Olivines and olivine coronas in mesosiderites     

C.E. Nehru  S.M. Zucker  G.E. Harlow  M. Prinz 《Geochimica et cosmochimica acta》1980,44(8):1103-1118

Olivines and their surrounding coronas in mesosiderites have been studied texturally and compositionally by optical and microprobe methods. Most olivine is compositionally homogeneous but some is irregularly zoned. It ranges from Fo_58–92 and shows no consistent pattern of distribution within and between mesosiderites. Olivine occurs as large single crystals or as partially recrystallized mineral clasts, except for two lithic clasts. One is in Emery, the other in Vaca Muerta, and they are both shock-modified olivine orthopyroxenites. $\text{FeO}\text{MnO}$ ratios in olivine exhibit a variety of differing trends and range from 22–46, most commonly 35–40. These values are lower than those in olivine from diogenites sensu stricto (45–50) and have therefore experienced a different history. Some of the olivine clasts could have coexisted with some of the large orthopyroxene clasts as equilibrium assemblages, but some could not. Much of the olivine may be derived from mesosiderite olivine orthopyroxenites, which differ from diogenites sensu stricto. More magnesian olivine may be a residue from one or more source rocks, with varying degrees of melting. These events probably occurred in a highly evolved and differentiated parent body.Fine-grained coronas surround olivine, except for those in impact-melt group mesosiderites (Simondium, Hainholz, Pinnaroo) and those without tridymite in their matrices (Bondoc, Veramin). Coronas consist largely of orthopyroxene, plagioclase, clinopyroxene, chromite, merrillite and ilmenite and are similar to the matrix, but lack metal and tridymite. Coronas contain abundant orthopyroxene but are unusually rich in chromite (up to 7%) and merrillite (up to 20%). The outer parts of the corona grade into the matrix, but have little or no metal and tridymite. Texturally the innermost part of the corona can be divided into three stages of development: I Radiating acicular; II Intermediate; III Granular. Stage I is the result of the greatest disequilibrium between olivine and matrix orthopyroxene and Stage III has the least disequilibrium. Coronas are the result of the reaction olivine + tridymite = orthopyroxene, probably because FeO (and MgO) diffuse from olivine to tridymite in the matrix. Absence of metal and concentration of chromite in the corona are probably the result of an FeO potential gradient away from the olivine. Merrillite concentrations are a result of P₂O₅ migration into the corona but are controlled by the availability of calcic pyroxene, or possibly plagioclase. Although the coronas are texturally similar to terrestrial and lunar counterparts, they are unique and represent different kinds of reactions marked by a large degree of intra-corona diffusion under dry conditions. Opaque oxide-silicate-metal buffer assemblages yield apparent equilibration conditions of about 840°C and fO₂ near 10^?20. Poikiloblastic pyroxene textures in some coronas suggest a closing of reaction systems between 900 and 1000°C and such systems may record a higher temperature stage of development.  相似文献   

L'Olivine dans les howardites: origine,et implications pour le corps parent de ces météorites achondritiques     

C Desnoyers 《Geochimica et cosmochimica acta》1982,46(4):667-680

Electron microprobe analyses have been performed on 300 olivine grains found in 11 howardites. The olivine compositions almost continuously range from Fa 8 to Fa 89 with two prominent populations at Fa 13 and Fa 30. The tail of the fayalite contents distribution may correspond to the succession of several small clusters of Fe-rich olivine grains. Most howardites have olivine populations in common that would result from the fragmentation of different rocks of the howardites parent body. The distribution of the olivine grains between several groups of different $\text{FeO}\text{MnO}$ ratios indicates olivine crystallization from distinct magmas. The chemical characteristics of the olivines of the pallasites, diogenites and mesosiderites are found among the olivines of the howardites and suggests a common parent body for these different types of meteorites. The differentiation model of the eucrites parent body proposed by Stolper (1977) is extended to the partial fusion of distinct assemblages silicates + metal which could proceed from recrystallizations, under different oxidation-reduction conditions, of a primordial chondritic material depleted in volatile elements.  相似文献   

