Volume 104, Issue 6

Calcite-graphite carbon isotope thermometry results are presented for 150 new marble samples across the western Central Metasedimentary Belt (CMB) of the Ontario segment of the Grenville Province. The results show a gradual increase in peak metamorphic temperature from under 500 °C in the Tudor Township area in the east to over 700 °C along the western margin of the CMB. Modestly elevated d 13 C values above 3‰ across all terranes in the study area suggest a common temporal, and perhaps depositional origin, for the ~1.3 Ma limestone protoliths. The preserved thermal gradient is consistent with variations in marble mineralogy and character. Based on published geochronology, we argue that these temperatures correspond to peak metamorphism during the 1090–1020 Ma Ottawan orogeny, with the exception of preserved contact aureoles most notably associated with the Tudor gabbro and the Cheddar granite. The lack of significant thermal discontinuities at terrane boundaries and other shear zones, including the Bancroft shear zone and the CMB boundary zone, indicate that the entire region including the adjacent Central Gneiss Belt remained largely intact during and after the Ottawan peak metamorphic event. Variations in deformation style of pre-existing igneous complexes appear to correspond to the Ottawan thermal conditions, which include crystalline thrust sheets at high temperature, and mildly foliated domes to undeformed plutons at decreasing temperatures. In light of the peak temperatures and smooth thermal gradient we show to be superimposed on the established Grenvillian architecture, much of the western CMB should be considered part of the allochthonous medium pressure belt, and separate from the Ottawan orogenic lid that defines the eastern CMB.

Minerals reveal the nature of the co-evolving geosphere and biosphere through billions of years of Earth history. Mineral classification systems have the potential to elucidate this rich evolutionary story; however, the present mineral taxonomy, based as it is on idealized major element chemistry and crystal structure, lacks a temporal aspect, and thus cannot reflect planetary evolution. A complementary evolutionary system of mineralogy based on the quantitative recognition of “natural kind clustering” for a wide range of condensed planetary materials with different paragenetic origins has the potential to amplify, though not supersede, the present classification system.

Combining occurrence rates of rocky exoplanets about sun-like stars, with the number of such stars that occupy possibly hospitable regions of the Milky Way, we estimate that at least 1.4 × 10 8 near-Earth-sized planets occupy habitable orbits about habitable stars. This number is highly imprecise to be sure, and it is likely much higher, but it illustrates that such planets are common, not rare. To test whether such rocky exoplanets might be geologically similar to Earth, we survey >4000 star compositions from the Hypatia Catalog—the most compositionally broad of such collections. We find that rocky exoplanets will have silicate mantles dominated by olivine and/or orthopyroxene, depending upon Fe partitioning during core formation. Some exoplanets may be magnesiowüstite- or quartz-saturated, and we present a new classification scheme based on the weight percent ratio (FeO+MgO)/SiO 2 , to differentiate rock types. But wholly exotic mantle mineralogies should be rare to absent; many exo-planets will have a peridotite mantle like Earth, but pyroxenite planets should also be quite common. In addition, we find that half or more of the range of exoplanet mantle mineralogy is possibly controlled by core formation, which we model using α Fe = Fe BSP /Fe BP , where Fe BSP is Fe in a Bulk Silicate Planet (bulk planet, minus core), on a cation weight percent basis (elemental weight proportions, absent anions) and Fe BP is the cation weight percent of Fe for a Bulk Planet. This ratio expresses, in this case for Fe, the fraction of an element that is partitioned into the silicate mantle relative to the total amount available upon accretion. In our solar system, α Fe varies from close to 0 (Mercury) to about 0.54 (Mars). Remaining variations in theoretical exoplanet mantle mineralogy result from non-trivial variations in star compositions. But we also find that Earth is decidedly non-solar (non-chondritic); this is not a new result, but appears worth re-emphasizing, given that current discussions often still use carbonaceous or enstatite chondrites as models of Bulk Earth. While some studies emphasize the close overlap of some isotope ratios between certain meteoritic and terrestrial (Earth-derived) samples, we find that major oxides of chondritic meteorites do not precisely explain bulk Earth. To allow Earth to be chondritic (or solar), there is the possibility that Earth contains a hidden component that, added to known reservoirs, would yield a solar/chondritic bulk Earth. We test that idea using a mass balance of major oxides using known reservoirs, so that the sum of upper mantle, metallic core, and crust, plus a hidden component, yields a solar bulk composition. In this approach, the fractions of crust and core are fixed and the hidden mantle component, F h , is some unknown fraction of the entire mantle (so if F DM is the fraction of depleted mantle, then F h + F DM = 1). Such mass balance shows that if a hidden mantle component were to exist, it must comprise >28% of Earth’s mantle, otherwise it would have negative abundances of TiO 2 and Al 2 O 3 . There is no clear upper limit for such a component, so it could comprise the entire mantle. But all estimates from F h = 0.28 to F h = 1.0 yield a hidden fraction that does not match the inferred sources of ocean island or mid-ocean ridge basalts, and would be geologically unusual, having higher Na 2 O, Cr 2 O 3 , and FeO and lower CaO, MgO, and Al 2 O 3 compared to familiar mantle components. We conclude that such a hidden component does not exist.

