Category Archives: Geochemistry, mineralogy, petrology and volcanology

Hot-spot track beneath the Greenland ice cap

Around 63 Ma ago, during the Palaeocene Epoch, major igneous activity broke out in what are now both sides of the North Atlantic Ocean. After initial sputtering it culminated massively between 57 and 53 Ma. Relics are to be seen in Baffin Island, West and East Greenland, the Faeroes and north-western parts of the British Islands, in the form of flood basalts, dyke swarms and scattered remnants of central volcanoes. Offshore drilling on the North Atlantic’s continental shelves suggests that the volcanism extended over 1.3 million km2 and blurted out around 6.6 million km3 of magma. Not for nothing have the products of this event been categorised as a Large Igneous Province. Its formation took place before the North Atlantic existed. It began to form as this precursor magmatic paroxysm waned.  Continued basaltic magma production created the ocean floor each side of the mid-Atlantic Ridge system to divide North America and Greenland from northern Europe. Sea floor spreading continues, rising above sea level in Iceland, which is underlain by a large mantle plume.

The plume beneath Iceland may have been present at a fixed position in the mantle for tens of million years. A hot spot over which plate movements have shifted lithosphere to be heated in a similar way to a sheet of paper dragged slowly over a candle flame. The Iceland plume may have left a hot-spot track similar to that involved in the Hawaiian island chain. The ocean floor to the east and west of Iceland is shallower and forms broad rides at right angles to the trend of the Mid-Atlantic Ridge system, judged to be such tracks that are still warm and buoyant after formation over the plume. But are there traces of earlier passage of drifting lithosphere over the plume. A way to detect older hot-spot tracks is through variations in geothermal heat flow through the continental surface, a linear pattern raising suspicions of such trace of passage. There is no sign to the east beneath Europe, so what about to the west. Greenland, being mainly blanketed in ice, is not a good place to conduct such a search as it would involve deep drilling through the ice at huge cost for each hole. But there is a roundabout way of obtaining geothermal information without even setting foot on Greenland’s icy wastes.

The geomagnetic field measured at the surface records anomalies in rock magnetisation in the solid Earth beneath. Near-surface variations due to large variations in rock types that comprise the continental crust appear as sharp, high frequency signals. Aeromagnetic surveys over Greenland are characterised by such noisy patterns because the subsurface geology is extremely complicated. However, the underlying upper mantle beneath all continents is geologically quite bland, but being uniformly rich in iron it contains a high proportion of magnetic minerals such as magnetite (Fe3O4). The upper mantle should therefore leave a signal in the surface geomagnetic field, albeit a commensurately bland one. Like radio signals that span a large range of wavelengths, Earth properties that vary spatially, such as the geomagnetic field, may be analysed using filters. Once the high-frequency geomagnetic features of the crust are filtered out what should remain is a signal that reflects the magnetic structure of the upper mantle. It should be more or less featureless, yet beneath Greenland it isn’t.

greenland hot spot

Estimated Curie depth variation below Greenland (left) converted to geothermal heat flow variation (right). (Credit: Martos et al. 2018; Figures 1b and 1c)

Magnetic anomalies are created by magnetisation induced in magnetic minerals in rocks by the Earth’s magnetic field. Yet minerals lose their ability to be magnetised at temperatures above a threshold known as the Curie point, which is 580 °C for magnetite, the most abundant magnetic mineral. Depending on the geothermal heat flow the Curie point is exceeded at some depth in the lithosphere. So magnetic anomalies can safely be assumed to be produced only by rocks above the so-called Curie depth. Yasmina Martos of the British Antarctic Survey (now at the University of Maryland) and scientists from Britain, the US and Spain used a complex procedure, including gravity data and a few direct measurements of heat flow below Greenland as well as filtered aeromagnetic data, to estimate the variation in Curie depth beneath the ice cap. (Martos, Y.M. et al. 2018. Geothermal heat flux reveals the Iceland hotspot track underneath Greenland. Geophysical Research Letters, v. 45, online publication; doi: 10.1029/2018GL078289). Using that as an inverse proxy for heat flow they were able to map the likely geothermal variation beneath the island. Rather than a random and narrow variation in depth, as would be expected for roughly uniform heat flow, the Curie depth varied in a non-random way by over 20 km, equivalent to roughly 20 mW m-2.

