High-field-strength element systematics of ancient basalts and their bearing on mantle depletion.

摘要

The high-field-strength elements (HFSE), being highly incompatible during mantle melting and immobile during alteration and metamorphism, are the best candidates for studying the mantle's chemistry using ancient basalts. This thesis reports results of two HFSE studies. First, the behaviour of the element tungsten (W) in the modern mantle is examined to better understand its distribution between the solid mantle and its derived melts. The study implications are extended to both Earth and early, proto-planetary differentiation. Second, an attempt is made to better constrain the degree of Paleoproterozoic mantle depletion, by combining Nb/Th and Nd and Pb isotope systematics.;The 1.9 Ga Flin Flon Belt is one of the best preserved expressions of Paleoproterozoic mafic volcanism, hosting chemically and geologically traceable arc and ocean floor basalts. In this study, ocean floor basalts with implied N-MORB-, E-MORB and OIB-like mantle sources that erupted predominantly in a back-arc basin environment, are examined. New major element, high-precision trace element and isotopic (Nd and Pb) data is provided for the ocean floor basalts. The impetus of the study is to best approximate the trace element, particularly Nb/Th, and isotopic composition of the contemporaneous depleted mantle in aim of better understanding the degree of depletion established by the Paleoproterozoic. By systematic examination of the basalt chemistry, it is possible to isolate samples that are influenced by subduction zone processes (low Nb/Th and Ta/W), and to identify a group of high Nb/Ta, high Nb/Th basalts. The chemical trends observed in the latter samples (e.g. anti-correlated Nb/Th and LOI) are best explained by post-depositional metamorphic dehydration, resulting in mobility of Th>Ta>Nb. This provides a hitherto unappreciated means of biasing the Nb/Th in ancient basalts to significantly higher ratios. The chemistry of the remaining samples provide evidence of mixing between long-term (ca. 450 Ma), isotopically distinct mantle reservoirs (Delta +3 epsilon Nd units) with relatively consistent Nb/Th ratios of 12.5 to 13.5. The average Nb/Th ratio of 13.0 +/- 0.9 for these ocean floor basalts is more precise, and lower, than previous averages of the 1.9 Ga depleted mantle (∼14.4). The magnitude of mantle depletion required by this ratio still justifies significant net crustal 'growth' from the Neoarchean (Nb/Th c. 11.1) to the Paleoproterozoic. Based on combined isotope and Th-Nb evidence, however, the lower-than-expected Nb/Th in 1.9 Ga ocean floor basalts may represent mixing of a less depleted (lower?) mantle reservoir into the highly-depleted post-Archean asthenosphere, possibly in response to establishment of whole mantle convection.;Tungsten is a moderately siderophile high-field-strength element that is hydrophile and widely regarded as highly incompatible during mantle melting. In an effort to extend empirical knowledge regarding the behaviour of W during the latter process, new high precision trace element data (W, Th, U, Ba, La, Sm) is presented that represent both terrestrial and planetary reservoirs: MORB (11), abyssal peridotites (8), eucrite basalts (3), and carbonaceous chondrites (8). A full trace element suite is described for Cordilleran Permian ophiolite peridotites (12) to better constrain the behaviour of W in the upper mantle. In addition, the long-term averages for a number of USGS (BIR-1, BHVO-1, BHVO-2, PCG1, DTS-1) and JGS (JA-3, JP-1) standard reference materials indicate that some of these are heterogeneous and contaminated with respect to W. The most significant finding is that many of the highly depleted upper mantle peridotites contain far higher W concentrations than expected. In the absence of convincing indications for alteration, re-enrichment or contamination, it appears that the W excess was caused by retention in an Os-Ir alloy phase, whose stability was dependent on fO2 of the mantle source region. This could help to account for the particularly low W content of N-MORB and implies that the melt behavior of W recorded in basaltic rocks is not an accurate representation of the melt source. These findings are relevant to the subsequent interpretation of W isotopic data for achondrites, where the fractionation of Hf from W during melting has been used to infer the Hf/W of the parent body mantle. This is illustrated by the differentiation chronology of the eucrite parent body (EPB), which has been modeled with a melt source with high Hf/W. An alternative scenario, with a low mantle Hf/W on the EPB, indicates a maximum core segregation age of 1.2 +/- 1.2 Myr after the closure of CAIs. A more prolonged time (ca. 2 Myr) between core formation and mantle fractionation is also indicated using this interpretation. This is consistent with most recent published chronologies of the EPB differentiation based on the 53Mn-53Cr and 26Al- 26Mg systems.