2010 Annual Science Report
Carnegie Institution of Washington Reporting | SEP 2009 – AUG 2010
Project 3: The Origin, Evolution, and Volatile Inventories of Terrestrial Planets
The origin and Sustenance of life on Earth strongly depends on the fact that volatile elements H-C-O-N where retained in sufficient abundance to sustain an ocean-atmosphere. The research in this project involves studies of how terrestrial planets form, why differences exist among the terrestrial planets, how volatiles behave deep within the Earth, and how volatiles and life influence the large and small scale composition of the near surface Earth.
3.1 Planet formation
CoI John Chambers progress on planet formation has evolved along two fronts. Over the last year, CoI Chambers conducted a detailed investigation of the range of circumstances that lead to successful planetesimal formation, looking at a range of disk properties such as mass and metallicity (dust-to-gas ratio). This work has shown that planetesimal formation via turbulent concentration is viable provided that another process partially enhances the dust-to-gas ratio initially. One unexpected result it that planetesimal formation then becomes more efficient with increasing distance from a star, the opposite of later stages of planetary growth. This suggests there may be a “sweet spot’” for efficient growth of large planets several astronomical units from a star, in the region now occupied by the Sun’s giant planets. This work has been published in Icarus.
A second project involves looking for possible signatures of terrestrial planets in orbit around other Sun-like stars. Recently, high-precision elemental abundances have been available for a number of Sun-like stars. The new abundance data show that the Sun is unusual. The solar photosphere is progressively depleted in refractory elements compared to most Sun-like stars, with the degree of depletion strongly correlated with each element’s condensation temperature. This depletion may be the result of refractory elements locked up in the Sun’s terrestrial planets, which suggests most Sun-like stars do not possess such planets. Chambers has investigated this scenario in more detail, beginning with an examination of the elements that make up the depletion pattern. This work shows that the solar convection zone is missing roughly 4 Earth masses of rocky material compared to a typical Sun-like star. This is comparable to the mass of rock contained in the terrestrial planets together with refractory-rich material that would have been ejected from the asteroid belt after the Sun’s giant planets formed. The elemental abundance pattern of the Sun can be brought into line with the average for Sun-like stars by adding rocky material that is an equal mixture of Earth-like and carbonaceous-chondrite-like material, suggesting that the solar depletion pattern was caused by the formation of rocky bodies throughout the inner solar nebula. Other Sun-like stars actually show a range of compositions (with the Sun lying near one end of the distribution). Most stars show refractory depletion patterns compared to the most refractory rich stars, and these are consistent with the presence of small terrestrial planets with a total mass between 0 and 10 Earth masses. CoI Chambers has also examined elemental abundance patterns for stars with known giant-planet companions. There is no apparent correlation between detected planetary companions and elemental depletion patterns, suggesting that the presence of giant and terrestrial planets in the same system is not strongly correlated.
3.2. The Inner Solar System: Constraints from Mercury and Mars
CoI Solomon is the Principal Investigator of the MESSENGER mission to Mercury, and CoI Nittler is a Participating Scientist on the MESSENGER team. As part of this NAI project, Solomon and Nittler are integrating the information derived from MESSENGER into a better understanding of the processes that led to the formation of the small, embryo-sized inner planets, including Mercury at about 5% of Earth’s mass and Mars at about 10%. That the bulk compositions, volatile abundances, magmatic histories, and magnetic field histories differ so strongly on these two bodies demonstrates the strongly stochastic nature of the planet-building process and probably some dependence on solar distance. Because all of these aspects of planetary evolution affect the spatial extent and temporal duration of zones of habitability at the planetary surface and within the shallow planetary subsurface, an improved understanding of the profound differences in the make-up and evolution of these two similar-size planets holds the promise of illuminating the general nature of planetary habitability on smaller Earth-like planets, including those in other planetary systems. For both Mars and Mercury, recent spacecraft observations make such a comparison particularly timely. The ongoing Mars Odyssey, Mars Express, Mars Exploration Rover, and Mars Reconnaissance Orbiter missions continue to build the spectacular data sets from imaging and geochemical and geophysical remote sensing, and the recently completed Phoenix mission augmented our understanding of water and other volatiles at high Martian latitudes. The MESSENGER mission completed its three flybys of Mercury in 2008 and 2009, and an entire year of orbital observations is planned for 2011-2012. The broad goal of this task is a comparative evaluation of bulk composition, volatile inventory, magmatic history, and core dynamo history on Mars and Mercury, with a focus on aspects of those processes (water availability and circulation, organic material inventory, internal energy, magnetospheric shielding) most strongly relevant to habitability in space and time.
