The IIG iron meteorites: Probable formation in the IIAB core

The addition of two meteorites to the iron meteorite grouplet originally known as the Bellsbank trio brings the population to five, the minimum number for group status. With Ga and Ge contents in the general “II” range, the new group has been designated IIG. The members of this group have low-Ni contents in the metal and large amounts of coarse schreibersite ((Fe,NI)₃P); their bulk P contents are 17-21 mg/g, the highest known in iron meteorites. Their S contents are exceptionally low, ranging from 0.2 to 2 mg/g. We report neutron-activation-analysis data for metal samples; the data generally show smooth trends on element-Au diagrams. The low Ir and high Au contents suggest formation during the late crystallization of a magma.Because on element-Au or element-Ni diagrams the IIG fields of the important taxonomic elements Ni, Ga, Ge and As are offset from those of the IIAB irons, past researchers have concluded that the IIG irons could not have formed from the same magma, and thus that the two groups originated on separate parent bodies. However, on most element-Au diagrams the IIG fields plot close to extensions of IIAB trends to higher Au concentrations.There is general agreement that immiscibility led to the formation of an upper S-rich and a lower P-rich magma in the IIAB core. We suggest that the IIG irons formed from the P-rich magma, and that schreibersite was a liquidus phase during the final stages of crystallization. The offsets in Ni and As (and possibly other elements) may result from solid-state elemental redistribution between metal and schreibersite during slow cooling. For example, it is well established that the equilibrium Ni content is >2× higher in late-formed relative to early-formed schreibersite. It is plausible that As substitutes nearly ideally for P in schreibersite at eutectic temperatures but becomes incompatible at low temperatures.[Wasson J. T., Huber, H. and Malvin, D. J. (2007) Formation of IIAB iron meteorites. Geochim. Cosmochim. Acta71, 760-781] argued that, in the most evolved IIAB irons, the amount of trapped melt was high. The high P contents of IIG irons also require high contents of trapped melt but the local geometry seems to have allowed the S-rich immiscible melt to escape as it formed. The escaping melt may have selectively depleted elements such as Au and Ge.     

Chemical classification of iron meteorites—X. Multielement studies of 43 irons,resolution of group IIIE from IIIAB,and evaluation of Cu as a taxonomic parameter

We report structural and compositional data leading to the classification of 41 iron meteorites, increasing the number of classified independent iron meteorites to 576. We also obtained data on a new metal-rich mesosiderite and on two new iron masses that are paired with previously studied irons. For the first time in this series we also report concentrations of Cr, Co, Cu, As, Sb, W, Re and Au in each of these 44 meteorites. We determined 7 of these elements (all except Sb) in 30 previously studied ungrouped or unusual irons, and obtained Cu data on 104 irons, 21 pallasites, and 3 meteorite phases previously studied by E. Scott. We show that Cu possesses characteristics well suited to a taxonomic element: a siderophile nature, a large range among all irons, and a low range within magmatic groups. For the first time we report the partial resolution of the C-rich group IIIE from its populous twin group IIIAB on element-Ni diagrams other than Ir-Ni. Cachiyuyal previously classified ungrouped and Armanty (Xinjiang) previously classified IIIAB are reclassified IIIE. Despite the addition of 3 new irons and the reanalysis of 3 previously studied irons the members of the set of 15 ungrouped irons having very low Ga (<3 μg/g) and Ge (<0.7 μg/g) contents remain individualists. The same is generally true for irons having $\text{100 ≤ Ni ≤ 180}\text{mg/g}$ and compositional similarities to IIICD, but A80104 increases the Garden Head trio to a quartet. Algoma is reclassified from ungrouped to IIICD-an and Hassi-Jekna and Magnesia from IIICD to IIICD-an. The metal of Horse Creek and Mount Egerton is compositionally closely related to metal from EH chondrites. We suggest that the P-rich Bellsbank trio irons formed in the IIAB core in topographic lows filled with an immiscible, P-rich second liquid.     

