Unusually abundant refractory inclusions from Sahara 97159 (EH3): a comparative study with other groups of chondrites

Sixty-eight refractory inclusions and fragments were found in two polished thin sections of the Sahara 97159 EH3 chondrite, indicative of the highest abundance of refractory inclusions (22/cm², or 0.06 vol.%) in enstatite chondrites studied to date. All of the inclusions are intensely altered, mainly producing feldspathoids and albite, CaO depletion and minor Ti-rich compounds, such as Ti-sulfides. The alteration assemblages and FeO-poor spinel suggest that the reactions took place under reducing and SiO₂-rich conditions. This is consistent with the redox state of the host enstatite chondrite. The presence of Ti sulfides and low FeO alteration phases distinguishes alteration of E chondrite refractory inclusions from that of carbonaceous and ordinary chondrites.Most of the inclusions are referred to as Type A-like (35) and spinel-rich (26), respectively. Assuming melilite has been altered, these inclusions could be analogues of individual concentrically zoned objects of fluffy melilite-spinel-rich (Type A) and spinel-pyroxene-rich inclusions from carbonaceous chondrites such as the Ningqiang (CV anomalous) and Y 81020 (CO3) chondrites. Two inclusions consist mainly of Ca-pyroxene, fine-grained alteration products (feldspathoids and albite) and spinel. They are probably altered fragments of Ca-pyroxene-plagioclase-rich (Type C) inclusions, assuming all plagioclase has been altered to produce the fine-grained groundmass. Five other inclusions are hibonite and/or corundum bearing, similar to those reported in carbonaceous chondrites. Abundance ratios of various types of the inclusions from Sahara 97159 are similar to those from Ningqiang and Y 81020.Most of the observations, including mineral assemblages, mineral chemistry, texture, bulk compositions, O isotopic compositions and REE patterns, of the Sahara inclusions suggest a common reservoir of refractory inclusions in enstatite, ordinary and carbonaceous chondrites. The apparent differences, such as absence of melilite and anorthite, rare Wark-Lovering rim and small size, can be explained by intense alteration due to large change of postformation environment of these inclusions, size sorting and collision during transfer. Hence, these differences are not inconsistent with the common reservoir model. Refractory inclusions in non-carbonaceous chondrites may put additional constraints on origins of refractory inclusions, and provide hints for a spatial relationship of their host meteorites.     

A reappraisal of the metamorphic history of EH3 and EL3 enstatite chondrites

The thermal history of a series of EH3 and EL3 chondrites has been investigated by studying the degree of structural order of the organic matter (OM) located and characterized in matrix areas by Raman micro-spectroscopy. By comparison with unequilibrated ordinary chondrites (UOCs) and CO and CV carbonaceous chondrites, the following petrologic types have been assigned to various E chondrites: Sahara 97096 and Allan Hills 84206: 3.1-3.4; Allan Hills 85170 and Parsa: 3.5; Allan Hills 85119: 3.7; Qingzhen, MacAlpine Hills 88136 and MacAlpine Hills 88184: 3.6-3.7. The petrologic type of Qingzhen is consistent with the abundance of the P3 noble gas component, a sensitive tracer of the grade of thermal metamorphism. The petrologic types are qualitatively consistent with the abundance of fine-grained matrix for the whole series. No significant effects of shock processes on the structure of OM were observed. However such processes certainly compete with thermal metamorphism and the possibility of an effect cannot be fully discarded, in particular in the less metamorphosed objects. The OM precursors accreted by the EH3 and EL3 parent bodies appear to be fairly similar to those of UOCs and CO and CV carbonaceous chondrites. Raman data however show some slight structural differences that could be partly accounted for by shock processes. The metamorphic history of EH3 and EL3 chondrites has often been described as complex, in particular regarding the combined action of shock and thermal metamorphism. Because OM maturity is mostly controlled by the temperature of peak metamorphism, it is possible to distinguish between the contributions of long duration thermal processes and that of shock processes. Comparison of the petrologic types with the closure temperatures previously derived from opaque mineral assemblages has revealed that the thermal history of EH3 and EL3 chondrites is consistent with a simple asteroidal onion shell model. Thermal metamorphism in enstatite chondrites appears to be fairly similar to that which takes place in other chondrite classes. The complex features recorded by mineralogy and petrology and widely reported in the literature appear to be mostly controlled by shock processes.     

