Origin of the Earth and Moon by A. E. Ringwood - Book Review

As awesome as our ancestors must have found the Biblical explanation of the Earth's origin, few are likely to have imagined anything as spectacular as the scenario for the formation of the Earth and moon described here. The author envisions the Earth accreting from a collection of objects ranging in size from dust particles up to small planets 1,000 km in radius. As the protoearth and its gravitational field grew, so too did the kinetic energy of the incoming objects and the violence of their impacts with the growing Earth. Vaporization of the volatile constituents of the incoming bodies during these impacts led to the growth of a massive primitive atmosphere that interacted with accreting material

Theoretical petrology

The central issues in petrology have remained remarkably unchanged in the last 50 years. In igneous petrology, the focus is on understanding the nature and cause of diversity in igneous rocks: on identifying primary magma types and constraints on the compositional and mineralogical characteristics, the physical conditions, and the evolutions of their source regions and on establishing the processes by which derivative magmas evolve from primary magmas. In metamorphic petrology, the major concern is with understanding the conditions and processes experienced by a rock in reaching its present state. In both igneous and metamorphic petrology, the ultimate goal is the integration of petrological constraints with those from other branches of earth science into regional and global theories of earth history. What has changed over the years, however, is the framework within which these issues are addressed: the backdrop provided by plate tectonics and geophysical constraints, the growing sophistication of chemical and physical models of rock systems, the ever increasing inputs from trace element and isotopic geochemistry, the sophistication and complexity of experimental approaches to petrological problems, and the growing body of detailed petrological studies of specific rock suites and associations from all over the world. What I will attempt in this report is to pinpoint and briefly review those areas of growing interest and emphasis in American efforts in petrology during the 1975–1978 quadrennium and the ways in which they were shaped by this framework

Geochemical Consequences of Melt Percolation: The Upper Mantle as a Chromatographic Column

As magmas rise toward the surface, they traverse regions of the mantle and crust with which they are not in equilibrium; to the extent that time and the intimacy of their physical contact permit, the melts and country rocks will interact chemically. We have modeled aspects of these chemical interactions in terms of ion-exchange processes similar to those operating in simple chromatographic columns. The implications for trace element systematics are straightforward: the composition of melt emerging from the top of the column evolves from close to that of the incipient melt of the column matrix toward that of the melt introduced into the base of the column. The rate of evolution is faster in the incompatible than the compatible elements and, as a result, the abundance ratios of elements of different compatibilities can vary considerably with time. If diffusion and other dispersive processes in the melt are negligible and if exchange between melt and solid rock is rapid, extreme fractionations may occur, and the change from initial to final concentration for each element can be through an abrupt concentration front. Integration and mixing of the column output in a magma chamber or dispersive processes within the column, in particular the incomplete equilibration between matrix and fluid due to the slow diffusion in the solid phases, may lead to diffuse fronts and smooth trace element abundance patterns in the column output. If the matrix material is not replenished, the chromatographic process is a transient phenomenon. In some geological situations (e.g., under island arcs and oceanic islands), fresh matrix may be fed continuously into the column, leading to the evolution of a steady state. Aspects of the geochemistry of ultramafic rocks, island arc lavas, and comagmatic alkaline and tholeiitic magmas may be explained by the operation of chromatographic columns

A Thermodynamic Model for Hydrous Silicate Melts

A simple thermodynamic model describing hydrous silicate melts has been applied to the systems albite-, diopside-, and silica-H_2O. The model is based on the assumption of ideal mixing of hydroxyl groups, H_2O molecules, and oxygens in the melt. Calculated and experimentally determined freezing-point depressions and H_2O solubilities for these systems are in agreement over substantial pressure and temperature intervals. The success of this model in accounting for observed phase equilibria of hydrous systems and its consistency with spectroscopic measurements of the concentrations of H-bearing species in glasses suggests that it accurately represents the interaction between H_2O and silicate melts at a molecular level

On the nature of pressure‐induced coordination changes in silicate melts and glasses

Progressive decreases in the Si‐O‐Si angles between corner‐shared silicate tetrahedra in glasses and melts with increasing pressure can lead to arrangements of oxygen atoms that can be described in terms of edge‐ or face‐shared octahedra. This mechanism of compression can account for the gradual, continuous increases in melt and glass densities from values at low pressure that indicate dominantly tetrahedral coordination of Si to values at several tens of GPa that suggest higher coordination. It also can explain the unquenchable nature of octahedrally coordinated Si in glasses, the absence of spectroscopically detectable octahedrally coordinated Si in glasses until they are highly compressed, the gradual and reversible transformation from tetrahedral to octahedral coordination in glasses once the transformation is detectable spectroscopically, and the fact that this transformation takes place in glass at room temperature. It may also have relevance to pressure‐induced transformations from crystalline to glassy phases, the difficulty in retrieving some metastable high pressure crystalline phases at low pressure, and the observed differences between the pressures required for phase transformations in shock wave experiments on glasses and crystals

Measurement of water in rhyolitic glasses; calibration of an infrared spectroscopic technique

A series of natural rhyolitic obsidians were analyzed for their total water contents by a vacuum extraction technique. The grain size of the crushed samples can significantly affect these analyses. Coarse powders must be used in order to avoid surface-correlated water. These analyses were used to calibrate infrared spectroscopic measurements of water in glass using several infrared and near-infrared absorption bands. We demonstrate that infrared spectroscopy can yield precise determinations of not only total dissolved water contents, but also the concentrations of individual H-bearing species in natural and synthetic rhyolitic glasses on spots as small as a few tens of micrometers in diameter

A Numerical Treatment of Melt/Solid Segregation: Size of the Eucrite Parent Body and Stability of the Terrestrial Low-Velocity Zone

Crystal sinking to form cumulates and melt percolation toward segregation in magma pools can be treated with modifications of Stokes' and Darcy's laws, respectively. The velocity of crystals and melt depends, among other things, on the force of gravity (g) driving the separations and the cooling time of the environment. The increase of g promotes more efficient differentiation, whereas the increase of cooling rate limits the extent to which crystals and liquid can separate. The rate at which separation occurs is strongly dependent on the proportion of liquid that is present. As a result, cumulate formation is a process with a negative feedback; the more densely aggregated the crystals become, the slower the process can proceed. In contrast, melt accumulation is a process with a positive feedback; partial accumulation of melt leads to more rapid accumulation of subsequent melt. This positive feedback can cause melt accumulation to run rapidly to completion once a critical stability limit is passed. The observation of cumulates and segregated melts among the eucrite meteorites is used as a basis for calculating the g (and planet size) required to perform these differentiations. The eucrite parent body was probably at least 10-100 km in radius. The earth's low velocity zone (LVZ) is shown to be unstable with respect to draining itself of excess melt if the melt forms an interconnecting network. A geologically persistent LVZ with a homogeneous distribution of melt can be maintained with melt fractions only on the order of 0.1% or less

Deep drilling into a Hawaiian volcano

Hawaiian volcanoes are the most comprehensively studied on Earth. Nevertheless, most of the eruptive history of each one is inaccessible because it is buried by younger lava flows or is exposed only below sea level. For those parts of Hawaiian volcanoes above sea level, erosion typically exposes only a few hundred meters of buried lavas (out of a total thickness of up to 10 km or more).Available samples of submarine lavas extend the time intervals of individual volcanoes that can be studied. However, the histories of individual Hawaiian volcanoes during most of their ~1-million-year passages across the zone of melt production are largely unknown