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

David W. Mittlefehldt 《Geochimica et cosmochimica acta》1979,43(12):1917-1935

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.  相似文献   

Ferromanganese oxide deposits from the Central Pacific Ocean,I. Encrustations from the Line Islands Archipelago     

Andrew C Aplin  David S Cronan 《Geochimica et cosmochimica acta》1985,49(2):427-436

Chemical and mineralogical analyses of a well-controlled suite of ferromanganese encrustations from the Line Islands Archipelago (Central Pacific) suggest that they represent purely hydrogenous deposits—i.e. they have formed through the slow accumulation of trace metal-enriched oxides directly from the water-column. Mineralogically they consist predominantly of δMnO₂ and amorphous FeOOHxH₂O. Compositionally, they are similar to δMnO₂ nodules from adjoining basinal areas but are enriched in both Mn (mean = 20.4%, max = 29.3%) and Co (mean = 0.55%, max = 1.57%). δMnO₂ is the most important trace metal bearing phase; strong associations are noted between it and Co, Mo, Ni, Zn, and Cd, whilst only Be is associated specifically with FeOOH. V, Sr and Pb are partitioned between the authigenic oxide phases, whilst Ti most probably occurs as TiO₂xH₂O. Cu is contained in both aluminosilicate contaminant phases and Fe oxide phases. These relations are considered to reflect the differing scavenging behaviour of Mn and Fe oxides in the water column.Crusts from ~1–2 km are enriched in Mn and the Mn-related elements and exhibit higher $\text{Mn}\text{Fe}$ ratios than deeper crusts, which are compositionally constant. The higher $\text{Mn}\text{Fe}$ ratios may result from a supply of Mn from continental borderland sediments at these depths, which is transported horizontally by advective-diffusive processes. Since manganophile elements are enriched relative to Mn in the 1–2 km crusts, it is considered that the supply of Mn is scavenged by existing oxides, is oxidised and effectively occludes them. A higher proportion of oxide particles thus exhibit Mn oxide scavenging properties in the 1–2 km depth zone. The increased vertical flux of Mn resulting from the supply at ~1–2 km is not reflected by higher $\text{Mn}\text{Fe}$ ratios in deeper crusts, so that the vertical flux of oxides is not simply related to the standing crop. The $\text{Mn}\text{Fe}$ ratios of the crusts thus reflect the composition of suspended oxides at similar depths.  相似文献   

The oxidation state of manganese in marine sediments and ferromanganese nodules     

James W Murray  Laurie S Balistrieri  Barbara Paul 《Geochimica et cosmochimica acta》1984,48(6):1237-1247

Marine sediments and ferromanganese nodules from the Pacific Ocean have been analyzed for the $\text{O}\text{Mn}$ ratio of solid manganese. We tested six chemical methods and concluded that the iodometric and oxalate methods were equivalent and were the best choice in terms of accuracy and precision on natural samples. We choose the iodometric method for most of our analyses because the oxalate procedure is a method of differences.The ferromanganese nodules that we analyzed were all from MANOP site H and had $\text{Mn}\text{Fe}$ ratios that ranged from 5.6 to 70. These nodules were invariably highly oxidized with $\text{O}\text{Mn}$ values ranging from 1.90 to 2.00. Our most precise analyses suggest that less than 1% of the total manganese is present as Mn(II).We also analyzed red clay and hemipelagic sediments from the eastern tropical Pacific (Baja borderland and MANOP site H) and carbonate ooze samples from the equatorial Pacific. These sediments are also highly oxidized ($\text{O}\text{Mn}\text{= 1.90 to 2.00}$) except when Mn(II) appears in the interstitial water. As dissolved Mn(II) increases the value of the $\text{O}\text{Mn}$ ratio in the solid phase decreases. The $\text{O}\text{Mn}$ ratio decreases to values as low as 1.40. This decrease appears to be due to a decrease in oxidized manganese by reduction, however, an increase in reduced manganese in the solid sediments by adsorption or MnCO₃ formation can not be ruled out in all cases.  相似文献   

