Petrographic and chemical characterization of igneous lithic clasts from mesosiderites and howardites and comparison with eucrites and diogenites

Combined petrographic, electron microprobe and instrumental neutron activation analysis (INAA) studies of igneous lithic clasts separated from mesosiderites and howardites and INAA investigation only of whole rock eucrites and diogenites have been performed to help elucidate the differentiation processes that occurred on asteroidal sized bodies. Although similar to eucrites in mineralogy and major element chemistry, trace element abundances in basaltic lithic clasts give evidence for more complex differentiation episodes than have been observed for eucrites. These complex fractionations include sequential melting and expulsion of liquid from the source region and remelting of cumulate materials, followed by a second fractional crystallization episode. Rare earth element (REE) abundances in a basaltic clast from Petersburg suggest that the source region which produced this melt was noticably different from that which produced the eucrites Pasamonte and Bereba.Pyroxenites from mesosiderites show slight enrichments in Sc and Mn when compared with average diogenites. This suggests that the pyroxenites in mesosiderites are not fragments of diogenites sensu stricto. A plagioclase clast from the Johnstown diogenite contains light REE abundances that are not in equilibrium with the pyroxene phase. This implies that some of the plagioclase in diogenites may be a foreign component not directly related to the diogenites. This component probably formed on the same parent body as the diogenites however.The characteristics which are inferred for the heat source are that it was spatially and temporally variable. This suggests that heating of the differentiated meteorite parent bodies may in part have been from outside the parent body.     

Magnetite compositions and oxygen fugacities of the Khibina magmatic system

Most titanomagnetite in the Khibina alkaline igneous complex, sampled through 500 m of a vertical cross-section, is represented by Ti-rich varieties. The ulvöspinel component is most commonly around 55 mol%, rarely reaching up to 80 mol%.

We calculated an f_O2–T diagram for magnetite + ilmenite + titanite + clinopyroxene + nepheline + alkali feldspar and magnetite + titanite+ clinopyroxene + nepheline + alkali feldspar phase assemblages at a hedenbergite activity of 0.2. The diagram shows that magnetites with 55 mol% of ulvöspinel crystallized at oxygen fugacities just slightly below the quartz–fayalite–magnetite buffer. More Ti-rich varieties crystallized at higher temperatures and slightly lower ΔQMF values, whereas more Ti-poor magnetites crystallized at or below about 650 °C.

Under the redox conditions estimated for the apatite-bearing intrusion of the Khibina complex (close to the QFM buffer), substantial quantities of methane may only form during cooling below 400 °C in equilibrium with magma. However, even at higher orthomagmatic temperatures and redox conditions corresponding to ΔQMF = 0, the hydrogen content in the early magmatic stage is not negligible. This hydrogen present in the gas phase at magmatic temperatures may migrate to colder parts of a solidifying magma chamber and trigger Fischer-Tropsch-type reactions there. We propose therefore, that methane in peralkaline systems may form in three distinct stages: orthomagmatic and late-magmatic in equilibrium with a melt and — due to Fischer-Tropsch-type reactions — post-magmatic in equilibrium with a local mineral assemblage.     

Apatite stability under different oxygen fugacities relevant to planetary bodies

Mineralogy and Petrology - Apatite is widely distributed in terrestrial and extraterrestrial environments and may therefore crystallize in relatively oxidized environments found here on Earth, and...     

Mineral assemblages and oxygen and sulphur fugacities in natural water-rock interaction processes

Several mineral assemblages capable of buffering oxygen and sulphur fugacity and theoretical calculations were used to determine the fugacities of these two gas species over a temperature range between 150° and 350°C. Computed values were compared with those calculated from the composition of the geothermal fluids of Larderello, Travale, Bagnore and Piancastagnaio (Italy), The Geysers (USA) and Cerro Prieto (Mexico). The possible hydrothermal mineral assemblages that best fit the field data are those containing minerals frequently found in many geothermal fields: epidote, chlorite, K-feldspar, quartz and pyrite. These assemblages can be used in place of the classical buffers based on iron oxides and sulphides.     

