Plate tectonics, sea-floor spreading, and continental drift: an introduction

The present ruling theory of geotectonics commonly known as the "new global tectonics”-- includes the concepts of plate tectonics, seafloor spreading, continental drift, and polar wondering. Recent seismic activity defines the positions and relative movements of rigid lithosphere plates. The geomagnetic time scale for polarity reversals seems to be calibrated to about 4 m.y. ago, and extrapolated to about 80 m.y. ago by correlation of oceanic magnetic anomalies with reversals and seafloor spreading. Seafloor spreading and the magnetic anomalies thus indicate the directions and roles of movements of lithosphere plates during the last 80 m.y. The continents drift with the lithosphere plates, and independent paleomagnetic evidence permits location of the relative positions of the continents and the poles to 500 m.y. ago, or more. The theory, which explains phenomena previously unexplainable, is supported by a mass of persuasive evidence. There is no doubt that the theory is a success, but it has been so successful that it has become a ruling theory, and subservience to a ruling theory never has served science well. There are data which do not seem to fit the theory. We should strive to keep open minds and to search for alternate solutions to fit all of the data. The record is clear: today's history was yesterday's model. Dare we conclude that at last we know the answers

The effect of carbon dioxide on phase relationships for synthetic lherzolite and harzburgite

The system CaO-MgO-SiO_2-CO_2 includes mineral assemblages corresponding to model lherzolite: forsterite(Fo) + orthopyroxene(Opx) + clinopyroxene(Cpx), and model harzburgite: Fo + Opx, as well as model websterite and wehrlite. When fully carbonated, the peridotites are converted to limestones: dolomite(Do) + magnesite(Mc) + quartz(Qz), or Mc + Qz. When partly carbonated, the peridotites are converted to carbonate-lherzolite and magnesite-harzburgite, which cannot coexist with CO_2. Available experimental and calculated reaction data are presented for carbonatc-lherzolite: (6) Opx + Do = Cpx + Fo + CO_2 and (6A) Opx + Cc = Cpx + Fo + CO_2, where Do is dolomite and its solid solution, and Cc is magnesium calcite; for magnesite-harzburgite: (3) MC+ En = Fo + CO_2; for websterite + carbonate: (0) Mc + Cpx = Do + Opx and (01) Do + Cpx = Cc + Opx; and for carbonate-wehrlite: (9)Do + Cpx = Fo + Cc + CO_2. Conditions for the occurrence of dolomite(stoichiome trjc)-lherzolite are evaluated. Comparison of fossil geotherms deduced from kimbe rlite nodules with the phase diagrams for model harzburgite and lherzolite, and solidus curves with H_2O present, indicates that partially melted lherzolite may coexist with solid magnesiteharzburgite between about 175 and 195 km depth. Dissociation of magnesite could disrupt the harzburgite nodules during eruption, distributing low-calcium garnet through kimberlite

The effect of H_2O and CO_2 on planetary mantles

The solidus for peridotite-H_2O-CO_2 is a divariant surface traversed by univariant lines that locate the intersections of subsolidus divariant surfaces for carbonation or hydration reactions occurring in the presence of H_2O-CO_2 mixtures. Vapor phase compositions are normally buffered to these lines; the buffering capacity of carbonates is much greater than that of amphibole and phlogopite. Near the buffered curve for the solidus of partly carbonated peridotite, extending to higher pressures and lower temperatures from an invariant point near 26 kb-1200°C, there is a temperature maximum on the peridotite-vapor solidus. On the CO_2 side of the maximum, above 26 kb, CO_2/H_2O is greater in liquid than in vapor, and liquids are SiO_2-poor; on the H_2O side of this maximum (including all pressures below 26 kb), H_2O/CO_2 is greater in liquid than in vapor, and liquids change from forsterite-normative to quartz-normative with increasing H_2O/CO_2 in vapor. Even traces of H_2O and CO_2, in minerals or vapor, lower mantle solidus temperatures through hundreds of degrees compared with the volatile-free solidus

Melting relations

The process of magma generation with subsequent uprise and intrusion or extrusion of magma is one of the fundamental processes in the evolution of the earth. Rheological and other physical properties change markedly wherever and whenever partial melting occurs. The melting relations of minerals and rocks can now be measured in the laboratory to pressures corresponding to depths of more than 250 km. The measurements provide limits for temperatures within the earth and a basis for extrapolation to greater depths. This report outlines experimental results for the melting of elements, minerals, and rocks under various conditions, in a dry state or in the presence of water and other volatile components. There is overlap with reports on experimental petrology in another section, but it is reduced to a minimum by limiting this review to melting curves and properties, without attention to the more detailed aspects of petrogenesis

The origin of kimberlite

A new diapiric model for kimberlite genesis takes into account recent interpretations of peridotite-CO_2-H_2O melting relationships. A minor thermal perturbation at depth might trigger release of reduced vapors with major components C-H-O. Where these volatile components cross the estimated solidus boundary near 260 km, partial melting occurs, the density inversion causes diapiric uprise along adiabats, and the partially melted diapirs begin to crystallize at 100 to 80-km depth, where they reach a temperature maximum (thermal barrier) on the solidus. The released vapor enhances the prospects for crack propagation through overlying lithosphere in tension, and this could produce an initial channel to the surface. Magma separation could then occur from progressively greater depths, releasing CO_2-under-saturated kimberlitic magma for independent uprise through the established conduit, quite unaffected by the thermal barrier on the solidus of peridotite-CO_2-H_2O. Cooler diapirs would cross the solidus at somewhat greater depth, solidifying to phlogopite-dolomite-peridotite with the release of aqueous solutions. These solutions are likely candidates for the mantle metasomatism commonly considered to be a precursor for the generation of kimberlites and other alkalic magmas. According to this model the metasomatism is a consequence of kimberlite magmatism rather than its precursory cause

