J. F. Hays

An interlaboratory comparison of piston-cylinder pressure calibration using the albite-breakdown reaction.

The pressure of the reaction albite=jadeite and quartz was measured at 600° C by workers in six geophysical laboratories for the purpose of comparing pressure calibration procedures for the solid-pressure piston-cylinder apparatus. All groups used the same starting mix of crystalline reactant and products and all obtained hydrothermal reversals of the equilibrium. Solid pressure media used included talc, NaCl, boron nitride, pyrophyllite, pyrex glass and crushable ceramic. Various means of calibration were used, including internal standardization by transitions in indicator substances and the piston-in, piston-out bracketing method.There was agreement among all groups—the average preferred value of 16.3 kilobars at 600° C is enclosed by all of the error brackets assigned by the various investigators. This average preferred value is lower by nearly two kilobars than the often-quoted extrapolation of Birch and LeComtes data (1960). It will be important for both petrology and high-pressure technology to test this result in a very high gas pressure apparatus.

The distribution of Fe and Mg between olivine and lunar basaltic liquids

Abstract We have examined the Fe and Mg distribution between coexisting olivine and lunar basaltic liquids produced by equilibrium partial melting of natural lunar samples. In agreement with the findings of Roeder and Emslie ( Contrib. Mineral. Petrol . 29 , 275–289) on terrestrial compositions, the logarithms of the conventional distribution coefficients, K ol - L Fe and K ol - L Fe - Mg , are nearly linear functions of inverse temperature; and the exchange coefficient, K D = K ol - L Fe - Mg , is nearly independent of temperature and composition within a given magma group. There are, however, small but significant differences in conventional and exchange distribution coefficients from one magma group to another, e.g. low-Ti vs high-Ti lunar basalts. It is possible to achieve slightly greater precision for the inverse temperature functions by including terms approximating silica activity in the conventional distribution coefficients. The term ( 2Si O ) L is apparently the best simple approximation for silica activity in olivine-saturated liquids based upon data for Fe, Mg, Mn, Ca, Ti and Cr. Pressure has noticeable effects upon Fe and Mg distribution between olivine and liquid only above 5 kbar. The excellent linear correlation of the logarithms of the distribution coefficients with inverse temperature allows calculation of approximate values of Δ H 0 for the reactions : 2 MgO L + SiO 2 L ai Mg 2 SiO 4 ol and 2 FeO L + SiO 2 L ai Fe 2 SiO 4 ol . Values obtained, approx −26 kcal/mole, are comparable with values of the heats of fusion of forsterite and fayalite calculated by Bradley ( Am. J. Sci. 260 , 550–554) and measured by Orr ( J . Am . Chem . Soc . 75 , 528–529). The exchange distribution coefficient for Fe and Mg, K D , is sensitive to large changes in liquid chemistry. Although K D is explicitly independent of silica activity, K D apparently changes with silica concentration. This change is a reflection of changes in the mixing properties of Fe and Mg in liquids with different chemistry and hence structure. Regular solution theory predicts that as the mixing properties of an element in a solution change, the most radical changes in activity coefficients occur in the range of dilute concentrations. Therefore, the distribution coefficients for trace elements will also be dependent upon large changes in liquid chemistry, even if corrections for silica and other liquid component activities are applied.

Crystallization history of lunar picritic basalt sample 12002 - Phase-equilibria and cooling-rate studies

Experimental crystallization of a lunar picrite composition (sample 12002) at controlled linear cooling rates produces systematic changes in the temperature at which crystalline phases appear, in the texture, and in crystal morphology as a function of cooling rate. Phases crystallize in the order olivine, chromium spinel, pyroxene, plagioclase, and ilmenite during equilibrium crystallization, but ilmenite and plagioclase reverse their order of appearance and silica crystallizes in the groundmass during controlled cooling experiments. The partition of iron and magnesium between olivine and liquid ( K D = 0.33) is independent of cooling rate (0.5° to 2000°C/hr), temperature (1325° to 600°C), and pressure (0 to 12 kb). Comparison of the olivine nucleation densities in the lunar sample and in the experiments indicates that the sample began cooling at about l°C/hr. Pyroxene size, chemistry, and growth instability spacings (“swallowtails”), as well as groundmass coarseness, all suggest that the cooling rate subsequently decreased by as much as a factor of 10 or more. The porphyritic texture of this sample, then, is produced at a decreasing, rather than a discontinuously increasing, cooling rate.

Origin of lunar feldspathic rocks

Melting experiments and petrographic studies of lunar feldspathic rocks reveal possible genetic relationships among several compositionally and mineralogically distinct groups of lunar rocks and soil fragments. Dry, low PO_2 partial melting of crustal anorthositic norites of the anorthositic-noritic-troctolitic (ANT) suite produces liquids of the KREEP-Fra Mauro basalt type; dry, low PO_2 partial melting of pink spinel troctolite (PST) produces liquids of the “very high alumina basalt” or microtroctolite type. Both ANT and PST are probable components of the primitive terra crust. If crystal fractionation in a cooling basaltic liquid could have produced such a crust, it would also produce a mafic interior capable of yielding mare basalts by later remelting at depth.

