Robin Brett

Melting relations in the Fe-rich portion of the system FezFeS at 30 kb pressure

Abstract The melting relations of FezFeS mixtures covering the composition range from Fe to Fe 67 S 33 have been determined at 30 kb pressure. The phase relations are similar to those at low pressure. The eutectic has a composition of Fe 72.9 S 27.1 and a temperature of 990°C. Solubility of S in Fe at elevated temperatures at 30 kb is of the same order of magnitude as at low pressure. Sulfur may have significantly lowered the melting point of iron in the upper mantle during the period of coalescence of metal prior to core formation in the primitive earth.

Chemical equilibration of the Earth's core and upper mantle

The oxygen fugacity (fO2) of the Earths upper mantle appears to lie somewhat above that of the iron-wustite buffer, its fO2 is assumed to have been similar to the present value at the time of core formation. In the upper mantle, the Fe-rich liquid protocore that would form under such conditions of fO2 at elevated temperatures would lie predominantly in the system Fe-S-O. Distribution coefficients for Co, Cu, Ni, Ir, Au, Ir, W, Re, Mo, Ag and Ga between such liquids and basalt are known and minimum values are known for Ge. From these coefficients, upper mantle abundances for the above elements can be calculated by assuming cosmic abundances for the whole Earth and equilibrium between the Fe-S-O protocore and upper mantle. These calculated abundances are surprisingly close to presently known upper mantle abundances; agreements are within a factor of 5, except for Cu, W, and Mo. Therefore, siderophile element abundances in the upper mantle based on known distribution coefficients do not demand a late-stage meteoritic bombardment, and a protocore formed from the upper mantle containing S and O seems likely. As upper mantle abundances fit a local equilibrium model, then either the upper mantle has not been mixed with the rest of the mantle since core formation, or else partition coefficients between protocore and mantle were similar for the whole mantle regardless of P, T, and fO2. The latter possibility seems unlikely over such a P-T range.

THE CRETACEOUS-TERTIARY EXTINCTION : A LETHAL MECHANISM INVOLVING ANHYDRITE TARGET ROCKS

The Chicxulub Crater, Yucatan, Mexico, is a leading contender as the site for the impact event that caused the Cretaceous-Tertiary (K-T) extinctions. A considerable thickness of anhydrite (CaSO4) forms part of the target rock. High temperatures resulting from impact would drive SO2 off from the anhydrite. Hundreds of billions of tonnes of sulfuric acid aerosol would thus enter the stratosphere and cause considerable cooling of the Earths surface, decrease photosynthesis by orders of magnitude, deplete the ozone layer, and permit increased UV radiation to reach the Earths surface. Finally, the aerosol would fall back to Earth as acid rain and devastate land and some lacustrine biota and near-surface marine creatures. The presence of anhydrite in the Chicxulub target rock may thus help explain the many extinctions observed at the K-T boundary.

Intrinsic oxygen fugacity measurements on seven chondrites, a pallasite, and a tektite and the redox state of meteorite parent bodies

Abstract Intrinsic oxygen-fugacity (fO2) measurements were made on five ordinary chondrites, a carbonaceous chondrite, an enstatite chondrite, a pallasite, and a tektite. Results are of the form of linear log f O 2 − 1 T plots. Except for the enstatite chondrite, measured results agree well with calculated estimates by others. The tektite produced fO2 values well below the range measured for terrestrial and lunar rocks. The lowpressure atmospheric regime that is reported to follow large terrestrial explosions, coupled with a very high temperature, could produce glass with fO2 in the range measured. The meteorite Salta (pallasite) has low fO2 and lies close to Hvittis (E6). Unlike the other samples, results for Salta do not parallel the iron-wustite buffer, but are close to the fayalite-quartz-iron buffer in slope. Minor reduction by graphite appears to have taken place during metamorphism of ordinary chondrites. fO2 values of unequilibrated chondrites show large scatter during early heating suggesting that the constituent phases were exposed to a range of fO2 conditions. The samples equilibrated with respect to fO2 in relatively short time on heating. Equilibration with respect to fO2 in ordinary chondrites takes place between grades 3 and 4 of metamorphism. Application of P − T − fO2 relations in the system C-CO-CO2 indicates that the ordinary chondrites were metamorphosed at pressures of 3–20 bars, as it appears that they lay on the graphite surface. A steep positive thermal gradient in a meteorite parent body lying at the graphite surface will produce thin reduced exterior, an oxidized near-surface layer, and an interior that is increasingly reduced with depth; a shallow thermal gradient will produce the reverse. A body heated by accretion on the outside will have a reduced exterior and oxidized interior. Meteorites from the same parent body clearly are not required to have similar redox states.