Formation of IIAB iron meteorites

Group IIAB is the third largest group of iron meteorites and the second largest group that formed by fractional crystallization; many of these irons formed from the P-rich portion of a magma consisting of two-immiscible liquids. We report neutron-activation data for 78 IIAB irons. These confirm earlier studies showing that the group has the largest known range in Ir concentrations (a factor of 4000) and that slopes are steeply negative on plots of Ir vs. Au or As (or Ni). High negative slopes imply relatively high distribution coefficients for Ir, Au, and As (but, with rare exceptions, remaining less than unity for the latter). IIAB appears to have had the highest S contents of any magmatic group of iron meteorites, consistent with its high contents of other volatile siderophiles, particularly Ga and Ge. Large fractions of trapped melt were present in the IIAB irons with the highest Au and As and lowest Ir contents. As a result, when these irons crystallized, the D_Au and D_As values can, with moderate accuracy, be estimated to have been roughly 0.53 and 0.46, respectively. These low values imply that the initial nonmetal (S + P) content of the magma was much lower than 170 mg/g, as estimated in earlier studies; our estimate is 75 mg/g. Our results are consistent with an initial P/S ratio of 0.25, similar to the ratio estimated for other magmatic groups. There is little doubt that incompatible S-rich and P-rich metallic liquids were involved during the formation of group IIAB. After 20% crystallization of our assumed starting composition the two-liquid boundary is encountered (at 72 mg/g S and 18 mg/g P). Initially the volume of S-rich liquid is very small, but continued crystallization increased the volume of this phase and decreased its P/S ratio while increasing this ratio in the P-rich liquid. Most crystallization of the IIAB magma would have occurred in the lower, P-rich portion of the core. However, metal was still a liquidus phase at the top of the core and, because both the immiscible liquids would have convected, they may have approached equilibrium throughout the very limited crystallization of the magma recorded in group IIAB. All IIAB irons contain trapped melt, and this melt will have had very different compositions depending on whether the liquid is S-rich (at the outer solid/liquid interface) or P-rich (at the inner interface). The P/S ratio in the melt trapped in the Santa Luzia iron is about 0.6 g/g, consistent with our modeling of Ir-Au and Ir-As trends implying that Santa Luzia formed in the lower, P-rich portion of the core after about 48% crystallization of the magma. Because the liquids were in equilibrium, the point at which immiscibility first occurred is not recorded by a dramatic change in the trends on element-Au diagrams; the main compositional effect is recorded in the P/S ratio of the trapped melt. The high-Au (>0.8 μg/g) irons for which large sections are available all contain skeletal schreibersite implying a relatively high (>0.3 g/g) P/S ratio; none of these irons could have crystallized from the S-rich upper layer of the core.     

The compositional classification of some Chinese iron meteorites

Based on structural observations and the concentrations of Cr, Co, Ni, Cu, Ga, Ge, As, Sb, Re, Ir, and Au by neutron-activation analysis we have classified 14 Chinese iron meteorites. Thirteen are members of the large groups IAB, IIICD, IIIAB and IVA. Leshan is an ungrouped iron meteorite that falls within the IIE field on some element-Ni diagrams, but is distinctly outside this field on plots of Cu, W, and Ir vs. Ni; it is very similar in composition to Techado, another ungrouped iron. The high Cu content of Leshan in consistent with other evidence indicating that Cu is a valuable parameter for classifying iron meteorites. IIICD Dongling appears not to be a new meteorite, but to be paired with Nantan; Dongling was recovered about 50 km from the location of the Nantan shower. In view of the fact that Yongning is highly oxidized, we assign it to group IAB but cannot rule out IIICD. IVA-An Longchang has many characteristics of IVA irons, but has been remelted, probably in a terrestrial setting. Five irons belong to group IVA, a remarkably large number. Three are identical in composition, and we suspect that the two from Hubei, Guanghua and Huangling, are paired. Thus this set of 14 irons includes 12 independent falls.     

Iron meteorites with low Ga and Ge concentrations—composition,structure and genetic relationships

Twenty-one iron meteorites with Ge contents below 1 μg/g, including nine belonging to groups IIIF and IVB, have been analyzed by instrumental neutron activation analysis (INAA) for the elements Co, Cr, As, Au, Re, Ir and W. Groups IIIF and IVB show positive correlations of Au, As and Co (IIIF only) with published Ni analyses, and negative correlations of Ir, Re, Cr (IVB only) and W (IIIF only) with Ni. On element-Ni plots, the gradients of the least squares lines are similar to those of many other groups, excluding IAB and IIICD. With the inclusion of a new member, Klamath Falls, group IIIF has the widest range of Au, As and Co contents of any group and the steepest gradients on plots of these elements against Ni. It is likely that these trends in groups IIIF and IVB were produced by fractionation of elements between solid and liquid metal, probably during fractional crystallization.It has been suggested that some of the 15 irons with <l μg/g Ge which lie outside the groups might be related. However, the INAA data indicate that no two are as strongly related as two group members. These low-Ge irons and the members of groups IIIF, IVA and IVB tend to have low concentrations of As, Au and P, low $\text{Co}\text{Ni}$ ratios and high Cr contents. The depletion of the more volatile elements probably results from incomplete condensation into the metal from the solar nebula.The structures of low-Ge irons generally reflect fast cooling rates (20–2000 K Myr^?1). When data for all iron meteorites are plotted on a logarithmic graph of cooling rate against Ge concentration and results for related irons are averaged, there is a significant negative correlation. This suggests that metal grains which inefficiently condensed Ge and other volatile elements tended to accrete into small parent bodies.     

