Volatile Siderophile Elements Research Articles

Distribution of siderophile elements in CR chondrites: evidence for evaporation and recondensation during chondrule formation

New data on the chemical composition of bulk samples, and of metallic and nonmagnetic fractions of six CR chondrites (Renazzo, Y793495, PCA91082, EET92042, Acfer 209 and El Djouf 001) are reported. It is shown that volatile siderophile element abundance patterns of metallic and nonmagnetic fractions of CR chondrites are complementary and volatility-dependent. In the metallic fraction CI-normalized abundances of Au, As, Sb and Ga decrease with increasing volatility, whereas in the nonmagnetic fraction abundances increase in the same sequence as in the metallic fraction. We argue that the siderophile element patterns of the metallic and nonmagnetic fractions reflect those of the chondrule and matrix fractions of CR chondrites, respectively, based on: (1) CR metal is mostly located inside chondrules and those metal grains outside chondrules probably were also derived from chondrules; (2) element partitioning within chondrules has been reset during chondrule formation; and (3) resetting of element distribution within chondrules occurred at sufficiently reducing conditions to allow partitioning of Au, As, Sb and Ga into CR metal.The complementary siderophile element patterns of CR chondrules and matrix are difficult to explain by gradual gas loss during condensation. The CI proportions of highly volatile elements in the bulk CR chondrites further argue against the possibility of loss of solids during condensation. Thus, the fractionation of volatile elements in CR chondrites is unlikely to be the result of gas-solid fractionation during condensation.The fractionation of volatile siderophile elements between CR chondrules and matrix requires evaporation of volatile elements during chondrule formation. The matrix pattern indicates recondensation of evaporated volatile elements. It appears, based on the composition of CR matrix, that the depletion and fractionation of moderately volatile elements in the bulk CR chondrites is due to formation of CR chondrites before complete recondensation of volatile elements which were evaporated during chondrule formation. This implies that agglomeration of CR chondrites proceeded simultaneously with chondrule formation.

Clues to the nature of the impacting bodies from platinum‐group elements (rhenium and gold) in borehole samples from the Clearwater East crater (Canada) and the Boltysh impact crater (Ukraine)

Abstract— Seven large (10 g) impact melt rock samples from boreholes from the Boltysh impact crater (Ukraine) and six samples from the East Clearwater crater (Canada) were analyzed for Os, Ir, Ru, Rh, Pd, Re and Au by the nickel sulfide technique in combination with neutron activation. Earlier analyses of Clearwater East impact melt rocks have shown that they are strongly enriched in Ir, Os, Pd and Re. In this work, I confirm earlier findings and demonstrate similarly high enrichments of Rh and Ru. The average Os/Ir, Ru/Ir, Pd/Ir, Rh/Ir and Ru/Rh ratios of the melt rock samples from Clearwater East are CI‐chondritic and yield an average Ir content of 25.2 ± 6.5 ng/g relative to an average upper crust concentration of 0.03 ± 0.02 ng/g Ir. The amount of meteoritic component corresponds to 4 to 7% of a nominal CI component for Clearwater East.The impact melt rock samples from a bore hole from Boltysh are low in Ir with an average of 0.2 ± 0.1 ng/g. The CI‐normalized abundances increase from the refractory to the more volatile siderophile elements (Os < Ir < Ru < Rh ∼ Pd ∼ Au ∼ Ni ∼ Co). Because of the low Ir anomaly and uncertainties in making corrections (correlations are weak) for indigenous siderophile elements, no clear projectile assignment can be made.

Highly siderophile elements (Re, Os, Ir, Ru, Rh, Pd, Au) in impact melts from three European impact craters (Sääksjärvi, Mien, and Dellen): Clues to the nature of the impacting bodies

Twenty-two large (10 g) impact melt samples from three Scandinavian craters (i.e., Sääksjärvi, Finland; and Mien and Dellen, Sweden) were analyzed for highly siderophile elements (HSE: platinum group elements, Rh, and Au) by the nickel sulfide technique in combination with neutron activation. The ten impact melt samples from Sääksjärvi are enriched in Ir and other highly siderophile elements (Ir = 2.48 ± 0.73 ng/g) relative to average upper crust concentrations (0.03 ± 0.02 ng/g Ir). The twelve Dellen and Mien samples are marginally enriched in Ir (0.48 ± 0.23 ng/g for Dellen, and 0.37 ± 0.23 ng/g for Mien). The amount of meteoritic component corresponds to 0.5% of a nominal CI component for Sääksjärvi, and about 0.1% for Mien and Dellen. The Sääksjärvi pattern is fractionated relative to CI-chondrites. Normalized abundances increase from the refractory to the more volatile siderophile elements (Ir < Ni). The trend is qualitatively similar to magmatic iron meteorites and corresponds to 0.4% of a nominal magmatic iron meteorite component (Tamarugal, IIIAB). For Mien and Dellen no projectile assignment can be made. All samples from the three impact craters have low Os Ir ratios compared to chondritic ratios. Either the projectile had low Os Ir ratios or Os was lost during the impact as volatile OsO 4. Based on the results of this study and on a compilation of literature data, average upper crustal abundances of the highly siderophile elements in the target area (Baltic shield) are estimated as 0.03 ng/g for Ir and Os, 1.1 ng/g for Ru, 0.38 ng/g for Rh, 2.0 ng/g for Pd, 8 μg/g for Co, and 37 μg/g for Cr. These data are representative of the upper crust. They allow, for the first time, reliable estimates of crustal abundances for Rh and Ru. Upper crustal abundances also show that Ir and Os are the most favourable elements for the identification of meteoritic signatures. Fractionated Os Ir ratios in magmatic iron meteorites or potential losses of Os during the impact leaves Ir as the best indicator element for meteoritic contamination. With Ir, extraterrestrial components as low as 2 × 10 4 × CI may be identified.

