50 Years Ago 100 Years

NEWS & VIEWS RESEARCH

Basalt Harzburgite Pyrolite Earth’s surface

100 200 Upper mantle 50 Years Ago 300

Depth (km) Later this year, in November, Nature 400 Hydrated pyrolite will be a hundred years old. For Transition much of the century, the journal 500 zone has contributed in several unrelated 600 ways to the development of science. From the beginning, of course, it has 700 been a professional journal, but at Lower mantle the beginning it was most especially 800 Dehydration melting a means by which the still small and informal profession of science could be kept aware of events within Figure 1 | Current theories for the recycling of oceanic crust in Earth’s mantle. Slabs of oceanic crust and of pressures from outside. The (which is formed mainly of a rock called basalt) and underlying mantle rocks (harzburgite) sink into early volumes of Nature contain an Earth’s mantle, which is often modelled as being formed from a hypothetical rock called pyrolite. Basalt entirely readable mixture of opinion, accumulates within and beneath the transition zone between the upper and lower mantle. In downwelling news — particularly the doings of regions, hydrated rocks in the transition zone are pushed into the lower mantle, where they release water universities, learned societies and bound in their minerals. The resulting aqueous fluids can trigger melting. This dehydration melting could 10,11 observatories — and occasionally of explain why seismic waves have low velocities in some regions at depths greater than 660 kilometres. argument … For a great many years, Gréaux et al.2 report that basaltic rocks in the mantle will also have slow seismic waves. The presence the correspondence from readers of basaltic rocks at depths greater than 660 km could therefore be an alternative explanation for the low seismic-wave velocities in these regions. was largely concerned with natural curiosities … Only by the turn of the century did the correspondence 600 kelvin, and this cubic form is probably in the case of CaPv, would stabilize the cubic columns become a vehicle for adopted at the high temperatures present in form. The authors went on to use the new data the announcement of important the mantle. However, the crystal structure of for cubic CaPv to improve estimates of the discoveries in science. CaPv spontaneously distorts to a tetragonal velocities of seismic waves travelling through From Nature 11 January 1969 form at lower temperatures. CaPv must there- rocks in Earth’s mantle. fore be held at high temperatures and pressures The boundary between Earth’s upper and to determine the physical properties of the lower mantle is marked by steep gradients in cubic phase. density and seismic-wave velocities at depths 100 Years Ago In their experiments, Gréaux et al. of around 660 kilometres. The pressures and synthesized CaPv, and analysed both the temperatures thought to prevail at this depth The Saxon State Railways are now cubic and the tetragonal forms at high pres- coincide with major changes in the mineral submitting their engine-drivers and sures and temperatures in a high-pressure assemblage of pyrolite — the hypothetical other responsible train officials to apparatus. They measured the time taken for rock that is often used as a model for the rocks certain tests in their psychometric ultrasonic waves to travel through the CaPv that constitute the bulk of the mantle. These laboratory at Dresden … [T]he at different pressure–temperature combina- mineral transformations increase the density tests comprise strength of will tions, and irradiated the samples with intense of pyrolite, so that basaltic materials become and endurance, and fatigue where X-rays to generate images and diffraction pat- buoyant at depths of between 660 km and there is physical strain. The Dubois terns. By combining these measurements, the 750 km (refs 4, 7, 8). Geodynamic simulations9 ergograph is used for the purpose, authors derived sound-wave velocities and show that the sequence of density changes the object being to trace a fatigue elastic moduli (which quantify the resistance in this region can effectively trap recycled curve. The forearm rests on the table; of a solid material to small, non-permanent oceanic crust. over the middle finger is run a catgut deformations) for CaPv. Gréaux and colleagues’ results suggest loop, which passes over a pulley, the Gréaux and colleagues found that the shear that seismic waves travel much more slowly other end of the gut supporting a modulus of cubic CaPv, which specifically through recycled oceanic crust than through weight of from 4 to 8 kg., according measures the resistance of the mineral to pyrolite at depths of between 660 km and to the suitability of the subject. reversible deformation caused by distortions 770 km. Consequently, the authors propose When the middle finger is bent the (shear deformation), is substantially lower that the presence of trapped basaltic rocks weight is raised, and when relaxed than estimates5,6 calculated using first-princi- could explain the reduction of seismic-wave again the weight is dropped, the ple computations. The difference reflects the velocities that has been observed locally at process of this motion being traced fact that the computed sound-wave velocities these depths (Fig. 1). The reduction was previ- on a recording drum … The system were significantly higher than those measured ously attributed10,11 to deep dehydration melt- has been said to give satisfactory in the experiments. The experimental findings ing — the process in which water is released results as regards the selection of highlight the importance of assessing the elas- from the crystal structures of hydrous miner- men for the proper posts. tic and acoustic properties of mantle minerals als, causing melting. The report last year of From Nature 9 January 1919 at relevant pressures and temperatures that, the discovery12 of a fragment (an inclusion)