Shock effects in “EH6” enstatite chondrites and implications for collisional heating of the EH and EL parent asteroids

Of the six chondrites that were listed as EH6 or EH6-an during the course of this study, we confirm the EH classification of Y-8404, Y-980211 and Y-980223 and the EH-an classification of Y-793225; two chondrites (A-882039 and Y-980524) are reclassified as EL (the former contains ferroan alabandite and both contain kamacite with ∼1 wt% Si). All of the meteorites contain euhedral enstatite grains surrounded by metal ± sulfide (although this texture is rare in Y-793225), consistent with enstatite crystallizing from a mixed melt. All contain enstatite with <0.04 wt% MnO; the three EH chondrites average 0.25 wt% Mn in troilite. (Literature data show that typical EH3-EH5 chondrites contain enstatite with 0.13-0.20 wt% MnO and troilite with 0.05-0.11 wt% Mn.) The three EH chondrites contain keilite [(Fe_>0.5,Mg_<0.5)S], which has been interpreted in the literature as a product of impact melting. Y-8404 and Y-980223 contain abundant silica (∼13 and ∼10 wt%, respectively), a rare phase in most enstatite chondrites. We suggest that all six meteorites have experienced impact melting; Mn was preferentially partitioned into sulfide during subsequent crystallization. The silica-rich samples may have become enriched in the aftermath of the impact by a redox reaction involving FeO and reduced Si. A-882039, Y-8404, Y-980211, Y-980223 and Y-980524 were incompletely melted; they contain rare relict chondrules and are classified as impact-melt breccias; Y-793225 is a chondrule-free impact-melt rock. If these EH and EH-an chondrites (which were previously listed as petrologic type 6) have, in fact, been impact melted, it seems plausible that collisional heating is generally responsible for EH-chondrite metamorphism. This is consistent with literature data showing that a large fraction (?0.7) of those chondrites classified EH5-7 and a significant fraction (?0.3) of those chondrites classified EH4 and EH4/5 possess textural and mineralogical properties suggestive of impact melting. In addition, ∼60% of classified EL6-7 chondrites (now including A-882039 and Y-980524) appear to have formed by impact melting. It thus seems likely that collisional heating is mainly responsible for EL- and EH-chondrite metamorphism.     

Response of silicate chemical composition variation on thermal metamorphism of ordinary chondrites and classification of petrologic types: the case of L chondrites from Grove Mountains,Antarctica

Acta Geochimica - Analysis of the thermal metamorphism of the ordinary chondrites is a key premise for gaining insights into the accretion and heating of rocky bodies in the early solar system....     

Chemical and physical studies of type 3 chondrites—V: The enstatite chondrites

We report instrumental neutron activation analysis determinations of 19 major, minor and trace elements in three enstatite chondrites. Based on these, and literature data on the bulk and mineral composition of enstatite chondrites, we discuss the history of the type 3 or unequilibrated enstatite chondrites, and their relationship with the other enstatite chondrites. The type 3 enstatite chondrites have E chondrite lithophile element abundances and their siderophile element abundances place them with the EH chondrites, well resolved from the EL chondrites. Moderately volatile chalcophile elements are at the low end of the EH range and Cr appears to be intermediate between EH and EL. We suggest that the type 3 enstatite chondrites are EH chondrites which have suffered small depletions of certain chalcophile elements through the loss of shock-produced sulfurous liquids. The oxygen isotope differences between type 3 and other enstatite chondrites is consistent with equilibration with the nebula gas ~30° higher than the others, or with the loss of a plagioclase-rich liquid. The mineral chemistry of the type 3 chondrites is consistent with either low temperature equilibration, or, in some instances, with shock effects.     