Silicate-bearing iron meteorites and their implications for the evolution of asteroidal parent bodies

Silicate-bearing iron meteorites differ from other iron meteorites in containing variable amounts of silicates, ranging from minor to stony-iron proportions (∼50%). These irons provide important constraints on the evolution of planetesimals and asteroids, especially with regard to the nature of metal–silicate separation and mixing. I present a review and synthesis of available data, including a compilation and interpretation of host metal trace-element compositions, oxygen-isotope compositions, textures, mineralogy, phase chemistries, and bulk compositions of silicate portions, ages of silicate and metal portions, and thermal histories. Case studies for the petrogeneses of igneous silicate lithologies from different groups are provided. Silicate-bearing irons were formed on multiple parent bodies under different conditions. The IAB/IIICD irons have silicates that are mainly chondritic in composition, but include some igneous lithologies, and were derived from a volatile-rich asteroid that underwent small amounts of silicate partial melting but larger amounts of metallic melting. A large proportion of IIE irons contain fractionated alkali-silica-rich inclusions formed as partial melts of chondrite, although other IIE irons have silicates of chondritic composition. The IIEs were derived from an H-chondrite-like asteroid that experienced more significant melting than the IAB asteroid. The two stony-iron IVAs were derived from an extensively melted and apparently chemically processed L or LL-like asteroid that also produced a metallic core. Ungrouped silicate-bearing irons were derived from seven additional asteroids. Hf–W age data imply that metal–silicate separation occurred within 0–10 Ma of CAI formation for these irons, suggesting internal heating by ²⁶Al. Chronometers were partly re-set at later times, mainly earlier for the IABs and later for the IIEs, including one late (3.60 ± 0.15 Ga) strong impact that affected the “young silicate” IIEs Watson (unfractionated silicate, and probable impact melt), Netschaëvo (unfractionated, and metamorphosed), and Kodaikanal (fractionated). Kodaikanal probably did not undergo differentiation in this late impact, but the similar ages of the “young silicate” IIEs imply that relatively undifferentiated and differentiated materials co-existed on the same asteroid. The thermal histories and petrogeneses of fractionated IIE irons and IVA stony irons are best accommodated by a model of disruption and reassembly of partly molten asteroids.     

Relationship between iron-meteorite composition and size: Compositional distribution of irons from North Africa

During the past three decades many iron meteorites have been collected from the deserts of North Africa. Almost all are now characterized, and the distribution among classes is found to be very different from those that were in museums prior to the collection of meteorites from hot and cold (Antarctica) deserts. Similar to the iron meteorites from Antarctica, the irons from Northwest Africa include a high fraction of ungrouped irons and of minor subgroups of group IAB. The different distribution is attributed to the small median size of the desert meteorites (∼1.3 kg in North African irons, ∼30 kg in non-desert irons). It appears that a sizable fraction of these small (several centimeter) masses constitute melt pockets produced by impacts in chondritic regoliths; they were never part of a large (meter-to-kilometer) magma bodies. As a result, a meter-size fragment ejected from the regolith of the asteroid may contain several of these small metallic masses. It may be that such stochastic sampling effects enhanced the fraction of IAB-sHL irons among the irons from Northwest Africa.The variety observed in small meteoroids is also enhanced because (relative to large) small fragments are more efficiently ejected from asteroids and because the orbital parameters of small meteoroids are more strongly affected by collisions and drag effects, they evolve to have Earth-crossing perihelia more rapidly than large meteoroids; as a result, the set of small meteoroids tends to sample a larger number of parent asteroids than does the set of larger meteoroids.     

Silica and pyroxene in IVA irons; possible formation of the IVA magma by impact melting and reduction of L-LL-chondrite materials followed by crystallization and cooling

Group IVA is a large magmatic group of iron meteorites. The mean Δ¹⁷O (=δ¹⁷O − 0.52·δ¹⁸O) of the silicates is ∼+1.2‰, similar to the highest values in L chondrites and the lowest values in LL chondrites; δ¹⁸O values are also in the L/LL range. This strongly suggests that IVA irons formed by melting L-LL parental material, but the mean Ni content of IVA irons (83 mg/g) is much lower than that of a presumed L-LL parent (∼170 mg/g) and the low-Ca pyroxene present in two IVA meteorites is Fs13, much lower than the Fs20-29 values in L and LL chondrites. Thus, formation from L-LL precursors requires extensive addition of metallic Fe, probably produced by reduction of FeS and FeO. Group IVA also has S/Ni, Ga/Ni, and Ge/Ni ratios that are much lower than those in L-LL chondrites or any chondrite group that preserves nebular compositions, implying loss of these volatile elements during asteroidal processing. We suggest that these reduction and loss processes occurred near the surface of the asteroid during impact heating, and resulted partly from reduction by C, and partly from the thermal dissociation of FeS and FeO with loss of O and S. The hot (∼1770 K) low-viscosity melt quickly moved through channels in the porous asteroid to form a core. Two members of the IVA group, São João Nepomuceno (hereafter, SJN) and Steinbach, contain moderate amounts of orthopyroxene and silica, and minor amounts of low-Ca clinopyroxene. Even though SJN formed after ∼26% crystallization and Steinbach formed after ∼77% crystallization of the IVA core, both could have originated within several tens of meters of the core-mantle interface if 99% of the crystallization occurred from the center outwards. Two other members of the group (Gibeon and Bishop Canyon) contain tabular tridymite, which we infer to have initially formed as veins deposited from a cooling SiO-rich vapor. The silicates were clearly introduced into IVA irons after the initial magma crystallized. Because the γ-iron crystals in SJN are typically about 5 cm across, an order of magnitude smaller than in IVA irons that do not contain massive silicates, we infer that the metal was in the γ-iron field when the silicates were injected. The SJN and Steinbach silicate compositions are near the low-Ca-pyroxene/silica eutectic compositions. We suggest that a tectonic event produced a eutectic-like liquid and injected it together with unmelted pyroxene grains into fissures in the solid metal core. Published estimates of IVA metallographic cooling rates range from 20 to 3000 K/Ma, leading to a hypothesized breakup of the core during a major impact followed by scrambling of the core and mantle debris [Haack, H., Scott, E.R.D., Love, S.G., Brearley, A. 1996. Thermal histories of IVA stony-iron and iron meteorites: evidence for asteroid fragmentation and reaccretion. Geochim. Cosmochim. Acta60, 3103-3113]. This scrambling model is physically implausible and cannot explain the strong correlation of estimated cooling rates with metal composition. Previous workers concluded that the low-Ca clinopyroxene in SJN and Steinbach formed from protopyroxene by quenching at a cooling rate of 10¹² K/Ma, and suggested that this also supported an impact-scrambling model. This implausible spike in cooling rate by a factor of 10¹⁰ can be avoided if the low-Ca clinopyroxene were formed by a late shock event that converted orthopyroxene to clinopyroxene followed by minimal growth in the clinopyroxene field, probably because melt was also produced. We suggest that metallographic cooling-rate estimates (e.g., based on island taenite) giving similar values throughout the metal compositional range are more plausible, and that the IVA parent asteroid can be modeled by monotonic cooling followed by a high-temperature impact event that introduced silicates into the metal and a low-temperature impact event that partially converted orthopyroxene into low-Ca clinopyroxene.     

