Oxygen Isotope Compositions of Minerals and Rocks and Chemical Alteration Patterns in Pillow Lavas from the Barberton Greenstone Belt, South Africa

Abstract

Oxygen isotope, major and trace element data have been obtained on a suite of rocks and minerals from the Barberton greenstone belt, South Africa. Analyses of relict orthopyroxene grains separated from a cumulate ultramafic rock indicate that δ¹⁸O values of the komatiite magmas were ~5.7. This value is similar to that of modern ocean floor basalts and indicates that source mantle material of the Archaean komatiites had the same isotopic composition as mantle material from which modern ocean ridge basalts are derived.

Oxygen isotope compositions of secondary minerals (antigorite and chrysotile) are similar to those of serpentine minerals from modern altered oceanic rocks. Similarly, low magnesium komatiitic meta-basalts (8–10% MgO) from Barberton have a similar range and mean of δ¹⁸O values to those of greenschist facies meta-basalts on the modern ocean floor.

Variations in the major and trace element contents between margins and cores of komatiitic basalt pillows show that the elements analysed can be divided into two groups. In the first group which includes Si, Ti, Al, Mg, P, Nb, Zr, Y, Co, V, Sc, and probably Cr there are no variations within analytical error. In the second group differences between margins and cores of pillows vary from barely detectable up to a few hundred percent. This group consists of the following elements arranged in order of increasing variability: — Fe(II), Mn, Ca, Ni, Ga, Fe(III), Na, Sr, K, S, Rb, Ba, Zn, and Cu. The patterns of variation of Fe(II) and S are particularly significant as they are similar to patterns observed in altered pillow lavas from the ocean floor and indicate that the Barberton pillows were subject to seawater alteration before being metamorphosed. The isotopic data and alteration patterns in the pillows provide strong evidence that the processes that occurred in the Archaean rocks were similar to those occurring today during alteration and metamorphism of submarine lavas.

Detailed modelling of δ¹⁸O variations in komatiite lava flows indicates that magmatic or primitive waters (δ¹⁸O = 5 to 7) could have been the major fluids responsible for the hydration of the lavas at temperatures of 240- 450 °C. Such an interpretation is consistent with the greenschist facies mineralogy of the Barber- ton rocks. However, waters with δ¹⁸O values of around 0 (Archaean seawater?) may have exchanged with the komatiites at temperatures as low as 100°C. Mixtures of Archaean seawater and deep-seated water could have exchanged with the komatiites at intermediate temperatures (100-450°C). Specifically a mixture of 75% seawater (δ¹⁸O = 0) and 25% deep-seated water could have given rise to the komatiite isotopic compositions if equilibration occurred at ~130°C. The available evidence is consistent with the interpretation that Archaean seawater had a δ¹⁸O of around zero, but further work is required to establish a reliable estimate of this value. Oxygen isotope compositions of rocks and minerals from the Komati Formation and the chemical alteration patterns observed in the pillow lavas are compatible with an ocean floor environment for extrusion, alteration and metamorphism of these ancient volcanic rocks.