The MESSENGER flybys of Mercury provided an abundance of new information pertinent to this task on the geological history, magnetic field, and volatile budget of the innermost planet. Images from the three flybys provided evidence for widespread volcanism. The ~1500-km-diameter Caloris basin was the focus for concentrations of volcanic centers, some displaying evidence for pyroclastic deposits, and smooth plains interior and exterior to the basin that postdate the basin-forming event. The largely volcanic smooth plains constitute ~40% of the surface area and span nearly the full range of visible–near-infrared spectral types seen on Mercury. Excavation of spectrally similar material by large craters and basins suggests that much of the upper crust of Mercury was emplaced by a succession of plains volcanic flows. The comparatively young, 290-km-diameter Rachmaninoff peak-ring basin is floored by inner smooth plains deposits that differ in color from and are lower in crater density than the peak ring, outer plains, and basin rim, indicating that the central plains are one of the youngest expanses of volcanic deposits on Mercury. A nearby irregular rimless depression ~30 km across surrounded by a high-reflectance halo of distinctive color ~200 km in diameter is a candidate for a volcanic vent amid what may be the largest expanse of pyroclastic deposits yet seen on Mercury. The former feature extends the known history of magmatism, and the latter provides another indication that Mercury’s interior may at least locally contain larger concentrations of volatiles than predicted by most models for Mercury’s formation.
Newly imaged lobate scarps and other tectonic landforms confirmed that Mercury contracted globally in response to interior cooling. Both the areal density and the typical relief on lobate scarps are greater than appreciated from Mariner 10 observations, an important constraint on thermal history and power available for a core dynamo. Also seen in flyby images was evidence for pervasive contractional and extensional deformation across the floors of the Caloris basin and the ~700-km-diameter Rembrandt basin, as well as concentric extensional faults within the peak ring of the comparatively young Raditladi and Rachmaninoff basins. The focusing of deformational and volcanic activity within major basins can be understood because basin formation amplified magma production at depth, by the removal of overburden pressure and the emplacement of impact energy as heat, and also changed the lithospheric stress state by removing pre-existing stress within the basin interior and modifying stress within a damage zone that extended to several basin radii. The first known example of extensional faulting unrelated to an impact basin, a family of narrow graben that crosscut an elevated block, may be the result of relaxation of topographic relief on a crustal plateau. These examples of extensional deformation constrain the relief of global compressional stress that accompanied impacts and large-scale faulting.
Reflectance spectra of Mercury’s surface obtained during the flybys show no evidence for FeO in surface silicates and a slope from visible to near-infrared wavelengths consistent with space weathering by some combination of micrometeoroid bombardment and sputtering by solar wind ions. The reflectance and color imaging observations provide fresh support for earlier inferences that Mercury’s surface material consists dominantly of iron-poor, calcium-magnesium silicates with a spatially varying admixture of spectrally neutral opaque minerals such as iron-titanium oxides. Analysis of the thermal neutron flux measured during the three flybys combined with calculations of the effects of the spacecraft on the spectrometer response indicate that Mercury’s surface material matches the neutron absorption characteristics of Luna 24 soil from Mare Crisium. Given that little of this Fe+Ti is in silicate phases, the measured neutron absorption is consistent with 7-18% ilmenite by weight, a range broadly consistent with that inferred from color and reflectance observations.
