Composition of terrestrial planets records planetary accretion, core–mantle and crust–mantle differentiation, and surface processes. Here we compare the compositional models of Earth and Mars to ...reveal their characteristics and formation processes. Earth and Mars are equally enriched in refractory elements (1.9 × CI), although Earth is more volatile-depleted and less oxidized than Mars. Their chemical compositions were established by nebular fractionation, with negligible contributions from post-accretionary losses of moderately volatile elements. The degree of planetary volatile element depletion might correlate with the abundances of chondrules in the accreted materials, planetary size, and their accretion timescale, which provides insights into composition and origin of Mercury, Venus, the Moon-forming giant impactor, and the proto-Earth. During its formation before and after the nebular disk's lifetime, the Earth likely accreted more chondrules and less matrix-like materials than Mars and chondritic asteroids, establishing its marked volatile depletion. A giant impact of an oxidized, differentiated Mars-like (i.e., composition and mass) body into a volatile-depleted, reduced proto-Earth produced a Moon-forming debris ring with mostly a proto-Earth's mantle composition. Chalcophile and some siderophile elements in the silicate Earth added by the Mars-like impactor were extracted into the core by a sulfide melt (∼0.5% of the mass of the Earth's mantle). In contrast, the composition of Mars indicates its rapid accretion of lesser amounts of chondrules under nearly uniform oxidizing conditions. Mars’ rapid cooling and early loss of its dynamo likely led to the absence of plate tectonics and surface water, and the present-day low surface heat flux. These similarities and differences between the Earth and Mars made the former habitable and the other inhospitable to uninhabitable.
Terrestrial planets (Mercury, Venus, Earth, and Mars) are differentiated into three layers: a metallic core, a silicate shell (mantle and crust), and a volatile envelope of gases, ices, and, for the ...Earth, liquid water. Each layer has different dominant elements (e.g., increasing iron content with depth and increasing oxygen content to the surface). Chondrites, the building blocks of the terrestrial planets, have mass and atomic proportions of oxygen, iron, magnesium, and silicon totaling ≥ 90% and variable Mg/Si (∼ 25%), Fe/Si (factor of ≥2), and Fe/O (factor of ≥ 3). What remains an unknown is to what degree did physical processes during nebular disk accretion versus those during post-nebular disk accretion (e.g., impact erosion) influence these planets final bulk compositions. Here we predict terrestrial planet compositions and show that their core mass fractions and uncompressed densities correlate with their heliocentric distance, and follow a simple model of the magnetic field strength in the protoplanetary disk. Our model assesses the distribution of iron in terms of increasing oxidation state, aerodynamics, and a decreasing magnetic field strength outward from the Sun, leading to decreasing core size of the terrestrial planets with radial distance. This distribution enhances habitability in our solar system and may be equally applicable to exoplanetary systems.
The composition of Mars Yoshizaki, Takashi; McDonough, William F.
Geochimica et cosmochimica acta,
03/2020, Letnik:
273
Journal Article
Recenzirano
Odprti dostop
Comparing compositional models of the terrestrial planets provides insights into physicochemical processes that produced planet-scale similarities and differences. The widely accepted compositional ...model for Mars assumes Mn and more refractory elements are in CI chondrite proportions in the planet, including Fe, Mg, and Si, which along with O make up >90% of the mass of Mars. However, recent improvements in our understandings on the composition of the solar photosphere and meteorites challenge the use of CI chondrite as an analog of Mars. Here we present an alternative model composition for Mars that avoids such an assumption and is based on data from Martian meteorites and spacecraft observations. Our modeling method was previously applied to predict the Earth’s composition. The model establishes the absolute abundances of refractory lithophile elements in the bulk silicate Mars (BSM) at 2.26 times higher than that in CI carbonaceous chondrites. Relative to this chondritic composition, Mars has a systematic depletion in moderately volatile lithophile elements as a function of their condensation temperatures. Given this finding, we constrain the abundances of siderophile and chalcophile elements in the bulk Mars and its core. The Martian volatility trend is consistent with ⩽7 wt% S in its core, which is significantly lower than that assumed in most core models (i.e., >10 wt% S). Furthermore, the occurrence of ringwoodite at the Martian core-mantle boundary might have contributed to the partitioning of O and H into the Martian core.
•Nominally lithophile elements are moderately chalcophile in highly reduced EC.•A separation of reduced sulfides produced RLE fractionation in EC chondrules.•The EC-dominated Earth models require RLE ...release from reduced sulfides to mantle.•Otherwise, the Earth’s building blocks condensed before a sulfide precipitation.•Nb might be partially siderophile, rather than chalcophile, in the Earth.