The nature of the Mohorovicic discontinuity, A compromise

The available experimental data and steady-state calculations make it difficult to explain the M discontinuity beneath both oceans and continents on the basis of the same phase change. The oceanic M discontinuity may be a chemical discontinuity between basalt and peridotite, and a similar chemical discontinuity may thus be expected beneath the continents. Since available experimental data place the basalt-eclogite phase change at about the same depth as the continental M discontinuity, intersections may exist between a zone of chemical discontinuity and a phase transition zone, the transition being either basalt-eclogite or feldspathic peridotite-garnet peridotite. Detection of the latter transition by seismic techniques may be difficult. The M discontinuity could therefore represent the basalt-eclogite phase change in some localities (e.g. mountain belts) and the chemical discontinuity in others (e.g. oceans and continental shields). Variations in the depth to the chemical discontinuity and in the positions of geoisotherms produce great flexibility in orogenetic models. Intersections between the two zones at depth could be reflected at the surface by major fault zones separating large structural blocks of different elevations

The System CaO-MgO-FeO-SiO_2 and its Bearing on the Origin of Ultrabasic and Basic Rocks

Experimental data in the system CaO-MgO-FeO-SiO_2 suggest that there may be a plateau on the liquidus and solidus of the multicomponent system basalt-peridotite. If this is so, fusion of peridotite would produce only basaltie magmas over a wide temperature range; when the temperature reached a value such that the liquid crossed the threshold of the plateau, there would be a rapid increase in the amount of fusion for small temperature increases, with the formation of picritic magmas; basaltic magmas containing suspended forsteritic olivine crystals could dissolve them if the temperature rose slightly above that of the plateau threshold; a high proportion of a picritic magma would crystallize in a small temperature interval, with the precipitation of forsteritic olivine that was only slightly zoned. These possibilities are compared with current theories, and it is concluded that several petrological axioms may require critical re-examination. An experimental procedure is outlined to determine the shape of the liquidus and solidus in the basalt-peridotite system

Joseph V. Smith 1928–2007 [Obituary]

Joseph V. Smith was born on the 30th of July 1928, in Derbyshire, England. He married Brenda Wallis at Crich, Derbyshire, on the 31st of August, 1951, moved to the USA, and their family grew with two daughters, Virginia and Susan. He retired in September 2005 as the Louis Block Professor Emeritus in Geophysical Sciences and the College at the University of Chicago. On Friday the 6th of April, 2007, at age 78, he died of pneumonia at Beth Israel Deaconess Medical Center in Boston. Parkinson’s disease had begun to take its cruel toll about five years earlier. He and his wife Brenda moved to Brookline in 2005 to be near their daughter, Virginia, and family, where he suffered a broken hip and several heart attacks before the final event. In the meantime, he continued to write an autobiographical book Living Safely which dealt with local and global problems facing our species. As Brenda said: “He was very strong and very stoic. He handled any difficulties in life the way he handled his illness.

Water-spouts on the Britannia Gletscher, north-east Greenland

Wiseman's (1963) letter to this Journal describing a water-spout on the Aletsch Gletscher reminded me of the water-spouts encountered by members of the British North Greenland Expedition (Simpson, 1955) near the snout of the Britannia Gletscher in the summer of 1954, and prompted me to exhume two photographs from my files (Figs. 1 and 2). These water-spouts were not intermittent like those described by Wiseman (1963) and Rucklidge (1956), but were continuous gushers lasting for several days, and forming an integral part of the drainage pattern of the glacier. They are thus more akin to the spouts described by Glen (1941), who stressed the role of crevasses in englacial and subglacial drainage and stated that sometimes the water carried in this way from higher levels "attains such a pressure that it literally bursts its way through the ice, sending up a small water-spout which may continue for as a long as an hour, then dying down into a more gentle fountain"

The Effect of 'Impure' Pore Fluids on Metamorphic Dissociation Reactions

Comparison of experimental data from the systems MgO-CO_2-H_2O (closed) and MgO-CO_2-A (simulating an open system) shows that the effects of H_2O and A on the dissociation of magnesite are almost identical; both behave as inert components reducing the partial pressure of CO_2. The dissociation temperature at constant total pressure is lowered according to the proportion of inert volatiles in the initial vapour phase. The dissociation is completed at one temperature (univariant) in an open system but in a closed system it proceeds through a temperature interval (divariant) because the vapour phase changes composition. The amount of dissociation remains small until the upper limit of the interval is reached. More complex dissociation reactions in the systems CaO-MgO-CO_2-H_2O and CaO-SiO_2-CO_2-H_2O are described; they follow similar patterns. Under closed or partially open metamorphic conditions non-reacting pore fluid components (inert) have to be treated as one additional component for application of the mineralogical phase rule. Comparison of the pattern of metamorphic parageneses with the patterns of reactions occurring under known experimental conditions may provide information about metamorphic processes. Metamorphic reactions can be represented within a petrogenetic model with axes P, T, and pore fluid composition varying between H_2O and CO_2