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Constructed on one continuous folded page, this book explores the layers of the Earth from human-made structures like sewers, subways, and archeological finds, down through various formations of rock, to the Earth's core and back up again.

The formation, storage and chemical differentiation of magma in the Earth’s crust is of fundamental importance in igneous geology and volcanology. Recent data are challenging the high-melt-fraction ‘magma chamber’ paradigm that has underpinned models of crustal magmatism for over a century, suggesting instead that magma is normally stored in low-melt-fraction ‘mush reservoirs’ 1 – 9 . A mush reservoir comprises a porous and permeable framework of closely packed crystals with melt present in the pore space 1 , 10 . However, many common features of crustal magmatism have not yet been explained by either the ‘chamber’ or ‘mush reservoir’ concepts 1 , 11 . Here we show that reactive melt flow is a critical, but hitherto neglected, process in crustal mush reservoirs, caused by buoyant melt percolating upwards through, and reacting with, the crystals 10 . Reactive melt flow in mush reservoirs produces the low-crystallinity, chemically differentiated (silicic) magmas that ascend to form shallower intrusions or erupt to the surface 11 – 13 . These magmas can host much older crystals, stored at low and even sub-solidus temperatures, consistent with crystal chemistry data 6 – 9 . Changes in local bulk composition caused by reactive melt flow, rather than large increases in temperature, produce the rapid increase in melt fraction that remobilizes these cool- or cold-stored crystals. Reactive flow can also produce bimodality in magma compositions sourced from mid- to lower-crustal reservoirs 14 , 15 . Trace-element profiles generated by reactive flow are similar to those observed in a well studied reservoir now exposed at the surface 16 . We propose that magma storage and differentiation primarily occurs by reactive melt flow in long-lived mush reservoirs, rather than by the commonly invoked process of fractional crystallization in magma chambers 14 . Magma storage and differentiation in the Earth’s crust mainly occurs by reactive melt flow in long-lived mush reservoirs, rather than by fractional crystallization in magma chambers, as previously thought.

The liquidus water content of a haplogranite melt at high pressure (P) and temperature (T) is important, because it is a key parameter for constraining the volume of granite that could be produced by melting of the deep crust. Previous estimates based on melting experiments at low P ([less than or equal to]0.5 GPa) show substantial scatter when extrapolated to deep crustal P and T (700-1000 °C, 0.6-1.5 GPa). To improve the high-P constraints on H.sub.2O concentration at the granite liquidus, we performed experiments in a piston-cylinder apparatus at 1.0 GPa using a range of haplogranite compositions in the albite (Ab: NaAlSi.sub.3O.sub.8)-orthoclase (Or: KAlSi.sub.3O.sub.8)-quartz (Qz: SiO.sub.2)-H.sub.2O system. We used equal weight fractions of the feldspar components and varied the Qz between 20 and 30 wt%. In each experiment, synthetic granitic composition glass + H.sub.2O was homogenized well above the liquidus T, and T was lowered by increments until quartz and alkali feldspar crystalized from the liquid. To establish reversed equilibrium, we crystallized the homogenized melt at the lower T and then raised T until we found that the crystalline phases were completely resorbed into the liquid. The reversed liquidus minimum temperatures at 3.0, 4.1, 5.8, 8.0, and 12.0 wt% H.sub.2O are 935-985, 875-900, 775-800, 725-775, and 650-675 °C, respectively. Quenched charges were analyzed by petrographic microscope, scanning electron microscope (SEM), X-ray diffraction (XRD), and electron microprobe analysis (EMPA). The equation for the reversed haplogranite liquidus minimum curve for Ab.sub.36.25Or.sub.36.25Qz.sub.27.5 (wt% basis) at 1.0 GPa is [Formula omitted] for [Formula omitted] wt% and [Formula omitted] is in °C. We present a revised [Formula omitted] diagram of liquidus minimum H.sub.2O isopleths which integrates data from previous determinations of vapor-saturated melting and the lower pressure vapor-undersaturated melting studies conducted by other workers on the haplogranite system. For lower H.sub.2O (<5.8 wt%) and higher temperature, our results plot on the high end of the extrapolated water contents at liquidus minima when compared to the previous estimates. As a consequence, amounts of metaluminous granites that can be produced from lower crustal biotite-amphibole gneisses by dehydration melting are more restricted than previously thought.

