Tag Archives: continental growth

So, when did plate tectonics start up?

Tiny, 4.4 billion year old zircon grains extracted from much younger sandstones in Western Australia are the oldest known relics of the Earth system. But they don’t say much about early tectonic processes. For that, substantial exposures of rock are needed, of which the undisputedly oldest are the Acasta gneisses 300 km north of Yellowknife in Canada’s North West Territories, which have an age of slightly more than 4 Ga. The ‘world’s oldest rock’ has been something of a grail for geologists and isotope geochemists who have combed the ancient Archaean cratons for 5 decades. But since the discovery of metasediments with an age of 3.8 Ga in West Greenland during the 1970s they haven’t made much headway into the huge time gap between Earth’s accretion at 4.54 Ga and the oldest known rocks (the Hadean Eon).

The Deccan Traps shown as dark purple spot on ...

Continental cratons (orange) where very-old rocks are likely to lurk. (credit: Wikipedia)

There have been more vibrant research themes about the Archaean Earth system, specifically the issue of when our planet settled into its modern plate tectonic phase A sprinkling of work on reconstructing the deep structural framework of Archaean relics has convinced some that opposed motion of rigid, brittle plates was responsible for their geological architecture, whereas others have claimed signs of a more plastic and chaotic kind of deformation of the outer Earth. More effort has been devoted to using the geochemistry of all the dominant rocks found in the ancient cratons, seeking similarities with and differences from those of more recent vintage. There can be little doubt that the earliest processes did form crust whose density prevented or delayed it from being absorbed into the mantle. Even the 4.4 Ga zircons probably crystallized from magma that was felsic in composition. Once trapped by buoyancy at the surface and subsequently wrapped around by similarly low density materials continental crust formed as a more or less permanent rider on the Earth’s deeper dynamics. But did it all form by the same kinds of process that we know to be operating today?

Plate tectonics involves the perpetual creation of rigid slabs of basalt-capped oceanic lithosphere at oceanic rift systems and their motions and interactions, including those with continental crust. Ocean floor cools as it ages and becomes hydrated by seawater that enters it. The bulk of it is destined eventually to oppose, head-to-head, the motions of other such plates and to deform in some way. The main driving force for global tectonics begins when an old, cold plate does deform, breaks, bends and drives downwards. Increasing pressure on its cold, wet basaltic top transforms it into a denser form: from a wet basaltic mineralogy (feldspar+pyroxene+amphibole) to one consisting of anhydrous pyroxene and garnet (eclogite) from which watery fluid is expelled upwards. Eclogite’s density exceeds that of mantle peridotite and compels the whole slab of oceanic lithosphere to sink or subduct into the mantle, dragging the younger parts with it. This gravity-induced ‘slab pull’ sustains the sum total of all tectonic motion. The water rising from it induces the wedge of upper mantle above to melt partially, the resulting magma evolves to produce new felsic crust in island arcs whose destiny is to be plastered on to and enlarge older continental masses.

Relics of eclogites and other high-pressure, low-temperature versions of hydrated basalts incorporated into continents bear direct and unchallengeable witness to plate tectonics having operated back to about 800 Ma ago. Before that, evidence for plate tectonics is circumstantial and in need of special pleading. Adversarial to-ing and fro-ing seems to be perpetual, between geoscientists who see no reason to doubt that Earth has always behaved in this general fashion and others who see room for very different scenarios in the distant past. The non-Huttonian tendency suggests an early, more ductile phase when greater radioactive heat production in the mantle produced oceanic crust so fast that when it interacted with other slabs it was hot enough to resist metamorphic densification wherever it was forced down. Faster production of magma by the mantle without slab-pull could have produced a variety of ‘recycling’ turnover mechanisms that were not plate-tectonic.