A lunar rock of deep crustal origin: sample 76535

Abstract Lunar sample 76535 is a coarse-grained troctolitic granulite exhibiting a texture indicative of long annealing times. It is composed of homogeneous crystals of plagioclase (58 per cent, An96), olivine (37 per cent, Fo88) and bronzite (4 per cent, En86). Chromian spinel-bronzite-diopside (Wo46En50Fs4) symplectic intergrowths commonly occur along olivine-plagioclase boundaries and as tiny inclusions within olivine grains. These symplectites apparently formed by a reaction of the type: OI + An + Chromite → Opx + Cpx + Al-Mg-chromite . The reaction is related to the experimentally determined reaction OI + An = Opx + Cpx + Sp of Kushiro and Yoder (1966). The enstatite content of the diopside coexisting with the bronzite indicates equilibration at about 1000°C. Thermodynamic calculations for 1000°C indicate that the symplectites formed at a minimum pressure of about 0.6 kb. Low alumina contents of the pyroxenes indicate equilibration near this minimum pressure. Clusters of the same assemblage found in the symplectic intergrowths, but containing accessory metal, troilite, Ca-phosphates, baddeleyite, plagioclase and/or K-feldspar occur sporadically throughout the rock. These apparent late stage products crystallized in the low temperature-high pressure region discussed above. Phase relations of co-existing metal phases indicate that the rock cooled at a few tens of degrees/my, corresponding to depths of 10–20 km below the lunar surface, in agreement with the above pressure estimate. We infer that 76535 represents an original cumulate deposited at a depth between about 10 and 30 km. The last liquid crystallized in the relatively high pressure-low temperature field opx + cpx + Al-Mg-chromite. Cooling was extremely slow and accompanied by extensive chemical and textural re-equilibration. Reaction to form the symplectites occurred during the late stages of re-equilibration.

The earth's core - Speculations on its chemical equilibrium with the mantle

Abstract A review of the literature indicates that a reasonable estimate of the composition of the earths core is iron with Ni0–5, Si10–25 (wt.%). Thermodynamic calculations and comparison of chondritic with terrestrial abundances indicate that 1 wt.% each of Mn, P, and Cr might also be present. A core of this composition was probably in chemical equilibrium with the mantle at the time of core formation because: 1. (1) The reactions 2Fe + SiO2 = 2FeO + Si and Fe2SiO4 + 2Ni = Ni2SiO4 + 2Fe proceed further to the right at the T and P values prevailing at the core-mantle boundary than at lower temperatures, thus supporting the presence of Si in the core and the relatively high Ni concentration of the mantle; 2. (2) the Fe 3+ Fe 2+ ratios in mantle materials indicate oxygen fugacity values close to that of the Fe-Fe1−xO buffer; and 3. (3) the apparent partitioning of Au and similar elements between the core and the mantle is close to that of pallasites. The anomalously high abundance of Cu in the upper mantle can be explained by enrichment through partial melting. Volcanic gases are not likely to represent the composition of volatile elements at the core-mantle boundary, and hence cannot be regarded as valid criteria of disequilibrium at the boundary. Available data on reaction kinetics suggest that a disequilibrium state would be unlikely during core formation.

Metal grains in Apollo 12 igneous rocks

Abstract The Apollo 12 igneous rocks we have examined contain abundant disseminated grains of metallic iron with a wide range of Ni and Co contents. The compositions of these metal grains vary to a degree not found in terrestrial igneous rocks or in meteorites. Ni and Co are generally higher in metal grains enclosed by the earliest-formed minerals (olivine and chromite) and are lower in those associated with later phases. The metal grains do not indicate the former presence of an immiscible metal liquid but have formed by reduction from the silicate melt. Formation of the metal grains accompanied, and may be a direct result of, the crystallization of the major phases under low oxygen partial pressures. The crystallization of chromite, for example, from a melt containing divalent chromium, may have resulted in the reduction of ferrous iron to metal. The presence of nickel-iron grains in lunar igneous rocks indicates that metal grains in lunar soil need not be exclusively of meteoritic origin.