The shallowest Curie depth and highest estimated heat flow occurs in East Greenland around Scoresby Sund where the largest sequence of Palaeocene flood basalts occur. It is also on a line perpendicular to the mid-Atlantic Rift system that meets the active Iceland plume. Running north-west from Scoresby Sund is a zone of locally high estimated heat flow. Martos et al. suggest that this is the track of Greenland’s motion over the Iceland hot spot from about 80 Ma to the period of maximum on-shore volcanism and the start of sea-floor spreading at around 50 Ma.

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Volcano heading for the sea

John Murray of The Open University, UK has been studying Europe’s largest active volcano Mount Etna on Sicily for most of his career. With a group of colleagues he installed high-precision GPS receivers at over 100 stations on the flanks of the mountain. This was to monitor any shifts in elevation and geographic position, which might be related to magmatic events within the volcano, such as inflation and contraction of the magma chamber. Measurements of position gathered annually since 2001 reveal a somewhat alarming picture (Murray, J.B. et al. 2018. Gravitational sliding of the Mt. Etna massif along a sloping basement. Bulletin of Volcanology, v. 80 online, open access; doi /10.1007/s00445-018-1209-1). The edifice is moving relentlessly ESE at 14 mm yr-1, on average, towards the Mediterranean Sea. Research by one of Murray’s co-authors, Benjamin van Wyk de Vries of the Université Clermont Auvergne, established that many volcanoes have associated signs of deformation due to their huge masses. Often, this is a matter of radial spreading that produces thrust-like faults at their base and in the basement on which they grew. In the case of Etna all the annual displacements on its flanks are skewed to the ESE. The researchers are able to show that this is not a case of flank instability that ultimately may result in lateral collapse but the entire volcano is slowly slipping sideways.

English: Mount Etna, Sicily, topped in snow It...

Mount Etna, Sicily, topped in snow (credit: Wikipedia)

An experimental mock up of the volcano– a cone and flanking layers of lava and pyroclastic rocks made of sand on a substrate of putty to represent underlying sedimentary strata – began to slide once it was tilted at a shallow angle. This suggests that the base of the volcano and igneous debris that it has emitted dips gently to the ESE. The underlying materials are poorly consolidated Quaternary sediments, which are likely to be rheologically weak. Geophysics shows that the NW side of the volcano rests on an almost horizontal plateau, the cone itself being above a spoon-like depression, probably produced by the cone’s mass, and the base dips seawards  in the SE sector. It is through this basement that magma makes its way to Etna’s summit vent system, probably along fractures.

The authors warn that such sliding volcanoes are prone to devastating sector collapse on the downslope side, although there are no signs that might be imminent. Yet it will almost certainly have an effect on eruptive activity as the magma conduits are continually changing. Future research needs to focus on periods when there is horizontal contraction on the volcano, as happens during lengthy periods of dormancy – the period for which there are data has been one of expansion.

Volcanism and sea level fall

Most volcanic activity stems from the rise of hot, deep rock, usually within the mantle. Pressure suppresses partial melting, so as hot rock rises the greater the chance that it will begin to melt without any rise in its temperature. That is the reason why mantle plumes are associated with many volcanic centres within plates. Extension at oceanic ridges allows upper mantle to rise in linear belts below rift systems giving rise to shallow partial melting, mid-ocean ridge basalts and sea-floor spreading. These aren’t the only processes that can reduce pressure to induce such decompression melting; any means of uplift will do, provided the rate of uplift exceeds the rate of cooling at depth. As well as tectonic uplift and erosion, melting of thick ice sheets and major falls in sea level may result in unloading of the lithosphere.

During Messinian Stage of the late Miocene up to 3 km of evaporitic salt was deposited in the deepest parts of the Mediterranean Basin. One mechanism might have been faster evaporation of seawater than its resupply from the Atlantic through the Straits of Gibraltar, similar to the way in which salts is deposited below the Dead Sea. But the salt layer beneath the modern Mediterranean Sea bed has interleaved riverine sediments containing fossils of land plants. The Straits had closed and the Mediterranean Sea evaporated away. From about 6 to 5.3 Ma ago sea level fell by 3 to 5 km, only returning to normal when the Straits reopened to launch the huge Zanclean flood, with which the Pliocene of southern Europe and North Africa commenced. A team from the Universities of Geneva, Orleans and Paris and the Instituto de Ciencias de la Tierra Jaume Almera in Barcelona has tested the hypothesis that the Messinian Crisis affected volcanic activity in the area (Sternai, P. et al. 2017. Magmatic pulse driven by sea-level changes associated with the Messinian salinity crisis. Nature Geoscience, v. 10 online; doi:10.1038/ngeo3032).