Chondrites are sediments of materials left over from the earliest stage of the solar system history. Based on their undifferentiated nature and less fractionated chemical compositions, chondrites are widely considered to represent the unprocessed building blocks of the terrestrial planets and their embryos. Models of chemical composition of the terrestrial planets generally find chondritic relative abundances of refractory lithophile elements (RLE) in the bulk bodies (“constant RLE ratio rule”), based on limited variations of RLE ratios among chondritic meteorites and the solar photosphere. Here, we show that ratios of RLE, such as Nb/Ta, Zr/Hf, Sm/Nd and Al/Ti, are fractionated from the solar value in chondrules from enstatite chondrites (EC). The fractionated RLE ratios of individual EC chondrules document different chalcophile affinities of RLE under highly reducing environments and a separation of RLE-bearing sulfides from silicates before and/or during chondrule formation. In contrast, the bulk EC have solar-like RLE ratios, indicating that a physical sorting of silicates and sulfides was negligible before and during the accretion of EC parent bodies. Likewise, if the Earth’s accretion was dominated by EC-like materials, as supported by multiple isotope systematics, physical sorting of silicates and sulfides in the accretionary disk did not occur. Alternatively, the Earth’s precursors were high-temperature nebular condensates that formed prior to the precipitation of RLE-bearing sulfides. A lack of Ti depletion in the bulk silicate Earth, combined with similar silicate-sulfide and rutile-melt partitioning behaviors of Nb and Ti, prefers a moderately siderophile behavior of Nb as the origin of the accessible Earth’s Nb depletion. Highly reduced planets that have experienced selective removal or accretion of silicates or metal/sulfide phases, such as Mercury, possibly yield fractionated, non-solar bulk RLE ratios.
Ultrarefractory (UR) phases in calcium-aluminum-rich inclusions (CAIs) could have formed at higher temperature compared to common CAI minerals and thus they potentially provide constraints on very ...high temperature processes in the solar nebula. We report a detailed characterization of the mineralogy, petrology and oxygen isotopic composition of an UR phase davisite (CaScAlSiO6) bearing CAI from a reduced type CV chondrite. The CAI is an irregular-shaped, compound inclusion that consists of five units that are composed of melilite + spinel + Al,Ti-rich pyroxene ± perovskite with various modal abundances of minerals and lithologies, and surrounded by the Wark-Lovering (WL) rim. Davisite occurs only in one lithological unit that consists of three chemically and isotopically distinct parts: (i) 16O-poor (-20‰⩽δ18O⩽0‰) regions with reversely-zoned melilite and davisite; (ii) 16O-rich (-50‰⩽δ18O⩽-40‰) regions consisting of unzoned, gehlenitic melilite, Al,Ti-rich diopside and spinel; and (iii) spinel framboids composed of 16O-rich spinel and 16O-poor melilite. Absence of secondary iron- and/or alkali-rich phases, occurrence of low-iron, manganese-enriched (LIME) olivine, and random distribution of the oxygen isotopic heterogeneity indicate that primitive chemical and isotopic compositions are preserved in the inclusion. The occurrence of chemical and isotopic heterogeneities with sharp boundaries in the CAI indicates formation of the inclusion by an aggregation of mineral assemblages formed and processed separately at different time and/or space in the solar nebula. Although isotope exchange between 16O-rich solids and 16O-poor gases prior to the final agglomeration of the CAI cannot be ruled out, we suggest that modification of chemical and isotopic composition of porous CAI precursors or aggregation of isotopically distinct mineral assemblages are alternative scenarios for the origin of oxygen isotopic heterogeneity in CAIs. In either case, coexistence of spatially and/or temporally distinct 16O-rich and 16O-poor gaseous reservoirs at the earliest stage of the solar system formation is required. The grain-scale oxygen isotopic disequilibrium in the CAI indicate that post-formation heating of the inclusion (i.e., the WL rim formation event) was short (e.g., ≲103 h at 1400 K; ≲105 h at 1100 K), which can be achieved by rapid outward transport of the CAI. High Ti3+/Titot ratios of pyroxene from CAI interior and the rim and LIME composition of the olivine rim document that the entire CAI formation process took place under highly reducing conditions.
Purpose
To evaluate the effect of combined use of ibandronate and eldecalcitol for 6 to 12 months on bone mineral density (BMD) and bone strength of the proximal femur in postmenopausal Japanese ...women with osteoporosis.
Methods
BMD and bone strength of the proximal femur were evaluated in 78 postmenopausal women (mean age, 73.6 years) who underwent treatment for osteoporosis with combined use of ibandronate and eldecalcitol for at least 6 months. BMD was measured at the lumbar spine, femoral neck, and total hip using dual-energy X-ray absorptiometry (DXA) at baseline and every 6 months thereafter. Hip structure analysis of the narrow neck and intertrochanter was performed by a radiologist using DXA images.