Differentiation history of the mesosiderite parent body: constraints from trace elements and manganese-chromium isotope systematics in Vaca Muerta silicate clasts

We report here the results of a study of trace element microdistributions and ⁵³Mn-⁵³Cr systematics in several basaltic and orthopyroxenitic clasts from the Vaca Muerta mesosiderite. Ion microprobe analyses of selected trace and minor element abundances in minerals of the silicate clasts indicate that, following igneous crystallization, these clasts underwent extensive metamorphic equilibration that resulted in intra- and inter-grain redistribution of elements. There is also evidence in the elemental microdistributions that these clasts were subsequently affected to varying degrees by alteration resulting from redox reactions involving the indigenous silicates and externally derived reducing agents (such as phosphorus, derived from the mesosiderite metal) at the time of metal-silicate mixing. Furthermore, our results suggest that the varying degrees of alteration by redox reactions recorded in the different clasts were most likely facilitated by different degrees of remelting induced by heating during the metal-silicate mixing event. After taking into account the effects of these postmagmatic secondary processes, comparison of the trace and minor element concentrations and distributions in minerals of basaltic and orthopyroxenitic clasts with those of noncumulate eucrites and diogenites, respectively, suggests that the primary igneous petrogenesis, including parent magma and source compositions, of Vaca Muerta silicates were similar to those of achondritic meteorites of the Howardite-Eucrite-Diogenite (HED) association. Internal ⁵³Mn-⁵³Cr isochrons obtained for two basaltic (pebble 16 and 4679) and two orthopyroxenitic (4659 and 4670) clasts show that chromium isotopes are equilibrated within each clast. Nevertheless, just as for noncumulate eucrites and diogenites, ⁵³Cr excesses in whole-rock samples of the basaltic clasts (∼1.01 ε in pebble 16; ∼1.07 ε in 4679) are significantly higher than in the orthopyroxene-rich clasts (∼0.62 ε in 4659; ∼0.53 ε in 4670). As in the case of the HED parent body, this suggests that Mn/Cr fractionation in the parent body of the Vaca Muerta silicate clasts occurred very early in the history of the solar system, when ⁵³Mn was still extant. However, the slope of the ⁵³Mn-⁵³Cr isochron defined by the whole-rock samples of Vaca Muerta clasts (corresponding to a ⁵³Mn/⁵⁵Mn ratio of 3.3 ± 0.6 × 10⁻⁶) is distinctly lower than that defined by the HED whole-rock samples (corresponding to a ⁵³Mn/⁵⁵Mn ratio of 4.7 ± 0.5 × 10⁻⁶), indicating that the global Mn/Cr fractionation event that established mantle source reservoirs on the parent body of the Vaca Muerta silicate clasts occurred ∼2 Ma after a similar event on the HED parent body.     

Crystal chemistry of a natural schorlomite and Ti-andradites synthesized at different oxygen fugacities

Ti-andradites were synthesized at a pressure of P(H₂O)=3 kbar and temperatures of 700–800° C. Oxygen fugacities were controlled by solid state buffers (Ni/NiO; SiO₂ + Fe/Fe₂SiO₄). The Fe²⁺-and Fe³⁺-distribution was determined by low temperature Mössbauer spectroscopy. The water content was measured by a solid's moisture analyzer. The chemical composition of the synthetic and the natural sample has been determined by electron microprobe. Ti-andradites from runs at high oxygen fugacities have Fe³⁺ on octahedral and tetrahedral sites; Ti-andradites from runs at low oxygen fugacities have tetrahedrally and octahedrally coordinated Fe²⁺ as well. These “reduced” garnets must also contain Ti³⁺ on octahedral sites. Charge balance is maintained due to substitution of O^2? by (OH)^? by two mechanisms: (SiO₄)^4? ? (O₄H₄)^4? and (Fe³⁺O₆)^9? ? (Fe²⁺O₅OH)^9?. FTIR spectra of the synthetic samples do show the presence of structurally bound (OH)^?. In a natural sample tetrahedrally and octahedrally coordinated Fe³⁺ are observed together with Fe²⁺ on all three cation sites of the garnet structure.     