Intrinsic oxygen fugacities of diogenites and mesosiderite clasts     

Roger H. Hewins  G.C. Ulmer 《Geochimica et cosmochimica acta》1984,48(7):1555-1560

Oxygen fugacities of diogenite and mesosiderite clast material were measured with the double ZrO₂ cell technique between 800° and 1150°C. The samples were taken from large clasts in the diogenites Johnstown (En₇₃) and Tatahouine (En₇₅), and the mesosiderites Estherville (En₈₁), West Point (Fo₈₈) and Emery (En₆₈). Fugacity values for all except Emery plot near the wüstite-iron buffer curve and are interpreted as indicating similar source regions and environments of crystallization for the two suites. Emery orthopyroxene records a lower fugacity, close to the fayalite-quartz-iron buffer curve, probably as a result of equilibration with the mesosiderite matrix assemblage. The similarity of redox conditions experienced by mesosiderite orthopyroxenite and diogenites is not sufficient to require a single parent body and, if the common achondrites were derived from Vesta, mesosiderites probably came from a different body.  相似文献   

Manganese carbonate overgrowths on foraminifera tests     

Edward A. Boyle 《Geochimica et cosmochimica acta》1983,47(10):1815-1819

Below the zone where manganese is remobilized as Mn²⁺(aq), reductively cleaned foraminifera in deep sea sediments have much higher $\text{Mn}\text{Ca}$ than those in core tops and sediment traps. $\text{Mn}\text{Ca}$ ranges from less than 20 × 10^?6 in and above the MnO₂ maximum to as high as 700 × 10^?6 in reducing Panama Basin sediments. The most plausible explanation for this enrichment is that the tests are coated with Mn carbonate overgrowths. These coatings can account for a significant proportion of the Mn in reduced deep-sea sediments. Uptake of manganous ion by carbonate may explain the absence of Mn nodules in areas of high carbonate accumulation. Extreme degrees of overgrowth can alter foram trace element values, but this artifact can be avoided by avoiding foraminifera with high Mn/Ca.  相似文献   

The behavior of Mg, Fe, and Ni during the replacement of olivine by orthopyroxene: experiments relevant to mantle metasomatism     

Ralf Milke  Rainer Abart  Lukas Keller  Dieter Rhede 《Mineralogy and Petrology》2011,103(1-4):1-8

Replacement of olivine by orthopyroxene is a frequently observed phenomenon in mantle metasomatism. In order to study element redistribution in SiO₂ metasomatism we synthesised orthopyroxene reaction rims at the contacts between forsterite-rich olivine and quartz. The orthopyroxene rims grew from the original quartz-olivine interface into both directions implying counterdiffusion of iron/magnesium and silicon. Following local equilibrium partitioning the X_Fe is lower in the orthopyroxene than in the reactant olivine at the olivine-orthopyroxene replacement front. The resulting local iron excess is compensated by formation of orthopyroxene with a higher X_Fe at the quartz-orthopyroxene interface, which is out of equilibrium with the reactant olivine. This is facilitated through short circuit diffusion along grain boundaries within the orthopyroxene rim. Due to the low capacity of orthopyroxene to accommodate Ni, this component is forced to diffuse back into the olivine producing a Ni enriched zone ahead of the replacement front. This leads to Ni contents in the orthopyroxene rim, which are higher than what is expected in equilibrium with the unaltered olivine. Taking quartz as a proxy for a silica rich fluid or liquid metasomatising agent, we conclude that the overall element fractionation between olivine and the silica rich phase may deviate from equilibrium partitioning so that the Fe and Ni concentrations in the orthopyroxene which is in contact with quartz are higher than in equilibrium with the reactant olivine. This indicates that kinetic fractionation is important for the chemical evolution of both the mantle rocks and the metasomatising agents.  相似文献   