REE and Other Trace Element Chemistry of Oldhamite（CaS） in the Qingzhen Chondrite（EH3） and Their Genetic Implications

Instrumental neutron activation analysis(INAA) of 14 single oldhamite grains separated from the Qingzhen chondrite (EH3) for refractory(La,Ce,Sm ,Eu,Yb,Lu,Ca,Sc,Hf, and Th),volatile (Na,Cr,Zn,Se,Br,etc.)and siderophile elements (Fe,Ni,Co,Ir,Au ,and As) revealed that oldhamite is highly rich in refractory elements.The mineral serves as the principal carrier of REE and contains about 80% of the REEs in the Qingzhen enstatite chondrite .Furthermore, the large enrichment of LREE relative to HREE is noticed in oldhamite from the Qingzhen .In general, the oldhamite from metal-sulfide assemblages is richer in REE than that from the matrix,i.e.,the earlier the oldhamite grains condensed, the richer they are in REE. Meanwhile.oldhamite is also rich in vol-atile elements such as Se,Br, etc.In terms of the distribution of trace elements in oldhamitc from the Qingzhen ,the chondrite is srggested to have resulted from high-temperature condensation of solar nebula.     

Chondrules in H3 chondrites: textures,compositions and origins

Major and minor element bulk compositions of 90 individual chondrules and 16 compound chondrule sets in unequilibrated (type 3) H-group chondrites were determined in polished thin sections by broad beam electron probe analysis and the chondrules were classified petrographically into six textural types (barred olivine, porphyritic olivine, porphyritic pyroxene, barred pyroxene, radiating pyroxene, fine-grained). Although analyses of individual chondrules scatter widely, the mean composition of each textural type (except barred pyroxene) is rather distinct, as verified by discriminant function analysis. Al₂O₃, TiO₂ and Na₃O are correlated in chondrules, but Al₂O₃ and CaO do not correlate. Compound chondrule sets were found to consist almost entirely of chondrules or partial chondrules of similar texture and composition.The data suggest that composition played a conspicuous role in producing the observed textures of chondrules, though other factors such as cooling rates and degrees of supercooling prior to nucleation were also important. If compound chondrules formed and joined when they were still molten or plastic, then the data suggest that chondrules of each textural type could have formed together in space or time. The correlation of Al₂O₃ and TiO₂ with Na₂O and not with CaO appears to rule out formation of chondrules by direct equilibrium condensation from the nebula. We conclude that the most reasonable model for formation of the majority of chondrules is that they originated from mixtures of differing fractions of high-, intermediate- and low-temperature nebular condensates that underwent melting in space. A small percentage of chondrules might have formed by impacts in meteorite parent-body regoliths.     

Determination of REE,Ba, Fe,Mg, Na and K in carbonaceous and ordinary chondrites

Precise determination of REE and Ba abundances in three carbonaceous (Orgueil Cl, Murchison C2 and Allende C3) and seven olivine-bronzite chondrites were carried out by mass spectrometric isotope dilution technique. Replicate analyses of standard rock and the three carbonaceous chondrites demonstrated the high quality of the analyses (accuracies for REE are ±1–2 per cent). Certain carbonaceous chondrite specimens showed small positive irregularities in Yb abundance. The Yb ‘anomaly’ (approximately + 5 per cent relative to the average of 10 ordinary chondrites) in Orgueil may relate to high temperature components. The REE pattern of Guareña (H6) exhibits comparatively extensive fractionation (about factor 2) with a negative anomaly for Eu (17 ± 1 percent) compared to the average H chondrite. This could be interpreted in terms of extensive thermal metamorphism leading to melting.Apart from absolute abundance differences, there appears to be small but recognizable fractionation among the average relative REE abundances of Cl, E, H and L chondrites. However, individual chondrites within these groups showed more or less fractionated REE patterns relative to each other. The distinction between H and L chondrites was well demonstrated in Eu-Sm correlation curves and absolute abundance differences of REE and major elements.Si-normalized atomic ratios of the REE abundances in different kinds of chondrites to those in Orgueil (Cl) chondrite were 0.58 (E), 0.75 (H), 0.81 (L), 1.07 (C2) and 1.32 (C3).     

