John R. Beckett

Discovery of bridgmanite, the most abundant mineral in Earth, in a shocked meteorite

Meteorites exposed to high pressures and temperatures during impact-induced shock often contain minerals whose occurrence and stability normally confine them to the deeper portions of Earth’s mantle. One exception has been MgSiO3 in the perovskite structure, which is the most abundant solid phase in Earth. Here we report the discovery of this important phase as a mineral in the Tenham L6 chondrite and approved by the International Mineralogical Association (specimen IMA 2014-017). MgSiO3-perovskite is now called bridgmanite. The associated phase assemblage constrains peak shock conditions to ~ 24 gigapascals and 2300 kelvin. The discovery concludes a half century of efforts to find, identify, and characterize a natural specimen of this important mineral. X-ray analysis identifies magnesium silicate perovskite, now known as bridgmanite, in a heavily shocked meteorite. [Also see Perspective by Sharp] A mineral name for mantle perovskite A rock from outer space finally puts a name to Earths most abundant mineral, frequently referred to as perovskite. Mineral names are only bestowed on specimens that are found in nature and characterized. Tschauner et al. isolate a magnesium silicate in the perovskite structure, now called bridgmanite, in the Tenham L6 chondrite meteorite (see the Perspective by Sharp). Bridgmanite formed in this meteorite during a high-pressure and -temperature shock event. Other minerals associated with bridgmanite allow the pressure-temperature conditions to be narrowly bound, giving insight into the shock process. The long-sought-after specimen finally puts to rest a confusing nomenclature of this dense deep mantle silicate. Science, this issue p. 1100; see also p. 1057

The origin of abyssal peridotites: a reinterpretation of constraints based on primary bulk compositions

We calculated primary bulk compositions for a global suite of abyssal peridotites using primary mineral modes and either analyzed or calculated phase compositions. The latter were obtained through correlations between reported mineral compositions and modal olivine contents. Both the modal data and the mineral compositions were averaged by dredge site, drill hole, or fracture zone (FZ) depending on the amount of available data. Our calculated abyssal peridotite compositions yield major-element oxide-MgO trends that are generally in good agreement with those based on compilations of ultramafic nodules and peridotite massifs. In particular, we find no statistically significant correlation between FeO* (total Fe as FeO) and MgO and, therefore, no evidence for significant olivine accumulation. Previous reports of a positive correlation reflect an artifact of the regressions used to calculate missing phase compositions and result in a relationship between the Mg# of olivine and modal olivine abundance that is inconsistent with observed variations in abyssal peridotites. There is a slight positive correlation between bulk FeO* and MgO if individual thin sections are used to derive the mineral composition versus modal olivine regressions, but the large grain sizes and heterogeneous distributions of phases within abyssal peridotities make it unlikely that individual thin section modes accurately reflect phase proportions in meter-sized dredge-haul samples. The variability of Na and Ti contents in pyroxenes from plagioclase-free abyssal peridotites suggests to us, as it has to other workers, that a majority of these samples interacted to varying degrees with small amounts of melt. On the other hand, lower bounds on Na and Ti contents in the pyroxenes at a given dredge site as a function of modal olivine content are broadly consistent with calculated partial melting residues. Thus, abyssal peridotites may retain information both on the original partial melting process and on concurrent or later interactions with partial melts from other sources.

Origin of opaque assemblages in C3V meteorites - Implications for nebular and planetary processes

Mineral phases from opaque assemblages (OAs) in Ca, Al-rich refractory inclusions (CAIs), chondrules and matrix in C3V meteoites were chemically analyzed and compared with experimentally determined phase equilibria and partitioning data in the Ni-Fe-S, Ni-Fe-S and Ni-Fe-O systems to estimate the temperature, sulfur fugacity (f_(S2)) and oxygen (f_(O2)) of OA formation. The kinetics of dissolution and exsolution of metallic phases in the Ni-Fe-Ru system were used to constrain the thermal history of OAss that occur in CAIs. Based on this work, we suggest that OAs formed after the crystallization of host CAIs by exsolution, sulfidation and oxidation of precursor alloys at low temperatures (~ 770 K) and higher than solar gas f_(S2) and f_(S2). Our model contrasts with previous models that call upon the formation of CAI OAs by aggregation of previously formed phases in the solar nebula prior to the crystallization of CAIs. Opaque assemblages in CAIs and chondrules probably originated as homogeneous alloys during melting of the silicate protions of CAIs and chondrules. The compositions of these precursor alloys reflect high-temperature and low-f_(O2) conditions in the early solar nebula. The similarities in the temperature, f_(S2) and f_(O2) of equilibrium for OAs that occur in CAIs, chondrules and matrix suggest that these three components of C3V meteorites share a common, late low-temperature history. The mineral phases in OAs do not preserve an independent history prior to CAI and chondrule melting and crystallization, but instead provide important information on the post-accretionary history of C3V meteorites and allow us to quantify the temperature, f_(S2) and f_(O2) of cooling planetary environments.