Modelling of two modes of continental crust formation suggests that before plate tectonics began operating, the Archean early Earth’s tectonic regime was governed by intrusive magmatism. Earth's primordial squishy lid The global geodynamic regime of early Earth, which was in operation before the onset of plate tectonics during the Archaean eon over 2.5 billion years ago, remains contentious. Antoine Rozel et al. use numerical models of global thermochemical convection, including magmatic processes, to show that a tectonics regime dominated by intrusive molten rock results in warm crustal geotherms and can reproduce the observed proportions of primordial continental crust. They therefore conclude that the early Archaean Earth operated globally in a 'Plutonic squishy lid' regime in which intrusions of magma solidified into igneous rock deep below Earth's surface, rather than in an ‘Io-like’ regime dominated by extrusive volcanism. The global geodynamic regime of early Earth, which operated before the onset of plate tectonics, remains contentious. As geological and geochemical data suggest hotter Archean mantle temperature 1 , 2 and more intense juvenile magmatism than in the present-day Earth 3 , 4 , two crust–mantle interaction modes differing in melt eruption efficiency have been proposed: the Io-like heat-pipe tectonics regime dominated by volcanism 5 , 6 and the “Plutonic squishy lid” tectonics regime governed by intrusive magmatism, which is thought to apply to the dynamics of Venus 7 , 8 , 9 . Both tectonics regimes are capable of producing primordial tonalite–trondhjemite–granodiorite (TTG) continental crust 5 , 10 but lithospheric geotherms and crust production rates as well as proportions of various TTG compositions differ greatly 9 , 10 , which implies that the heat-pipe and Plutonic squishy lid hypotheses can be tested using natural data 11 . Here we investigate the creation of primordial TTG-like continental crust using self-consistent numerical models of global thermochemical convection associated with magmatic processes. We show that the volcanism-dominated heat-pipe tectonics model results in cold crustal geotherms and is not able to produce Earth-like primordial continental crust. In contrast, the Plutonic squishy lid tectonics regime dominated by intrusive magmatism results in hotter crustal geotherms and is capable of reproducing the observed proportions of various TTG rocks. Using a systematic parameter study, we show that the typical modern eruption efficiency of less than 40 per cent 12 leads to the production of the expected amounts of the three main primordial crustal compositions previously reported from field data 4 , 11 (low-, medium- and high-pressure TTG). Our study thus suggests that the pre-plate-tectonics Archean Earth operated globally in the Plutonic squishy lid regime rather than in an Io-like heat-pipe regime.

The composition of natural calcium silicate perovskite, the fourth most abundant mineral in the Earth, found within a diamond indicates an origin from oceanic crust subducted deeper than 700 kilometres into the Earth’s mantle. Rock recycling revealed in a diamond Within the Earth's transition zone and lower mantle, the high-pressure perovskite-structured polymorph of calcium silicate (CaSiO 3 ) is thought to be the main host of calcium, as well as the heat-producing elements potassium, uranium and thorium. Despite being considered as the fourth most abundant mineral in the Earth, it has never been found in nature. Fabrizio Nestola and co-authors document the perovskite-structured polymorph of CaSiO 3 included within a diamond from Cullinan kimberlite mined in South Africa. The authors conclude that the bulk composition of material within the diamond is consistent with derivation from basaltic oceanic crust subducted to pressures equivalent to those present at the depths of the uppermost lower mantle, providing additional evidence for the recycling of oceanic crust and carbon from the surface to lower-mantle depths. Laboratory experiments and seismology data have created a clear theoretical picture of the most abundant minerals that comprise the deeper parts of the Earth’s mantle. Discoveries of some of these minerals in ‘super-deep’ diamonds—formed between two hundred and about one thousand kilometres into the lower mantle—have confirmed part of this picture 1 , 2 , 3 , 4 , 5 . A notable exception is the high-pressure perovskite-structured polymorph of calcium silicate (CaSiO 3 ). This mineral—expected to be the fourth most abundant in the Earth—has not previously been found in nature. Being the dominant host for calcium and, owing to its accommodating crystal structure, the major sink for heat-producing elements (potassium, uranium and thorium) in the transition zone and lower mantle, it is critical to establish its presence. Here we report the discovery of the perovskite-structured polymorph of CaSiO 3 in a diamond from South African Cullinan kimberlite. The mineral is intergrown with about six per cent calcium titanate (CaTiO 3 ). The titanium-rich composition of this inclusion indicates a bulk composition consistent with derivation from basaltic oceanic crust subducted to pressures equivalent to those present at the depths of the uppermost lower mantle. The relatively ‘heavy’ carbon isotopic composition of the surrounding diamond, together with the pristine high-pressure CaSiO 3 structure, provides evidence for the recycling of oceanic crust and surficial carbon to lower-mantle depths.

The history of the growth of continental crust is uncertain, and several different models that involve a gradual, decelerating, or stepwise process have been proposed 1 – 4 . Even more uncertain is the timing and the secular trend of the emergence of most landmasses above the sea (subaerial landmasses), with estimates ranging from about one billion to three billion years ago 5 – 7 . The area of emerged crust influences global climate feedbacks and the supply of nutrients to the oceans 8 , and therefore connects Earth’s crustal evolution to surface environmental conditions 9 – 11 . Here we use the triple-oxygen-isotope composition of shales from all continents, spanning 3.7 billion years, to provide constraints on the emergence of continents over time. Our measurements show a stepwise total decrease of 0.08 per mille in the average triple-oxygen-isotope value of shales across the Archaean–Proterozoic boundary. We suggest that our data are best explained by a shift in the nature of water–rock interactions, from near-coastal in the Archaean era to predominantly continental in the Proterozoic, accompanied by a decrease in average surface temperatures. We propose that this shift may have coincided with the onset of a modern hydrological cycle owing to the rapid emergence of continental crust with near-modern average elevation and aerial extent roughly 2.5 billion years ago. The use of triple-oxygen-isotope data from continental shales spanning the past 3.7 billion years suggests that continental crust with near-modern average elevation and extent emerged about 2.5 billion years ago.

