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

• Published in: Chemical Geology
• Main Authors: Heinonen, J.S., Jennings, Eleanor S., Riley, T.R.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Elsevier 2015
• Subjects: Earth and Planetary Sciences; Ahlmannryggen; Antarctic; Saunders; Vestfjella; Antarc*
• Online Access: https://eprints.bbk.ac.uk/id/eprint/21351/; https://eprints.bbk.ac.uk/id/eprint/21351/1/21351.pdf; https://doi.org/10.1016/j.chemgeo.2015.06.015

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. Aluminium-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 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.