Early Differentiation Processes Recorded By 142Nd and 182W in Eoarchean Rocks from Isua
Tuesday, 16 December 2014: 5:00 PM
The earliest phases of Earth’s evolution can be investigated using the short-lived 182Hf-182W and 146Sm-142Nd isotopic systems. Tungsten is siderophile while Hf is lithophile, so metal-silicate segregation greatly fractionates the Hf/W ratio of both planetary mantles and cores. Both daughter nuclides, W and Nd, are more incompatible than the parent nuclides Hf and Sm, so modification to Hf/W and Sm/Nd ratios in the silicate Earth can also be caused by crystal-liquid fractionation. However, because of the short half-lives of 182Hf and 146Sm (8.9 Ma and 103 Ma, respectively), variations in 182W and 142Nd can only be produced, respectively, during the first ~ 50 Ma and ~ 500 Ma of Earth’s history. We will present data from Eoarchean mantle-derived samples from the Isua supracrustal belt. The mantle source(s) of these rocks was characterized by 182W and 142Nd excesses, relative to terrestrial standards and modern rocks, of up to 15 ppm and 13 ppm, respectively. The 182W/184W and 142Nd/144Nd ratios in the Isua rocks are not correlated, suggesting that, because of the different time scales for 142Nd and 182W growth, two or more events are required to account for the isotopic enrichments. The enrichments in 182W can be explained by Hf/W fractionation after a mean core formation age of ~30 Ma followed by mantle differentiation between 45-67 Ma. The 142Nd excesses, coupled with 147Sm-143Nd systematics, suggest that the mantle source of these rocks differentiated ~100 Ma after Solar system formation. Samples from the Isua Supracrustal belt, with ages between 3.8 Ga and 3.3 Ga, show gradual diminution of 142Nd anomalies until they are no longer resolved by ~ 3.3 Ga. By contrast, there is no diminishment of the 182W variability with time in the Isua suite. Tungsten concentrations of the samples studied range between 0.1 ppm and 3 ppm. The high W concentrations in some of these samples likely reflect the mobility of W in hydrous fluids and the modification of the original W of the rocks. The W isotopic composition of the Archean mantle through time is estimated from the samples with W concentrations expected for typical mantle melts of a source with bulk silicate Earth-like (13 ppb) W concentrations.