Augite in Komatiitic Basalts: A Study of the Mobility of Trace Elements During Rock Alteration
Master of Science (MS)
Geology and Geophysics
Komatiitic basalts are ultramafic rocks formed from volcanic liquids containing less than 18 wt.% magnesium oxide that can be linked, spatially, geochemically and texturally to komatiites. Archean komatiites and komatiitic basalts are critical to models of early Earth’s mantle evolution. Major and trace elements in these rocks are poorly preserved due to early sub-seafloor alteration with superimposed regional metamorphism: in particular the alkali, alkaline earths, and other elements with low valence numbers. Current analytical methods focus on analyzing whole rock, fresh olivines and melt inclusions to evaluate elemental concentrations and for petrogenetic studies. Although these methods establish elemental concentrations, REE, Sr, Na and Ba should be better represented by evaluating trace elements in fresh augites from komatiitic basalts, which contain moderate levels of trace element concentrations that olivines lack. In this study trace elements were analyzed within samples from two different formations (Mendon and Weltevreden) within the Barberton Greenstone Belt, South Africa. These samples contain fresh augites from komatiitic basalts and were analyzed using LA-ICP-MS. Results suggest that REE in augites from the freshest komatiitic basalts closely match the predicted values calculated from bulk rock analyses when distribution coefficients and Rayleigh fractionation are taken into account. Zr and Ti, exhibit anomalies within the augites that correspond to anomalies found in bulk rock data. This supports the contention that both the REE and HFSE remain a closed system within these rocks. However, Na, Sr and Ba were mobilized within the bulk rock during alteration. Values for Na are particularly difficult to establish from bulk rock data. Even when they are nearly constant within a suite of flows, their values are far from those predicted from augite analyses. Sr has negative anomalies in these rocks, and this is confirmed to be a trait acquired from the magmas through augite analyses. These data confirm two important components of komatiite petrogenesis: 1) fractionation of a high pressure phase to modify the HFSE, and 2) assimilation of felsic material during crustal ascent. Important radiogenic systems, such as Rb-Sr, Nd-Sm, and Lu-Hf, should be evaluated using augite proxies from single mineral SIMS, LA-ICP_MS, or mineral separations.
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Morabito, Alesha C., "Augite in Komatiitic Basalts: A Study of the Mobility of Trace Elements During Rock Alteration" (2015). LSU Master's Theses. 2760.