Highly aluminous glasses in lunar soils and the nature of the lunar highlands.

Abstract Approximately 25 per cent of the glasses in two Apollo 14 soil samples and in the soils at two levels in the Luna 16 core have compositions equivalent to anorthositic gabbro. Reassessment of the non-mare glass components in the Apollo 11 and 12 soils shows that glasses with the composition of anorthositic gabbro are common to both; gabbroic anorthosite glasses are less common, and anorthositic glasses, rare. Anorthositic gabbro glasses have the same major element composition at all four sites, and resemble the Surveyor 7 analysis from a ‘highland’ site. Thus, strong presumptive evidence exists that material with this specific composition is abundant in the lunar highlands.

Cohenite: its occurrence and a proposed origin*

Abstract Cohenite is found almost exclusively in meteorites containing from 6 to 8 wt.% Ni. On the basis of phase diagrams and kinetic data it is proposed that cohenite cannot form in meteorites having more than 8 wt.% Ni and that any cohenite which formed in meteorites having Ni content lower than 6 wt.% decomposed during cooling. A series of isothermal sections for the system Fe Ni C has been constructed between 750 and 600°C from published information on the three constitutent binary systems. The diagrams indicate that the presence of a few tenths of a per cent carbon in a Ni Fe alloy may reduce the temperature at which kamacite separates from taenite by more than 50°C. Hence C in iron meteorites may be partly responsible for the postulated supercooled nucleation of kamacite in meteorites proposed by recent authors. Cohenite found in meteorites probably formed over the temperature range 650-610°C. For compositions approximating those of metallic meteorites, the greater the C or Ni content of the alloy, the lower the temperature of formation of cohenite. The presence of cohenite in meteorites indicates neither high nor low pressures of formation. However, the absence of cohenite in meteorites containing the assemblage metal + graphite requires low pressures during cooling. Such meteorites therefore cooled in parent bodies of asteroidal size, or near the surface of large bodies.

Petrology of a portion of the Mare Fecunditatis regolith.

Abstract 1300 microprobe analyses of glasses, pyroxenes, feldspars, oxides, olivines, troilite and metal in two 0.025 g samples of the Luna 16 return were made in order to characterize the Mare Fecunditatis regolith. Pyroxenes display a very wide compositional range, extreme fractionation, and metastable crystallization. Solid solution of Ti, Al, and Cr is appreciable and most pyroxenes plot along an Al:Ti line 2:1, similar to Apollo 11 clinopyroxenes. Orthopyroxenes are very rare. Zoning in plagioclase is varied but not extensive; compositions from An75 to An100 are dominant. The compositional distribution is indistinguishable from Apollo 12 and 11 low-K basaltic plagioclases. No potassic feldspars were found. Ilmenite is the dominant oxide phase, with minor ulvospinel, rare chrome spinel and spinel. The latter resemble Apollo 14 pink spinels. Olivines range from Fo75 to Fo11 but the majority range from Fo60 to Fo70 thus more iron-rich than olivines from other maria. On the basis of preferred compositions, a tentative classification of glasses has been made. Twenty-three percent of the glasses are Al-rich, Fe, Cr-poor, have Ca/Al ratios similar to many Apollo 14 glasses and are considered to have a non-mare origin. Their compositions are essentially the same as that of the high Al component at all Apollo landing sites. Glasses equivalent in composition to Fra Mauro basalts (KREEP) and to granite are extremely rare. The majority of glasses, mare-derived, are substantially higher in Fe, Ti, and Cr and lower in Ca and Al. They are divisible into a major group, Fecunditatis type A basaltic glasses, with less than 5% TiO2, and a smaller group, Fecunditatis type B basaltic glasses, with more than 5% TiO2. The type A glasses are richer in Al, and lower in Fe than glasses at the Apollo 11 or 12 sites. Type B glasses are similar to the high Fe basaltic glasses from the Apollo 11 regolith. If the type A glasses reflect the characteristic basalts at the Mare Fecunditatis site, then these are intermediate in major element chemistry between Apollo 11 and 12 basalts and the aluminous non-mare basaltic rocks.