Hydrous, silica-rich melts migrating through the mantle are preserved as glass inclusions in mantle minerals in xenoliths from Philippine arc lavas. These melts, with chemistries that indicate an origin by very low degrees of melting of the subducted ocean crust, have altered their host peridotites, yielding a metasomatized mantle. This 'fertilized' mantle is the source region of the arc magmas, which share continuous chemical trends with the melt inclusions, reflecting mixing and/or varying degrees of melting. These observations provide direct evidence for the importance of slab–mantle interactions in the genesis of island-arc magmas.

Characterising the extent of volcanism at the Galapagos spreading centre using deep tow sediment profiler records

Phase equilibria modelling of rocks from Western Australia confirms that the ancient continental crust could have formed by multistage melting of basaltic ‘parents’ along high geothermal gradients—a process incompatible with modern-style subduction Tim Johnson et al perform phase equilibria modelling of the Coucal basalts from Western Australia and confirm their suitability as parent rocks of the Archaean continental crust The authors suggest that these early crustal rocks were produced by 20–30 per cent melting along high geothermal gradients They conclude that the production and stabilization of the first continents required a protracted, multistage process When coupled with the high geothermal gradients, this suggests that the continents did not form by subduction Instead it favours a 'stagnant lid' regime in the early Archaean eon in which a single, rigid plate lay over the mantle The geodynamic environment in which Earth’s first continents formed and were stabilized remains controversial1 Most exposed continental crust that can be dated back to the Archaean eon (4 billion to 25 billion years ago) comprises tonalite–trondhjemite–granodiorite rocks (TTGs) that were formed through partial melting of hydrated low-magnesium basaltic rocks2; notably, these TTGs have ‘arc-like’ signatures of trace elements and thus resemble the continental crust produced in modern subduction settings3 In the East Pilbara Terrane, Western Australia, low-magnesium basalts of the Coucal Formation at the base of the Pilbara Supergroup have trace-element compositions that are consistent with these being source rocks for TTGs These basalts may be the remnants of a thick (more than 35 kilometres thick), ancient (more than 35 billion years old) basaltic crust4,5 that is predicted to have existed if Archaean mantle temperatures were much hotter than today’s6,7,8 Here, using phase equilibria modelling of the Coucal basalts, we confirm their suitability as TTG ‘parents’, and suggest that TTGs were produced by around 20 per cent to 30 per cent melting of the Coucal basalts along high geothermal gradients (of more than 700 degrees Celsius per gigapascal) We also analyse the trace-element composition of the Coucal basalts, and propose that these rocks were themselves derived from an earlier generation of high-magnesium basaltic rocks, suggesting that the arc-like signature in Archaean TTGs was inherited from an ancestral source lineage This protracted, multistage process for the production and stabilization of the first continents—coupled with the high geothermal gradients—is incompatible with modern-style plate tectonics, and favours instead the formation of TTGs near the base of thick, plateau-like basaltic crust9 Thus subduction was not required to produce TTGs in the early Archaean eon

Earth's first stable continents did not form by subduction

Adakites have a distinct chemistry that links them to melting of a mafic source at high pressure. They have been attributed to melting of subducted oceanic crust or melting of the mafic crustal roots of thick continental arcs, and are an important contrast to mantle wedge melting as a means of generating continental crust. We report the first direct evidence for the generation of adakitic melts in mafic lower continental crust, in an exhumed Cretaceous arc in the South Island of New Zealand. The lower crustal Pembroke Granulite has the bulk chemistry and partial melting textures involving peritectic garnet appropriate for a source region for an adakitic melt. The melt migrated from the area through a fracture network now filled with trondhjemitic veins. Emplacement of the melt was in the upper crust of the Cretaceous section, illustrated by the presence of coeval adakites in the upper crustal Nelson-Westland region.

Direct observation of adakite melts generated in the lower continental crust, Fiordland, New Zealand.

Tectonic evolution of convergent plate margins and its geological effects

Notwithstanding current heavy dependence on gas-fired electricity generation in the Eastern African Power Pool (EAPP), hydropower is expected to play an essential role in improving electricity access in the region. Expansion planning of electricity infrastructure is critical to support investment and maintaining balanced consumer electricity prices. Variations in water availability due to a changing climate could leave hydro infrastructure stranded or result in underutilization of available resources. In this study, we develop a framework consisting of long-term models for electricity supply and water systems management, to assess the vulnerability of potential expansion plans to the effects of climate change. We find that the most resilient EAPP rollout strategy corresponds to a plan optimised for a slightly wetter climate compared to historical trends. This study demonstrates that failing to climate-proof infrastructure investments can result in significant electricity price fluctuations in selected countries (Uganda & Tanzania) while others, such as Egypt, are less vulnerable.