Relationships among intrinsic properties of ordinary chondrites: Oxidation state, bulk chemistry, oxygen-isotopic composition, petrologic type, and chondrule size

The properties of ordinary chondrites (OC) reflect both nebular and asteroidal processes. OC are modeled here as having acquired nebular water, probably contained within phyllosilicates, during agglomeration. This component had high Δ¹⁷O and acted like an oxidizing agent during thermal metamorphism. The nebular origin of this component is consistent with negative correlations in H, L, and LL chondrites between oxidation state (represented by olivine Fa) and bulk concentration ratios of elements involved in the metal-silicate fractionation (e.g., Ni/Si, Ir/Si, Ir/Mn, Ir/Cr, Ir/Mg, Ni/Mg, As/Mg, Ga/Mg). LL chondrites acquired the greatest abundance of phyllosilicates with high Δ¹⁷O among OC (and thus became the most oxidized group and the one with the heaviest O isotopes); H chondrites acquired the lowest abundance, becoming the most reduced OC group with the lightest O isotopes.Chondrule precursors may have grown larger and more ferroan with time in each OC agglomeration zone. Nebular turbulence may have controlled the sizes of chondrule precursors. H-chondrite chondrules (which are the smallest among OC) formed from the smallest precursors. In each OC region, low-FeO chondrules formed before high-FeO chondrules during repeated episodes of chondrule formation.During thermal metamorphism, phyllosilicates were dehydrated; the liberated water oxidized metallic Fe-Ni. This caused correlated changes with petrologic type including decreases in the modal abundance of metal, increases in olivine Fa and low-Ca pyroxene Fs, increases in the olivine/pyroxene ratio, and increases in the kamacite Co and Ni contents. As water (with its heavy O isotopes) was lost during metamorphism, inverse correlations between bulk δ¹⁸O and bulk δ¹⁷O with petrologic type were produced.The H5 chondrites that were ejected from their parent body ∼7.5 Ma ago during a major impact event probably had been within a few kilometers of each other since they accreted ∼4.5 Ga ago. There are significant differences in the olivine compositional distributions among these rocks; these reflect stochastic nebular sampling of the oxidant (i.e., phyllosilicates with high Δ¹⁷O) on a 0.1-1 km scale during agglomeration.     

Chemical and physical studies of type 3 chondrites—I: Metamorphism related studies of Antarctic and other type 3 ordinary chondrites

Thermoluminescence sensitivity measurements have been made on 18 unequilibrated ordinary chondrites; 12 finds from Antarctica, 5 non-Antarctic finds and 1 fall. The TL sensitivities of these meteorites, normalized to Dhajala, range from 0.034 (St. Mary's County) to 2.3 (Allan Hills A78084), and, based primarily on these data, petrologic type assignments range from 3.3 (St. Mary's County) to 3.9 (Allan Hills A78084). Although the very low levels of metamorphism experienced by types 3.0 to ～3.4 evidently cause large changes in TL sensitivity, the new data demonstrate that they are unable to cause any appreciable homogenization of silicate compositions. We have therefore slightly revised the silicate heterogeneity ranges corresponding to the lower petrologic types.We have discovered that the temperature of maximum TL emission and the broadness of the major TL peak, vary systematically with TL sensitivity; as TL increases these parameters first decrease and then increase. Several mechanisms which could account, partially or completely, for the relationship between TL sensitivity and metamorphism are discussed. Those which involve the formation of feldspar—the TL phosphor in equilibrated meteorites—seem to be consistent with the trends in peak temperature and peak width since experiments on terrestrial albite show that the TL peak broadens and moves to higher temperatures as the stable form changes from the low (ordered) state to the high (disordered) state. (The post-metamorphism equilibration temperature of type ～3.5 meteorites would then correspond to the transformation temperature for the high to low form of meteorite feldspar.) Other factors which may be involved are obscuration of the TL by carbonaceous material, changes in the composition of the phosphor and changes in the identity of the phosphor.     