MESSENGER showed that Mercury’s internal magnetic field is dominantly dipolar with a vector moment closely aligned with the spin axis, and no evidence for crustal magnetic anomalies has been found to date. Both results support the inference that Mercury’s magnetic field is the product of a dynamo in the planet’s fluid outer core. Mercury’s magnetosphere was markedly different during each of the MESSENGER flybys. At the time of the first flyby, the interplanetary magnetic field (IMF) had a northward component, the magnetosphere was comparatively steady, and there was little energy input from the solar wind. During the second flyby, the IMF was southward and solar wind energy input was much higher, with magnetic reconnection rates ~10 times greater than typical at Earth. During the third flyby, the IMF direction was variable, and MESSENGER found evidence for “loading” and “unloading” of magnetic energy in the tail at timescales (1-3 min) much shorter than at Earth (1-3 hr). The tail energy is so intense during loading events that the ability of Mercury’s dayside magnetosphere to shield the surface from solar wind ions is substantially curtailed.
In supporting theoretical work, Postdoctoral Associate Natalia Gómez-Pérez has shown that because Mercury’s internal magnetic field is weak, the interaction of the planet’s magnetosphere with the IMF may affect the internal dynamics and the overall behavior of the liquid core. We have hypothesized that Mercury’s core dynamo was stabilized in a weak-field state early in Mercury’s history, when the solar wind was much stronger than today, and has been maintained in that state to the present by magnetospheric feedback. A prediction of this scenario is that secular variation should occur more rapidly for Mercury’s internal field than would be expected for some other models for the planet’s weak field.
In work led by Postdoctoral Associate Thomas Ruedas, new numerical simulations of the thermochemical evolution of the mantle of Mars were completed. The simulations combine a parameterized model of composition and thermoelastic properties of mantle material with a two-dimensional, anelastic, compressible convection and melting algorithm in a spherical-annulus geometry. Results of the models may be compared with chemical and geophysical observations provided by spacecraft missions to Mars. Some models reproduce observed surface concentrations of K and Th and yield crustal thicknesses around 100 km, depths for ancient Curie temperatures for magnetite and hematite between 50 and 100 km, and mechanical lithosphere thicknesses that have increased from less than 50 km at ~3.8 Ga to 150-250 km now. Generally, models with a Mg# of about 75, radionuclide contents similar to those of standard cosmochemical models, and a large core tend to explain observations best. However, the present models indicate that convection is strongly layered and that no long-lived, whole-mantle plumes exist. Under these model assumptions, the existence of Tharsis and Elysium cannot be explained as sustained sites of upwelling from the core-mantle boundary.
Task 3.3 CHON elements in Terrestrial Planetary Interiors
CoI Mysen focused on characterization of reduced COHN volatile in terrestrial and planetary interiors during early stages of planetary formation when redox conditions were such that reduced C-H and N-H volatiles were likely stable together with H2 and H2O. To this end, he determined experimentally the solubility and solution mechanisms of reduced volatiles in the system C-O-H-N in silicate melt compositionally in the range between haplobasalt (NBO/Si=1) and haploandesite (NBO/Si=0.4) to 2.5 GPa at 1400˚C with the hydrogen fugacity controlled with the range between magnetite-hematite + H2O [fH2 (MH)] and iron wüstite + H2O buﬀer [fH2 (IW)] and Mo-MoO2 + H2O buffer (fH2 (MMO). Experiments were conducted with quenched materials and in-situ with the sample at high temperature and pressure using the hydrothermal diamond anvil cell (HDAC). Raman and FTIR spectroscopies were employed as structural tools for studies of glasses, melts, and fluids. Further, hydrogen, carbon, and nitrogen isotope fractionation between melt and fluid were determined and interpreted with the aid of the structural data.
