THE INFLUENCE OF ALTERATION ON THE TRACE-ELEMENT AND ND ISOTOPIC COMPOSITIONS OF KOMATIITES

To investigate the effects of hydrothermal alteration and metamorphism on the chemical and isotopic compositions of komatiites, we studied samples from the Alexo and Texmont regions in the 2.7-Ga Abitibi belt of Canada and from the Weltevreden, Mendon and Komati Formations of the 3.2-3.5-Ga Barberton belt of South Africa. Particular emphasis was placed on multiple samples from individual layered spinifex-olivine cumulate flows, the argument being that if these flows showed variations in the ratios of elements incompatible with olivine, then these variations were most likely due to chemical mobility during alteration. Data for rare-earth (REE) and high-field-strength elements (HFSE) reveal anomalies (non-chondritic HFSE/REE ratios), particularly in the most altered samples. After taking into account the limits of analytical precision, these anomalies are attributed to element mobility, not to fractionation of high-pressure minerals, as suggested by other authors for Abitibi komatiites. In Barberton komatiites three separate processes produced HFSE anomalies: element mobility during alteration, crust assimilation in some samples, and majorite fractionation. The effects of the latter process were recognized from systematic relationships between HFSE/REE and Al/Ti that coincided with calculated majorite fractionation trends. Initial Nd isotopic compositions of the komatiites were determined by analysis of magmatic pyroxenes. The initial 6(Nd), of pyroxene from the 2.7-Ga Alexo komatiite is + 3.8, slightly higher than in whole rocks (+ 0.6 to + 3.5, average similar to + 2.5). In the carbonatized Texmont samples, the range in initial 6(Nd) is far greater (- 6.2 to + 8.8). The variable initial 6(Nd)-values in the rocks are attributed to isotopic exchange of Nd with surrounding rocks during early alteration, and fractionation of Sm/Nd during later events. Pyroxene from 3.3-Ga Barberton komatiitic basalt had initial 6(Nd) of + 2.3.