Natural analogues for the transuranic actinide elements: An investigation in Minas Gerais,Brazil

A highly weathered deposit of thorium and rare earth elements located near the summit of a hill in the state of Minas Gerais, Brazil, is being studied as an analogue for a radioactive waste repository that sometime in the distant future may be eroded to the surface or intruded by groundwater Thorium serves as an analogue for Pu⁴⁺, and La³⁺ as an analogue for Cm³⁺ and Am³⁺ The mobilization rate constants of the analogue elements by groundwater are so slow (of the order of 10⁻⁹ per year) as to suggest that essentially complete radioactive decay of the transuranic actinides would occur in place even under the relatively unfavorable conditions that exist at a site such as this     

Local structural environment of calcium in garnets: A combined structure-refinement and XANES investigation

Comparative crystal-chemical analysis of natural garnets allowed the compositional gap at Gr 45÷70% in the (Py, Alm)-Gr join to be related to the presence of two different structural arrangements around the X site in pyralspitic and ugranditic composition, respectively. A combined investigation (single-crystal X-ray structure-refinement (SREF), Ca K-edge XANES spectroscopy and multiple-scattering calculation of XANES spectra) carried out on a series of natural garnets in the (Py, Alm)-Gr join, allowed the structural modifications of the Ca local environment at increasing grossular content to be followed. The differences in the experimental XANES spectra as a function of the Ca content were shown to depend only on the configuration of the ligand shells surrounding the absorbing atom, and not on the nature of adjacent dodecahedral cations. Both experimental XANES spectra and their theoretical simulations confirm that Ca at the X site, when less abundant than (Fe+Mg), is not in the same structural configuration as in grossular, but adapts itself to that imposed by the dominant (Fe, Mg) cations. In the same way, (Fe, Mg) adapt themselves to the X-site configuration observed in grossular when Ca is the dominant cation. The deformation of the X site due to progressive Ca → (Fe, Mg) substitution is gradual but not linear along the solid solution; it shows different slopes on the two sides of the observed (Py, Alm)-Gr compositional gap. Best performances in reproducing the XANES spectra were obtained in the framework of the one-electron fullmultiple-scattering theory, using the real Hedin-Lundqvist exchange-correlation potential and clusters of 83 atoms (i.e. for a coordination sphere with 7 Å radius from the central Ca absorber); smaller clusters resulted inadequate in simulating some XANES features, indicating that a high number of single and multiple scatteringpaths contribute in garnets to the first part of the absorption spectrum.     