From the record of salt precipitation, Pietro Sternai and colleagues, reckon that the main phase of unloading of the Mediterranean Basin began at around 5.6 Ma. Allowing for loading by the thick evaporites they calculated that the effect of the loss of water mass was equivalent to an unloading of 15 MPa in the deeper Eastern Mediterranean and 10 MPa in the west. Using standard pressure-temperature melting curves for the upper mantle, they then estimated that any magma chambers affected by the decrease in pressure could yield up to 17% more melt. Radiometrically dated lavas and igneous dykes within the Mediterranean region became more frequent and the number of events more than doubled during the time of main salt deposition.

https://upload.wikimedia.org/wikipedia/commons/thumb/6/64/DenglerSW-Stromboli-20040928-1230x800.jpg/1200px-DenglerSW-Stromboli-20040928-1230x800.jpg

Stromboli, one of the most active volcanoes in the Mediterranean Basin (credit: Wikipedia)

In May 2017 a study of subglacial volcanoes in West Antarctica based on radar mapping of the solid surface identified 138, 91 of them previously unknown (van Wyk de Vries et al. 2017. A new volcanic province: an inventory of subglacial volcanoes in West Antarctica.  Geological Society, London, Special Publication 461) They lie within a buried rift system and are covered by thick ice. Only one volcano in Antarctica is known to be active, Erebus, which is part of the cluster. Most of the news items stemming from the publication mentioned the possibility that the buried volcanic tract could be adding to the instability of the West Antarctic Ice Sheet through heating up its base. The WAIS is the ice sheet most feared to collapse seawards leading to a rise of about 3 m in global sea level. If the 2 km thick WAIS did slide off its underlying crust it might possibly trigger reactivation of the volcanic cluster.

Wildfires and climate at the K-Pg boundary

It is now certain that the Cretaceous-Palaeogene boundary 66 Ma ago coincided with the impact of a ~10 km diameter asteroid that produced the infamous Chicxulub crater north of Mexico’s Yucatán peninsula. Whether or not this was the trigger for the mass extinction of marine and terrestrial fauna and flora – the flood basalts of the Deccan Traps are still very much in the frame – the worldwide ejecta layer from Chicxulub coincides exactly with the boundary that separates the Mesozoic and Cenozoic Eras. As well as shocked quartz grains, anomalously high iridium concentrations and glass spherules the boundary layer contains abundant elemental carbon, which has been widely ascribed to soot released by vegetation that went up in flames on a massive scale. Atmospheric oxygen levels in the late Cretaceous were a little lower than those at present, or so recent estimates from carbon isotopes in Mesozoic to Recent ambers suggest (Tappert, R. et al. 2013. Stable carbon isotopes of C3 plant resins and ambers record changes in atmospheric oxygen since the Triassic. Geochimica et Cosmochimica Acta, v. 121, p. 240-262,) – other estimates put the level substantially above that in modern air. Whatever, global wildfires occurred within the time taken for the Chicxulub ejecta to settle from the atmosphere; probably a few years. It has been estimated that about 700 billion tonnes of soot were laid down, suggesting that most of the Cretaceous terrestrial biomass and even a high proportion of that in soils literally went up in smoke.

Charles Bardeen and colleagues at the University of Colorado, Boulder, have modelled the climatic and chemical effects of this aspect of the catastrophe (Bardeen, C.G. et al. 2017. On transient climate change at the Cretaceous−Paleogene boundary due to atmospheric soot injections. Proceedings of the National Academy of Sciences; doi:10.1073/pnas.1708980114). Despite the associated release of massive amounts of CO2 and water vapour by both the burning and the impact into seawater, giving increased impetus to the greenhouse effect, the study suggests that fine-grained soot would have lingered as an all enveloping pall in the stratosphere. Sunlight would have been blocked for over a year so that no photosynthesis would have been possible on land or in the upper ocean, the temperatures of the continent and ocean surfaces would have dropped by as much as 28 and 11 °C respectively to cause freezing temperatures at mid-latitudes. Moreover, absorption of solar radiation by the stratospheric soot layer would have increased the temperature of the upper atmosphere by several hundred degrees to destroy the ozone layer. Consequently, once the soot cleared the surface would have had a high ultraviolet irradiation for around a year.