Orthopyroxene-olivine assemblages in diogenites and mesosiderites

Diogenites contain equilibrated orthopyroxene-olivine assemblages. Mn is very regularly partitioned between olivine and orthopyroxene in pallasites, diogenites and synthetic eucrite melts, with an $\text{FeO}\text{MnO}$ partition ratio for olivine versus orthopyroxene of 1.6 by weight over a very wide range of FeO contents. In contrast to diogenites, Fe and Mn are not regularly partitioned between the olivine and orthopyroxene of mesosiderites and these minerals were not in equilibrium. Mesosiderite olivine differs from diogenite olivine in $\text{Fe}\text{Mn}$ and $\text{Ca}\text{Mn}$ ratios. Lack of olivine-orthopyroxene equilibrium suggests that olivine in mesosiderites was derived not from a pyroxenite component analogous to diogenites but from dunites.     

Petrogenetic relationships between diogenites and olivine diogenites: Implications for magmatism on the HED parent body

Diogenites are orthopyroxenites and harzburgites that are petrogenetically associated with basaltic magmatism linked to the earliest stages of asteroidal melting on the parent body for the howardite-eucrite-diogenite (HED) meteorites. There are several models proposed for their origin: (1) accumulation of orthopyroxene (OPX) + chromite (CHR) ± olivine (OL) during the crystallization of a magma ocean during the initial stages of asteroidal differentiation, (2) accumulation of OPX + CHR ± OL during the crystallization of compositionally distinct basaltic magmas emplaced into the crust of the HED parent body, and (3) the orthopyroxenites formed by the crystallization of basaltic magmas within the HED parent body crust, whereas the harzburgites represent the mantle of the HED parent body. Although OL and OPX experienced varying degrees of subsolidus reequilibration (1100-700 °C), their minor and trace element characteristics appear to partially preserve magmatic signatures that provide insights into distinguishing among different models for the origin of diogenites. The OPX exhibits a continuous and very systematic variation in incompatible elements such as Al, Ti, Zr, Y, and Yb. Polymict diogenites (i.e. Roda, EET 79002) can contain distinct lithologies with both different incompatible element characteristics and different model abundances of OL. There appears to be no relationship between the appearance and abundance of OL and the incompatible element characteristics of the OPX. The OL exhibits a range in Mg# and systematic variations Ni, Co, Ni/Co, and Mn. For examples, low Ni/Co appears to be closely associated with the harzburgites and Ni and Mn exhibit a negative correlation. Surprisingly, incompatible element concentrations in OPX are not negatively correlated to Ni concentrations in OL. The continuous nature of the minor and trace element characteristics of the OPX and OL is consistent with the all the diogenite lithologies forming through a single process such as crystallization within a magma ocean or a series of layered intrusions. Further, the range in incompatible element variability in the OPX, the Ni and Co systematics in the OL, and the association of distinctly different lithologies within polymict diogenites are most consistent with the diogenites representing lithologies from diverse layered intrusions. Alternatively, they may represent crystallization products of a magma ocean much more complex than has been thus far proposed (i.e. multiple MOs). There are some distinct differences between diogenites and the OL-rich achondrite QUE 93148 that was also analyzed during this study. These differences (such as Ni/Co in OL, estimated conditions of f_O2) suggest that QUE 93148 is closely related to main-group pallasites rather than the parent bodies for the HED meteorites.     

A Mössbauer and X-ray diffraction study of annites synthesized at different oxygen fugacities and crystal chemical implications

A refined set of Mössbauer parameters (isomer shifts, quadrupole splittings, Fe²⁺/Fe³⁺ ratios) and lattice parameters were obtained from annites synthesized hydrothermally at pressures between 3 and 5 kbars, temperatures ranging from 250 to 780° C and oxygen fugacities controlled by solid state buffers (NNO, QMF, IM, IQF). Mössbauer spectra showed Fe²⁺ and Fe³⁺ on both the M1 and the M2 site. A linear relationship between Fe³⁺ content and oxygen fugacity was observed. Towards low Fe³⁺ values this linear relationship ends at ≈10% of total iron showing that the Fe³⁺ content cannot be reduced further even if more reducing conditions are used. This indicates that in annite at least 10% Fe²⁺ are substituted by Fe³⁺ in order to match the larger octahedral layer to the smaller tetrahedral layer. IR spectra indicate that formation of octahedral vacancies plays an important role for charge balance through the substitution 3 Fe²⁺ → 2 Fe³⁺ + ?_(oct).     