Main-group pallasites: chemical composition, relationship to IIIAB irons, and origin

We used neutron activation to characterize the metal of 33 main-group pallasites (PMG), widely held to be samples of a core-mantle interface. Most PMG cluster in a narrow range of metal and silicate compositions, but 6 are assigned to an anomalous subset (PMG-am) because of their deviant metal compositions, and 4 others to another anomalous subset (PMG-as) because of their appreciably higher olivine Fa contents. Metal compositions in all PMG are closely related to those in evolved IIIAB irons, and are generally consistent with their formation in the IIIAB parent asteroid. On element-Au diagrams for incompatible elements the normal PMG plot near an extrapolation of IIIAB trends to higher Au concentrations. On element-Au plots of compatible elements such as Ir or Pt the loci of PMG spread out over a broader region explainable by mixing of evolved IIIAB magma with early-crystallized core or mantle-residue solids.Two features of PMG require special models: (1) Ga and Ge contents are generally high (≈1.5×) compared to the IIIAB-based mixing model: and (2) the FeO/(FeO + MgO) ratios span a surprisingly wide range, from 0.11-0.14 in normal PMG to 0.16 to 0.18 in PMG-as This range is larger than expected in a cumulate layer at the base of a mantle. We suggest that both features may be related to the interaction of PMG precursors with a highly evolved magmatic gas phase, and that some or all of these anomalies may have resulted from vapor deposits in voids near the core-mantle interface.An important boundary condition for understanding the detailed PMG origin at the core-mantle interface is the large difference between the solidus temperature of Fa11 olivine (≈2000 K) and the liquidus temperature of an evolved IIIAB melt containing >100 mg/g S and some P (≈1600 K). Following the mixing event that formed the PMG it is therefore reasonable that there would have been olivine rubble floating on top of the IIIAB-like magma, but with appreciable void space present just above the upper level reached by the magma. These voids would have contained gases released from the magma during its flow into the PMG region. We suggest that Ga and Ge, the two most volatile siderophiles in our element set, were added to PMG metal from the magmatic gas. We also suggest that the magmatic gas was oxidizing and that the PMG having high olivine fayalite contents formed in regions where the ratio of void to olivine was high, and that some metallic Fe was oxidized and entered the olivine (or the phosphoran olivine). In support of the latter idea is the observation that both Ni and Co are elevated in the PMG-as (Fa≥16) compared to values predicted by IIIAB trends.We analyzed two Eagle-Station pallasites (PES); after correction for weathering effects in Cold Bay, its composition is found to closely resemble that of Eagle Station but to represent a more evolved composition (i.e., lower Ir, higher Au). Vermillion and Yamato 8451 have been called pyroxene pallasites but have metal compositions (unrelated to those of the PMG or PES) that are too different from each other to even allow assignment to the same grouplet.     

Structure and formation of the San Cristobal meteorite,other IB irons and group IIICD