Oxidation of Mn(II): Initial mineralogy,oxidation state and ageing     

James W Murray  John G Dillard  Rudolf Giovanoli  Harald Moers  Werner Stumm 《Geochimica et cosmochimica acta》1985,49(2):463-470

The initial solid phase oxidation products formed during the oxidation of aqueous Mn(II) at 25°C were studied as a function of time. The analyses included morphology (TEM), mineralogy (x-ray diffraction), $\text{O}\text{Mn}$ ratio (iodometric method), oxidation state of manganese (XPS), and dissolved manganese. The initial solid formed under our conditions was Mn₃O₄ (hausmannite) which converted completely to γMnOOH (manganite) after eight months. βMnOOH (feitknechtite) appeared to be an intermediate in this transformation. The $\text{O}\text{Mn}$ ratio was initially 1.37 and increased to 1.49 over the same time span. Throughout the course of this study the XPS analyses showed that the surface of the solids (<50 Å) was dominated by Mn(III). The solution pH and dissolved manganese concentrations were consistent with disproportionation and oxidation reactions that favor the transformation of Mn₃O₄ to γMnOOH but not to γMnO₂.  相似文献   

The formation of todorokite and birnessite in sea water pumped from under ground     

N Takematsu  Y Sato  S Okabe 《Geochimica et cosmochimica acta》1984,48(5):1099-1106

Manganese oxides precipitated from aerated well sea water at the Marine Science Museum, Tokai University, have been analyzed chemically and mineralogically. The $\text{O}\text{Mn}$ ratios are lower in todorokite than in birnessite but these minerals have similar contents of minor transition metals, which can be taken up additionally from sea water after the precipitation of Mn oxides. On the basis of these results, the genesis of Mn minerals is discussed in relation to marine Mn nodules.  相似文献   

On the thermal and redox history of the Yamato diogenite Y-74013     

Amalbikash Mukherjee  T. A. Viswanath 《Journal of Earth System Science》1986,95(3):381-395

The Yamato diogenite, Y-74013, shows a high degree of textural equilibrium with the apparent crystallization sequence: troilite and metal → orthopyroxene → plagioclase. The position of the large chromite crystals in this sequence is unclear. Except chromite, all other minerals have composition similar to common orthopyroxene achondrites. The chromite is more magnesian than in common diogenites, strongly zoned and, on the whole, intermediate in composition between chromites of diogenites and pallasites. Texture, mineral composition data and an equilibrium thermodynamic analysis of the mineral association strongly indicate that the chromite crystallized earlier than the silicates at a much higher temperature (possibly above 1100°C) and rapidly grew in a medium which was progressively enriched in Mg, Al and Ti. But the chromite failed to reach chemical equilibrium, even at its outermost rim, with the orthopyroxene. The calculated equilibrium log fO₂ of the Yamato diogenite, ?20·21 to ?11·08 for temperatures between 880°C and 1500°C is well within the normal oxygen fugacity range of pyroxene achondrites.  相似文献   

Rare gases and ³⁶Cl in stony-iron meteorites: cosmogenic elemental production rates,exposure ages,diffusion losses and thermal histories     

F Begemann  H.W Weber  E Vilcsek  H Hintenberger 《Geochimica et cosmochimica acta》1976,40(3):353-368