High pressure and high temperature fluid fugacities

High-temperature and high-pressure shock-wave data on fluids (pure species) have been combined with low-temperature and low-pressure data to generate a “corresponding state” equation in the virial format in reduced pressure and temperature for many species. The equation is then modified to obtain a similar equation of state for H₂O. The fugacities of the pure species in the CHO system can be calculated to a temperature of 3000 K and to a pressure of 1 megabar. However, dissociation of the pure species may invalidate the data over certain pressure-temperature ranges.     

东马珠穆沁旗中铁陨石表面的角砾状橄榄石的特征

东乌珠穆沁旗中铁陨石中有二种橄榄石。一种是陨石中基质橄榄石，另一种是镶嵌在陨石表面的角砾状橄榄石。电子探针成分分析结果表明，两种橄榄石中的FeO和MnO比值，包体矿物种类，包体铁纹石和镍纹石中Fe,Ni的含量等，均有较大区别。陨石中基质橄榄石矿物是本陨石中原物质，而角砾状橄榄石是宇宙中物质。他们是两块自由翱于宇宙中的物质碰撞混合而形成的东乌珠穆旗陨石中这种现象。     

Metasomatic alterations of olivine inclusions in the Budulan mesosiderite

Mesosiderites are thermal metamorphic breccias consisting of fragments of pyroxene-plagioclase rocks and FeNi metal. The silicate constituent of mesosiderites has a chemical and oxygen isotopic composition analogous to those of meteorites of the HED group: howardites, eucrites, and diogenites. The hypothesis currently most widely accepted for the genesis of mesosiderites is the impact mixing of the material of a differentiated asteroid and an iron meteorite. In contrast to many other classes of meteorites, mesosiderites exhibit no traces of metasomatic processes. The Budulan mesosiderite is the first meteorite of this type in which traces of metasomatism under the effect of an anhydrous fluid were detected. The metasomatic alterations are manifested as chemical zoning of olivine, aggregates of secondary minerals, and the mobilization and redeposition of iron and nickel in the form of metals and sulfides. These alterations were most probably caused by a reaction of olivine with S- and/or CO-bearing gases of endogenic or supergenic provenance. Traces of such metasomatic alterations were previously found in some meteorites and lunar rocks, and these processes could likely play a certain role in the differentiation of chondritic bodies.     

Aubrites and diogenites: Trace element clues to their origin

We have analyzed by RNAA 25 aubrite and 9 diogenite samples for 13 to 29 siderophile, volatile, and lithophile trace elements. Both meteorite classes show a typically igneous siderophile element pattern, with Ir, Os, Re, Ge more depleted than Au, Ni, Pd, Sb. But aubrites tend to have about 10 × higher abundances (10^?3 ? 10 ^{? 4} × Cl for the first 4 and 10^?2?10^?3 × Cl for the last 4 siderophiles), apparently reflecting smaller metal/silicate distribution coefficients at lowerf(O₂), or less complete segregation of metal. Se is surprisingly abundant in aubrites (up to 0.4 × Cl), but Te is less so ($\text{Se}\text{Te}\text{? 5 × Cl}$), apparently due to its stronger siderophile character. Other volatiles (Ag, Zn, In, Cd, Bi, T1) show depletions intermediate between lunar dunite and the Earth's mantle.Of 7 aubrites analyzed for REE (Ce, Nd, Eu, Tb, Yb, Lu), 6 are depleted in REE (0.08?0.5 × Cl) and 5 show negative Eu anomalies (the exceptions are Bishopville and Mt. Egerton silicate). This supports an igneous origin, as already noted by Boynton and Schmitt (1972). No samples of the complementary, basaltic and feldspathic rocks have been found thus far, but one of our samples of Khor Temiki dark is a candidate for the basalt. It is 5?7 × enriched in REE and only slightly less so in Rb, Cs, and U. Though shocked and enriched in siderophiles to ~0.05 × Cl, it apparently represents a new meteorite class.Three diogenites analyzed for REE show very diverse patterns, from strongly depleted in light REE for Tatahouine (Ce = 0.01 × Cl) to flat for Garland (~2.5 × Cl). The data confirm the trends found by Fukuokaet al. (1977) as well as their interpretations.Factor analysis shows several parallel groupings for aubrites and diogenites: siderophiles (Re, Ir, Os, Pd, Ge), chalcophiles (Se, Te), volatiles (Ag, In, Tl) and incompatibles (U, REE, and Cs or Rb). But there are some differences for elements such as Ni, Sb, Cd, Bi, Au, and Zn, most of which behave more sensibly in aubrites than in diogenites.Several element pairs that differ greatly in volatility (Cs-U, Ge-Ir) correlate closely in aubrites, in approximately Cl-chondrite proportions. These correlations, and other lines of evidence, suggest strongly that aubrites originated by igneous processes in their parent body, not by direct nebular condensation. The source material may have resembled EL chondrites in oxidation state and depletion of refractories, metal, and volatiles.     