Petrology and oxygen isotope compositions of chondrules in E3 chondrites

Chondrules in E3 chondrites differ from those in other chondrite groups. Many contain near-pure endmember enstatite (Fs_<1). Some contain Si-bearing FeNi metal, Cr-bearing troilite, and, in some cases Mg, Mn- and Ca-sulfides. Olivine and more FeO-rich pyroxene grains are present but much less common than in ordinary or carbonaceous chondrite chondrules. In some cases, the FeO-rich grains contain dusty inclusions of metal. The oxygen three-isotope ratios (δ¹⁸O, δ¹⁷O) of olivine and pyroxene in chondrules from E3 chondrites, which are measured using a multi-collection SIMS, show a wide range of values. Most enstatite data plots on the terrestrial fractionation (TF) line near whole rock values and some plot near the ordinary chondrite region on the 3-isotope diagram. Pyroxene with higher FeO contents (∼2-10 wt.% FeO) generally plots on the TF line similar to enstatite, suggesting it formed locally in the EC (enstatite chondrite) region and that oxidation/reduction conditions varied within the E3 chondrite chondrule-forming region. Olivine shows a wide range of correlated δ¹⁸O and δ¹⁷O values and data from two olivine-bearing chondrules form a slope ∼1 mixing line, which is approximately parallel to but distinct from the CCAM (carbonaceous chondrite anhydrous mixing) line. We refer to this as the ECM (enstatite chondrite mixing) line but it also may coincide with a line defined by chondrules from Acfer 094 referred to as the PCM (Primitive Chondrite Mineral) line (Ushikubo et al., 2011). The range of O isotope compositions and mixing behavior in E3 chondrules is similar to that in O and C chondrite groups, indicating similar chondrule-forming processes, solid-gas mixing and possibly similar ¹⁶O-rich precursors solids. However, E3 chondrules formed in a distinct oxygen reservoir.Internal oxygen isotope heterogeneity was found among minerals from some of the chondrules in E3 chondrites suggesting incomplete melting of the chondrules, survival of minerals from previous generations of chondrules, and chondrule recycling. Olivine, possibly a relict grain, in one chondrule has an R chondrite-like oxygen isotope composition and may indicate limited mixing of materials from other reservoirs. Calcium-aluminum-rich inclusions (CAIs) in E3 chondrites have petrologic characteristics and oxygen isotope ratios similar to those in other chondrite groups. However, chondrules from E3 chondrites differ markedly from those in other chondrite groups. From this we conclude that chondrule formation was a local event but CAIs may have all formed in one distinct place and time and were later redistributed to the various chondrule-forming and parent body accretion regions. This also implies that transport mechanisms were less active at the time of and following chondrule formation.     

The origin and history of ordinary chondrites: A study by iron isotope measurements of metal grains from ordinary chondrites

Chondrules and chondrites provide unique insights into early solar system origin and history, and iron plays a critical role in defining the properties of these objects. In order to understand the processes that formed chondrules and chondrites, and introduced isotopic fractionation of iron isotopes, we measured stable iron isotope ratios ⁵⁶Fe/⁵⁴Fe and ⁵⁷Fe/⁵⁴Fe in metal grains separated from 18 ordinary chondrites, of classes H, L and LL, ranging from petrographic types 3-6 using multi-collector inductively coupled plasma mass spectrometry. The δ⁵⁶Fe values range from −0.06 ± 0.01 to +0.30 ± 0.04‰ and δ⁵⁷Fe values are −0.09 ± 0.02 to +0.55 ± 0.05‰ (relative to IRMM-014 iron isotope standard). Where comparisons are possible, these data are in good agreement with published data. We found no systematic difference between falls and finds, suggesting that terrestrial weathering effects are not important in controlling the isotopic fractionations in our samples. We did find a trend in the ⁵⁶Fe/⁵⁴Fe and ⁵⁷Fe/⁵⁴Fe isotopic ratios along the series H, L and LL, with LL being isotopically heavier than H chondrites by ∼0.3‰ suggesting that redox processes are fractionating the isotopes. The ⁵⁶Fe/⁵⁴Fe and ⁵⁷Fe/⁵⁴Fe ratios also increase with increasing petrologic type, which again could reflect redox changes during metamorphism and also a temperature dependant fractionation as meteorites cooled. Metal separated from chondrites is isotopically heavier by ∼0.31‰ in δ⁵⁶Fe than chondrules from the same class, while bulk and matrix samples plot between chondrules and metal. Thus, as with so many chondrite properties, the bulk values appear to reflect the proportion of chondrules (more precisely the proportion of certain types of chondrule) to metal, whereas chondrule properties are largely determined by the redox conditions during chondrule formation. The chondrite assemblages we now observe were, therefore, formed as a closed system.     