The main implication of the modelling is a collapse in both green terrestrial vegetation and oceanic phytoplankton; most of the food chain would have been absent for long enough to wipe out those animals that depended on it entirely. While an enhanced greenhouse effect and increased acidification of the upper ocean through CO2 emissions by the Deccan flood volcanism would have placed gradually increasing and perhaps episodic stresses on the biosphere, the outcome of the Chicxulub impact would have been immediate and terrible.

More on mass extinctions and impacts here and here

The late-Ordovician mass extinction: volcanic connections

The dominant feature of Phanerozoic stratigraphy is surely the way that many of the formally named major time boundaries in the Stratigraphic Column coincide with sudden shifts in the abundance and diversity of fossil organisms. That is hardly surprising since all the globally recognised boundaries between Eras, Periods and lesser divisions in relative time were, and remain, based on palaeontology. Two boundaries between Eras – the Palaeozoic-Mesozoic (Permian-Triassic) at 252 Ma and Mesozoic-Cenozoic (Cretaceous-Palaeogene) at 66 Ma – and a boundary between Periods – Triassic-Jurassic at 201 Ma – coincide with enormous declines in biological diversity. They are defined by mass extinctions involving the loss of up to 95 % of all species living immediately before the events. Two other extinction events that match up to such awesome statistics do not define commensurately important stratigraphic boundaries. The Frasnian Stage of the late-Devonian closed at 372 Ma with a prolonged series of extinctions (~20 Ma) that eliminated  at least 70% of all species that were alive before it happened. The last 10 Ma of the Ordovician period witnessed two extinction events that snuffed out about the same number of species. The Cambrian Period is marked by 3 separate events that in percentage terms look even more extreme than those at the end of the Ordovician, but there are a great many less genera known from Cambrian times than formed fossils during the Ordovician.

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Faunal extinctions during the Phanerozoic in relation to the Stratigraphic Column.

Empirical coincidences between the precise timing of several mass extinctions with that of large igneous events – mainly flood basalts – suggest a repeated volcanic connection with deterioration of conditions for life. That is the case for four of the Famous Five, the end-Ordovician die-off having been ascribed to other causes; global cooling that resulted in south-polar glaciation of the Gondwana supercontinent and/or an extra-solar gamma-ray burst (predicated on the preferential extinction of Ordovician near-surface, planktonic fauna such as some trilobite families). Neither explanation is entirely satisfactory, but new evidence has emerged that may support a volcanic trigger (Jones, D.S. et al. 2017. A volcanic trigger for the Late Ordovician mass extinction? Mercury data from south China and Laurentia. Geology, v. 45, p. 631-634; doi:10.1130/G38940.1). David Jones and his US-Japan colleagues base their hypothesis on several very strong mercury concentrations in thin sequences in the western US and southern China of late Ordovician marine sediments that precede, but do not exactly coincide with, extinction pulses. They ascribe these to large igneous events that had global effects, on the basis of similar Hg anomalies associated with extinction-related LIPs. Yet no such volcanic provinces have been recorded from that time-range of the Ordovician, although rift-related volcanism of roughly that age has been reported from Korea. That does not rule out the possibility as LIPs, such as the Ontong Java Plateau, are known from parts of the modern ocean floor that formed in the Mesozoic and Cenozoic. Ordovician ocean floor was subducted long ago.

The earlier Hg pulses coincide with evidence for late Ordovician glaciations over what is now Africa and eastern South America. The authors suggest that massive volcanism may then have increased the Earth’s albedo by blasting sulfates into the stratosphere. A similar effect may have resulted from chemical weathering of widely exposed flood basalts which draws down atmospheric CO2. The later pulses coincide with the end of Gondwanan glaciation, which may signify massive emanation of volcanic CO2 into the atmosphere and global warming. Despite being somewhat speculative, in the absence of evidence, a common link between the Big Five plus several other major extinctions and LIP volcanism would quieten their popular association with major asteroid and/or comet impacts currently being reinvigorated by drilling results from the K-Pg Chicxulub crater offshore of Mexico’s Yucatan Peninsula.

Steam-bath Earth

The Earth’s mantle probably contained a significant amount of water from the start. Its earliest history was one of intense bombardment, including the impact that formed the Moon. Together with the conversion of gravitational potential energy to heat while the core was settling out from the mantle, impacts would have kept its overall temperature high enough to prevent water vapour from condensing on the surface until such heat input ceased and heat loss by radiation allowed the surface rapidly to cool. The atmosphere would have been rich in water vapour. Evidence from zircons that are the earliest tangible materials yet recovered hint at the formation of Zr-rich magmas – probably granitic in the broad sense – about 100 Ma after the Moon-forming event (see EPN July 2001: Zircons’ window on the Hadean). Yet no trace of substantial granitic rocks that old have ever been found.