Partitioning of Cu, Ni, Au, and platinum-group elements between monosulfide solid solution and sulfide melt under controlled oxygen and sulfur fugacities

We have performed six experiments in which we equilibrated monosulfide solid solution (mss) with sulfide melt in evacuated silica capsules containing solid buffers to fix oxygen and sulfur fugacity, at temperatures of 950°C, 1000°C and 1050°C at bulk concentrations of ∼50 ppm for each of the PGE and Au, 5% Ni, and 7% Cu. Concentrations of O, S, Fe, Ni and Cu were determined by electron microprobe, whereas precious metal concentrations were determined by laser-ablation inductively-coupled mass spectrometry. Partition coefficients of all elements studied show minimal dependences on oxygen fugacity from the IW to the QFM buffers when sulfur fugacity is fixed at the Pt-PtS buffer. Cu, Pt, Pd and Au are strongly incompatible and Ru remains moderately to strongly compatible under all conditions studied. At all oxygen fugacities, at the Pt-PtS sulfur buffer, Ir and Rh remain highly compatible in mss. In the single run at both low oxygen and low sulfur fugacity Ir and Rh were found to be strongly incompatible in mss. At QFM and Pt-PtS the partition coefficient for Ni shows weak temperature dependence, ranging from 0.66 at 1050°C to 0.94 at 950°C. At lower oxygen and sulfur fugacity Ni showed much more incompatible behavior. Comparison with the compositions of sulfide ores from the Lindsley deposit of Sudbury suggests that the sulfide magma evolved under conditions close to the QFM and Pt-PtS buffers. The compatible behavior observed for Ni, Ir and Rh at Lindsley and most other magmatic sulfide deposits hosted by mafic rocks requires equilibration of mss and sulfide liquid at moderately high sulfur fugacity and low temperatures near to the solidus of the sulfide magma. We argue that this constraint requires that the sulfide magma must have evolved by equilibrium crystallization, rather than fractional segregation of mss as is commonly supposed.     

Solubilities of nitrogen and noble gases in silicate melts under various oxygen fugacities: implications for the origin and degassing history of nitrogen and noble gases in the earth