Resilience of the Eastern African electricity sector to climate driven changes in hydropower generation

Adakitic rocks are intermediate-acid magmatic rocks characterized by enrichment in light rare-earth elements, depletion in heavy rare-earth elements, positive to negligible Eu and Sr anomalies, and high La/Yb and Sr/Y ratios. Cenozoic adakitic rocks generated by partial melting of subducted oceanic crust (slab) under eclogite-facies conditions (i.e., the original definition of “adakite”) occur mainly in Pacific Rim volcanic arcs (intra-oceanic, continental, and continental-margin island arcs), whereas those generated by partial melting of thickened lower crust occur mainly in Tethyan Tibetan collisional orogens. In volcanic arcs, adakitic melts derived from the melting of subducted oceanic crust metasomatize the mantle wedge to form a unique rock suite comprising adakite-adakite-type high-Mg andesite-Piip-type high-Mg andesite-Nb-rich basalt-boninite. This suite differs from the basalt-andesite-dacite-rhyolite suite formed from mantle wedge metasomatized by fluids derived from subducted oceanic crust. Previously published data indicate that partial melting of mafic rocks can generate adakitic magmas under pressure, temperature, and hydrous conditions of 1.2–3.0 GPa, 800–1000°C, and 1.5–6.0 wt.% H2O, respectively, leaving residual minerals of garnet and rutile with little or no plagioclase. Cenozoic Au and Cu deposits occur proximally to adakitic rocks, with host rocks of some deposits actually being adakitic rocks. Adakitic rocks thus have important implications for both deep-Earth dynamics and Cu-Au mineralization/exploration. Although studies of Cenozoic adakitic rocks have made many important advances, there remain weaknesses in some important areas such as their tectonic settings, petrogenesis, magma sources, melt-mantle interactions of pre-Cenozoic adakitic rocks, and their relationship with the onset of plate tectonics and crustal growth. Future research directions are likely to involve (1) the generation of adakitic magmas by experimental simulations of partial melting of different types of rock (including intermediate-acid rocks) and magma fractional crystallization at different temperatures and pressures, (2) the relationship between magma reservoir evolution and the formation of adakitic rocks, (3) the tectonic setting and petrogenesis of pre-Cenozoic adakitic rocks and related geodynamic processes, (4) interactions between slab melts and the mantle wedge, (5) the formation of Archean adakitic tonalite-trondhjemite-granodiorite and its link to the onset of plate tectonics and crustal growth, and (6) the relationship between the formation of adakitic rocks and metal mineralization in different tectonic settings.

Adakitic rocks at convergent plate boundaries: Compositions and petrogenesis

Serpentinization produces H2 and synthesizes CH4 that can support nonphotosynthetic habitability on Earth and possibly beyond. Although fluids on Earth and beyond that may participate in serpentinization are extensively saline with elevated levels of chlorine, chloride minerals are generally not considered to be a crucial component of serpentinization. In this study, we report the formation of kuliginite, a recently discovered hydroxychloride mineral with the ideal formula of Fe3Mg(OH)6Cl2, during internal serpentinization within olivine‐hosted fluid inclusions in ultramafic rocks from New Caledonia, southwest Pacific. It is inferred that kuliginite may form during serpentinization by Cl‐rich saline fluids under certain conditions on celestial bodies, which is largely unrevealed due to our insufficient knowledge about kuliginite. As a mineral containing ∼57 wt.% FeO that is mainly ferrous, kuliginite, even in minor amounts, may play an important role in iron partitioning during serpentinization, with a great influence on the production of H2 and synthesis of CH4. In a word, stabilization of kuliginite retards iron oxidation, whereas further hydration of kuliginite supplements canonical pathways of iron oxidation. In addition, as a hydroxychloride, kuliginite may have a significant weight in the chlorine budget of serpentinite. Therefore, kuliginite in the serpentinized fluid inclusions presented in this study may have general implications for serpentinization in certain saline environments on Earth and beyond, which needs to be further explored in the future.

Hydroxychloride‐Bearing Fluid Inclusions in Ultramafic Rocks From New Caledonia: Implications for Serpentinization in Saline Environments on Earth and Beyond

Long Zhang

Papers

Adakitic rocks at convergent plate boundaries: Compositions and petrogenesis

Hydroxychloride‐Bearing Fluid Inclusions in Ultramafic Rocks From New Caledonia: Implications for Serpentinization in Saline Environments on Earth and Beyond