Crystallisation temperatures of the most Mg-rich magmas of the Karoo LIP on the basis of Al-in-olivine thermometer

• Published in: Chemical Geology
• Main Authors: Heinonen, Jussi S., Jennings, Eleanor S., Riley, Teal R.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Elsevier 2015
• Subjects: Antarctic; Saunders; Vestfjella; Ahlmannryggen; Antarc*
• Online Access: http://nora.nerc.ac.uk/id/eprint/510161/; https://nora.nerc.ac.uk/id/eprint/510161/1/Heinonen%202015%20-%20Crystallisation%20temperatures%20of%20the%20most%20Mg-rich%20magmas%20AAM.pdf

Calculating reliable temperatures of Mg-rich magmas is problematic because melt composition and KD(Fe-Mg)ol-liq, the key parameters of many traditional thermometers, are difficult to constrain precisely. The recently developed Al-in-olivine thermometer [Coogan, L.A., Saunders, A.D., Wilson, R.N., 2014. Aluminum-in-olivine thermometry of primitive basalts: Evidence of an anomalously hot mantle source for large igneous provinces. Chemical Geology 368, 1–10] circumvents these problems by relying on the temperature-dependent exchange of Al between olivine and spinel crystallising in equilibrium with each other. This thermometer is used to re-evaluate the crystallisation temperatures of the most Mg-rich magma type identified from the Karoo large igneous province (LIP), known as the Vestfjella depleted ferropicrite suite. Previous temperature estimates for the suite were based on olivine-melt equilibria and indicated anomalously high crystallisation temperatures in excess of 1600 °C. We also present crystallisation temperatures for another Antarctic Karoo magma type, Group 3 dykes from Ahlmannryggen, which are derived from a pyroxene-rich mantle source. Our high-precision analysis of Al in olivine-spinel pairs indicate crystallisation temperatures from 1391 ± 42 °C to 1481 ± 35 °C for the Vestfjella depleted ferropicrite suite (Fo88–92) and from 1253 ± 64 °C to 1303 ± 40 °C for the Group 3 dykes (Fo79–82). Although the maximum temperature estimates for the former are over 100 °C lower than the previously presented estimates, they are still ~ 200 °C higher than those calculated for mid-ocean ridge basalts using the same method. Although exact mantle potential temperatures are difficult to estimate, the presented results support elevated sub-Gondwanan upper mantle temperatures (generated by a mantle plume or internal mantle heating) during the generation of the Karoo LIP.