Solubility experiments for nitrogen and noble gases (Ar and Ne) in silicate melts were conducted using two experimental configurations: one was conducted at 1 atmospheric pressure, T =1300°C and oxygen fugacity (fO₂) of IW + 0.9 (i.e., 0.9 log units higher than the iron-wüstite buffer) and the other at high pressures (P_total ∼ 2 × 10⁸ Pa), 1500°C and fO₂ ∼ IW + 6. For the former experiment, isotopically labeled-nitrogen (¹⁵N¹⁵N-enriched) was used to distinguish dissolved nitrogen from contaminating atmospheric or organic nitrogen and to examine dissolution mechanisms of nitrogen in silicate melts. The results obtained for the two series of experiments are consistent with each other, suggesting that Henry's law is satisfied for fN₂ of up to ∼250 atm (2.5 × 10⁷ Pa). The results are also consistent with our earlier results (Miyazaki et al., 1995) obtained at highly oxidizing conditions (fO₂ ∼ IW + 10). All these results support physical dissolution of nitrogen as N₂ molecules in silicate melts for fO₂ from ∼IW + 10 down to ∼IW. The observed solubility (Henry's constant) of nitrogen (3-5 × 10⁻⁹ mol/g/atm) is comparable to that of Ar (2-4 × 10⁻⁹ mol/g/atm), and much lower than that of Ne (11-14 × 10⁻⁹ mol/g/atm) at 1300°C. A preliminary experiment was also performed for partitioning of nitrogen and noble gases between clinopyroxene (cpx) and basaltic melt using a piston cylinder-type apparatus at 1.5 GPa and at 1270 to 1350°C. The obtained cpx/melt partition coefficient of nitrogen is 0.06, slightly lower than those of noble gases (∼0.1 for Ne to Xe), suggesting that nitrogen is as incompatible as or even slightly more incompatible than noble gases. The present results imply that a large nitrogen/Ar fractionation would not be produced by magmatic processes. Therefore, the two orders of magnitude difference between the N₂/³⁶Ar ratios in the Earth's atmosphere (∼10⁴) and that in the mantle (∼10⁶) must be explained by some other processes, such as incomplete segregation of metal blobs into the core and their later oxidation.     

Intrinsic oxygen Fugacity measurements on chromites from the Bushveld Complex and their petrogenetic significance

Oxygen Fugacity measurements were carried out on chromites from the Eastern Bushveld Complex (Maandagshoek) and are compared with former measurements on chromites from the western Bushveld Complex (Zwartkop Chrome Mine). These results together with those of Hill and Roeder (1974) yield the following conditions of formation for the massive chromitite layers: Western Bushveld Complex (Zwartkop Chrome Mine) $$\begin{gathered} Layer{\text{ }}T(^\circ C) p_{O_2 } (atm) \hfill \\ LG3{\text{ 1160}} - {\text{1234 10}}^{ - {\text{5}}} - 10^{ - 7.6} \hfill \\ LG4{\text{ 1175}} - {\text{1200 10}}^{ - 6.35} - 10^{ - 7.20} \hfill \\ LG6{\text{ 1162}} - {\text{1207 10}}^{ - 6.20} - 10^{ - 7.50} \hfill \\ \hfill \\ \end{gathered} $$ Eastern Bushveld Complex (Farm Maandagshoek) $$\begin{gathered} {\text{LXI 1115}} - {\text{1150 10}}^{ - 7.80} - 10^{ - 8.80} \hfill \\ ( = {\text{Steelpoort Seam)}} \hfill \\ {\text{LX 1125 10}}^{ - 8.25} \hfill \\ {\text{V 1120 10}}^{ - 8.55} \hfill \\ {\text{LII 1120 10}}^{ - 8.0} - 10^{ - 8.60} \hfill \\ \end{gathered} $$ The comparison of the data shows, that the chronitite layers within each particular sequence were formed under approximately identicalp _{o 2}- andT-conditions. The chromites from the western Bushveld Complex, however, were formed at higher temperatures and higher oxygen fugacities than the chromites from the eastern Bushveld Complex. Fromp _{o 2}-T-curves of disseminated chromites and the temperatures derived above, the following conditions of formation for the host rocks were obtained: Western Bushveld Complex $$T = 1200^\circ {\text{C; }}p_{{\text{o}}_{\text{2}} } = 10^{ - 7.25} - 10^{ - 7.50} $$ Eastern Bushveld Complex $$T = 1125^\circ {\text{C; }}p_{{\text{o}}_{\text{2}} } = 10^{ - 8.50} - 10^{ - 9.25} $$ Consequently, the host rocks in the Zwartkop-Chrome-Mine, were formed under higher temperatures and higher oxygen fugacities than the host rocks at Maandagshoek. The rock sequence in the Zwartkop-Chrome-Mine therefore originated in an earlier stage of the differentiation of the Bushveld magma. Comparison of the chromites from the host rocks with the chromites from massive layers supports Ulmer's (1969) thesis that an increase of the oxygen fugacity is responsible for the formation of massive chromitite layers. The values in this investigation show that increases of only about 0.5–1.0 log units are necessary to enhance chromitite layer formation.     

