Trace element distribution and isotopic composition of Archean Greenstones

Abstract

Trace element abundances and Sr isotopic compositions in Archean and modern volcanic rocks are reviewed. K, Rb, Ba and perhaps Sr are more mobile than rare earth and some transition elements in alteration and low-grade metamorphism. Data of K, Rb, Sr and Ba from individual greenstone samples usually show significant variations. However, average values of these elements from a large set of samples tend to show a rather consistent feature. Geochemical arguments derived from these elements are also in general agreement with those from more refractory REE. Alteration effects on REE patterns are observed, but most Archean volcanic rocks appear to possess their original magmatic patterns. Archean low-K tholeiites and high-Mg komatiites are characterized by their generally flat REE patterns of about 2.5 to 15 times chondritic abundances. Their ( La Sm ) N ratios range from 0.7 to 1.3 and are significantly higher than those of typical MORB (0.4 to 0.7). The data of peridotitic komatiites have been used to estimate the REE abundances in the Archean upper mantle. The two times chondritic abundances thus derived is similar to those estimated for the modern upper mantle. For andesitic rocks, most Archean REE patterns can find their modern analogues, but in some cases Archean patterns (with severe HREE depletion) have rare modern analogues. Archean siliceous volcanic rocks commonly show severe HREE depletion. This cannot be explained as due to fractional crystallization of basaltic magmas, rather, it suggests a separate melting event in which garnet plays an important role in the source region. The source is hence likely an eclogite or garnet amphibolite in the mantle due to lithosphere subduction or that converted from a thick pile of basaltic rocks of the same greenstone belt. The application of trace element abundances to identification of the tectonic settings for Archean volcanic rocks is not very successful. This is mainly due to the following factors: (1) insufficient understanding of geochemical characteristics of trace elements or simply due to indiscriminate LIL and REE patterns in modern volcanic rocks from different tectonic settings, (2) insufficient understanding of geochemical behaviors of some transition elements during partial melting and crystal fractionation under different P, T, X conditions, (3) evolutionary change of the upper mantle composition through time, (4) existence of heterogeneities in both the Archean and the modern upper mantle, and finally, (5) insufficient understanding in tectonic styles during the Archean. Available Sr isotopic data for Archean volcanic rocks show that they are essentially evolved along a path with Rb Sr ratio = 0.026–0.034 . Most modern oceanic island volcanic rocks and continental basalts also have Sr isotopic composition evolved along the same path. Some Archean basalts, e.g. 2.7 b.y. Minnesota, show an early depletion of Rb relative to Sr. Likewise, the modern ocean ridge basalts show a significant depletion of Rb relative to Sr. This depletion probably took place about 2 b.y.ago. Evidence from both Nd and Sr isotopes suggests that, counting MORB sources apart, the upper mantle has evolved essentially with a constant Rb Sr and Sm Nd ratios. Trace element abundances also show a gross constancy in Archean and modern volcanic rocks. This seems to militate against the consequences of partial melting processes operating in the upper mantle of non-infinite trace element reservoir. It may, however, be explained as due to the interplay of (1) replenishment of trace elements from lower part of the mantle, and (2) recycling of trace elements in the crust-upper mantle system.