Garnet-Clinopyroxene Assemblages in the Earth's Mantle

Abstract

For over 100 years eclogites have played a vital role in the development of partial melting models, crustal-mantle systems and geodynamic models involving subduction/recycling processes. However their origin remains controversial from one involving subducted basaltic protoliths to polybaric fractionates of basaltic melts with the added complexity of post-formational metamorphic processes. This thesis presents new chemical data on eclogite and pyroxenite samples from varied geological settings: a classic craton [Kaapvaal Craton (e.g., Roberts Victor, Kimberley and Bultfontein pipes)], circum-cratonic localities (Chino Valley – USA) and oceanic environments (Malaita, New Zealand and Hawaii). Apart from petrographic data, mineral chemistry has been constrained using the electron microprobe and laser ablation ICP-MS for major, minor and trace element data, laser ablation for oxygen isotope data and ICP-MS and TIMS for radiogenic isotopes (Sm-Nd, Lu-Hf and Sr). Garnet pyroxenites have normalised trace element patterns and O isotopes consistent with derivation from silicate melts or by reaction between melts and peridotite. On-craton eclogites and garnet clinopyroxenites have normalised trace element patterns and heterogeneous O isotopes indicative of lower temperature processes either as (a) a “basaltic” protolith altered at low temperatures and subducted to form eclogite or (b) an eclogite retrograded to garnet pyroxenite pre- or syn-entrainment. Modeling of Hf-Nd-Sr isotopes indicates that the Kaapvaal eclogites were derived from Archean protoliths whose isotopic signatures have been disturbed by extensive metasomatism. Four aspects of the thesis are noteworthy: (1) the extreme Hf-Nd-Sr heterogeneity shown by eclogites and pyroxenites relative to MORB and OIB, (2) the heavy O isotope ratios for on- and off-craton samples from Chino Valley and Lovedale, (3) the unique Archean (3.15 Ga) low Hf, Nd and Sr reservoir represented by a Roberts Victor eclogite, and (4) the Lu-Hf system is more robust than the Sm-Nd system.