Don Baker and Kassandra Sofonio of McGill University in Montreal, Canada have considered processes other than partial melting or fractional crystallisation that may have been possible during the earliest Hadean. In particular they have looked at one thought once to be a contender in the genesis of granite and latterly sidelined (Baker, D.R. & Sofonio, K. 2017. A metasomatic mechanism for the formation of Earth’s earliest evolved crust. Earth and Planetary Science Letters, v. 463, p. 48-55; http://dx.doi.org/10.1016/j.epsl.2017.01.022 ). They heated powdered artificial samples that chemically resembled the Earth’s original silicate mantle in sealed double capsules – an inner part containing the silicate powder and an outer one containing water. The capsules were held at around 727°C for a time and then quenched. The outer part of each capsule was found to be a glass of roughly granite composition. The experimental design ensured that superheated water diffused across the inner-outer capsule wall. So the ‘granite’ must have formed by a metasomatic process – essentially preferential solution of its component elements in supercritical water – the experimental temperature being insufficient to partially melt the ultramafic charge in the inner capsule.

Baker and Sofonio conclude that degassing of this metasomatic fluid – silicate-rich ‘steam’ – may have produced substantial masses of sialic crust on the Earth’s surface. Removal of material produced in such a manner would also have extracted trace elements with an affinity for granite from the early mantle – so-called incompatible elements. The subsequent recycling of such granitic blobs back into the mantle may explain geochemical signs in >500 Ma younger Archaean crust – produced by ‘normal’ igneous processes – of incompatible-element enriched reservoirs in the Early mantle.

Archaean continents derived from Hadean oceanic crust

As DNA is to tracing  human evolution and migration, so various isotope systems are to the evolution of the Earth. One of the most fruitful is the samarium-neodymium (Sm-Nd) system. The decay of 147Sm to 143Nd is used in dating rocks across the full range of Earth history, given coeval rocks with a suitable range of Sm/Nd ratios, because the decay has a long half life (1.06 x 1011 years). However, samarium has another radioactive isotope 147Sm with a half life that is a thousand times shorter (1.06 x 108 years). So it remains only as a minute proportion of the total Sm in rocks, most having decayed since it was formed in a pre-Solar System supernova. But its daughter isotope 142Nd is present in easily measurable quantities, having accumulated from 147Sm decay over the first few hundred million years of Earth’s history; i.e. during the Hadean and earliest Archaean Eons. It is this fact that allows geochemists to get an indirect ‘handle’ on events that took place in the Earth’s earliest, largely vanished history. The principle behind this approach is that when an ancient rock undergoes partial melting to produce a younger magma the rock that crystallizes from it inherits the relative proportions of Nd isotopes of its source and thereby carries a record of the earlier history.

English: An outcrop of metamorphosed volcanose...

Metamorphosed volcanosedimentary rocks from the Porpoise Cove locality, Nuvvuagittuq supracrustal belt, Canada. Possibly the oldest rocks on Earth. (credit: Wikipedia)

The eastern shore of Hudson Bay in Canada hosts the oldest tangible geology known, in form of some metamorphosed basaltic rocks dated at 4200 Ma old known as the Nuvvuagittuq Greenstone Belt – the only known Hadean rocks. They occur in a tiny (20 km2) patch associated with gneisses of tonalite-trondjhemits-granodiorite composition that are dated between 3760 and 3350 Ma. Engulfing both are younger (2800 to 2500 Ma) Archaean plutonic igneous rocks of felsic composition. Jonathan O’Neil and Richard Carlson of the University of Ottawa, Canada and the Carnegie Institution for Science, Washington DC, USA respectively, measured proportions of Nd isotopes in both sets of felsic igneous rocks (O’Neil, J. & Carlson, R.W. 2017. Building Archean cratons from Hadean mafic crust. Science, v. 355, p. 1199-1202; doi:10.1126/science.aah3823).