Two melting regimes during Paleogene flood basalt generation in East Greenland: combined REE and PGE modelling

Previously published platinum group element (PGE) and rare earth element data (REE) from a sample suite of the Palaeogene flood basalts of the East Greenland rifted margin are used to approximate primary magma compositions by numerical models of mantle melting. Both high-Ti and low-Ti basalts are found intercalated in the coastal section “the Sortebre Profile” in central East Greenland, and the apparent lack of mixing between the two series indicates coexistence of two geographically separated melting regions and plumbing systems during continental breakup above the Palaeogene Iceland plume. The lavas show little or no sign of crustal contamination and the limited variation in La/Sm and Cu/Pd ratios can be interpreted to reflect mantle source composition and melting processes. Numerical modelling indicate that the low-Ti series formed by F~20% melting in a columnar melting regime from a slightly depleted upper mantle source with a relatively normal S-content (~180 ppm S). In contrast, the high-Ti series formed by much lower degrees of melting (F~6%) in a spreading-related, triangular melting regime from a relatively S-poor (~100 ppm S) source. The low-Ti suite was S-undersaturated at the stage of melt segregation from a shallow mantle source due to the high degree of melting. In contrast, the high-Ti suite probably formed from a S-poor source where some low degree melt batches were S-saturated at the stage of deep segregation in distal parts of the triangular melting regime. This suite shows a geochemical high pressure garnet-signature and adiabatic decompression could therefore have played a role in keeping the mantle-derived S in solution before Fe-enrichment related to fractional crystallisation also increased the S-capacity of these melts. An erratum to this article can be found at     

REE Abundaces in Megacrysts and Host Basalts：REE Behavior of Magma at High Pressures

Based on REE abundances in megacrysts and host basalts and their equilibrium conditions,it has proved that megacrysts may have been produced from the magma derived from the host rocks or of more basic composition.The REE ratios of megacrysts to host rocks may be taken as partition coefficients when both are equilibrium with each other.The crystal fractionation of megacrysts has caused the evolution of REE in the magma.It is obvious that some host basalts are the product of magma evolution after crystal fractionation.According to REE abundances in the host rocks and the partition coefficients between crystal and liquid,the history of crystal fractionation of magma can be traced.     

Chemical and physical studies of type 3 chondrites—VI: Siderophile elements in ordinary chondrites

The abundances of Fe, Ni, Co, Au, Ir, Ga, As and Mg have been determined by instrumental neutron activation analysis in 38 type 3 ordinary chondrites (10 of which may be paired) and 15 equilibrated chondrites. Classification of type 3 ordinary chondrites into the H, L and LL classes using oxygen isotopes and parameters which reflect oxidation state (Fa and Fs in the olivine and pyroxene and Co in kamacite) is difficult or impossible. Bulk compositional parameters, based on the equilibrated chondrites, have therefore been used to classify the type 3 chondrites. The distribution of the type 3 ordinary chondrites over the classes is very different from that of the equilibrated chondrites, the LL chondrites being more heavily represented. The type 3 ordinary chondrites contain 5 to 15 percent lower abundances of siderophile elements and a compilation of the present data and literature data indicates a small, systematic decrease in siderophile element concentration with decreasing petrologic type. The type 3 ordinary chondrites have, like the equilibrated ordinary chondrites, suffered a fractionation of their siderophile elements, but the loss of Ni in comparison with Au and Ir is greater for the type 3 chondrites. These siderophile element trends were established at the nebula phase of chondritic history and the co-variation with petrologic type implies onion-shell structures for the ordinary chondrite parent bodies. It is also clear that the relationship between the type 3 and the equilibrated ordinary chondrites involves more than simple, closed-system metamorphism.