San Cristobal is an unusual group IB ataxite with 25 per cent Ni, composed of taenite grains 2–3 cm in diameter and silicate-troilite-graphite nodules concentrated on the grain boundaries. Silicate compositions are typical of group IAB: olivine Fa_3.3, orthopyroxene Fs_6.9 and feldspar Ab₈₈. Plagioclase shows peristerite unmixing, previously unrecorded in meteorites, and occasional K-rich feldspar grains have an unusual antiperthite exsolution. Brianite Na₂CaMg(PO₄)₂ and haxonite (Fe, Ni)₂₃C₆ are common in nodules and matrix, respectively, while cohenite is rare. Part of the matrix contains a pearlitic kamacite precipitate instead of the usual oriented platelets.San Cristobal has extreme concentrations of many elements; e.g. the highest published Ag, Cu, In and Sb contents and the lowest Mo and Pt in irons. These data and the mineralogy show that San Cristobal has many characteristics of both groups IB and IIID, but that it fits group IB trends better. Ratios of refractory element abundances to those in Cl chondrites (both normalized to Ni) decrease through IB from l in IA to 0.03 in San Cristobal, but the other siderophilic elements have a small range of abundance ratios, 0.5–2, throughout IAB. We suggest that IB grains either formed in a part of the solar nebula where refractories had been previously removed, or else failed to equilibrate with a refractory-rich, high-temperature condensate. After condensation of the volatiles, Fe was partially removed, perhaps by oxidation. Group IIICD seems to have experienced similar fractionations. Unlike other iron meteorite groups, neither IAB nor IIICD appears to have been fully molten.     

Mesosiderites—I. Compositions of their metallic portions and possible relationship to other metal-rich meteorite groups

The metal from 17 mesosiderites has been analyzed for Ni, Ga, Ge and Ir by the techniques of atomic-absorption spectrometry and neutron activation. Most mesosiderite metal samples fall in a narrow compositional range: Ni, 7·0–9·0 per cent; Ga, 13–16 ppm; Ge, 47–58 ppm; and Ir, 2·4–4·4 ppm. Most of those falling outside these ranges belong to Powell's (1971) least-metamorphosed type. Mesosiderite metal falls in the same general composition range as IIIAB irons, IIIE irons, pallasites and H-group chondrite metal. There are distinct differences in detail, however, and firm evidence for a close genetic relationship between any of these groups and the mesosiderites is lacking. Metallic portions of Weekeroo-type irons tend to have slightly higher Ni, Ga, Ge and Ir contents than found in mesosiderite metal, and the two groups tend to form a single trend on all plots. The Weekeroo-type silicates closely resemble mesosiderites in terms of orthopyroxene composition and oxygen-isotope ratio. We interpret these similarities to indicate that the silicate and metallic portions of these two groups are closely related; if the mesosiderite silicates and metal were initially formed in separate parent bodies, these were of similar composition and formed at about the same distance from the Sun.     

Silicate inclusions in group IAB irons and a relation to the anomalous stones Winona and Mt Morris (Wis)

Silicates are found in many group IAB irons; in some cases as abundant angular cm-sized inclusions and in other cases as smaller fragments or single grains in troilite or graphite nodules. The mineralogy of the silicates is chondritic—olivine, pyroxene, albitic plagioclase—as is the bulk composition. The degree of oxidation of the olivine and pyroxene is intermediate between E and H chondrites (Fa 1–8, Fs 4–9). IAB inclusions have ages of about 4.5 Gyr, I¹²⁹-Xe¹²⁹ formation intervals in the ranges of chondrites and contain planetary-type rare gases.Samples of San Cristobal, Campo del Cielo, Mundrabilla and Woodbine were examined by microprobe and bulk inclusions from Campo del Cielo, Copiapo, Landes and Woodbine were analyzed by instrumental and radiochemical neutron activation analysis. Nonvolatile lithophilic and siderophih'c elements in Copiapo, Landes and Woodbine have approximately chondritic abundances. The chondritic level of lithophiles indicates the inclusions have not undergone igneous differentiation while the chondritic levels of siderophiles is evidence the metal is native to the inclusions and not matrix metal injected into the silicates. The two Campo del Cielo inclusions analyzed have roughly chondritic abundances of lithophiles but have fractionated rare earth patterns and widely varying amounts and abundances (relative to Ni) of siderophiles. These inclusions appear to have experienced some partial melting. Siderophile ratios for the inclusions have some differences when compared to matrix metal. One Campo del Cielo inclusion contains kamacite (5.5% Ni) with over 1000 μg Ge.Three-isotope O analyses by Clayton and coworkers of parts of the same or neighboring inclusions to those analyzed chemically place the inclusions slightly below the terrestrial fractionation line of clayton et al. (1976) and rule out the possibility of the inclusions being trapped fragments of one of the ordinary chondrite groups.The IAB silicates formed probably in a similar manner as chondrite groups but in a different region of the nebula and they record the O₂ fugacity and O isotopic composition of that location. They later became trapped in the metal-rich matrix probably as the result of collisions producing the breccialike texture. The relationship of the silicates to the kamacite-taenite structure of the metal requires that the metal-silicate mix have been heated to over 1000 K for an extended period.Two anomalous stony meteorites, Winona and Mt. Morris (Wis), are similar to IAB inclusions in mineralogy, bulk composition, $\text{FeO}\text{(FeO + Mg)}$ ratio of the silicates, and chromite composition and are possibly related to the IAB silicates. Winona also has an age of 4.6 Gyr and contains planetary-type rare gases. Microprobe data are reported for the major minerals of these anomalous meteorites. Although attempts to detect IAB levels of Ge in the metal phases were not successful, the weight of the evidence favors a relationship between these meteorites and IAB     

Chemical classification of iron meteorites—IX. A new group (IIF), revision of IAB and IIICD,and data on 57 additional irons

