An estimate of 1.9Ga mantle depletion using the high-field-strength elements and Nd-Pb isotopes of ocean floor basalts, Flin Flon Belt, Canada

摘要

The 1.9. Ga Flin Flon Belt is one of the best preserved Paleoproterozoic mafic volcanic provinces, hosting chemically and geologically distinguishable ocean floor and arc basalts. The ocean floor basalts, previously interpreted to have predominantly erupted in a back-arc basin, apparently tapped N-MORB-, E-MORB-, and OIB-like mantle sources. These ocean floor basalts are revisited in this study with a new major element, high-precision trace element, and isotopic (Nd and Pb) data set (～60 samples). The main impetus behind the study was to combine trace element, particularly Nb/Th, and isotopic compositions to better approximate the contemporaneous depleted mantle, so as to establish more precisely the degree of depletion in the Paleoproterozoic. Through systematic examination of the basalt chemistry, samples influenced by subduction zone processes (low Nb/Th and Ta/W) were isolated first. The Ta/W ratio of the ocean floor basalts is higher than the primitive mantle value and provides strong evidence that the depletion imprint due to the preferential extraction of W in subduction zones was well established by the Paleoproterozoic.Next, a group of unexpectedly high Nb/Ta and high Nb/Th basalts was identified, whose chemical trends (e.g., anti-correlated Nb/Th and LOI) are best explained by post-depositional metamorphic dehydration, resulting in mobility of Th. >. Ta. >. Nb.The chemistries of the remaining, least subduction and dehydration modified, samples provide evidence of mixing between long-term (ca. 450Ma), isotopically distinct mantle reservoirs (Δ+3e{open}_(Nd) units) with relatively consistent Nb/Th ratios of 12.5-13.5. The average Nb/Th ratio of 13.0±0.9 for these ocean floor basalts is more precise, and lower, than previous estimates for the 1.9Ga depleted mantle (～14.4), which included samples that had suffered preferential Th>Nb loss. The magnitude of mantle depletion required by the new Nb/Th ratio of 13 still justifies significant net crustal growth from the Neoarchean (Nb/Th c. 11.1) to the Paleoproterozoic.When the Archean and Paleoproterozoic Th-Nb evidence is combined with insight from Pb and Nd isotopes, it appears that the mass of the depleted portion of the mantle increased between 2.6 and 2.0. Ga. Namely, the data are best explained with a model in which less depleted (lower~) mantle material was mixed into the quite strongly depleted post-Archean upper asthenosphere. This could possibly indicate the onset and establishment of whole mantle convection.