Ian D. MacGregor

Oxygen isotope ratios in eclogites from kimberlites.

The oxygen isotope compositions (δ18O) of eclogitic xenoliths from the Roberts Victor kimberlite range from 2 to 8 per mil relative to SMOW (standard mean ocean water). This surprising variation appears to be due to fractional crystallization: the eclogites rich in oxygen-18 represent early crystal accumulates; the eclogites poor in oxygen-18 represent residual liquids. Crystal-melt partitioning probably exceeded 3 per mil and is interpreted to be pressure-dependent. Anomalous enrichment of oxygen-18 in cumulate eclogites relative to ultramafic xenoliths suggests that crystal-melt partitioning increased after melt-formation but prior to crystallization.

Chromite spinels from ultramafic xenoliths

Abstract The spinels in ultramafic xenoliths from kimberlites and alkali olivine basalts show a wide range of compositional variation, far in excess of the ranges shown by their coexisting silicate phases. The chemical variation of the spinels is a function of the host magma, texture and depth of origin. The spinels occur in five textural types: 1. (a) euhedral spinels restricted only to the kimberlites; 2. (b) spinels intergrown with silicates commonly found in kimberlites; 3. (c) exsolved spinel from orthopyroxene in xenoliths from both alkali olivine basalt and kimberlite; 4. (d) interstitial spinel texture generally restricted to xenoliths from alkali olvine basalts; 5. (e) spinels in kelyphitic rims around garnet in xenoliths from kimberlites. The chemistry of the spinels varies systematically with texture. The highest Cr (Cr + Al) ratios and lowest Mg (Mg + Fe 2+ ) ratios occur in the euhedral spinels, whereas the spinels showing intergrowth and exsolution textures are compositionally intermediate. Spinels from kimberlite xenoliths have a higher and wider range of the ratio Fe 3+ (Cr + Al + Fe 3+ ) than the spinels from alkali olivine basalt xenoliths. The ratio Cr (Cr + Al + Fe 3+ ) increases with pressure or depth of origin of the xenoliths. The Al2O3 content of the spinels varies sympathetically with the Al2O3 content of the coexisting orthopyroxenes, the value being lowest for euhedral spinel-orthopyroxene pairs in kimberlite xenolith and highest for interstitial spinel-orthopyroxene pairs in alkali basalt xenolith. The wide range of chemistry of the spinels and their correspondence with geologic environment suggest that the chromite spinels from ultramafic xenoliths are particularly sensitive minerals in examining the environmental conditions of the Earths upper mantle.

Thermal Structure of the Lithosphere: A Petrologic Model

A preliminary evaluation of the thermal history of the upper mantle as determined by petrologic techniques indicates a general correspondence with theoretically derived models. The petrologic data supply direct information which may be used as an independent calibration of calculated models, serve as a base for evaluating the assumptions of the theoretical approach, and allow more careful selection of the variables describing mantle thermal properties and processes. Like the theoretical counterpart, the petrological approach indicates that the lithosphere is dominated by two thermal regimes: first, there is a continental regime which cools at rates of the order of 109 years and represents the longterm cooling of the earth. Secondly, superimposed on the continental evolution is the thermal event associated with the formation of an oceanic basin, and which may be thought of as a 108 year convective perturbation on the continental cycle. Of special interest is petrologic evidence for a sudden steepening of the thermal gradients across the lithosphere-asthenosphere boundary not seen in the theoretical models. The unexpected change of slope points to the need for a critical reevaluation of the thermal processes and properties extant in the asthenosphere. The potential of the petrologic contribution has yet to be fully realized. For a start, this article points to an important body of independent evidence critical to our understanding of the earths thermal history.

Petrologic and thermal structure of the upper mantle beneath South Africa in the cretaceous

Abstract Assignment of equilibration temperatures and pressures to ultramafic xenoliths from South African kimberlites illustrates that they have been derived from a wide range of depths. The distribution predicates against a cognate hypothesis and indicates that the xenoliths are accidental fragments of the mantle transported to the surface by the kimberlite magma. Suites of xenoliths from single localities show well-defined trends marking Cretaceous geothermal gradients. In general the gradients are comparable to the steady-state model of Clark and Ringwood (1964). The gradients are steepest in the South West Africa region and decrease successively in the Lesotho and Kimberley regions. In all regions there is a sudden steepeing of the interpreted geothermal gradient correlating with presence of intensively sheared xenoliths. It appears that the associated change of slope and textural type marks the top of the low-velocity zone under the shield, giving lithospheric thicknesses which increase from 140 km to 180 and 195 km in the South West Africa, Lesotho and Kimberley regions, respectively. The maximum depth of xenoliths in any locality suggests that kimberlites are derived from minimum depths of 150, 190 and 200 km in the South West Africa, Lesotho and Kimberley regions, respectively. In general, spinel-bearing xenoliths give place to spinel-plus-garnet- and garnet-bearing xenoliths with increasing depth. The boundaries are diffuse and dependent on compositional variations. Major textural breaks correlate with the disappearance of phlogopite and a peridotite H 2 O CO 2 saturated solidus. The phlogopite dehydration and melting reactions mark boundaries of increasing deformation, suggesting that these reactions are critical in defining the rheid properties of the mantle.