Crystal chemical effects on the partitioning of trace elements between mineral and melt - An experimental study of melilite with applications to refractory inclusions from carbonaceous chondrites

The partitioning behavior of the trace elements Be, Sc, Ba, La, Ce, and Tm between melilite and liquid has been determined using stepwise integration of a Rayleigh fractionation equation for ion microprobe analyses of synthetic zoned melilite crystals. Distribution coefficients between melilite and liquid (D^(Mel/L)_i) were determined over the entire range of melilite + spinel crystallization for one bulk composition corresponding to that of an average Type B inclusion from the Allende C3V carbonaceous chondrite. Beryllium is incompatible in gehlenitic meliliters (e.g.,D^(Mel/L)B_e = 0.5 for X_(ak) = 0.3) but compatible in akermanitic melilites (e.g., D^(Mel/L)B_e = 1.9 for X_(Ak) = 0.75). Barium (D^(Mel/L)B_a = 0.04-0.05), Sc (D^(Mel/L)S_c = 0.01-0.02), La (D^(Mel/L)L_α ≤ 0.07-0.29), Ce (D^(Mel/L)C_e ≤ 0.05-0.21), and Tm (D^(Mel/L)T_m ≤ 0.03-0.14) are all incompatible in melilites in the range Ak25-Ak75. Variations in D^(Mel/L)i as a function of X_(Ak) are explicable in terms of a simple thermodynamic treatment of trace element partitioning based on crystal chemical considerations. Characteristic trace element zoning patterns are predicted for early crystallizing melilite from meliliterich Type B inclusions. For example, concentrations of the REEs in melilite should continually decrease with increasing degrees of crystallization, opposite the behavior normally expected of an incompatible element. Concentrations of Be should rise with increasing degrees of crystallization even when the element is compatible, again opposite the behavior normally expected of compatible elements. In general, zoning patterns of trace elements in melilite from Type B inclusions are consistent with those predicted for fractional crystallization from a melt in a closed system. Trace element zoning patterns in meteoritic melilite crystals constrain the origin and thermal histories of Ca-, Al-rich inclusions from carbonaceous chondrites.

Volatilization kinetics of silicon carbide in reducing gases: an experimental study with applications to the survival of presolar grains in the solar nebula

The volatilization kinetics of single crystal α-SiC, polycrystalline β-SiC, and SiO_2 (cristobalite or glass) were determined in H_2-CO_2, CO-CO_2, and H_2-CO-CO_2 gas mixtures at oxygen fugacities between 1 log unit above and 10 log units below the iron-wustite (IW) buffer and temperatures in the range 1151 to 1501°C. Detailed sets of experiments on SiC were conducted at 2.8 and 6.0 log units below IW (IW-2.8 and IW-6.0) at a variety of temperatures, and at 1300°C at a variety of oxygen fugacities. Transmission electron microscopic and Rutherford backscattering spectroscopic characterization of run products shows that the surface of SiC exposed to IW-2.8 is characterized by a thin (<1 μm thick), continuous layer of cristobalite. SiC exposed to IW-6.0 lacks such a layer (or its thickness is <0.01 μm), although some SiO_2 was found within pits and along incised grain boundaries. In H_2-CO_2 gas mixtures above ∼IW-3, the similarity of the SiC volatilization rate and of its dependence on temperature and fO_2 to that for SiO_2 suggests that SiC volatilization is controlled by volatilization of a SiO_2 layer that forms on the surface of the SiC. With decreasing log fO_2 from ∼IW-3 to ∼IW-6, the SiC volatilization rate is constant at constant temperature, whereas that for SiO_2 increases. The independence of the SiC volatilization rate from the gas composition under these conditions suggests that the rate-controlling step is a solid-solid reaction at the internal SiC/SiO_2 interface. For gas compositions more reducing than ∼IW-6, the SiC volatilization rate increases with decreasing fO_2, with both bare SiC surfaces and perhaps silica residing in pits and along incised grain boundaries contributing to the overall reaction rate. If the volatilization mechanism and reaction rate in the solar nebula were the same as in our H_2-CO_2 experiments at IW-6.0, then estimated lifetimes of 1-μm-diameter presolar SiC grains range from several thousand years at ∼900°C, to ∼1 yr at 1100°C, ∼1 d at 1300°C, and ∼1 h at 1400°C. The corresponding lifetimes for 10-μm SiC grains would be an order of magnitude longer. If the supply of oxidants to surfaces of presolar SiC grains were rate limiting—for example, at T > 1100°C for P^(tot)= 10^(−6) atm and sticking coefficient = 0.01, then the calculated lifetimes would be about 10 yr for 10-μm-diameter grains, essentially independent of temperature. The results thus imply that presolar SiC grains would survive short heating events associated with formation of chondrules (minutes) and calcium-, aluminum-rich inclusions (days), but would have been destroyed by exposure to hot (≥900°C) nebular gases in less than several thousand years unless they were coated with minerals inert to reaction with a nebular gas.