Origin of ureilites inferred from a SIMS oxygen isotopic and trace element study of clasts in the Dar al Gani 319 polymict ureilite

Secondary ion mass spectrometer (SIMS) oxygen isotope analyses were performed on 24 clasts, representing 9 clast types, in the Dar al Gani (DaG) 319 polymict ureilite with precisions better than 1‰. Olivine-rich clasts with typical ureilitic textures and mineral compositions have oxygen isotopic compositions that are identical to those of the monomict ureilites and plot along the CCAM (Carbonaceous Chondrite Anhydrous Mineral) line. Other igneous clasts, including plagioclase-bearing clasts, also plot along the CCAM line, indicating that they were derived from the ureilite parent body (UPB). Thus, we suggest that some of the plagioclase-bearing clasts in the polymict ureilites represent the “missing basaltic component” produced by partial melting on the UPB.Trace element concentrations (Mg, K, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Rb, Sr and Ba) in ureilitic plagioclase and glass from 13 clasts were obtained by using the SIMS high mass resolution method. The trace element contents of the plagioclase generally show monotonic variations with anorthite content (mol%) that are consistent with partial melting and fractional crystallization. Incompatible trace element concentrations (K, Ti, and Ba) are low and variable for plagioclase with An > 40, indicating that the parental magmas for some clasts were derived from a depleted source. We performed partial melt modeling for CI and CM precursor compositions and compared the results to the observed trace element (K, Ba, and Sr) abundances in the plagioclase. Our results indicate that (1) the UPB evolved from a alkali-rich carbonaceous chondritic precursor, (2) parent melts of porphyritic clasts could have formed by 5-20% equilibrium partial melting and subsequent fractional crystallization, and (3) parent melts of the incompatible trace element-depleted clasts could be derived from fractional melting, where low degree (<10%) partial melts were repeatedly extracted from their solid sources.Thus, both the oxygen isotopic and trace element compositions of the plagioclase bearing clasts in DaG-319 suggest that the UPB underwent localized low degree-partial melting events. The partial melts could have been repeatedly extracted from the precursor, resulting in the formation of the olivine-pigeonite monomict ureilites as the final residue.     

Classification,formation, and transport mechanisms of mud clasts

Mud clasts are common in non-marine to marine sedimentary records, however, why lack a widely accepted classification scheme? We propose that it is the relative balance of volumetric abundance, sorting, roundness, and grain size that controls the texture and fabric of mud clasts. Nine distinct types of mud clasts are identified in the study based on quantitatified properties, and fall into two groups coarse-grained and fine-grained. The generation of mud clasts can be assigned to failure, erosion, and/or bioturbation of muddy sediment. These clasts are transported within fluid flows including Newtonian fluids, non-Newtonian fluids, and Bingham plastics (gravity flow and turbidity flow), showing various physical characteristics depended upon the density and viscosity of flows. Newtonian flows with less density and viscosity commonly form mud clasts with mature textures. In non-Newtonian (gravity-driven) flows, mud clasts are normally transported in laminar flows with high density and viscosity, developing matrix-supported mud clasts with immature textures. The study of classification, formation, and transport mechanisms of mud clasts has implications for identifying and interpreting sedimentary environments.     

Experiments on Dehydration—Melting of the Khondalite Series in the Northern Segment of Helan Mountain, Ⅲ—The Evolution of Oxygen,Water,and Hydrogen Fugacities

Based on the experiments on dehydration-melting of solid samples of Al-rich gneiss (H029) and biotite granulitite (H013), the fugacities of O₂, H₂O and H₂ have been calculated. It is recognized that the fugacities of O₂, H₂O and H₂ vary regularly, but the fugacity of H₂O shows a tendency of abrupt increasing at about 700°C and 800°C. According to the above fact, the melting mechanism of biotite can be well documented. Under relatively low temperatures (< 750°C), part of the water can be liberated and induce plagioclase to melt, which may mark the beginning of migmatization. At high temperatures (> 800°C), biotite can be dissociated and a larger amount of water can be released, which would result in a bigger degree of melting, hence leading to the formation of granitic magma.