Structural observations and concentrations of Ni, Ga, Ge and Ir allow the classification of 57 iron meteorites in addition to those described in the previous papers in this series; the number of classified independent iron meteorites is now 535. INAA for an additional six elements indicates that five previously studied irons having very high $\text{Ge}\text{Ga}$ ratios are compositionally closely related and can be gathered together as group IIF. A previously unstudied iron, Dehesa, has the highest $\text{Ge}\text{Ga}$ ratio known in an iron meteorite, a ratio 18 × higher than that in CI chondrites. Although such high $\text{Ge}\text{Ga}$ ratios are found in the metal grains of oxidized unequilibrated chondrites, their preservation during core formation requires disequilibrium melting or significant compositional and temperature effects on metal/silicate distribution constants and/or activity coefficients. In terms of $\text{Ge}\text{Ga}$ ratios and various other properties group IIF shows genetic links to the Eagle Station pallasites and $\text{CO}\text{CV}$ chondrites. Klamath Falls is a new high-Ni, low-Ir member of group IIIF that extends the concentration ranges in this group and makes these comparable to the ranges in large igneous groups such as IIIAB. Groups IAB and IIICD have been revised to extend the lower Ni boundary of group IIICD down to 62 mg/g. The iron having by far the highest known Ni concentration (585 mg/g), Oktibbeha County, is a member of group IAB and extends the concentration ranges of all elements in this nonmagmatic group. Morasko, a IAB iron associated with a crater field in Poland, is paired with the Seeläsgen iron discovered 100 km away. All explosion craters from which meteorites have been recovered were produced by IAB and IIIAB irons.     

Iron meteorites: Crystallization,thermal history,parent bodies,and origin

We review the crystallization of the iron meteorite chemical groups, the thermal history of the irons as revealed by the metallographic cooling rates, the ages of the iron meteorites and their relationships with other meteorite types, and the formation of the iron meteorite parent bodies. Within most iron meteorite groups, chemical trends are broadly consistent with fractional crystallization, implying that each group formed from a single molten metallic pool or core. However, these pools or cores differed considerably in their S concentrations, which affect partition coefficients and crystallization conditions significantly. The silicate-bearing iron meteorite groups, IAB and IIE, have textures and poorly defined elemental trends suggesting that impacts mixed molten metal and silicates and that neither group formed from a single isolated metallic melt. Advances in the understanding of the generation of the Widmanstätten pattern, and especially the importance of P during the nucleation and growth of kamacite, have led to improved measurements of the cooling rates of iron meteorites. Typical cooling rates from fractionally crystallized iron meteorite groups at 500–700 °C are about 100–10,000 °C/Myr, with total cooling times of 10 Myr or less. The measured cooling rates vary from 60 to 300 °C/Myr for the IIIAB group and 100–6600 °C/Myr for the IVA group. The wide range of cooling rates for IVA irons and their inverse correlation with bulk Ni concentration show that they crystallized and cooled not in a mantled core but in a large metallic body of radius 150±50 km with scarcely any silicate insulation. This body may have formed in a grazing protoplanetary impact. The fractionally crystallized groups, according to Hf–W isotopic systematics, are derived originally from bodies that accreted and melted to form cores early in the history of the solar system, <1 Myr after CAI formation. The ungrouped irons likely come from at least 50 distinct parent bodies that formed in analogous ways to the fractionally crystallized groups. Contrary to traditional views about their origin, iron meteorites may have been derived originally from bodies as large as 1000 km or more in size. Most iron meteorites come directly or indirectly from bodies that accreted before the chondrites, possibly at 1–2 AU rather than in the asteroid belt. Many of these bodies may have been disrupted by impacts soon after they formed and their fragments were scattered into the asteroid belt by protoplanets.     

Implications of εNd—La／Nb,Ba／Nb,Nb／Th Diagrams to Mantle Heterogeneity—Classification of Island—arc Basalts and Decomposition of EMII Component

A group of εNd/Nb,Ba/Nb,Nb/Th diagrams are used to study mantle heterogeneity.Island-arc basalts(IAB) are distributed in a triangle of these diagrams. Three end-member components (the MORB-type depleted mantle, the fluid released from subducted oceanic crust and the sediments from the continental crust) of the source of IAB may be displayed in these diagrams. Two types of IAB are identified .They are of the two-component type (with little continental sediments), such as the basalts from Aletians and New Britain ,and the three-compeonent type, such as those from Sunda, Lesser Antilles and Andes. In addition ,the EMII type mantle-derived rocks may also be divided into two groups. One is exemplified by continental flood basalts and some peridotite xenoliths, similar to IAB, with high La/Nb and Ba/Nb and low Nb/Th ratios, The other includes the Samoa-type oceanic island basalts, with low La/Nb and Ba/Nb and high Nb/Th ratios. The corresponding two sub-components of EMII are EMIIM, which is related to the metasomatism of lithosphere mantle by fluids released from the subducted oceanic crust, and EMIISR, related to the intervention of recycling continental sediments into the convective mantle.     