In C-O-H-silicate melt systems at the fH2(IW) carbon exists in fluids and melts in (C+H)-bearing complexes. Methane solubility in melts, calculated as CH4, increases from 0.2 wt% to 0.5 wt% in the melt NBO/Si-range 0.4 to 1.0. The solubility is not signiﬁcantly pressure-dependent, probably because fH2(IW) in the 1–2.5 GPa range does not vary greatly with pressure. From HDAC experiments conducted in-situ in the 500˚-800˚C and 860-1435 MPa temperature and pressure range, respectively, the dominant species in the fluid are CH4+H2O together with minor amounts of molecular H2 and an undersaturated hydrocarbon species. In coexisting melt, CH4 – groups linked to the silicate melt structure via Si-O-CH3 bonding may dominate and possibly coexists with molecular CH4. A schematic solution mechanism is, Si-O-Si+CH4⇌Si-O-CH3+H-O-Si. Carbon isotope fractionation between methane-saturated melts and (H2 + CH4) ﬂuid varied by 14 ‰ in the 0.4 – 1.0 NBO/Si-range of these melts where the dominant (C..H)-bearing complexes are methyl groups, CH3 and CH4, and a complex or functional group that includes entities with double- or triple-bonded carbon. Their abundance ratio is positively correlated with NBO/Si. The 14 ‰ carbon isotope fractionation change between ﬂuid and melt is because of the speciation changes of carbon in the melt.
In C-O-N-silicate systems, the N solubility decreases from 0.98 to 0.28 wt% in the melt NBO/Si-range from 0.4 to 1.18 at fH2(IW) and decreases by about 50% between fH2(IW) and fH2(MH). The H solubility at fH2(IW) is insensitive to NBO/Si and averages 0.76±0.28 wt% and 0.48±0.07 wt% H in (N+H)-saturated and in N-free and H-saturated melts, respectively. The H solubility in the melts decreases by at least ~70% between fH2(IW) and fH2(MH). Under the most reducing conditions, fH2(IW), nitrogen occurs in silicate melts and N-O-H fluids in reduced (N+H)-bearing complexes only. In melts, these are amine groups, NH2, bonded to oxygen, and molecular NH3. In coexisting fluids, NH3 is the only N-bearing species. In fluids and melts, decreasing hydrogen fugacity leads to oxidation of nitrogen to form increasingly abundant molecular N2 in melts and fluids so that at fH2 (MH), the only N-bearing species is N2 with all hydrogen as H2O and OH-groups. Under the most reducing conditions, N and H isotope ratios are systematic functions of the abundance ratio of structurally bound N and H (as NH2— and OH—groups bonded to Si4+) relative to molecular H2, N2, and NH3 in the melts. The NH2-/NH3 and OH-/H2 abundance ratios vary by ~55 and ~500% between NBO/Si=1.18 and 0.4 relative to the values at NBO/Si=0.4. In this same NH2-/NH3 abundance ratio range, the δ15N of (N+H)-saturated melts, relative to that of melts with NBO/Si=0.4, varies by ~2‰, whereas the δD varies by ~87‰. In N-free melts, the δD varies by ~12 ‰. Changing abundance of volatiles dissolved in silicate melts in molecular form and as structural complexes that form bonds as a function of the silicate melt structure is, therefore, an important factor that can affect stable isotope fractionation during melting and crystallization at high pressure and temperature.
Oxygen is the most abundant element in the Earth. Its chemical reactivity (reduction and oxidation) plays a critical role in many geological processes in the deep Earth. CoI’s Goncharov and Hemley have been performing extensive laboratory experiments and theoretical simulations that reveal unique properties of oxygen at elevated pressures including a rich phase diagram, numerous anomalies in thermal, magnetic, and optical properties, and transformation to a conducting state at high pressures and temperatures. However, the knowledge of the phase diagram and the stable phases of oxygen at simultaneous conditions of high temperatures and high pressures (P-T) is limited below a thousand degrees kelvin. Experiments employing X-ray diffraction and Raman spectroscopy in laser and resistively heated diamond anvil cells, reveal that familiar molecular high-pressure phase ε-O2, which consists of (O2)4 clusters, reversibly transforms in the pressure range of 44 to 90 GPa and temperatures above 1000 K to a new phase that is isostructural to a phase reported previously at lower pressures and temperatures. The melting curve increases monotonically up to the maximum pressures studied (~60 GPa). The structure factor of the fluid measured as a function of pressure to 58 GPa shows continuous changes toward molecular dissociation. These results provide a basis for understanding of the chemical and physical transformations in the Earth’s lower mantle.