One thing that geochemists have discovered is that the composition of Archaean continental crust is very different from that produced in later times. In 1985 Ross Taylor and Scott McLennan, then of the Australian National University, hit on the idea of using shales of different ages as proxies for the preceding continental crust from which they had been derived by long erosion. Archaean and younger shales differed in such a way that suggests that after 2.5 Ga (the end of the Archaean) vast amounts of feldspar were extracted from the continent-forming magmas. This left the later Precambrian and Phanerozoic upper crust depleted in the rare-earth element europium, which ended up in a mafic, feldspar-rich lower crust. On the other hand, no such mass fractionation had left such a signature before 2.5 Ga. Another ANU geochemist, now at the University of Maryland, Roberta Rudnick has subsequently carried this approach further, culminating in a recent paper (Tang, M., Chen, K and Rudnick, R.L. 2016. Archean upper crust transition from mafic to felsic marks the onset of plate tectonics. Science, v. 351, p. 372-375). This uses nickel, chromium and zinc concentrations in ancient igneous and sedimentary rocks to track the contribution of magnesium (the ‘ma’ in ‘mafic’) to the early continents. The authors found that between 3.0 to 2.5 Ga continental additions shifted from a dominant more mafic composition to one similar to that of later times by the end of the Archaean. Moreover, this accompanied a fivefold increase in the pace of continental growth. Such a spurt has long been suspected and widely suggested to mark to start of true plate tectonics: but an hypothesis bereft of evidence.

A better clue, in my opinion, came 30 years ago from a study of the geochemistry of actual crustal rocks that formed before and after 2.5 Ga (Martin, H. 1986. Effect of steeper Archean geothermal gradient on geochemistry of subduction-zone magmas. Geology, v. 14, p. 753-756). Martin showed that plutonic Archaean and post-Archaean felsic rocks of the continental crust lie in distinctly different fields on plots of their rare-earth element (REE) abundances. Archaean felsic plutonic rocks show a distinct trend of enrichment in light REE relative to heavy REE as measures of the degree of partial melting decreases, whereas the younger crustal rocks show almost constant, low values of heavy REE/light REE whatever the degree of melting. The conclusion he reached was that while in the post Archaean the source was consistent with modern subduction processes – i.e. partial melting of hydrated peridotite in the mantle wedge above subduction zones – but during the Archaean the source was hydrated, garnet-bearing amphibolite of basaltic composition, in the descending slab of subducted oceanic crust. Together with Taylor and McLennan’s lack of evidence for any fractional crystallization in Archaean continental growth, in contrast to that implicated in Post-Archaean times.

The geochemistry forces geologists to accept that a fundamental change took place in the generation and speed of continental growth at the end of the Archaean, marking a shift from a dominance of melting of oceanic, mafic crust to one where the upper mantle was the main source of felsic, low-density magmas. Yet, no matter how much we might speculate on indirect evidence, whether or not subduction, slab-pull and therefore plate tectonics dominated the Archaean remains an open question.

More on continental growth and plate tectonics

Charting the growth of continental crust

Česky: Budynáž nedaleko obce Kangerlussuaq, zá...

Archaean gneisses from West Greenland (Photo credit: Wikipedia)

When continents first appeared; the pace at which they grew; the tectonic and magmatic processes responsible for continental crust, and whether or not crustal material is consumed by the mantle to any great extent have been tough issues for geologists and geochemists to ponder on for the last four decades. Clearly, continental material was rare if not absent in the earliest days of the solid Earth, otherwise Hadean crust should have been found by now. Despite the hints at some differentiated, high silica rocks that may have hosted >4 billion-year old zircon crystals from much younger sediments, the oldest tangible crust – the Acasta Gneiss of northern Canada – just breaks the 4 Ga barrier: half a billion years short of the known age of the Earth (http://earth-pages.co.uk/2008/11/01/at-last-4-0-ga-barrier-broken/). Radiometric ages for crustal rocks steadily accumulated following what was in the early 1970s the astonishing discovery by Stephen Moorbath and colleagues at Oxford University and the Geological Survey of Greenland of a 3.8 billion year age for gneisses from West Greenland.  For a while it seemed as if there had been great pulses that formed new crust, such as one between 2.8 and 2.5 Ga (the Neoarchaean) separated by quieter episodes. Yet dividing genuinely new material coming from the mantle from older crust that later thermal and tectonic events had reworked and remelted required – and still does – lengthy and expensive radiometric analysis of rock samples with different original complements of radioactive isotopes.