Metal and silicate portions from 13 mesosiderites, one pallasite, Bencubbin (“unique”) and Udei Station (‘iron with silicate inclusions’) have been analysed for their content of He, Ne and Ar; in most cases ³⁶Cl could be determined as well. ³⁶Cl-³⁶Ar cosmic ray exposure ages fall between 10 and 160 Myr. Half of the metal samples show a deficit of spallogenic ³He (up to 30%) which we ascribe to a loss of tritium. The observed depletion of ³He in the silicates is correlated with their mineralogical composition: feldspar has lost its ³He in all cases, pyroxene definitely in one and possibly in five others, while olivine has been affected in only two meteorites. The thermal histories during their exposure to the cosmic radiation have been different for different meteoroids. Nevertheless, with the exception of Veramin, the data are compatible with the assumption of a continuous diffusion loss during a considerable fraction of the exposure era. For Veramin, however, an episodic event late in the exposure history is required. The exceptionally high ${}^{39}\text{Ar}{}^{36}\text{Cl}$ ratio in the metal, which is due to a high ³⁹Ar activity, indicates that the event occurred during the last 500,000 years or so and resulted in an extremely excentric orbit (large aphelion).Production rates of ^38,39Ar from Ca and ^21,22Ne from Mg are given. The ratio $\text{P}{}^{38}{}_{\mathrm{Ca}}\text{P}{}^{21}{}_{\mathrm{Mg}}$ is close to unity. The ratios $\text{P}{}^{38}{}_{\mathrm{Ca}}\text{P}{}^{38}{}_{\mathrm{Fe}}$ vary between 20 and 50, and are not correlated with the absolute production rate of ³⁸Ar from metal. The ${}^{22}\text{Ne}{}^{21}\text{Ne}$ production ratio from Mg is found to be close to but below unity.Of the mesosiderites only Veramin shows unambiguous evidence for primordial rare gases with larger amounts and a higher ${}^{20}\text{Ne}{}^{36}\text{Ar}$ ratio in the olivine, suggesting in situ fractionation to have at least been partly responsible for the abundance pattern found. Bencubbin contains large amounts of strongly fractionated primordial gases, but again part of the fractionation may have occurred in situ. Udei Station shows an excess of (3.5 ± 0.6) × 10^?10 cm³ STP ¹²⁹Xe/g in the non-magnetic portion.  相似文献   

Partitioning of Ni between olivine and silicate melt: The ‘Henry's Law problem’ reexamined     

Michael J Drake  John R Holloway 《Geochimica et cosmochimica acta》1981,45(3):431-437

The partitioning of Ni between olivine and silicate melt has been investigated experimentally at atmospheric pressure in air. Beta track autoradiography using ⁶³Ni and direct microprobe analysis of polished run products were employed. At constant temperature and bulk composition, the $\text{olivine}\text{liquid}$ partition coefficient for Ni in the system Di₇₀Fo₂₅Qtz₅ remains independent of concentration from approximately 10 ppm to 40,000 ppm Ni in olivine. Similar experiments by Mysen (1979) in the system Jd₈₀Fo₂₀ utilizing beta track autoradiography alone indicated that Henry's Law was followed only in the concentration interval of approx. 10–1000 ppm Ni in olivine. Above 1000 ppm, olivine/liquid partition coefficients decreased monotonically to about half of the value observed below 1000 ppm. We have performed experiments in the system Jd₈₀Fo₂₀, but are unable to replicate Mysen's results. While in agreement with Mysen below 1000 ppm Ni in olivine, we do not observe the decrease in partition coefficient value at higher concentrations. We conclude from our reversed experiments that, at constant temperature and bulk composition, the $\text{olivine}\text{liquid}$ partition coefficient for Ni in the system Jd₈₀Fo₂₀ remains independent of concentration from approx. 10–60,000 ppm Ni in olivine. Attempts to resolve these differing conclusions by changing experimental techniques have been unsuccessful.  相似文献   