The structure, the elastic properties, and the Raman frequencies of the zircon and reidite polymorphs of ZrSiO 4 were calculated as a function of hydrostatic pressure up to 30 GPa using HF/DFT ab initio calculations at static equilibrium (0 K). The softening of a silent (B 1u ) mode of zircon leads to a phase transition to a “high-pressure–low-symmetry” (HPLS) ZrSiO 4 polymorph with space group I 42 d and cell parameters a = 6.4512 Å, c = 5.9121 Å, and V = 246.05 Å 3 (at 20 GPa). The primary coordination of SiO 4 and ZrO 8 groups in the structure of zircon is maintained in the high-pressure phase, and the new phase deviates from that of zircon by the rotation of SiO 4 tetrahedra and small distortions of the ZrO 8 dodecahedra. The new polymorph is stable with respect to zircon at 20 GPa and remains a dynamically stable structure up to at least 30 GPa. On pressure release, the new phase reverts back to the zircon structure and, therefore, cannot be quenched in experiments. In contrast, the transformation from zircon to reidite is reconstructive in nature and results in a first-order transition with a volume and density change of about 9%. The calculated energies from the DFT simulations yield an equilibrium transition pressure of 9.13(1) GPa at 0 K. Simulations of the Raman spectra of the three polymorphs at 20 GPa show how they can be distinguished. In particular, the peak due to the lowest-energy A 1 mode with a calculated wavenumber of 94 cm –1 is diagnostic of the HPLS phase because it does not overlap with any of the peaks of zircon or reidite.

The usefulness of the Mn/Fe ratios of olivine and pyroxene to identify a sample's host parent body is well established. Although there is an overarching, defining slope for each planetary body, there is some scatter, or "dispersion" around the defining slope. This dispersion reveals important facts relating to the planetary body. The source regions of the three main types of lunar basalts (very high-Ti, low-Ti, and very low-Ti) have f O2 values near IW-1 or below, and all iron is either ferrous or metallic. The dispersion in the Mn/Fe ratios of pyroxene from the Moon is largely caused by differences in the Ti and Al concentrations in the mantle source regions and the resulting differences in Ti activity of the primary basaltic melts derived from those sources. Ti displaces ferrous iron in the pyroxene Ml site (in a coupled substitution with Al for Si in the tetrahedral site), and therefore, with increasing Ti activity the Mn/Fe ratio in pyroxene increases in all three suites studied. For lunar mare basalts, the effect of Ti activity on the occupancy of the pyroxene Ml site, and crystallization sequence differences among high-Ti, low-Ti, and VLT basalts account for almost all of the observed dispersion in the Mn/Fe ratios.

As part of a project to investigate the volatilities of so-called “moderately volatile elements” such as Zn, In, Tl, Ga, Ag, Sb, Pb, and Cl during planetary formation, we began by re-calculating the condensation temperatures of these elements from a solar gas at 10- 4 bar. Our calculations highlighted three areas where currently available estimates of condensation temperature could be improved. One of these is the nature of mixing behavior of many important trace elements when dissolved in major condensates such as silicates, Fe-rich metals, and sulfides. Nonideal solution of the trace elements can alter (generally lower) condensation temperatures by up to 500 K. Second, recent measurements of the halogen contents of CI chondrites (Clay et al. 2017) indicate that the solar system abundance of chlorine is significantly overestimated, and this affects the stabilities of gaseous complexes of many elements of interest. Finally, we have attempted to improve on previous estimates of the free energies of chlorine-bearing solids since the temperature of chlorine condensation has an important control on the condensation temperatures of many trace elements. Our result for the 50% condensation temperature of chlorine, 472 K is nearly 500 K lower than the result of Lodders (2003), and this means that the HCl content of the solar gas at temperatures <900 K is higher than previously estimated. We based our calculations on the program PHEQ (Wood and Hashimoto 1993), which we modified to perform condensation calculations for the elements H, O, C, S, Na, Ca, Mg, Al, Si, Fe, F, Cl, P, N, Ni, and K by free energy minimization. Condensation calculations for minor elements were then performed using the output from PHEQ in conjunction with relevant thermodynamic data. We made explicit provision for nonidealities using information from phase diagrams, heat of solution measurements, partitioning data and by using the lattice strain model for FeS and ionic solids and the Miedema model for solutions in solid Fe. We computed the relative stabilities of gaseous chloride, sulfide, oxide, and hydroxide species of the trace elements of interest and used these, as appropriate in our condensation calculations. In general, our new 50% condensation temperatures are similar to or, because of the modifications noted above, lower than those of Lodders (2003).