The oldest gneisses contained relative proportions of 142Nd commensurate with them having been formed by partial melting of the Hadean mafic rocks about a few hundred million years after they had been erupted to form the oldest known crust; no surprise there. However, the dominant components of the local continental crust that are about a billion years younger also contain about the same relative proportions of 142Nd. A reasonable conclusion is that the Archaean continental crust of NE Canada formed by repeated melting of mafic crust of Hadean age over a period of 1.5 billion years. The modern Earth continually replenishes its oceanic crust over about 200 Ma due to plate tectonics. During the Archaean mantle dynamics would have been driven faster by much higher internal heat production. Had this involved simply faster plate tectonics the outermost skin of mafic crust would have been resorbed into the mantle even faster. By the end of the Archaean (2500 Ma) barely any Hadean crust should have been available to produce felsic magmas. But clearly at least some did linger, adding more weight to the idea that plate tectonics did not operate during the Hadean and Archaean Eons. See Formation of continents without subduction below.

Formation of continents without subduction

The formulation of the theory of plate tectonics provided plausible explanations for the growth of continental crust over time, among many other fundamental Earth processes. Briefly expressed, once basalt capped oceanic lithosphere is forced downwards at plate boundaries where plates move towards one another, beyond a certain penetration cool, moist basalt undergoes a pressure-controlled change of state. Its chemical constituents reassemble into minerals more stable under elevated pressure. In doing so, one outcome involves dehydration reactions the other being that the bulk composition is recast mainly in the form of high-pressure pyroxene and the mineral garnet: the rock eclogite. The density of the basaltic cap increases above that of the mantle. Gravity acts to pull the subducting slab downwards, this slab-pull force being the main driver of plate motions globally. Water vapour and other fluids shed by dehydration reactions rise from the subducted slab into the wedge of overlying mantle to change its conditions of partial melting and the composition of the magma so produced. This is the source of arc magmatism that persists at the destructive plate margin to increase the volcanic pile’s thickness over time. When magma is able to pond at the base of the new crust its fractional crystallisation produces dense cumulates of high-temperature mafic silicates and residual melt that is both lighter and more enriched in silica. Residual magma rises to add to the middle and upper crust while the cumulate-rich lower crust becomes less gravitationally stable, eventually to spall downwards by delamination. Such a process helps to explain the bulk low density of continental crust built up over time together with the freeboard of continents relative to the ocean floor: a unique feature of the Earth compared with all other bodies in the Solar System. It also accounts for the vast bulk of continental crust having remained at the surface since it formed: it rarely gets subducted, if at all.

One suggested model for pre-plate tectonic continent formation (credit, Robert J Stern https://speakingofgeoscience.org/2013/04/28/when-did-plate-tectonics-begin-on-earth-and-what-came-before/)

Tangible signs that such subduction was taking place in the past – eclogites and other high-pressure, low-temperature metamorphosed basalts or blueschists – are only found after 800 Ma ago. Before that time evidence for plate tectonics is circumstantial. Some geologists have argued for a different style of subduction in earlier times, plates under riding others at low angles. Others have argued for a totally different style of tectonics in Earth early history, marked by changes in bulk chemical composition of the continental crust at the Archaean-Proterozoic boundary. A new twist comes from evidence in the Archaean Pilbara Craton of Western Australia (Johnson, T.E. et al. 2017. Earth’s first stable continents did not form by subduction. Nature, v. 543, p. 239-242; doi:10.1038/nature21383). The authors found that basalts dated at about 3.5 Ga have trace-element geochemistry with affinities to the primitive basalts of island arcs. That makes them a plausible source for slightly younger felsic plutonic rocks with a tonalite-trondhjemite-granodiorite (TTG) compositional range (characteristic of Archaean continental crust). If the basalts were partially melted to yield 30% of their mass as new magma the melt composition would match that of the TTG crust. This would be feasible at only 30 km depth given a temperature increase with depth of at least 25° C per kilometre; more than the average continent geothermal gradient today but quite plausible with the then higher heat production by less decayed radioactive isotopes of uranium, thorium and potassium 3.5 Ga ago. This would have required the basalts to have formed a 30 km thick crust. However, the basalts’ geochemistry requires their generation by partial melting of earlier more mafic basalts rather than directly from the mantle. That early Archaean mantle melting probably did generate vast amounts such primary magma is generally acknowledged and confirmed by the common occurrence of komatiitic lavas with much higher magnesium content than common basalts of modern constructive margins. In essence, Johnson et al. favour thermal reworking of primitive Archaean crust, rather than reworking in a plate tectonic cycle.

More on continental growth and plate tectonics

See also

When did Plate Tectonics begin on Earth, and what came before?