Task 3.4 Primary carbon in Martian meteorites and terrestrial analogues
Former Carnegie/NAI Fellow (now at Univ. New Mexico) Francis McCubbin working with CoI Andrew Steele focused on abundance and distribution of volatiles in the system COH in martian and lunar interiors. Whereas it is fairly well known that the Earth contains from several hundred and up to 1000 ppm H2O in its interior; little is known regarding the interior water contents of other planetary bodies in our solar system. Understanding the water content of a planetary interior will aid not only in understanding the thermal and magmatic evolution of that planet, but also its potential for hosting habitable environments for putative life. Therefore, one of the primary goals of this study was to determine the water contents of both the martian and lunar interiors. The martian and lunar interiors were chosen because they are the only large solar system bodies from which we have samples, and sample analysis is the most robust way of estimating the water content of a planetary interior.
Prior to the present study, Mars was reported to have a very dry interior, with only 1-37 ppm H2O. These values were primarily derived by the analysis of hydrous minerals, which were reported to be relatively dry. However, many of the reported mineral compositions were in violation of crystal chemical rules, and further analyses were required. McCubbin and coworkers showed that the water contents in the hydrous mineral grains were higher than previous estimates by at least a factor of four. Furthermore, the water content of the magmatic source region for the magma that crystallized the hydrous minerals was found to range from at least 140-250 ppm H2O, which is within the same order of magnitude as many estimates for the water content of the terrestrial interior.
For the last 40 years (since the Apollo missions to the Moon), the Moon has been widely reported as anhydrous, and estimates of water in the lunar interior have ranged from nothing to less than 1 ppb H2O. This anhydrous nature was supported by the near absence of hydrous minerals in many lunar rocks, however there was one mineral (apatite) that commonly contains water that was present in lunar rocks. In a study by McCubbin in collaboration with CoI Andrew Steele and others, provided qualitative evidence to the effect that there is at least some water within this mineral from lunar rocks. Subsequently, the amount of water was quantified in a few lunar samples using Secondary ion mass spectrometry. From the apatite water contents, a range of water contents for the lunar interior was calculated, from approximately 64 ppb to 5 ppm H2O. It was also shown that, while much drier than the Earth and Mars, the Moon did contain significant amounts of water within its interior.
The three studies reported here have shown that water is significantly more abundant in our neighboring solar system bodies than previously thought, and this could make for a much more habitable solar system than previously thought. Although water is very important for sustaining and perhaps forming life, it is not as rare a substance as once thought.
Previous studies of carbon in lunar rocks have not identified discrete carbon phases, except for carbides and solar wind implanted carbon. Although condensed organic phases coating lunar fines have been reported. CoI Steele and collaborators have discovered discrete multiple micron sized graphite phases within an Apollo 17 impact breccia. This sample was collected from land-slide material at Taurus-Littrow. 72255 is an aphanitic impact-melt breccia with a dark, fine-grained equigranular crystalline matrix containing larger clasts. The youngest material contained in the sample is dated to ~3.84 Ga, which is the age assigned to the Serenitatis impact basin. We conducted two and three dimensional Confocal Raman Imaging Spectroscopy (CRIS) on a thin section and on fresh fracture surfaces of sample 72255 In all, we imaged ~0.5 mm2 area of thin section 72255,89, with over 68 occurrences of subsurface (2 to 8 μm within the surface) graphite blebs between ~2 to 6 μm in diameter. Seven occurrences of subsurface graphite grains, one of which was mapped in 3-D, occur between 3 and 8 μm within the section. All instances of graphitic carbon were restricted to the 0.1 mm2 area analyzed around an area of darkened Aphinitic melt. No graphite phases are found in the 0.4 mm2 area of lighter material analyzed, although common carbon contamination phases occur all across the thin section. It therefore appears that carbonaceous material from impacts at the time of the Late Heavy Bombardment (LHB) and at a time when life may have been emerging on Earth, does survive on the Moon.