Recent studies have shown that α-damage in titanite influences He diffusivity and thus the closure temperature of the (U-Th)/He system in titanite. We compare different methods for measuring the α‑dose in titanite by Raman spectroscopy. Raman spectra of randomly oriented titanite fragments from the Archean Karelian domain in eastern Finland along with some well-studied young titanites and U-Pb standard reference materials were analyzed and related to the concentration of α-emitting elements (U and Th) that generated damage in the respective grains. Automated curve-fitting was performed by the IFORS software and different curve-fitting protocols were tested and compared. The Raman bands at 424 and 465 cm -1 show a good correlation of full-width at half maximum (FWHM) and position with the α-dose. However, these bands are not always present because titanite is highly anisotropic implying that Raman spectra are sensitive to orientation. The intensity-weighted mean FWHM (iw-FWHM) of all Raman bands of a spectrum proves to be the most robust measure of the α-dose. A simplified fitting approach considering 15 peaks is sufficient to describe the accumulated α-dose. For α-doses below 5 × 1016α/g the iw-FWHM is independent of α-dose and ranges from 25 to 50 cm -1 . Above this value the iw-FWHM increases linearly with increasing α-dose up to 3 × 1018α/g. The linear correlation can be described as iw-FWHM[cm -1 ] ≈ 39(±1.2)[cm -1 ] + 3.84(+0.61,–0.26) × 10 -17 [cm -1 /(α/g)] × α-dose[α/g]. The approach provides a pre-selection method to optimize the range of α-doses of titanite crystals to be dated by (U-Th)/He thermochronology.

The substitution of carbonate in apatites has been voraciously explored for more than 60 yr. However, the characterization of the sites of carbonate substitution in apatite by the frequently used identification method, infrared spectroscopy, is still incompletely understood. In a significant departure from previous studies, a recent IR study concluded that most of the carbonate in apatites resides in the channels, at least in apatites prepared at high temperature and pressure. A series of A- and AB-carbonated calcium and strontium apatites have been prepared by aqueous precipitation and by carbonation with CO2 at 800 °C. The type of carbonate substitution—A-(substitution for hydroxide in the channel) or B- (substitution for phosphate)—was determined from the carbonate asymmetric stretch ( v 3) and out-of-plane bend ( v 2) regions in the IR spectra. The IR v 3 and v 2 regions were analyzed by peak-fitting using both four- and six-peak models for the v 3 region. A correlation of the band position of the high-frequency A-type carbonate band frequency with weight percent carbonate was observed for the calcium apatites, whereas a correlation of the band positions of both the low- and high-frequency B-type carbonate bands with carbonate weight percent occurs for the strontium apatites. The high-frequency member of the A-type carbonate v 3 region for the calcium apatites showed the greatest variation with a change in the composition (A- or B-type) of carbonate. The lower frequency observed for the A-type v 3 band of those calcium apatites with the greatest carbonate content suggested the importance of carbonate clusters in the unit cell of highly carbonated apatites. Correlations of band frequencies with sodium content were weaker than those for carbonate, even though carbonate and sodium were correlated with each other in the calcium apatites. Analogous observations on the IR band frequencies in potassium-containing carbonated apatites, in which the potassium content is low, also suggest that carbonate content is the primary driver for the interactions that produce an overlap of A-type peaks with those in the B-type region. This conclusion is further strengthened by the lack of a correlation of the frequencies of A-type carbonate in strontium apatites in which the carbonate content is low. A comparison of the IR carbonate v 3 region of the calcium apatites with that of apatites prepared under high pressure and temperature indicates that this region is very similar for both types of syntheses. Thus, previous conclusions about the distribution of A- and B-type carbonate likely should be valid for most synthetic calcium apatites formed under a wide range of temperature and pressure conditions.

