There is widespread fascination about the origins and evolution of the Earth, so a great deal of work has gone into unravelling its history. But our planet is a dynamically active body – huge convection currents within the Earth, generated by internal heat, have stirred up the interior for most of its four-and-a-half billion year history. As hot, solid material rises to the surface, melts and then spreads out laterally to form new crust, oceans and continents are pushed around the globe as if on a slow conveyor belt. Elsewhere, cold, dense material sinks back into the interior where it becomes reheated and the whole process starts all over again.
The key issue is whether most of the mantle has been recycled or whether a significant volume still retains its original composition
This continuous recycling of material from the mantle, into the crust and back again, results in the original composition of the Earth’s interior being significantly changed as various elements get left behind in the crust during its formation. This ‘processing’ has led geologists to believe that the Earth’s interior, particularly the mantle which lies beneath the crust, is now well mixed and that there are no rocks left at the surface from the time when the planet first formed. So is it possible for geologists to find samples that tell them about the early stages of the Earth’s history? And if they can obtain samples, is there anything in them geologists can analyse that might carry signals of the processes that occurred billions of years ago? One approach taken at the University’s Department of Earth Sciences has been to undertake experiments at suitable temperatures and pressures to reproduce conditions deep in the Earth. Another is to carry out high-precision analyses of the isotope composition of elements from different portions of the Earth.
The key issue is whether most of the mantle has been recycled through the outer layers of the Earth, or whether a significant volume still retains its original composition. A couple of years ago, Bernie Wood and George Helffrich in the department marshalled geochemical and geophysical arguments in support of the whole mantle having been processed by crust formation and recycling. But more recently, in a study from the Azores, Simon Turner and Chris Hawkesworth, together with colleagues at the Open University, found some of the strongest evidence yet for mantle material that had been melted in an old event and then remained undisturbed for billions of years.
The islands of the Azores sit either side of the major mountain ridge that runs down the centre of the Atlantic ocean. Volcanoes like the Azores are striking manifestations of molten rocks from within the Earth that have reached the surface. In some cases the hot lava that pours forth may have been melted from shallow source reservoirs. In others, such as the Azores, melting takes place in response to the upward movement of a hot plume of material originating deep within the interior, perhaps from depths of several thousand kilometres. Thus volcanoes provide a snapshot of their source regions in the mantle in both space and through time, allowing us to map out lateral and vertical chemical variations within the Earth.
The lavas on the Azores are melts of material that came from great depths in the Earth, but what is very surprising is that they include evidence for some of the oldest material found within the convecting and, by inference, ‘well stirred’ part of the Earth. By analysing the isotope ratios of different elements found in rocks from places like the Azores, we can unravel the history of those elements in different samples. For example, we can distinguish whether those elements have spent time in the Earth’s crust or mantle, whether they contain contributions from the Earth’s core, or whether they have remained largely undisturbed throughout Earth history. Furthermore, because many isotopes are produced by radioactive decay, we can tell when the chemical changes took place.
The isotope ratios from the Azores lavas indicate that the area from which the lavas were derived contained mantle material left behind when new oceanic crust was extracted from it about two and a half billion years ago. This section of mantle then remained undisturbed for billions of years, gradually sinking to great depths, before rising in a plume to be melted once more in the generation of the volcanic rocks of the Azores. The fact that such old material could reside in the Earth’s interior for so long without being destroyed by convection has made geologists question their current understanding of these processes. These results will therefore inform new physical models about the efficiency of convection within the Earth.
The strongest evidence yet for mantle material that has remained undisturbed for billions of years
Thus isotopes offer unique insights into the origins of different components, and how things have changed with time in the Earth. Research at Bristol is leading the way in applying new isotope systems to understanding how the Deep Earth evolved. For example, lithium isotopes are highly sensitive to interaction with seawater. Consequently, the signature from rocks that were once in contact with the sea on the ocean floor, but subsequently returned to the mantle, can be detected using lithium isotope analyses on volcanic rocks derived from the mantle. It has now been demonstrated for the first time that the variation in a number of isotopes in volcanic rocks is linked to the amount of recycled crustal material they contain. This is stunning proof that such fragments of old crust are present in the Earth’s mantle. In a complementary study the isotopes of hafnium and tungsten are being used to isolate contributions from the Earth’s core in the source of the Hawaiian volcanoes.
By analysing these different systems there is a real opportunity to further our understanding of the various processes that have contributed to shaping the evolution of the deep Earth and its present composition – despite its apparent inaccessibility.