Task 3.5 Terrestrial Evolution
CoI Shirey examined geotectonics and carbon from the deep mantle in the early Earth from the perspective of Mesoarchean to Proterozoic eclogites and diamonds. Progress was made during FY2 primarily in the study of diamonds. Two synthesis/review papers were published; one describing the occurrence of natural macrodiamonds throughout Earth history and the other comparing the creation and depletion history of the Slave and Kaapvaal cratons. The former summarizes how diamond parageneses have changed versus time and the later points out how the Kaapvaal craton has been so depleted of sulfur early in its history that its lithosphere harbors virtually no peridotitic sulfide-bearing diamonds while the Slave craton has not seen the same process.
A major study of the sulfide bearing diamonds from the Proterozoic Ellendale lamproite, Kimberley craton, Western Australia was completed and published. A Proterozoic Re-Os isochron was obtained on exceptionally small (e.g. 1 microgram) sulfides that gave a depleted initial 187Os/188Os indicating reactivation of Archean lithosphere and incorporation of some of the lithospheric Os during this process.
Significant progress was made in the area of future collaborations, procurement of new diamond suites for study and expansion of lab capabilities. A field excursion to the Fuxian kimberlite field, in concert with the Sloan-Deep Carbon Observatory China workshop was made by Shirey and DTM ion probe specialist Jianhua Wang to look at the Pipe 50 kimberlite and to potentially look at any new diamonds that have been uncovered by ongoing state geological survey prospecting. Diamonds form this craton are important because of their unique C and N isotopic compositions and their ability to record the composition and history of the Archean lithosphere (largely removed since the Mesozoic). Shirey gave two talks; one invited talk (“Terrestrial macro-diamonds: tracers of ancient carbon-rich fluids delivered geologically to the deep continental lithosphere”) to the Sloan-DCO workshop on the role of diamond petrology in understanding the Earth’s deep igneous carbon cycle and the other to Brigade 6 (the state geological survey) on the ages diamonds and their formation. It is hoped that this trip will yield collaborations with Dr. Hongfu Zhang and his group so that future study of Pipe 50 and Mengyin diamonds will be possible.
With NAI travel support, co-I Shirey attended the 5th International Archean Symposium in Perth, Western Australia to give an invited talk (“_Isotopic constraints on the formation of Archean mantle and crust_”). This once-every-decade meeting, attended by a worldwide audience of 300-400, focusses exclusively on Archean geology, geochronology, ore forming processes, and geotectonics and is important for keeping up with the latest research directions. The symposium was followed by a field trip that made a time transect of geological occurrences across the Yilgarn craton from the 4.4 billion year old zircon-containing quartz-pebble conglomerates of the Jack Hills, through to the Neoarchean Coodardy granites. While not directly related to diamond occurrences, this trip was essential to frame questions related to the Archean operation of the carbon cycle in the latest geological understanding of the early Earth.
A new collaboration was initiated with the high pressure igneous petrology group at the University of Bristol (Michael Walter, Simon Kohn, Galina Bulanova and Christopher Smith) and the University of Brasilia (Deborah de Passos D’Arujao). This will bring the potential to work on new diamond samples from the Zimbabwe craton and Amazonian craton, of which the later craton includes rare superdeep diamonds from the Juina field, Brazil and komatiite-hosted diamonds from the Dachine area, French Guyana. Smith, Bulanova, and Kohn visited the Carnegie Institution in September to begin the collaboration. Some 60 diamonds from their collection were laser cut and a subset of them were polished to reveal inclusion detail. Normal and confocal Raman spectroscopy was carried out on the about 8 superdeep diamonds from the Collier 4 pipe of the Juina field revealing new detail on microinclusion abundance, composition and residual strain in these superdeep diamonds. During this visit, small diamond sample handing was substantially improved.