A ‘recipe’ for Earth’s accretion, without water

The Earth continues to collect meteorites, the vast majority of which are about as old as our planet; indeed many are slightly older. So it has long been thought that Earth originally formed by gravitational accretion when the parental bodies of meteorites were much more abundant and evenly distributed. Meteorites fall in several classes, metallic (irons) and several kinds that contain silicate minerals, some with a metallic component (stony irons) others without, some with blebs or chondrules of once molten material (chondrites) and others that do not (achondrites), and more subtle divisions among these general groups. In the latter half of the 20th century geochemists and cosmochemists became able to compare the chemical characteristics of different meteorite classes with that of the Sun –from its radiation spectrum – and those of different terrestrial rocks – from direct analysis. The relative proportions of elements in chondrites turned out to match those in the Sun – inherited from the gas nebula from which it formed – better than did other classes. The best match with this primitive composition turned out to be the chemistry of carbonaceous chondrites that contain volatile organic molecules and water as well as silicates and sulfides. The average chemistry of one sub-class of carbonaceous chondrites (C1) has been chosen as a ‘standard of standards’ against which the composition of terrestrial rocks are compared in order that they can be assessed in terms of their formative processes relative to one another. For a while carbonaceous chondrites were reckoned to have formed the bulk of the Earth through homogeneous accretion: that is until analyses became more precise at increasingly lower concentrations. This view has shifted …

Geochemistry is a complex business(!), bearing in mind that rocks that can be analysed today predominantly come from the tiny proportion of Earth that constitutes the crust. The igneous rocks at the centre of wrangling how the whole Earth has evolved formed through a host of processes in the mantle and deep crust, which have operated since the Earth formed as a chemical system. To work out the composition of the primary source of crustal igneous rocks, the mantle, involves complex back calculations and modelling. It turns out that there may be several different kinds of mantle. To make matters worse, those mantle processes have probably changed considerably from time to time. To work back to the original formative processes for the planet itself faces the more recent discovery that different meteorite classes formed in different ways, different distances from the Sun and at different times in the early evolution of the pre-Solar nebula. Thankfully, some generalities about chemical evolution and the origin of the Earth can be traced using different isotopes of a growing suite of elements. For instance, lead isotopes have revealed when the Moon formed from Earth by a giant impact, and tungsten isotopes narrow-down the period when the Earth first accreted. Incidentally, the latest ideas on accretion involve a series of ‘embryo’ planets between the Moon and Mars in size.

An example of an E-type Chondrite (from the Ab...

An example of an enstatite chondrite (from the Abee fall) in the Gallery of Minerals at the Royal Ontario Museum. (Photo credit: Wikipedia)

Calculating from a compendium of isotopic data from various types of meteorite and terrestrial materials, Nicolas Dauphas of the University of Chicago has convincingly returned attention to a model of heterogeneous accretion of protoplanetary materials from different regions of the pre-Solar nebula (Dauphas, N. 2017. The isotopic nature of the Earth’s accreting material through time. Nature, v. 541, p. 521-524; doi:10.1038/nature20830). His work suggests that the first 60% of Earth’s accretion involved materials that were a mixture of meteorite types, half being a type known as enstatite chondrites. These meteorites are dry and contain grains of metallic iron-nickel alloy and iron sulfides set in predominant MgSiO3 the pyroxene enstatite. The Earth’s remaining bulk accumulated almost purely from enstatite-chondrite material. A second paper in the same issue of Nature (Fischer-Gödde, M. & Kleine, T. 2017. Ruthenium isotopic evidence for an inner Solar System origin of the late veneer. Nature, v. 541, p. 525-527; doi:10.1038/nature21045) reinforces the notion that the final addition was purely enstatite chondrite.

This is likely to cause quite a stir: surface rocks are nothing like enstatite chondrite and nor are rocks brought up from the upper mantle by volcanic activity or whose composition has been back-calculated from that of surface lavas; and where did the Earth’s water at the surface and in the mantle come from? It is difficult to escape the implication of a mantle dominated by enstatite chondrite From Dauphas’s analysis, for lots of other evidence from Earth materials seem to rule it out. One ‘escape route’ is that the enstatite chondrites that survived planetary accretion, which only make up 2% of museum collections, have somehow been changed during later times.  The dryness of enstatite chondrites and the lack of evidence for a late veneer of ‘moist’ carbonaceous chondrite in these analyses cuts down the options for delivery of water, the most vital component of the bulk Earth and its surface.  Could moister meteorites have contributed to the first 60% of accretion, or was  post-accretion cometary delivery to the surface able to be mixed in to the deep mantle? Nature’s News & Views reviewer, Richard Carlson of the Carnegie Institution for Science in Washington DC, offers what may be a grim outlook for professional meteoriticists: that perhaps “the meteorites in our collection are not particularly good examples of Earth’s building blocks” (Carlson, R.W. 2017. Earth’s building blocks. Nature, v. 541, p. 468-470; doi:10.1038/541468a).

