Dissipation of Tungsten-182 Anomalies in the Archean Upper Mantle: Evidence from the Black Hills, South Dakota, USA

Abstract

Most Eoarchean rocks are characterized by positive μ182W anomalies averaging ∼ +13 ppm (where μ182W values are the part per million difference in 182W/184W between a sample and a laboratory reference material presumed to be representative of the bulk silicate Earth, BSE). Prior studies have concluded that the positive 182W anomalies in the upper mantle disappeared by the end of the Archean, yet the timing, nature, and causes of the inferred transition remain poorly understood. In this study, we obtained SmNd mantle extraction model ages (TDM) and μ182W values for Neoarchean and Paleoproterozoic granitic and metasedimentary rocks from the Black Hills, South Dakota, USA. The rocks examined have TDM model ages ranging between ∼3.6 Ga and 2.3 Ga, permitting the tracing of the evolution of 182W in the upper mantle precursors to these rocks, during the purported period of isotopic transition. Of these crustal rocks, the 2.55 Ga Little Elk Granite, with an average TDM age of ∼3.2 Ga, is characterized by a well resolved positive anomaly (μ182W = +8.2 ± 3.1, 2SE). This observation is consistent with a modest diminution in the upper mantle μ182W value relative to the Eoarchean average. By contrast, the 2.60 Ga Bear Mountain Granite, with a slightly younger average TDM age of ∼3.0 Ga, exhibits no resolved anomaly. Most other individual rocks examined also lack resolved anomalies, although the group average μ182W value of +3.3 ± 1.3 (2SE) for the 1.72 Ga Harney Peak Granite, with the majority of TDM model ages ranging from ∼2.7 to 2.3 Ga, may indicate a small positive anomaly for their Neoarchean to Paleoproterozoic upper mantle precursors.The Black Hills rocks provide new evidence for the uneven dissipation during the Mesoarchean through Paleoproterozoic of the positive μ182W value that typified the Eoarchean upper mantle. When the new data are combined with data from prior studies, it becomes evident that the transition to a “modern” BSE W isotopic composition was not the result of a smooth linear decrease, but rather an irregular trend. Nevertheless, the collective data indicate that the positive anomalies in the upper mantle were nearly eliminated by the beginning of the Proterozoic, presumably by mantle mixing processes. The diminution in the scale of 182W anomalies during the Archean is similar to that of 142Nd, and requires a global-scale process that most likely involved vertical and/or lateral mixing within the mantle. The reasons for the delay in the initiation of this mixing process until the end of the Eoarchean, and the completion of mantle mixing by ∼2.4 Ga, awaits exploration via geodynamical modeling of different mixing scenarios.