Comparative petrology of silicates in the Udei Station (IAB) and Miles (IIE) iron meteorites: Implications for the origin of silicate-bearing irons

The textures and mineral chemistries of silicate inclusions in the Udei Station (IAB) and Miles (fractionated IIE) iron meteorites were studied using optical and electron microscopy, SEM, EMPA, and LA-ICP-MS techniques to better understand the origin of silicate-bearing irons. Inclusions in Udei Station include near-chondritic, basaltic/gabbroic, feldspathic orthopyroxenitic, and harzburgitic lithologies. In Miles, most inclusions can be described as feldspathic pyroxenite or pyroxene-enriched basalt/gabbro. The trace-element compositions of both orthopyroxene and plagioclase grains are similar in different lithologies from Udei Station; whereas in different inclusions from Miles, the compositions of orthopyroxene grains are similar, while those of clinopyroxene, plagioclase, and especially Cl-apatite are variable. Orthopyroxene in Miles tends to be enriched in REE compared to that in Udei Station, but the reverse is true for plagioclase and clinopyroxene.The data can be explained by models involving partial melting of chondritic protoliths, silicate melt migration, and redox reactions between silicate and metal components to form phosphate. The extent of heating, melt migration, and phosphate formation were all greater in Miles. Silicates in Miles were formed from liquids produced by ∼30% partial melting of a chondritic precursor brought to a peak temperature of ∼1250 °C. This silicate melt crystallized in two stages. During Stage 1, crystallizing minerals (orthopyroxene, clinopyroxene, chromite, and olivine) were largely in equilibrium with an intercumulus melt that was evolving by igneous fractionation during slow cooling, with a residence time of ∼20 ka at ∼1150 °C. During Stage 2, following probable re-melting of feldspathic materials, and after the silicate “mush” was mixed with molten metal, plagioclase and phosphate fractionally crystallized together during more rapid cooling down to the solidus. In Udei Station, despite a lower peak temperature (<1180 °C) and degree of silicate partial melting (∼3-10%), silicate melt was able to efficiently separate from silicate solid to produce melt residues (harzburgite) and liquids or cumulates (basalt/gabbro, feldspathic orthopyroxenite) prior to final metal emplacement. Olivine was generally out of equilibrium with other minerals, but orthopyroxene and plagioclase largely equilibrated under magmatic conditions, and clinopyroxene in basalt/gabbro crystallized from a more evolved silicate melt.We suggest that a model involving major collisional disruption and mixing of partly molten, endogenically heated planetesimals can best explain the data for IAB and fractionated IIE silicate-bearing irons. The extent of endogenic heating was different (less for the IABs), and the amount of parent body disruption was different (scrambling with collisional unroofing for the IAB/IIICD/winonaite body, more complete destruction for the fractionated IIE body), but both bodies were partly molten and incompletely differentiated at the time of impact. We suggest that the post-impact secondary body for IAB/IIICD/winonaite meteorites was mineralogically zoned with Ni-poor metal in the center, and that the secondary body for fractionated IIE meteorites was a relatively small melt-rich body that had separated from olivine during collisional break-up.     

I-Xe dating of silicate and troilite from IAB iron meteorites

The IAB iron meteorites may be related to the chondrites: siderophile elements in the metal matrix have chondritic abundances, and the abundant silicate inclusions are chondritic both in mineralogy and in chemical composition. Silicate and troilite (FeS) from IAB irons were analyzed by the I-Xe technique. Four IAB silicate samples gave well-defined I-Xe ages [in millions of years relative to Bjurböle; the monitor error (± 2.5 Myr) is not included]: ?3.7 ± 0.3 for Woodbine, ?0.7 ± 0. 6 for Mundrabilla, +1.4 ± 0.7 for Copiapo, and +2.6 ± 0.6 for Landes. The (¹²⁹Xe/¹³²Xe)_trapped ratios are consistent with previous values for chondrites, with the exception of Landes which has an extraordinary trapped ratio of 3.5 ± 0.2. Both analyses of silicate from Pitts gave anomalous I-Xe patterns.Troilite samples were also analyzed: Pitts troilite gave a complex I-Xe pattern, which suggests an age of +17 Myr; Mundrabilla troilite defined a good I-Xe correlation, which after correction for neutron capture on ¹²⁸Te gave an age of ?10.8 ± 0.7 Myr. Thus, surprisingly, low-melting troilite substantially predates high-melting silicate in Mundrabilla.Abundances of Ga, Ge, and Ni in metal from these meteorites are correlated with I-Xe ages of the silicate; meteorites with older silicates have greater Ni contents. No model easily accounts for this result as well as other properties of IAB irons; nevertheless, these results, taken at face value, overall favor a nebular formation model (e.g. Wasson, 1970, Icarus 12, 407–423). The great age of troilite from Mundrabilla suggests that this troilite formed in a different nebular region from the silicate and metal, and was later mechanically mixed with these other phases.The correlation between the trace elements in the metal and the I-Xe ages of the silicate provides one of the first known instances in which another well-defined meteoritic property correlates with I-Xe ages. In addition, almost all the ¹²⁹Xe in Mundrabilla silicate (etched in acid) was correlated with ¹²⁸Xe. These two results further support the validity of the I-Xe dating method.     