The major-element chemistry of suspended matter in the Amazon Estuary     

E.R. Sholkovitz  N.B. Price 《Geochimica et cosmochimica acta》1980,44(2):163-171

Suspended matter from the surface waters of the Amazon Estuary were collected during May and June 1976 on the ‘R/V Alpha Helix’, and their major-element compositions (Al, Si, Ti, K, Mg, Ca, P, Fe and Mn) were measured.Between salinities of 0 and 10%. the suspended material, predominantly terrigenous in derivation, decreases in load from 500 to 3 mg/l, but has a chemical composition which remains essentially constant. With the onset of a large amount of biological productivity at approximately 10%. salinity, there are large increases in the ratios of $\text{Si}\text{Al}$, $\text{P}\text{Al}$, $\text{Ca}\text{Al}$, $\text{Mg}\text{Al}$, $\text{Ti}\text{Al}$ and $\text{Mn}\text{Al}$ which are maintained at higher salinities. Calculations of “excess” concentrations of elements held in the non-terrigenous components of the suspended material further support our main conclusion that Si, P, Ca, Mg, Ti and Mn are incorporated into the skeletal and organic phases of marine phytoplankton (predominately diatoms) of the Amazon Estuary. The data suggest, but with less certainty, that Fe and K follow the above elements.This study has demonstrated that the chemical composition of river-introduced suspended matter can be significantly altered by biological activity within estuarine waters as can be the geochemical cycle of inorganic elements.  相似文献   

Some constraints on the thermodynamic mixing properties of Fe-Mg orthopyroxenes and olivines     

Richard O. Sack 《Contributions to Mineralogy and Petrology》1980,71(3):257-269

Existing data on the temperature and composition dependence of the Fe²⁺-Mg²⁺ distribution between Fe-Mg olivine and orthopyroxene, the intra-crystalline distribution of Fe²⁺ and Mg²⁺ between M1 and M2 sites in orthopyroxene, and macroscopic activity-composition relations in olivine and orthopyroxene are shown to be inconsistent with generally accepted thermodynamic formulations which assume that the non-configurational Gibbs energy of orthopyroxene is independent of the degree of long-range ordering of Fe²⁺ and Mg⁺ between M1 and M2 sites. These data are interpreted in terms of the constraints they provide on the size of Bragg-Williams type energy, entropy, and volume terms for olivine and orthopyroxene. The apparent equilibrium constant for Fe-Mg exchange between olivine and orthopyroxene is shown to be a potentially useful ‘geothermometer’ for olivine-orthopyroxene assemblages with olivines with mole fraction of Fe₂SiO₄ component less than 0.2 or greater than 0.6. A provisional calibration of this ‘geothermometer’ is presented.  相似文献   

Effect of Silica Activity on OH- IR Spectra of Olivine: Implications for Low-aSiO2 Mantle Metasomatism     

MATVEEV  S.; O'NEILL  H. ST. C.; BALLHAUS  C.; TAYLOR  W. R.; GREEN  D. H. 《Journal of Petrology》2001,42(4):721-729

Hydrogen solubility and hydroxyl substitution mechanism in olivineat upper-mantle conditions are not only a function of pressure,temperature, water fugacity and hydrogen fugacity, but are alsoinfluenced by silica activity. Olivine synthesized in equilibriumwith magnesiowüstite displays hydroxyl stretching bandsin the wavenumber range from 3640 to 3430 cm^–1. In contrast,olivine in equilibrium with orthopyroxene shows absorption bandsin a narrower wavenumber range from 3380 to 3285 cm^–1.The two fundamentally different spectra are assigned to hydroxylin tetrahedral and octahedral sublattices, respectively. Olivinein equilibrium with orthopyroxene is also less capable of incorporatinghydroxyl, relative to olivines in equilibrium with magnesiowüstite,by about a factor of ten. A comparison of spectra obtained aspart of this study with hydroxyl spectra of natural mantle olivinesshows that the latter display hydroxyl stretching patterns reminiscentof equilibrium with magnesiowüstite, although undoubtedlyolivine in the Earth’s mantle coexists with orthopyroxene.This may be attributed to a metasomatic overprint by a low-silicafluid and/or melt that was in reaction relationship with orthopyroxene.A likely metasomatic agent is a carbonatitic melt. When carbonatiticmelts decompose to oxides and CO₂, they may temporarily imposea low-aSiO₂ environment inherited by the olivine structure.If this suggestion proves true, Fourier transform IR spectroscopymay be used to fingerprint metasomatic episodes in the lithosphericmantle. KEY WORDS: FTIR spectrometry; olivine; mantle; metasomatism; water  相似文献   