The structural and metamorphic evolution of the lower crust has direct effects on the lithospheric response to plate tectonic processes involved in orogeny, including subsidence of sedimentary basins, stability of deep mountain roots and extension of high-topography regions. Recent research shows that before orogeny most of the lower crust is dry, impermeable and mechanically strong 1 . During an orogenic event, the evolution of the lower crust is controlled by infiltration of fluids along localized shear or fracture zones. In the Bergen Arcs of Western Norway, shear zones initiate as faults generated by lower-crustal earthquakes. Seismic slip in the dry lower crust requires stresses at a level that can only be sustained over short timescales or local weakening mechanisms. However, normal earthquake activity in the seismogenic zone produces stress pulses that drive aftershocks in the lower crust 2 . Here we show that the volume of lower crust affected by such aftershocks is substantial and that fluid-driven associated metamorphic and structural transformations of the lower crust follow these earthquakes. This provides a ‘top-down’ effect on crustal geodynamics and connects processes operating at very different timescales. During continent collision and associated mountain building, a surprisingly large volume of the lower crust is shown to be affected by earthquake aftershocks, producing a top-down effect on crustal geodynamics.

Inorganic aerosol composition was measured in the southeastern United States, a region that exhibits high aerosol mass loading during the summer, as part of the 2013 Southern Oxidant and Aerosol Study (SOAS) campaign. Measurements using a Monitor for AeRosols and GAses (MARGA) revealed two periods of high aerosol nitrate (NO.sub.3 .sup.−) concentrations during the campaign. These periods of high nitrate were correlated with increased concentrations of supermicron crustal and sea spray aerosol species, particularly Na.sup.+ and Ca.sup.2+, and with a shift towards aerosol with larger (1 to 2.5 μm) diameters. We suggest this nitrate aerosol forms by multiphase reactions of HNO.sub.3 and particles, reactions that are facilitated by transport of crustal dust and sea spray aerosol from a source within the United States. The observed high aerosol acidity prevents the formation of NH.sub.4 NO.sub.3, the inorganic nitrogen species often dominant in fine-mode aerosol at higher pH. Calculation of the rate of the heterogeneous uptake of HNO.sub.3 on mineral aerosol supports the conclusion that aerosol NO.sub.3 .sup.− is produced primarily by this process, and is likely limited by the availability of mineral cation-containing aerosol surface area. Modeling of NO.sub.3 .sup.− and HNO.sub.3 by thermodynamic equilibrium models (ISORROPIA II and E-AIM) reveals the importance of including mineral cations in the southeastern United States to accurately balance ion species and predict gas-aerosol phase partitioning.

More than a third of mid-ocean ridges have a spreading rate of less than 20 millimetres a year 1 . The lack of deep imaging data means that factors controlling melting and mantle upwelling 2 , 3 , the depth to the lithosphere–asthenosphere boundary (LAB) 4 , 5 , crustal thickness 6 – 9 and hydrothermal venting are not well understood for ultraslow-spreading ridges 10 , 11 . Modern electromagnetic data have greatly improved our understanding of fast-spreading ridges 12 , 13 , but have not been available for the ultraslow-spreading ridges. Here we present a detailed 120-kilometre-deep electromagnetic joint inversion model for the ultraslow-spreading Mohns Ridge, combining controlled source electromagnetic and magnetotelluric data. Inversion images show mantle upwelling focused along a narrow, oblique and strongly asymmetric zone coinciding with asymmetric surface uplift. Although the upwelling pattern shows several of the characteristics of a dynamic system 3 , 12 – 14 , it probably reflects passive upwelling controlled by slow and asymmetric plate movements instead. Upwelling asthenosphere and melt can be traced to the inferred depth of the Mohorovičić discontinuity and are enveloped by the resistivity (100 ohm metres) contour denoted the electrical LAB (eLAB). The eLAB may represent a rheological boundary defined by a minimum melt content. We also find that neither the melt-suppression model 7 nor the inhibited-migration model 15 , which explain the correlation between spreading rate and crustal thickness 6 , 16 – 19 , can explain the thin crust below the ridge. A model in which crustal thickness is directly controlled by the melt-producing rock volumes created by the separating plates is more likely. Active melt emplacement into oceanic crust about three kilometres thick culminates in an inferred crustal magma chamber draped by fluid convection cells emanating at the Loki’s Castle hydrothermal black smoker field. Fluid convection extends for long lateral distances, exploiting high porosity at mid-crustal levels. The magnitude and long-lived nature of such plumbing systems could promote venting at ultraslow-spreading ridges. An inversion model for the ultraslow-spreading Mohns Ridge, combining controlled source electromagnetic and magnetotelluric data, reveals passive mantle upwelling controlled by slow and asymmetric plate movements.