Results
Respectively for the lumbar spine, femoral neck, and total hip, BMD significantly increased (from baseline) by 4.54%, 2.31%, and 1.56% at 6 months and by 5.92%, 3.02%, and 2.70% at 12 months. In hip structure analysis, most parameters improved significantly. Respectively for the narrow neck and intertrochanter, BMD significantly increased (from baseline) by 2.37% and 2.71% at 6 months and by 3.46% and 3.52% at 12 months; cross-sectional area significantly increased by 1.83% and 3.39% at 6 months and by 2.91% and 3.46% at 12 months; section modulus significantly increased by 2.42% and 4.11% at 6 months and by 4.84% and 3.26% at 12 months; cortical thickness significantly increased by 2.49% and 3.33% at 6 months and by 3.73% and 3.37% at 12 months; and buckling ratio significantly decreased by 2.97% and 2.57% at 6 months and by 3.86% and 2.99% at 12 months.
Conclusion
Combined use of ibandronate and eldecalcitol for 6 months significantly improved bone strength of the proximal femur in postmenopausal Japanese women with osteoporosis.
The purpose of this study was to clarify the effects of a 1-year treatment with either alendronate or risedronate on the proximal femoral geometry among Japanese women with osteoporosis by hip ...structure analysis. Postmenopausal women who had taken at least 90% of their prescription for alendronate (35 mg/week, 94 patients) or risedronate (17.5 mg/week, 181 patients) for 1 year were retrospectively analyzed. In the alendronate treatment group, bone mineral density (BMD), cross-sectional area (CSA), section modulus and average cortex significantly increased by 0.81, 1.35, 2.23 and 0.97% at the narrow neck and increased by 2.19, 2.28, 2.85 and 1.11% at the intertrochanteric, respectively. Buckling ratio at the intertrochanteric significantly decreased by 2.50%. The CSA, section modulus and average cortex at the shaft significantly increased at 1 year. In the risedronate treatment group, the CSA, section modulus and average cortex at the narrow neck significantly increased by 0.80, 0.95 and 0.89%, respectively. BMD, CSA, section modulus, and average cortex at the intertrochanteric significantly increased by 1.61, 0.88, 2.05 and 0.79%, respectively, and buckling ratio significantly decreased by 1.53%. BMD, CSA, section modulus, and average cortex at the shaft significantly increased. The percent change of section modulus was significantly correlated with that of BMD, CSA and average cortex and negatively correlated with that of buckling ratio at all regions in both treatment groups. Statistically significant differences between the alendronate and risedronate groups were seen for section modulus in the narrow neck and CSA in the intertrochanteric. In conclusion, Japanese osteoporotic women treated by either alendronate or risedronate showed significant improvements of geometry in proximal femur within 1 year.
Purpose.
To assess correlations between a bone resorption marker and the structural geometry of the proximal femur in raloxifene-treated postmenopausal women with osteoporosis.
Methods.
45 ...postmenopausal, osteoporotic women aged 57 to 79 (mean, 67) years underwent raloxifene treatment (60 mg/day) for 12 months. Serum type-I collagen crosslinked N-telopeptide (sNTX) as the bone resorption marker was measured at baseline and 6 months, whereas hip structure analysis (HSA) parameters of the femur (neck, intertrochanter, and shaft) were measured at baseline and 12 months using dual energy X-ray absorptiometry. The HSA parameters included areal bone mineral density (BMD), inner diameter, mean cortical thickness, cross-sectional area, section modulus, and buckling ratio. Correlations between sNTX and HSA parameters were analysed using Pearson's R.
Results.
At baseline, sNTX correlated inversely with BMD, cross-sectional area, mean cortical thickness, and section modulus, and positively with buckling ratio in the intertrochanter and shaft (but not the neck). These correlations were significant both crude and adjusted for age. After 12 months of raloxifene treatment, HSA parameters improved significantly for the intertrochanter and shaft only. Of the 3 femoral sites, only the change in sNTX at month 6 correlated positively with changes in the inner diameter at the intertrochanter at month 12 (r=0.303, p=0.045 adjusted for age). Changes in sNTX did not correlate with changes in any other parameters.
Conclusion.
The surrogate markers for hip fracture including sNTX, BMD, and HSA parameters correlated significantly with each other. Improvement (decrease) of sNTX indicated reduction in bone resorption.
Abstract Two-dimensional (2D) material is drawing considerable attention as a promising thermoelectric material. This study establishes the formation method of renewed Ca-intercalated group IV 2D ...materials, Ca(Ge 1− x Sn x ) 2 crystals including germanene-based 2D layers. The solid phase epitaxy allows us to form epitaxial Ca(Ge 1− x Sn x ) 2 on Si. Atomic force microscopy reveals that the Ca(Ge 1− x Sn x ) 2 has island structures. X-ray diffraction proved the epitaxial growth of the Ca(Ge 1− x Sn x ) 2 island structures and the increase of the c -axis lattice constant with Sn content increase. The formation of this renewed intermetallic compound including group IV 2D layer opens an avenue for high performance thermoelectric generator/Si.
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