected as the least altered samples to estimate their source mantle because the infiltration of metamorphic fluids/melts increases the light REE contents relative to the middle and heavy REE contents and more severely disturbs other trace element contents than Zr contents. The least altered mafic rocks have chondritic trace element relative abundances with negative Nb and Ta anomalies. This implies that the Nb and Ta were partitioned into the metallic core to form a Nb and Ta-deficit primitive mantle.
The least altered mafic rocks show a positive correlation on a 187Re/188Os vs 187Os/188Os diagram, yielding a formation age of 4272±300 Ma. The age is consistent with the field occurrence of mafic rocks because they were intruded by orthogneisses with 4.03 to 3.6 Ga ages. The highly radiogenic initial 187Os/188Os ratio suggests that their source material was a pre-late veneer mantle with a high Re/Os ratio. However, geochemical signatures, which the initial 187Os/188Os ratio overlaps with a chondritic value within the error and their highly siderophile element (HSE) abundances are similar to those of modern basalts, indicate that their source mantle had modern mantle-like high HSE contents, implying that the late veneer event and later homogenization took place before 4.27 Ga.
Their 142Nd/144Nd ratios are identical to those of a modern mantle with a suprachondritic value. Their chondritic initial 143Nd/144Nd ratio and REE patterns, and suprachondritic 142Nd/144Nd ratios indicate that their source material had a chondritic Sm/Nd ratio and a suprachondritic (modern mantle value) 142Nd/144Nd ratio at 4.27 Ga. This is the first evidence that > 3.7 Ga source mantle had a modern mantle-like (normal) 142Nd/144Nd ratio. Two possible scenarios account for the Hadean mantle with the primitive mantle-like trace element contents and normal 142Nd/144Nd ratio at 4.27 Ga. One is that the early mantle convection was rapid enough to homogenize large-scale mantle heterogeneity due to a magma ocean until 4.27 Ga. The Acasta mafic rocks was derived from the homogenized primitive mantle, whereas the 3.8-3.7 Ga Isua mafic rocks with excess μ142Nd values were formed from a shallow depleted mantle possibly due to progressive extraction of primitive crusts. This scenario is supported by the HSE contents and 187Re-187Os isotopes. Another model suggests that the extent of a magma ocean was limited and non-melting primitive parts remained in the deep mantle, and that the Acasta mafic rocks were formed from the deep primitive mantle, whereas the Isua mafic rocks were derived from the early differentiated upper mantle.
The mafic rocks mainly consist of fine to coarse-grained hornblende and plagioclase with small amount of quartz, chlorite, garnet and ilmenite. They commonly suffered from amphibolite to upper amphibolite facies metamorphism. The mafic rocks are subdivided into three groups based on the rare earth element (REE) patterns: highly variable light REE-enriched pattern, flat REE pattern and slightly light REE-enriched pattern with positive Eu anomaly, respectively. The samples, which have flat REE patterns and whose high field strength element (HFSE) and REE contents are well correlated with immobile Zr contents, were selected as the least altered samples to estimate their source mantle because the infiltration of metamorphic fluids/melts increases the light REE contents relative to the middle and heavy REE contents and more severely disturbs other trace element contents than Zr contents. The least altered mafic rocks have chondritic trace element relative abundances with negative Nb and Ta anomalies. This implies that the Nb and Ta were partitioned into the metallic core to form a Nb and Ta-deficit primitive mantle.
The least altered mafic rocks show a positive correlation on a 187Re/188Os vs 187Os/188Os diagram, yielding a formation age of 4272±300 Ma. The age is consistent with the field occurrence of mafic rocks because they were intruded by orthogneisses with 4.03 to 3.6 Ga ages. The highly radiogenic initial 187Os/188Os ratio suggests that their source material was a pre-late veneer mantle with a high Re/Os ratio. However, geochemical signatures, which the initial 187Os/188Os ratio overlaps with a chondritic value within the error and their highly siderophile element (HSE) abundances are similar to those of modern basalts, indicate that their source mantle had modern mantle-like high HSE contents, implying that the late veneer event and later homogenization took place before 4.27 Ga.
Their 142Nd/144Nd ratios are identical to those of a modern mantle with a suprachondritic value. Their chondritic initial 143Nd/144Nd ratio and REE patterns, and suprachondritic 142Nd/144Nd ratios indicate that their source material had a chondritic Sm/Nd ratio and a suprachondritic (modern mantle value) 142Nd/144Nd ratio at 4.27 Ga. This is the first evidence that > 3.7 Ga source mantle had a modern mantle-like (normal) 142Nd/144Nd ratio. Two possible scenarios account for the Hadean mantle with the primitive mantle-like trace element contents and normal 142Nd/144Nd ratio at 4.27 Ga. One is that the early mantle convection was rapid enough to homogenize large-scale mantle heterogeneity due to a magma ocean until 4.27 Ga. The Acasta mafic rocks was derived from the homogenized primitive mantle, whereas the 3.8-3.7 Ga Isua mafic rocks with excess μ142Nd values were formed from a shallow depleted mantle possibly due to progressive extraction of primitive crusts. This scenario is supported by the HSE contents and 187Re-187Os isotopes. Another model suggests that the extent of a magma ocean was limited and non-melting primitive parts remained in the deep mantle, and that the Acasta mafic rocks were formed from the deep primitive mantle, whereas the Isua mafic rocks were derived from the early differentiated upper mantle.