Animation of how the Solar System may have formed.

Oceans of magma, Moon formation and Earth’s ‘Year Zero’

That the Moon formed and Earth’s geochemistry was reset by our planet’s collision with another, now vanished world, has become pretty much part of the geoscientific canon. It was but one of some unimaginably catastrophic events that possibly characterised the early Solar System and those around other stars. Since the mantle geochemistry of the Earth’s precursor was fundamentally transformed to that which underpinned all later geological events, notwithstanding the formation of the protoEarth about 4.57 Ga ago, I now think of the Moon-forming event as our homeworld’s ‘Year Zero’. It was the ‘beginning’ of which James Hutton reckoned there was ‘no vestige’. Any modern geochemist might comment, ‘Well, there must be some kind of signature!’, but what that might be and when it happened are elusive, to say the least. One way of looking for answers is, as with so many thorny issues these days, to make a mathematical model. James Connelly and Martin Bizzarro of the University of Copenhagen, Denmark, have designed one based on the fact that one of the volatile elements that must have been partially ‘blown off’ by such a collision is lead and, of course, that is an element with several isotopes that are daughters of long-term decay of radioactive uranium and thorium (Connelly, J.N. & Bizzarro, M. 2016. Lead isotope evidence for a young formation age of the Earth–Moon system. Earth and Planetary Science Letters, v. 452, p. 36-43. doi:10.1016/j.epsl.2016.07.010).

Artist’s impression of the impact of a roughly Mars-size planet with the proto-Earth to form an incandescent cloud, from part of which the Moon formed.

Artist’s impression of the impact of a roughly Mars-size planet with the proto-Earth to form an incandescent cloud, from part of which the Moon formed. A NASA animation of lunar history can be viewed here.

Loss of volatile daughter isotopes of Pb produced by the decay schemes of highly refractory isotopes of U and Th would have reset the U-Pb and Th-Pb isotopic systems and therefore the radiogenic ‘clocks’ that depend on them in the same way as melting or high-temperature metamorphism resets the simpler 87Rb-87Sr decay scheme. Each radioactive U isotope has a different decay rate that produces a different Pb isotope daughter (235U Þ 207Pb; 238U Þ 206Pb, so it is possible to devise means of using present-day values of ratios between Pb isotopes, such as 207Pb/206Pb, 206Pb/204Pb and 207Pb/204Pb, to work back to such a ‘closure’ time. In short, that is the approach used by Connelly and Bizzarro. The most complicated bit of that geochemical ruse is estimating values of the ratios for the Earth’s modern mantle and for the Solar system in general – a procedure based on what we can actually measure: lots of mantle-derived basalts and lots of meteorites. Cutting out some important caveats, the result of their model is quite a surprise: ‘Year Zero’ on their account was between 4426 and 4417 Ma years ago, which is astonishingly precise. And it is pretty close to the measured age of the of lunar Highland anorthosites – products of fractional crystallisation of the Moon’s early magma ocean – and also to that of the oldest zircons on Earth. But is also about 60 Ma later than previous estimates

The Connelly and Bizzarro paper follows hard on the heels of another with much the same objective  (Snape, J.F. and 8 others 2016. Lunar basalt chronology, mantle differentiation and implications for  determining the age of the Moon. Earth and Planetary Science Letters, v. 451, p. 149-158. doi.org/10.1016/j.epsl.2016.07.026). Once again omitting a great deal of argument, Snape and colleagues end up with an age for the isotopic resetting of the lunar mantle of 4376 Ma to the nearest 18 Ma; i.e. an age significantly different from that arrived at by Connelly and Bizzarro. So the answer to the question, ‘When was there a vestige of a beginning?’ is, ‘It depends on the model’… Thankfully, neither estimate for ‘Year Zero’ has much bearing on the big, practical questions, such as, ‘When did life form?’, ‘Has there always been plate tectonics?’

More on the origin of the early Solar System and formation of the Earth-Moon system