Magma Oceanography and the Early Evolution of the Earth

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-3~------N~EWWSSJA~N~D~V~IEEW~S:------~N~A~ru~~~v~oL~.~~~~~~nE~MB~E~R~I~~ Piscataway) have isolated and character Geophysics ized tubulin mutants from Aspergillus, and identified multiple species of tubulins in dicating multigenic control of tubulin pro Magma oceanography and the duction and MT polymerization. Various types of MT can be distin guished by their sensitivity to nocodazole early evolution of the Earth as well as to taxol (M. DeBrabander, from Richard W. Carlson Janssen Pharmaceutica Research Labora tories, Beerse). The pattern of assembly A RECENT paper by Anne Hofmeister in mantle. In contrast, if the initial compo and disassembly varies strongly depending Journal of Geophysical Research 1 presents sition of the ocean is more similar to a par on the particular combination of drugs us a model for the evolution of a terrestrial tial melt of the mantle containing more Ca ed, indicating different target sites for the 'magma ocean' created by heat from the and AI than the bulk Earth, the increased two drugs. J. Hyams (University College) impact of accreting planetesimals. Magma pressure gradient of the Earth will allow the showed that methyl benzimidazol car ocean models were applied to the Moon early and prolonged crystallization of the bamate can be used as an inhibitor of MT almost immediately after the first samples dense Mg, Fe, AI silicate, garnet. A major polymerization in yeast. Thus, mitotic in were returned by Apollo 11 (ref. 2) but it is effect of garnet crystallization is that the hibitors for cell types which are less sensitive only very recently that similar models have deep garnet-bearing layers will be denser to the mitotic poisons used in higher plant been seriously discussed for the early than the underlying solid mantle and will and animal cells are now available. Earth. sink back into it. In this case, only a 30-km W. Cande (University of California, During a planet's formation a large frac thick plagioclase-rich crust will remain as Berkeley) has isolated and characterized a tion of its energy is transformed to heat in evidence of a magma ocean episode. cytoplasmic dynein-like ATPase from sea the interior. The heat is lost by solid-state In neither of the terrestrial magma ocean urchin eggs which binds to different types convection but, depending on how effi models 1 does this crust actually come close of MT by bundling them in large parallel ciently this takes place, additional energy to matching the composition of the crust arrays, in which many cross-links can be received from the accretion of plane preserved in the oldest sections of the con found. The characteristics of the ATPase tesimals may be sufficient to cause sub tinents. This failure of a simple shallow are consistent with its being a microtubule stantial melting. Given an adequate magma ocean model to match the associated protein (MAP) capable of cross amount of melting, magma will coalesce to geological evidence is compounded further linking MTs in a Mg2 +- and ATP form a 'magma ocean' which, in the extreme because, unlike rocks on the Moon, rocks dependent manner. By careful ultrastruc case, could be of global scale and several of the Earth's crust lack the chemical and tural analyses and a computer-controlled hundred kilometres deep. As the magma isotopic signatures expected to result from program, C. Jensen (University of ocean cools and crystallizes it can produce an early global molten episode. Auckland) demonstrated a highly ordered dramatic physical and chemical effects that While the visible geological evidence for pattern of side arms and cross-bridges. will control the future dynamic evolution a terrestrial magma ocean is lacking, or at Periodicities of 10-14, 20-24 and 48 nm are of the planet. least not obvious, the physical reality of a found along parallel MTs. Assuming side If the Moon did in fact undergo a magma very hot early Earth is difficult to avoid. arms and cross-bridges represent MAPs, ocean episode, then the Earth, because of Whether this implies that the Earth man then either MAPs cannot be bound to all its larger size and hence greater accretional aged to rid itself of its initial heat without tubulin subunits in the MT; or different energy, should have melted to an even undergoing extensive melting is uncertain. MAPs may have different binding sites, thus greater extent. Understanding how a Other possibilities include erasure of the forming the different periodicities. terrestrial magma ocean would behave is evidence for a magma ocean episode by MAPs can show different patterns of complicated by the fact that the phase rela convective mixing of the crust and mantle distribution within different cells: whereas tionships for the crystallization of silicate over Earth history, or that different and MAP2 exhibits a restricted cellular magmas at the higher pressures present in unexpected fractionation paths were distribution in the brain, MAP1 is widely the Earth are not available. This limits caused by the crystallization of unusual distributed and prominent in the mitotic modelling of a terrestrial magma ocean to a high-pressure phases from a deep ocean. spindle (R. Vallee, Worcester Foundation perhaps unrealistically shallow depth of Another intriguing paper3 notes that because for Experimental Biology, Shrewsbury). about 120 km. Nevertheless, Hofmeister's silicate liquids are more compressible than Different types of MT can be characterized work draws some interesting conclusions solids, there may be a depth in the mantle, by their associated MAPs. It is possible regarding the minimum size (laterally estimated to be about 150 km, where li that the diversity of MTs is due to their global), the time required for crystal quids become denser than solids. In this MAPs rather than to their tubulins. lization (< 10,000 yr; a factor of several event the terrestrial magma ocean would be Novel mechanisms for chromosome hundred less than required for the Earth to confined to great depth and its subsequent movement were not proposed. Looking form from accreting planetesimals) and evolution would be more difficult to back at all the data presented at the the crystallization sequence of a 120-km predict. workshop, the attractive notion that one deep terrestrial magma ocean. Whatever the ultimate answer, attempts unique mechanism is responsible for the To a first approximation, the crystal to understand the evolution of the Earth translocation of chromosomes seems no lization sequence of a terrestrial magma starting from a given set of initial physical longer tenable. A large body of data has ocean that is initially similar in compo conditions provide a welcome and much shown that sliding, assembly-disassembly sition to the bulk Earth will resemble that needed approach to understanding the ear and lateral interaction or zipping (to men believed to have occurred on the Moon. ly evolution ofthe solid Earth. 0 tion only the three main hypotheses) are all Put simply, the crystallization produces a feasible. Clearly a combination of different layer cake of relatively dense Mg- and Fe Richard W. Carlson is in the Carnegie mechanisms could be responsible for the rich minerals (olivine, pyroxene) overlain Institution of Washington, Department of chromosome transport. The workshop has by a thick crust of low-density plagioclase Terrestrial Magnetism, 5241 Broad Branch also shown that we may also have to take (a Ca, AI silicate). Plagioclase, being less Road, NW Washington, DC 20015. into consideration mechanisms which have dense than the magma it crystallizes from, not yet been discussed at all. 0 will actually float to the surface of the I. Hofm