ALBERT

Description: The start of plate tectonics on Earth is one of the most controversial issues in modern geology, with proposed timings covering almost the entire history of our planet. On page 2387 of this issue (vol. 100, 2015), Blichert-Toft and co-authors report Sm-Nd and Lu-Hf isotopic and lithophile trace element data for early Archean komatiites from the Barberton Greenstone Belt (GB) in South Africa, and argue for the onset of plate tectonics on Earth as early as 3.5 Ga. The studied komatiites show a large decoupling of the two isotopic systems and lithophile trace element signatures that are most consistent with deep-water, pelagic sediments being present in the lower-mantle source of these lavas. Their conclusions have far-reaching implications for advancing our understanding of how the Earth system operated in the distant geological past.

Description: It is widely accepted that stabilization of the continental crust requires the presence of sub-continental lithospheric mantle. However, the degree of melt depletion required to stabilize the lithosphere and whether widespread refertilization is a significant process remain unresolved. Here, major and trace element, including platinum group elements (PGE), characterization of 40 mantle xenoliths from 13 localities is used to constrain the melt depletion, refertilization and metasomatic history of lithospheric mantle underneath the micro-continent Zealandia. Our previously published Re–Os isotopic data for a subset of these xenoliths indicate Phanerozoic to Paleoproterozoic ages and, reinterpreted with the new major and trace element data presented here, demonstrate that a large volume (〉2 million km 3 ) of lithospheric mantle with an age of 1·99 ± 0·21 Ga is present below the much younger crust of Zealandia. A peritectic melting model using moderately incompatible trace elements (e.g. Yb) in bulk-rocks demonstrates that these peridotites experienced a significant range of degrees of partial melting, between 3 and 28%. During subsolidus equilibration clinopyroxene gains significant rare earth elements (REE), which then leads to the underestimation of the degree of partial melting by ≤12% in fertile xenoliths. A new approach taking into account the effects of subsolidus re-equilibration on clinopyroxene composition effectively removes discrepancies in the calculated degree of melting and provides consistent estimates of between 4 and 29%. The estimated amount of melting is independent of the Re–Os model ages of the samples. The PGE patterns record simple melt depletion histories and the retention of primary base metal sulfides in the majority of the xenoliths. A rapid decrease in Pt/Ir N observed at c . 1·0 wt % Al 2 O 3 is a direct result of the exhaustion of sulfide in the mantle residue at c . 20–25% partial melting and the inability of Pt to form a stable alloy phase. Major elements preserve evidence for refertilization by a basaltic component that resulted in the formation of secondary clinopyroxene and low-forsterite olivine. The majority of xenoliths show the effects of cryptic metasomatic overprinting, ranging from minor to strong light REE enrichments in bulk-rocks (La/Yb N = 0·16–15·9). Metasomatism is heterogeneous, with samples varying from those with weak REE enrichment and notable positive Sr and U–Th anomalies and negative Nb–Ta anomalies in clinopyroxene to those that have extremely high concentrations of REE, Th–U and Nb. Chemical compositions are consistent with a carbonatitic component contributing to the metasomatism of the lithosphere under Zealandia. Notably, the intense metasomatism of the samples did not affect the PGE budget of the peridotites as this was controlled by residual sulfides.

Description: Major-element, lithophile trace element, and Sm–Nd and U–Pb zircon isotopic data are presented for Palaeoproterozoic mid-ocean ridge basalt (MORB)-type tholeiitic dikes ranging in age from 2140 ± 3 to 2126 ± 5 Ma studied at six localities within three terranes in the Karelian Craton, eastern Fennoscandian Shield. All the studied dikes have remarkably uniform geochemical and isotope characteristics. They are tholeiitic basalts with low contents of large ion lithophile elements, high field strength elements, and rare earth elements (REE), nearly flat chondrite-normalized REE patterns [(La/Sm) n = 0·9–1·2, (Gd/Yb) n = 1·0–1·2], and positive Ti, Nb, and Zr anomalies in the primitive mantle-normalized diagrams. The dikes also show relatively uniform initial Nd isotope compositions, with Nd values ranging from +1·4 to +3·0, despite the occurrence of these dikes within Archaean terranes with different crustal history. According to the results of U–Pb (zircon) and Sm–Nd internal isochron dating the crystallization age of the dikes is constrained to be c . 2·14 Ga. The studied MORB-type tholeiitic dikes are probably comagmatic with Palaeoproterozoic MORB-type basalts that have previously been recognized in the Karelian Craton, and might represent relicts of their magma feeder system. The uniformity of ages and geochemical and isotope characteristics of the MORB-type dikes and volcanic rocks suggest that they are probably related to a common magmatic event. This event was near-contemporaneous with the eruption of high-Ti plume-related basalts and intrusion of dikes in the c . 2·1 Ga Jatulian continental flood basalt province. Geochemical modelling indicates that the chemical and isotopic compositions of the dikes are best explained by derivation of their parental magmas by partial melting of a uniformly depleted mantle source in the spinel peridotite stability field, followed by fractional crystallization and minor (〈6%) assimilation of continental crustal material. This suggests that magma-storage processes in upper crustal chambers were very short-lived; this could be the result of rapid extension and fast transport of melts through the relatively thin lithosphere of the Karelian Craton. Indirect evidence for the formation of the studied dikes in an extensional tectonic setting is provided by the established presence of extensional tectonics in the eastern part of the Fennoscandian Shield at c . 2·1 Ga associated with the opening of the Lapland–Kola and Svecofennian oceans. The studied continental MORB-type tholeiites, therefore, may play an important role as indicators of the timing of continental breakup. Palaeoproterozoic MORB-type tholeiitic dikes and basalts show significant geochemical similarities to Phanerozoic syn-breakup continental flood basalts of the North Atlantic and Afar provinces; this adds further support to the indicative role of continental MORB-type tholeiites in the reconstruction of continental breakup processes in the Precambrian.

Description: Zealandia is a largely submerged, continental fragment in the southwest Pacific, generally considered to be derived from East Gondwana, but whose origins, age, structure, and relationships with other continental masses are poorly known. To explore the development of this microcontinent, a suite of mantle xenoliths was assembled from 12 localities throughout New Zealand, an emergent part of Zealandia. The 187 Re- 188 Os isotopic systematics of the xenoliths yield model ages (T RD2 ) between 0 and 2.3 Ga. Six samples from the newly defined Waitaha domain, South Island, have a narrow range of T RD2 ages from 1.6 to 1.9 Ga, in agreement with an aluminochron model age for this mantle domain of ca. 1.95 Ga, and with a three-point Re-Os isochron age of 2.26 ± 0.10 Ga. These ages are 〉500 m.y. older than T RD2 ages preserved in other regions of mantle lithosphere from the eastern margin of Gondwana (e.g., southeastern Australia and Marie Byrd Land, Antarctica) and 〉1 b.y. older than the oldest crustal rocks exposed in New Zealand. Thus, the lithospheric mantle of Zealandia has a complex age structure, including a region of Paleoproterozoic cratonic mantle with a minimum extent of ~45,000 km 2 . This ancient mantle resided at the margins of several supercontinents during the past ~2 b.y., attesting to the durability of subcontinental lithospheric mantle domains, even when decoupled from overlying contemporaneous crust and in an oceanic setting distanced from stable cratonic nuclei.