Origin of titaniferous lunar basalts

Delineation of low pressure phase equilibria in the composition space relevant to titaniferous lunar basalts demonstrates a significant degree of control by those equilibria on the compositions of the basalts. The existence of two distinct chemical groups of basalts (high and low-K) which cannot be related one to the other by fractional crystallization at any pressure, suggests that melting is responsible for the two groups. Consideration of the pressure shift required to produce the differences between groups constrains magma segregation to have occurred in the outer 150 km of the Moon. It is difficult to relate low-Ti and high-Ti basalts to the same source region. The preferred source region of high-Ti basalts, based on phase equilibrium considerations, is a late ilmenite-rich cumulate produced from the residual liquid of the primordial differentiation of the outer portions of the Moon. This ilmenite-rich layer is sandwiched between the lunar feldspathic crust and a complementary mafic cumulate.

A petrogenetic model of the relationships among achondritic meteorites

The basaltic achondrite, shergottite, nakhlite, and chassignite meteorites appear to define a petrological and geochemical sequence. Assuming that they developed from basaltic liquids produced by low pressure partial melting of plagioclase peridotites, their petrological and chemical distinctions can be understood in terms of the compositional differences between their source periodites. The source regions of basaltic achondrite magmas were alkali-poor, metal-bearing peridotites in which pigeonite and/or orthopyroxene was the only pyroxene. By simultaneously increasing the ratio of high-Ca pyroxene to low-Ca pyroxene, the alkali content of the feldspar, the oxidation state, and the overall volatile content of the basaltic achondrite source peridotite, peridotites capable of yielding the parent liquids of the shergottites can be produced. Further increases can produce peridotites capable of yielding the parent liquids of the nakhlites and chassignites. Addition of a volatile-rich component to the volatile-poor type of peridotite required for the source regions of the eucrites appears to be capable of producing the required series of peridotites. Alternatively, progressive volatile-loss from a volatile-rich material, possibly of roughly cosmic composition, could have produced this sequence of peridotites. A simple two-component model of planetary compositions is, to a first approximation, consistent with the petrology and chemistry of these igneous meteorite groups.

Dependence of growth rate of quartz in fused silica on pressure and impurity content

We have measured the effects of pressure, temperature, and some variations in impurity content on the growth rate u of quartz into fused silica. Under all conditions the growth rate was interface controlled and increased exponentially with pressure with an activation volume averaging −21.2 cm3/mole. The activation enthalpy for all specimens extrapolated to a zero pressure value of 64 kcal/mole, within the experimental uncertainty. At a given stoichiometry the effect of hydroxyl content on growth rate is described entirely by a linear term COH in the prefactor of the equation for the growth rate. The effect of chlorine impurity can be described similarly. Also u is increased as the ideal stoichiometry is approached from the partially reduced state.

The mechanism of growth of quartz crystals into fused silica

It is proposed that the growth of quartz crystals into fused silica is effected by a mechanism involving the breaking of an Si‐O bond and its association with an OH group, followed by cooperative motion of the nonbridging oxygen and the hydroxyl group which results in the crystallization of a row of several molecules along a crystalline‐amorphous interfacial ledge. This mechanism explains, at least qualitatively, all the results of our earlier experimental study of the dependence of quartz crystal growth upon applied pressure: large negative activation volume; single activation enthalpy below Si‐O bond energy; growth velocity constant in time, proportional to the hydroxyl and chlorine content, decreasing with increasing degree of reduction, and enhanced by nonhydrostatic stresses; lower preexponential for the synthetic than for the natural silica.

Crystal Growth Kinetics of Boron Oxide Under Pressure

We have measured the crystal growth rate u of B2O3‐I in the amorphous phase, as it varied over five orders of magnitude with changes in temperature and pressure. We eliminated the crystal nucleation barrier by seeding the surface of boron oxide glass with crystals. u became measurable only when the pressure exceeded a threshold level near 10 kbar. Using the published thermodynamic information on the B2O3 system and a crude free‐energy model for the crystal and glass phases, we account qualitatively for our results with the theory of crystal growth limited by the rate of two‐dimensional nucleation of monolayers. The constants for the prefactor, activation energy, activation volume, and ledge tension are determined by fitting. By adjusting the thermodynamic parameters to a set of values that are well within the ranges delineated by their experimental uncertainties, we account quantitatively for the measured growth rates from 300 to 500 °C and from 0 to 30 kbar with the following relation: u(T,P)=(785 m/s)[‖...

Plagioclase flotation and lunar crust formation

Anorthitic plagioclase floats in liquids parental to the lunar highlands crust. The plagioclase enrichment that is characteristic of lunar highlands rocks can be the result of plagioclase flotation. Such rocks would form a gravitationally stable upper crust on their parental magma.