Secular change and the onset of plate tectonics on Earth

Plate tectonics: What, where, why, and when?

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

Mesoarchean (ultra)-high temperature and high-pressure metamorphism along a microblock suture: Evidence from Earth's oldest khondalites in southern India

\n Determining the mineralogical changes occurring in subducted oceanic crust is key to understanding short- and long-term geochemical cycles. Although numerous studies have explored the mineral assemblages that form in mid-ocean ridge basalt (MORB) at different depths below the Earth’s surface, it is widely recognized that seafloor hydrothermal alteration of the uppermost portion of the oceanic crust can change its composition between a ridge and a trench prior to subduction. In this study, we use petrological modelling to explore the effects of different types of pre-subduction hydrothermal alteration on the phase changes that occur during seafloor alteration of MORB-like compositions during subduction along an average Phanerozoic geotherm. We consider a representative composition of altered oceanic crust, as well as extreme end-member scenarios (pervasive spilitization, chloritization, and epidotization). Our models show that epidotization and chloritization of MORB strongly affects phase equilibria at different depths, whereas spilitization and an average style of alteration produce relatively fewer changes on the mineral assemblage to those expected in a pristine MORB. Devolatilization of MORB during subduction occurs mostly in the forearc region, although the type and extent of alteration strongly control the depth and magnitude of fluid released. Altered compositions carry significantly more H2O to sub- and postarc depths than unaltered compositions; the H2O carrying capacity of unaltered and altered compositions is further enhanced during subduction along colder geotherms. Extremely localized areas affected by epidotization can transport up to 22 times more H2O than unaltered MORB and up to two times more than average altered oceanic crust compositions to depths beyond the arc. Regardless of the extent and style of alteration, the stability of hydrous phases, such as epidote and phengite (important trace element carriers), is expanded to greater pressure and temperature conditions. Thus, hydrothermal alteration of the subducted oceanic slab-top represents a viable, and probably common, mechanism that enhances geochemical recycling between the Earth’s hydrosphere and shallow interior.

Petrological Implications of Seafloor Hydrothermal Alteration of Subducted Mid-Ocean Ridge Basalt

Arc volcanism and trace-element recycling are controlled by the devolatilization of oceanic crust during subduction. The type of fluid—either aqueous fluids or hydrous melts—released during subduction is controlled by the thermal structure of the subduction zone. Recent thermomechanical models and results from experimental petrology argue that slab melting occurs in almost all subduction zones, although this is not completely supported by the rock record. Here we show via phase equilibrium modeling that melting of either fresh or hydrothermally altered basalt rarely occurs during subduction, even at water-saturated conditions. Melting occurs only along the hottest slab-top geotherms, with aqueous fluids being released in the forearc region and anatexis restricted to subarc depths, leading to high-SiO2 adakitic magmatism. We posit that aqueous fluids and hydrous melts preferentially enhance chemical recycling in “hot” subduction zones. Our models show that subducted hydrothermally altered basalt is more fertile than pristine basaltic crust, enhancing fluid and melt production during subduction and leading to a greater degree of chemical recycling. In this contribution, we put forward a petrological model to explain (the lack of) melting during the subduction of oceanic crust and suggest that many large-scale models of mass transfer between Earth’s surface and interior may require revision.

Oceanic slab-top melting during subduction: Implications for trace-element recycling and adakite petrogenesis

Archean (4.0-2.5 Ga) tonalite-trondhjemite-granodiorite (TTG) terranes represent fragments of Earth's first continents that formed via high-grade metamorphism and partial melting of hydrated basaltic crust. While a range of geodynamic regimes can explain the production of TTG magmas, the processes by which they separated from their source and acquired distinctive geochemical signatures remain uncertain. This limits our understanding of how the continental crust internally differentiates, which in turn controls its potential for long-term stabilization as cratonic nuclei. Here, we show via petrological modeling that hydrous Archean mafic crust metamorphosed in a non-plate tectonic regime produces individual pulses of magma with major-, minor-, and trace-element signatures resembling-but not always matching-natural Archean TTGs. Critically, magma hybridization due to co-mingling and accumulation of multiple melt fractions during ascent through the overlying crust eliminates geochemical discrepancies identified when assuming that TTGs formed via crystallization of discrete melt pulses. We posit that much Archean continental crust is made of hybrid magmas that represent up to ~ 40 vol% of partial melts produced along thermal gradients of 50-100 °C/kbar, characteristic of overthickened mafic Archean crust at the head of a mantle plume, crustal overturns, or lithospheric peels.

/pdf/archean-continental-crust-formed-by-magma-hybridization-and-2jeij56wgm.pdf

Archean continental crust formed by magma hybridization and voluminous partial melting.

Trace elements in zircon are a promising proxy with which to quantitatively study Earth’s long-term lithospheric processes and its geodynamic regimes. The zircon Eu anomaly reflects the crystallization environment of its felsic or intermediate parental magma. In particular, it provides insight into the water content, magmatic redox conditions, and the extent of plagioclase fractionation in the source rock or its occurrence as a cogenetic crystallizing phase from the magma. We performed a statistical analysis of Eu anomalies from a compilation of detrital zircons over geologic time and found a major decrease in Eu anomaly ca. 2.5 Ga and an important increase ca. 0.9 Ga. Coupled with thermodynamic modeling, we suggest that these variations could be due to long-term change in the chemical system of the mafic source from which the intermediate to felsic melt and derived zircons were produced. The 2.5 Ga drop was likely associated with an enrichment in incompatible elements in the mafic source, which extended the pressure-temperature field of plagioclase stability as a cogenetic melt phase. We interpret the 0.9 Ga rise to record increasing hydration of magmagenetic sites due to the general development of cold subduction systems, which would delay and/or suppress the saturation of plagioclase in hydrous magmagenetic sites.

/pdf/europium-anomalies-in-detrital-zircons-record-major-1s0e1i3g.pdf

Juan David Hernández-Montenegro

Papers

Oceanic slab-top melting during subduction: Implications for trace-element recycling and adakite petrogenesis

Archean continental crust formed by magma hybridization and voluminous partial melting.

Europium anomalies in detrital zircons record major transitions in Earth geodynamics at 2.5 Ga and 0.9 Ga

Fractionation of iron isotopes between strongly peraluminous granites and their sedimentary sources: a case study of the Archean Ghost Lake batholith

PRIMARSMELT: Calculating primary magma compositions for Mars