Discriminating four tectonic settings: Five new geochemical diagrams for basic and ultrabasic volcanic rocks based on log — ratio transformation of major-element data

We present five new discriminant function diagrams based on an extensive database representative of basic and ultrabasic rocks from four tectonic settings of island arc, continental rift, ocean-island, and mid-ocean ridge. These diagrams were obtained after log_e-transformation of concentration ratios of major-elements — a technique recommended for a correct statistical treatment of compositional data. Higher % success rates (overall values from ∼ 83 to 97%) were obtained for proposing these new diagrams as compared to those (∼82 to 94%) obtained from the discriminant analysis of the raw major-element concentration data (i.e., without the log_e-transformation and without taking ratios of the compositional data, but using exactly the same database to provide an unbiased comparison), suggesting that such a data transformation constitutes a statistically correct and recommended technique. The new diagrams also resulted in less mis-classification of basic and ultrabasic rocks from known tectonic settings than the diagrams obtained from the raw data. The use of these highly successful new discriminant function diagrams is illustrated using Miocene to Recent basic and ultrabasic rocks from three areas of Mexico with complex or controversial tectonic settings (Mexican Volcanic Belt, Los Tuxtlas volcanic field, and Eastern Alkaline Province), as well as older rocks from three areas (Deccan, Malani, and Bastar) of India. Additionally, the major-element data from two ‘known’ continental arc settings are used to show that they are similar to those from the island arc setting. Continental rift setting is inferred for all Mexican cases and for one cratonic area of India (Malani) and an IAB setting for the Bastar craton. The Deccan flood basalt province of India is used to warn against an indiscriminate use of those discrimination diagrams that do not explicitly include the likely setting of the area under evaluation. An Excel template is also provided for an easy application of these new diagrams for discriminating the four settings considered in this work.     

Gold solubility in oxidized and reduced, water-saturated mafic melt

We have performed experiments to evaluate Au solubility in natural, water-saturated basaltic melts as a function of oxygen fugacity. Experiments were carried out at 1000 °C and 200 MPa, and oxygen fugacity was controlled at the fayalite-magnetite-quartz (FMQ) oxygen fugacity buffer and FMQ + 4. All experiments were saturated with a metal-chloride aqueous solution loaded initially as a 10 wt% NaCl eq. fluid. The stable phase assemblage at FMQ consists of basalt melt, olivine, clinopyroxene, a single-phase aqueous fluid, and metallic Au. The stable phase assemblage at FMQ + 4 consists of basalt melt, clinopyroxene, magnetite-spinel solid solution, a single-phase aqueous fluid, and metallic Au. Silicate glasses (i.e., quenched melt) and their contained crystalline material were analyzed by using both electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Measured Au concentrations in the quenched melt range from 4.8 μg g⁻¹ to 0.64 μg g⁻¹ at FMQ + 4, and 0.54 μg g⁻¹ to 0.1 μg g⁻¹ at FMQ. The measured solubility of Au in olivine and clinopyroxene was consistently below the LA-ICP-MS limit of detection (i.e., 0.1 μg g⁻¹). These melt solubility data place important limitations on the dissolved Au content of water-saturated, Cl- and S-bearing basaltic liquids at geologically relevant fO₂ values. The new data are compared to published, experimentally-determined values for Au solubility in dry and hydrous silicate liquids spanning the compositional range from basalt to rhyolite, and the effects of melt composition, oxygen fugacity, pressure and temperature are discussed.     

A AB-complex iron meteorite containing low-Ca clinopyroxene: northwest Africa 468 and its relationship to lodranites and formation by impact melting

Northwest Africa 468 (NWA 468) is a new ungrouped, silicate-rich member of the IAB complex of nonmagmatic iron meteorites. The silicates contain relatively coarse (∼300 μm) grains of low-Ca clinopyroxene with polysynthetic twinning and inclined extinction. Low-Ca clinopyroxene is indicative of quenching from high temperatures (either from protoenstatite in a few seconds or high-temperature clinoenstatite in a few hours). It seems likely that NWA 468 formed by impact melting followed by rapid cooling to ≤660°C. After the loss of a metal-sulfide melt from the silicates, sulfide was reintroduced, either from impact-mobilized FeS or as an S₂ vapor that combined with metallic Fe to produce FeS. The O isotopic composition (Δ¹⁷O = −1.39‰) indicates that the precursor material of NWA 468 was a metal-rich (e.g., CR) carbonaceous chondrite. Lodranites are similar in bulk chemical and O isotopic composition to the silicates in NWA 468; the MAC 88177 lodranite (which also contains low-Ca clinopyroxene) is close in bulk chemical composition. Both NWA 468 and MAC 88177 have relatively low abundances of rare earth and plagiophile elements. Siderophiles in the metal-rich areas of NWA 468 are similar to those in the MAC 88177 whole rock; both samples contain low Ir and relatively high Fe, Cu, and Se. Most unweathered lodranites contain ∼20 to 38 wt.% metallic Fe-Ni. These rocks may have formed in an analogous manner to NWA 468 (i.e., by impact melting of metal-rich carbonaceous chondrite precursors) but with less separation of metal-rich melts from silicates.