CoIs Hazen and Sverjensky continued to examine mineral evolution, which frames mineralogy in an historical context. These studies are based on the premise that the geo- and biospheres have coevolved through a sequence of deterministic and stochastic events. Three eras of mineral evolution – (1) planetary accretion, (2) crust and mantle reworking, and (3) biologically mediated mineralogy – each saw dramatic changes in the diversity and distribution of Earth’s near surface minerals. An important implication of this model is that different terrestrial planets and moons achieve different stages of mineral evolution. From a planetary perspective, the concept of mineral evolution allows each terrestrial body in the solar system to be placed in a broader mineralogical context. Mineral evolution provides an intellectual framework for identifying mineralogical targets in the search for extraterrestrial life.
Our efforts in mineral evolution research have focused on two main approaches. On the one hand we examine specific groups of minerals in detail. For example, our study of uranium and thorium minerals focused on the 4.5 billion year history of the parageneses and near-surface distributions of the approximately 250 known U and Th species. This history can be divided into four phases. The first, from ~4.5 to 3.5 Ga, involved successive concentrations of uranium and thorium from their initial uniform trace distribution into magmatic-related fluids from which the first U4+ and Th4+ minerals, uraninite (UO2), thorianite (ThO2) and coffinite (USiO4), precipitated in the crust. The second period, from ~3.5 to 2.2 Ga, saw the formation of large low-grade concentrations of detrital uraninite (containing several weight percent Th) in the Witwatersrand-type quartz-pebble conglomerates deposited in a highly anoxic fluvial environment. Earth’s third phase of uranium mineral evolution, during which most known U minerals first precipitated from reactions of soluble uranyl (U6+O2)2+ complexes, followed the Great Oxidation Event (GOE) at ~2.2 Ga and thus was mediated indirectly by biologic activity (Figure 1). The fourth phase of uranium mineralization began approximately 400 million years ago, as the rise of land plants led to non-marine organic-rich sediments that promoted new sandstone-type ore deposits. The near-surface mineralogy of uranium and thorium thus provides a measure of a planet’s geotectonic and geobiological history. In addition to this study, research is in progress on the minerals of Be and B (with Edward Grew, University of Maine), Mo and W (with Robert Downs and Melissa McMillan, University of Arizona), carbonate minerals (with Linda Kah, University of Tennessee), and several other elements.
We have also focused on clay mineral evolution, particularly with respect to changes in atmospheric composition. We find surprising correlations between relative abundances of the clay minerals kaolinite, montmorillonite and illite in shales global model changes in O2 or CO2 throughout the Phanerozoic (Figure 2). Furthermore, significant changes in patterns of these correlations at ~350 and ~200 Ma correspond to major biological events relevent to the weathering zone, including the rise of deep-rooted vascular plants and their associated mycorrhizal fungi. The present study supports recent reports that mycorrhizal evolution and function played an important role in biological weathering and the long-term carbon cycle.
A third major effort to develop a “Mineral Evolution Data Base has been initiated recently, thanks in part to a generous grant from the Carnegie Institution. Progress in mineral evolution depends on the development of a comprehensive database of mineral localities of known ages and geologic settings. Such a database, employing Mindat.org website as a platform, will incorporate software to correlate a range of mineral occurrences and properties versus time, and will thus facilitate studies of the changing diversity, distribution, associations, and characteristics of individual minerals as well as mineral groups. The Mineral Evolution Database thus holds the prospect of revealing mineralogical indicators that elucidate important geophysical, geochemical, and biological events in Earth history.
PROJECT INVESTIGATORS:John Chambers
PROJECT MEMBERS:George Cody
RELATED OBJECTIVES:Objective 1.1
Formation and evolution of habitable planets.
Sources of prebiotic materials and catalysts
Earth's early biosphere.