Thermodynamic properties of the Pt-Fe system

Abstract We determined activity-composition relationships for the Pt-Fe system by equilibrating Fe-oxides with Pt-Fe alloys at temperatures in the range of 1200-1400 °C and oxygen fugacities from 1.6 to 7.7 log units above the iron-wüstite (IW) buffer. The system is characterized by strong negative deviations from ideality throughout the investigated temperature range (e.g., γalloyFe<0.02 for XalloyFe <0.3). Our data are consistent with an asymmetric regular solution of the form: RT ln γalloyFe=[WG1+2(WG2-WG1)XalloyFe](XalloyPt)2 where WG1 = -138.0 ± 3.3 kJ/mol and WG2 = -90.8 ± 24.0 kJ/mol (1σ). Based on experiments at 1200-1400 °C, variations in the activity coefficients at a given composition are consistent with ln γ alloyFe(T1) / ln γalloyFe(T2) = T2 / T1. The Pt-Fe alloy composition in equilibrium with a FeO-bearing silicate liquid can be obtained from: where ΔG0r is the standard state free energy for the reaction 2Fealloy + O2gas +SiO2liq = Fe2SiOliq4 . We obtained values of αalloyFe from our model and used the program MELTS together with the thermodynamic properties of these elements to evaluate activities of SiO2 and Fe2SiO4 components in the liquid and ΔG0r . We provide sample calculations showing how to predict the optimum Fe concentrations for pre-saturation of Pt-bearing containers to reduce Fe loss from the charge during experiments on magmatic liquids at high temperatures and pressures from 1 atm to 40 kbar.

Ti^(3+) in meteoritic and synthetic hibonite

Electron spin resonance has been used to make the first direct determination of Ti^(3+) in synthetic hibonite and hibonite from inclusion SH-7 of the Murchison C2 chondrite. Ti^(3+) concentrations range from 0.02 to 0.64 wt% in synthetic blue hibonite and 0.35–0.44 wt% in hibonite from SH-7. No Ti^(3+) could be detected in orange hibonite, supporting the earlier conclusion that the orange-to-blue transition is associated with the presence of Ti^(3+). At constant temperature and oxygen fugacity, Ti^(3+)/Ti^(4+) in synthetic hibonite increases with decreasing V but is not strongly dependent on bulk Ti. At the concentration levels encountered in meteoritic hibonite, Fe and Cr contents do not have a significant effect on the amount of Ti^(3+). In both synthetic and meteoritic hibonite, Ti^(3+) occupies a 5-coordinated crystallographic site, which is consistent with the formation of doubly ionized oxygen vacancies. At low oxygen fugacities, essentially all Ti^(4+) on the five-fold Al-site has been reduced to Ti^(3+). Hibonite from SH-7 equilibrated with a gas that could have been as reducing as a gas of solar composition. This is consistent with other estimates based on mineral equilibria of high temperature oxygen fugacities in Ca-Al-rich inclusions. With the possible exception of Mo-W depletions, indicators based on bulk trace element concentrations in CAIs are inconclusive. There is considerable evidence that as CAIs cooled to lower temperatures, they experienced conditions significantly more oxidizing than those of a solar gas, perhaps in planetary environments.