Fingerprinting hydrogen storage sites in olivine at high temperature and high pressure is fundamental to understand water distribution and its impact on the upper mantle. We carried out in situ high-temperature and high-pressure IR spectroscopic investigations on hydrogen storage sites in the natural olivine and synthetic Fe-free forsterite. Based on in situ observations of hydrogen in both the natural olivine and synthetic Fe-free forsterite at high temperatures and high pressures, we find that hydrogen does not transfer between storage sites with increasing temperature, but displays disordering at temperatures over 600 °C. In contrast, pressure can induce re-configuration of hydrogen storage sites corresponding to the 3610 and 3579 cm –1 bands. Hydrogen storage sites also exhibit disordering at high pressure. In addition, the dehydrogenation experiments of the natural olivine indicate interacts of hydrogen storage sites. Protons released from titanium-clinohumite defects move to pure Si vacancies, and also to Mg vacancies coupling with trivalent cations. This study is the first attempt to fingerprint hydrogen storage sites in olivine at high temperature and high pressure using in situ IR spectroscopy. The implications of the temperature- and pressure-induced disordering and re-configuration of hydrogen storage sites are discussed. The disordering and re-configuration of hydrogen storage sites at high temperature and high pressure favor better understanding of the water effects on physical properties of olivine. The interactions of hydrogen storage sites during dehydrogenation warn that some hydrogen if observed in dehydrated mantle-derived samples may not be original and also make hydrogen diffusivity complex.

The ascent of hydrous, silica-rich magmas from the lower crust drives volcanic eruptions, builds the upper crust, and concentrates metals such as Cu, Au, and Mo into ore deposits. Owing to the negative slope of the melting curve for granitic materials in the presence of water, it has long been assumed that water-saturated magmas move into the subsolidus field and freeze upon ascent; therefore, for magma to rise it must be water-undersaturated at a temperature well above the solidus. This assumption ignores the considerable energy released by crystallization. Here I show that if magma ascent is treated as an adiabatic, reversible process, then water-saturated magma can rise to the surface, following the solidus to shallow depth and higher temperature as it undergoes modest crystallization and vapor exsolution. Decompression heating is an alternative to magma recharge for explaining pre-eruptive reheating seen in many volcanic systems and accounts for paradoxical growth of quartz during a heating event. The viscosity increase that accompanies vapor exsolution as magma rises to shallow depth explains why silicic magmas tend to stop in the upper crust rather than erupting, producing the observed compositional dichotomy between plutonic and volcanic rocks.

Clinoenstatite (Mg 2 Si 2 O 6 ) undergoes a well-known phase transition from a low-pressure form (LPCEN, space group P 2 1 / c ) to a high-pressure form (HPCEN, space group C 2/ c ) at ~6 GPa. High-pressure structure refinements of HPCEN were carried out based on single-crystal X‑ray diffraction experiments between 9.5 and 35.5 GPa to determine its P-V equation of state and structural evolution over an expanded pressure range relevant to pyroxene metastability. The best-fit isothermal equation of state to our data combined with the five data points between 5.34 and 7.93 GPa from Angel and Hugh-Jones (1994) yields a second-order Birch-Murnaghan equation with K T0 = 121(2) GPa and V 0 = 403.9(5) Å 3 (with K ʹ T0 = 4 implied). Further reduction of misfit upon fitting a third-order Birch-Murnaghan equation is not significant at the 90% confidence level. At ~45 GPa, a transition from HPCEN to a P 2 1 / c -structured polymorph (HPCEN2) was observed, which is isostructural to the P 2 1 / c phase recently observed in diopside (CaMgSi 2 O 6 ) at 50 GPa (Plonka et al. 2012) and in clinoferrosilite (Fe 2 Si 2 O 6 ) at 30–36 GPa (Pakhomova et al. 2017). Observation of HPCEN2 in Mg 2 Si 2 O 6 completes the third apex of the pyroxene quadrilateral wherein HPCEN2 is found, facilitating a broader view of clinopyroxene crystal chemistry at conditions relevant to metastability in the Earth’s mantle along cold subduction geotherms.

Synchrotron-based high-pressure single-crystal X‑ray diffraction experiments were conducted on synthetic clinopyroxenes at room temperature to a maximum pressure of 40 GPa. We studied two crystals with different compositions. A Na-Ti-pyroxene with formula (Na 0.86 Mg 0.14 )(Mg 0.57 Ti 0.43 )Si 2 O 6 synthesized at P = 7 GPa and T = 1700 °C, and a Na-pyroxene with composition (Na 0.886 Mg 0.085 Fe 0.029 ) (Si 0.442 Mg 0.390 Fe 0.168 )Si 2 O 6 synthesized at P = 15 GPa and T = 1500 °C. These phases were found to be monoclinic with the space group C 2/ c and exhibit K To of 106.8(2), 121.8(4) GPa, respectively. Na-Tipyroxene is more compressible than Fe-bearing Na-Mg-Si-pyroxene, likely due to the fact that the FeO 6 octahedron is significantly more rigid than MgO 6 at high pressure. The formation of Na-rich pyroxenes in the deep mantle is related to crystallization of low-degree alkaline carbonate-silicate melts formed when the crust and mantle interact during the slab descent and its stagnation in the transition zone.