Mesosiderites and howardites: igneous formation and possible genetic relationships     

David W Mittlefehldt  Chen-Lin Chou  John T Wasson 《Geochimica et cosmochimica acta》1979,43(5):673-688

We report neutron activation data for major, minor and trace elements determined in whole rock howardites and silicates from mesosiderites. Compositions of howardites and mesosiderites are similar, and intermediate between those of eucrites and diogenites. Relative to howardites mean mesosiderite abundances are slightly nearer the diogenites. Literature data indicate that mesosiderites have a higher normative silica component than howardites. It appears that this partly results from a higher content of a highly evolved igneous component, and partly from in situ reduction of FeO to Fe followed by magnetic separation of metal prior to analysis. Removal of a portion of the FeO in this manner yields a higher normative SiO₂ component for the nonmagnetic fraction. Petrographic observations demonstrate the formation of SiO₂ which may have resulted from a combination of various factors including accretion of a reducing agent together with the Fe-Ni metal, extensive reaction during the long cooling period, and catalysis by the finely divided metal.In the mesosiderites Mincy, Lowicz and Veramin the light rare earth elements (REE) are enriched. The resulting REE pattern is qualitatively similar to that in terrestrial basalts thought to have been formed by small degrees of partial melting. Of several partial melting models tested, the best match to the REE patterns is provided by one involving ~2–4% partial melting of a source containing low REE abundances. It appears that the light REE enrichment is not associated with the hypothetical silica enriched igneous phase.Since numerous properties separate mesosiderite silicates from howardites, it is clear that they are not composed of precisely the same material. Whether or not they originated on the same parent body is unresolved. If parent body regoliths were mixed vertically and horizontally on a planet-wide basis, then separate bodies would be required.  相似文献   

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

C.K. Shearer  Paul Burger  J.J. Papike 《Geochimica et cosmochimica acta》2010,74(16):4865-4880

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.  相似文献   

Metasomatic alterations of olivine inclusions in the Budulan mesosiderite     

C. A. Lorenz  M. A. Nazarov  F. Brandstaetter  Th. Ntaflos 《Petrology》2010,18(5):461-470

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.  相似文献   

Rare earth and trace element geochemistry of a fragment of jurassic seafloor,Point Sal,California     

Martin Menzies  Douglas Blanchard  Joyce Brannon  Randy Korotev 《Geochimica et cosmochimica acta》1977,41(10):1419-1430

A suite of rocks from the Point Sal ophiolite, California, were analyzed for rare earth elements (REE), Sc, Co, Na₂O, Cr, Zn and FeO. The lavas all have either flat or slightly light REE (LREE) depleted profiles relative to chondrites. The lavas contain smectite or greenschist facies mineralogy and some have radiogenically enriched ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios. This is interpreted as evidence of basaltseawater interaction (Hopsonet al., 1975; Davis and Lass, 1975). The smectite and zeolite bearing lavas that have been exposed to seawater for prolonged periods have anomalous Ce abundances. At higher grades of metamorphism, the lavas show no marked changes in light REE. The plutonic igneous rocks vary from early cumulus dunite to late stage, noncumulus diorite. All the plutonic rocks are light REE depleted with total REE abundance varying by a factor of 100 × between the dunites and diorites. Analyses of clinopyroxene and hornblende separates indicate that these two minerals strongly influence the REE characteristics of the early cumulates and late stage fractionates, respectively.In general, REE contents are: hornblende > clinopyroxene > plagioclase > orthopyroxene > olivine. Estimates of the REE compositions of parental lavas were obtained by calculating the REE contents of liquids in equilibrium with early cumulate clinopyroxenes. This reveals that the parent to the stratiform sequence was more depleted in light REE than the parent to the lava pile.  相似文献