One approach to dating has been to separate tiny grains of zircon from igneous and metamorphic rocks and date them using the U-Pb method as a route to the age at which the rock formed, but that too was slow and costly. Yet zircons, being among the most intransigent of Earth materials, end up in younger sedimentary rocks after their parents have been weathered and eroded. It was an investigation of what earlier history a sediment’s zircons might yield that lead to the discovery of grains almost as old as the Earth itself (http://earth-pages.co.uk/2011/12/21/mistaken-conclusions-from-earths-oldest-materials/ http://earth-pages.co.uk/2005/05/01/zircon-and-the-quest-for-life%E2%80%99s-origin/). That approach is beginning to pay dividends as regards resolving crustal history as a whole. Almost 7000 detrital zircon grains separated from sediments have been precisely dated using lead and hafnium isotopes. Using the age distribution alone suggests that the bulk of continental crust formed in the Precambrian, between 3 and 1 Ga ago, at a faster rate than it formed during the Phanerozoic. However, that assumes that a zircon’s radiometric age signifies the time of separation from the mantle of the magmas from which the grain crystallised. Yet other dating methods have shown that zircon-bearing magmas also form when old crust is remelted, and so it is important to find a means of distinguishing zircons from entirely new blocks of crust and those which result from crustal reworking. It turns out that zircons from mantle-derived crust have different oxygen isotope compositions from those which crystallised from remelted crust.

U-Pb ages of detrital zircons from sediments o...

An example of ages of detrital zircons from sediments, in this case from five Russian rivers (credit: Wikipedia)

Bruno Dhuime and colleagues from St.Andrew’s and Bristol universities in the UK measures hafnium model ages and δ18O  values in a sample of almost 1400 detrital zircons collected across the world from sediments of different ages (Dhuime, B. et al. 2012. A change in the geodynamics of continental growth 3 billion years ago. Science, v. 335, p. 1334-1336). Plotting δ18O  against Hf model age reveals two things: there are more zircons from reworked crust than from mantle-derived materials; plotting the proportion of new crust ages to those of reworked crust form 100 Ma intervals through geological time reveals dramatic changes in the relative amounts of ‘mantle-new’ crust being produced. Before 3 Ga about three quarters of all continental crust emerged directly from the mantle. Instead of the period from 3 to 1 Ga being one of massive growth in the volume of the crust, apparently the production rate of new crust fell to about a fifth of all crust in each 100 Ma time span by around 2 Ga and then rose to reach almost 100% in the Mesozoic and Cenozoic. This suggests that the late Archaean and most of the Proterozoic were characterised by repeated reworking of earlier crust, perhaps associated with the repeated formation and break-up of supercontinents by collision orogeny and then tectonic break up and continental drift.

Dhuine and colleagues then use the record of varying new crust proportions to ‘correct’ the much larger database of detrital zircon ages. What emerges is a well-defined pattern in the rate of crustal growth through time. In the Hadean and early Archaean the net growth of the continents was 3.0 km3 yr-1, whereas throughout later time this suddenly fell to and remained at 0.8 km3 yr-1. Their explanation is that the Earth only came to be dominated by plate tectonic processes mainly driven by slab-pull at subduction zones after 3 Ga. Subduction not only produces mantle-derived magmas but inevitably allows continents to drift and collide, thereby leading to massive deformation and thermal reworking of older crust in orogenic belts and an apparent peak in zircon ages. The greater rate of new crust generation before 3 Ga may therefore have been due to other tectonic processes than the familiar dominance of subduction. Yet, since there is convincing evidence for subduction in a few ancient crustal blocks, such as west Greenland and around Hudson’s Bay in NE Canada, plate tectonics must have existed but was overwhelmed perhaps by processes more directly linked to mantle plumes.

More on continental growth can be found here