Measurement of oxygen fugacities under reducing conditions: non-Nernstian behavior of Y2O3-doped zirconia oxygen sensors

A calibration procedure is presented for the use of a Y_2O_3-stabilized zirconia (YSZ) oxygen sensor in 1 atm gas-mixing furnaces in the temperature range 1200–1500°C and 0–8 orders of magnitude below the iron-wustite (IW) buffer. Corrections to the Nernst equation were obtained by measuring apparent oxygen fugacities of gases in equilibrium with graphite (equilibrated with pure CO vapor), Cr + Cr_2O_3, and Ta + Ta_2O_5. Under reducing conditions, fO_2s calculated using the ideal form of the Nernst equation are erroneously high, by <0.1 log units at IW but by nearly three log units for Ta-Ta_2O_5 at 1000°C. The deviations between measured emfs and those calculated assuming Nernstian behavior of the electrolyte in the oxygen sensor reflect mixed ionic-electronic conduction. Measured emfs under reducing conditions are readily corrected for this effect via experimentally determined values of P_θ, the oxygen fugacity at which electronic conduction constitutes half of the total conductivity. For the oxygen sensors used in this study, log P_θ(± 0.20,lσ)3.70(±0.72)-32.95 ± 1.15 X10^3 T(K). Even under conditions more reducing than a gas of solar composition (f_(O2) = 10^(−18) at 1200°C), YSZ oxygen sensors can be used to determine absolute values of the oxygen fugacity to within ±0.2 log units.

The origin of type C inclusions from carbonaceous chondrites

Type C inclusions are plagioclase-rich, Ca-, Al-rich inclusions found in carbonaceous chondrites. They formed as solid condensates which were later melted in an event that destroyed the original condensate grains. Neither the melting event nor secondary alteration had a significant effect on bulk composition. Two stages in the condensation history can be discerned on the basis of major element bulk compositions. As in type A inclusions, the condensate phase assemblage originally consisted of melilite + spinel + perovskite ± hibonite. Type Cs were, however, significantly enriched in spinel relative to unaltered portions of most type As. In contrast to type As, condensate grains of spinel and melilite in type Cs reacted partially with a coexisting gas to produce anorthite + diopside. One-half to two-thirds of the silica now in type C inclusions was introduced by this process. The reactions involving melilite and spinel that are predicted by equilibrium condensation calculations for a cooling gas of solar composition did not occur, probably due to kinetic constraints. Type Cs may be related to Al-rich chondrules in ordinary chondrites by the addition of olivine and albite. Bulk compositions of Al-rich chondrules in enstatite chondrites are consistent with the addition of orthopyroxene and albite to type C inclusions. Thus, types A and C inclusions and Al-rich chondrules could represent sequences of condensates removed from interaction with the primitive solar nebula at progressively lower temperatures. If Al-rich chondrules represent the high-temperature component in chondritic material, then type C inclusions rather than the more common subgroups of CAIs could be the true parents of chondrites.

The partitioning of Na between melilite and liquid: part II. applications to Type B inclusions from carbonaceous chondrites

The zoning of Na in the melilite of Type B1 Ca-, Al-rich inclusions from carbonaceous chondrites reflects a combination of factors including the maximum temperature, T_(max), for melting, bulk composition, crystal dimensions and the degree of diffusive relaxation. We used experimentally determined partition coefficients for Na between melilite and liquid together with modes and compositions of phases in meteoritic inclusions to calculate Na zoning produced in melilite by crystallization from Na-bearing liquids. We then used a simple model for the diffusive relaxation of an originally igneous zoning profile to constrain the thermal history. For Allende Type B1 inclusion TS-34, the melilite compositions are consistent with an initial melting event (T_(max) ~1450°C) during which essentially Na-free melilite crystallized, followed by the introduction of Na during a hiatus, a second melting event with T_(max) ~1290°C in which Na-bearing melilite with X^(Mel)_Al ≥ 0.47 grew on relict cores of melilite from the first melting event, and partial diffusive relaxation. For the Type B1 inclusion Vigarano 1623-8, previously published bulk and melilite compositions lead to the conclusion that T_(max) did not exceed ~1280°C during the last major melting event. A survey of literature data for other inclusions suggests that Na zoning in melilite may be a generally useful indicator of thermal history and that Type B1 inclusions were commonly subjected to multiple melting events with intervening periods of alteration. At any given value of X^(Mel)_(Ak) in TS-34, calculated melilite/liquid partition coefficients for Na are lower than values determined experimentally for melilite of similar composition because of liquid composition effects. The difference is more than a factor of two for X^(Mel)_(Ak) > 0.5, and it is likely, based on the activities of major oxides in the melt, that liquid composition effects are also important for other elements. On the other hand, modeling of diffusive relaxation for Na in melilite suggests that the diffusion coefficients are not strongly dependent on either X^(Mel)_(Ak) or Na concentration.