Water-present eclogite melting to produce Earth's early felsic crust

Abstract

The geochemistry of well preserved Archaean Tonalite–Trondhjemite–Granodiorite (TTG) rocks, such as the trondhjemites of the Meso-Archaean Barberton granitoid–greenstone terrain in South Africa, provides insight into the origins of Earth's early felsic continental crust. This is particularly well demonstrated by the high pressure (HP)-type of Archaean TTG magmas, where the geochemistry requires that they are formed by high-pressure melting of a garnet-rich eclogitic source. This has consequently been interpreted as evidence for the formation of these magmas by anatexis of the upper portions of slabs within Archaean subduction zones. Most of the experimental data relevant to TTG genesis has been generated by studies of fluid-absent melting of metabasaltic sources. However, water drives arc magmatism within Phanerozoic subduction zones and thus, understanding the behaviour of water in Archaean subduction zones, may have considerable value for understanding the genesis of HP-type Paleo- to Meso-Archaean trondhjemites. Consequently, this study investigates the role of high-pressure water-present melting of an eclogite-facies starting material, in the production of Paleo- to Meso-Archaean HP-type TTG melts. Water-saturated partial melting experiments were conducted between 1.9 and 3.0GPa; and, 870°C and 900°C. The melting reaction is characterised by the breakdown of the jadeite molecule in the clinopyroxene, together with quartz and water, to form melt in conjunction with a less sodic clinopyroxene: Qtz+Cpx1+Grt1+H2O=Melt+Cpx2+Grt2 and produced melt compositions that, if allowance is made for the low Mg# and almost K2O-free character of the starting material, are an excellent major element match with HP-type TTG compositions. In two of the experimental run products, melt segregated efficiently from residual crystals, allowing for the measurement of a full range of trace elements, via Laser Ablation Inductively Coupled Plasma Mass Spectroscopy (LA-ICP-MS). The experimental glasses produced are Light Rare Earth Element (LREE) (24–44ppm La; 50–86ppm Ce) and Zr (101–228ppm) enriched; and Heavy Rare Earth Element (HREE) (0.4ppm Yb; 50.9 La/Ybn), Y (2.2–3.2ppm; 82–142 Sr/Y), Sm (2.1–4.3ppm; 40.9–50.8 Zr/Sm) and Nb (1.7–2.8ppm) depleted compared to the DSE4 starting material. Thus, the REE element compositions of the experimental glasses are also similar to HP-type TTG compositions, and in more particular they are strikingly similar to Meso-Archaean Barberton trondhjemites. Additionally, we propose that due to Cpx being a major reactant, Ni and Cr contents (4–29ppm Ni and 43ppm Cr) of the glasses which where analysed for these elements, are within the compositional range displayed by HP-type Archaean TTG. This study suggests that the geochemistry of HP-type Archaean trondhjemites may reflect high pressure water-present partial melting of an eclogite facies metabasaltic source. Consequently, we propose that water-present melting of an eclogitic source is a viable mechanism for the genesis of this component of Paleo- to Meso-Archaean TTG crust.