Eclogites, Pyroxene Geotherm, and Layered Mantle Convection

Temperatures of equilibration for the majority (81 percent) of the eclogite xenoliths of the Roberts Victor kimberlite pipe in South Africa range between 1000� and 1250�C, falling essentially on the gap of the lower limb of the subcontinental inflected geotherm derived from garnet peridotite xenoliths. In view of the Archean age (>2.6 x 109 years) of these eclogites and their stratigraphic position on the geotherm, it is proposed that the inflected part of the geotherm represents the convective boundary layer beneath the conductive lid of the lithospheric plate. The gradient of 8 Celsius degrees per kilometer for the inflection is characteristic of a double thermal boundary layer and suggests layered convection rather than whole mantle convection for the earth.

Mineralogy, Petrology, and Surface Features of Lunar Samples 10062,35, 10067,9, 10069,30, and 10085,16

The primary rocks are a sequence of titanium-rich basic volcanics, composed of clinopyroxene, plagioclase, and ilmenite with minor olivine, troilite, and native iron. The soil and microbreccias are respectively loose and compacted mixtures of fragments and aggregates of similar rocks, minerals, and glassy fragments and spheres. Impact events are reflected by the presence of shock metamorphosed rock fragments, breccias, and glasses and their resulting compaction to form complex breccias, glass-spattered surfaces, and numerous glass-lined craters. Chemistry of the glasses formed by the impact events is highly variable, and the high iron and nickel content of a few moundlike features suggests that at least some of the projectiles are iron and nickel-rich meteorites.

Petrogenesis of the Mount Albert Ultramafic Massif, Quebec

Ultramafic massifs that occur in the Earth9s crust are, in a complex way, samples of the mantle. They may be classified into a number of different types (Wyllie, 1970; Moores, 1973), one of which is the high-temperature peridotite massifs characterized by high-temperature contact metamorphic aureoles. One such example is the Mount Albert ultramafic intrusion in the Gaspe Peninsula of Quebec, Canada. The high-temperature peridotites are composed of interlayered harzburgite, dunite, and lherzolite. The mineralogy is generally simple and lacks the cryptic layering generally found in comparable mineral systems in cumulate layered mafic-ultramafic intrusions (Wager and, Brown, 1967; Jackson, 1971). Compositional variations do occur, however, and the question arises as to whether they are related to the mechanism of intrusion or inherited mantle properties. Textural and structural evidence (Darot, 1973) indicate that some high-temperature peridotites have been intruded as solid material. The present study focuses on the mechanism of intrusion and the results of intrusion on the variations of chemistry hetween coexisting phases and observed textural and structural features.

Empirical geothermometers and geothermobarometers for spinel peridotite phase assemblages

Experimental synthesis of spinel peridotite phase assemblages for a range of compositions that mimic natural samples is used to derive a set of empirical geothermometers and geothermobarometers represented by multiple linear regression best-fit surfaces that link the variables of temperature, pressure, and composition. The calibrated geothermometers use reactions that govern the solubility of Al and Cr in both pyroxenes and the Mg–Fe exchange between silicates and spinel. Geothermobarometers map the Mg–Fe exchange between coexisting olivine and clinopyroxene and pyroxenes and Ca–Mg exchange between coexisting pyroxenes. Application of the geothermometers and geothermobarometers to suites of naturally occurring samples indicates that while reactions governing the Cr and Al solubility and solvus of orthopyroxene give useful estimates of ‘original’ mantle temperatures and pressures, respectively, comparable reactions for clinopyroxene yield estimates that are variably dependent on the transport phase of the sample suites. Temperature and pressure estimates from reactions governing Mg and Fe exchange between silicates and spinel and coexisting silicates are all sensitive to the later transport stage of the samples.

Natural Distribution of Metals and Some Economic Effects

The concentration of metals into ore bodies amenable to exploitation results from the integration of geological processes occurring over billions of years. The heritage of wealth represents a nonrenewable resource which is eventually depleted, usually in periods of less than a few hundred years. Examples from the industrialized nations show a typical cycle of production rising to a peak and eventually dropping to zero. On a world scale, a comparable history is predicted, and estimates indicate that few metals have total reserves that can extend beyond 200 years. The limit in metal resources poses a threat to the consumer-oriented societies now dominant and increasing in the world. Technical advice can predict the shortages and adequately pose short term solutions. However, viewed from a longer time base, the only solution to eventual metal shortages is through a major restructuring of the socio-economic base of consumer-oriented societies.

New visiting scientists in NSF's Earth sciences

The National Science Foundations Division of Earth Sciences has hired two new rotators to serve as program directors, as part of the ongoing visiting scientists program. The new directors are Jonathan Fink in Geochemistry and Petrology, and L. Douglas James in Hydrological Sciences. Fink has exchanged roles for 1 year with NSFs John Snyder, who is on sabbatical at Arizona State University. Finks current research includes studies of how the Theological properties of magma govern the emplacement of volcanic domes and lava flows, and the gravitational control on their mass movements. This research extends to the mechanisms of igneous intrusion and interpretation of volcanic features in extraterrestrial and submarine environments.