Fractionation of refractory siderophile elements in metal from the Rose City meteorite

Abstract— An ∼4 × 9 × 12‐mm concentration of metal (dubbed RC1) situated between silicate melt and a relict chondritic clast in the Rose City H5 impact‐melt breccia is compositionally heterogeneous. Approximately 65 wt% of RC1 is enriched in the refractory siderophile elements, Os and Ir, by 30–40% relative to bulk H chondrite metal; ∼20 wt% is depleted in these elements by 31–35%; and 15 wt% is depleted by a considerably greater amount (75%). Common and volatile siderophile elements are essentially unfractionated in all three regions; W is fractionated to only a moderate degree. The compositions of the different regions of RC1 are similar to those of previously analyzed metal nodules and veins in shocked but unmelted ordinary chondrites. All of these objects probably formed by a complex process involving vaporization of chondritic material, rapidly followed by oxidation of W to form volatile oxides, fractional condensation of refractory siderophile elements, transport of the residual vapor (containing common and volatile siderophile elements as well as W oxide) and condensation of this vapor in fractures and voids or on metallic liquid substrates. The common occurrence of vugs in shock‐heated chondrites and the pervasiveness of vaporization effects recorded in metal masses and veins underscores the important role of superheating in the formation of impact breccias.

Tin in mantle-derived rocks: Constraints on Earth evolution

We have analyzed tin together with other trace elements in a variety of mantle-derived rocks by isotope dilution-spark source mass spectrometry. Tin concentrations in oceanic basalts (mid-ocean ridge basalts, or MORB; oceanic island basalts, or OIB; island arc volcanics, or IAV) vary from 0.4 to 6 ppm. Tin is a moderately siderophile element which behaves like the moderately incompatible and lithophile element Sm during igneous processes in the mantle. The Sn Sm ratio is therefore similar in various reservoirs of the silicate portion of the Earth at the present day. To investigate possible secular variations of siderophile/chalcophile element abundances, we have also analyzed komatiites and basalts with ages ranging from Archean to Tertiary. Rocks having ages up to 3.4 Ga yield Sn Sm ratios comparable to those of the oceanic basalts. These results indicate that core growth during the last 3.4 Ga was negligibly small. In addition, the Sn Sm ratios in different mantle reservoirs are identical within the uncertainty of the data scatter. This indicates that tin was homogenized within the mantle after core formation. We conclude that the value of Sn Sm = 0.32 found in mantlederived rocks is representative of the primitive mantle because the estimated continental-crustal abundances of Sn and Sm yield a similar Sn Sm ratio. This results in Sn = 0.12 ± 0.01 ppm for the primitive mantle abundance, in agreement with that determined from fertile peridotite xenoliths (0.15 ± 0.02 ppm). To determine an absolute depletion factor for the volatile siderophile element tin in the primitive mantle, we have also analyzed four carbonaceous chondrites, including the CI chondrites Orgueil and Ivuna. We obtain a CI-value of tin of 1.62 ± 0.03 ppm. Tin is depleted in the Earth's primitive mantle by a factor of 33 ± 3; the volatility corrected depletion factor is similar to other moderately siderophile elements ( Newsom, 1990).

The origin of the moon and the early history of the earth—A chemical model. Part 1: The moon

The chemical implications of a “giant impact” model for the origin of the Moon are examined, both for the Moon and for the Earth. The Impactor is taken to be an approximately Mars-sized body. It is argued that the likeliest bulk chemical composition of the Moon (including a small Ni-rich metallic core) is quite similar to that of the Earth's mantle, and that this composition may be explained in detail if about 80% of the Moon came from the primitive Earth's mantle after segregation of the Earth's core. The other 20% of the Moon is modelled as coming from (a) the Impactor, which is constrained to be an oxidized, probably undifferentiated body of roughly CI chondritic composition (on a volatile free basis) and (b) a late stage veneer, with a composition and oxidation state similar to that of the H-group ordinary chondrites. This latter component is the source of all the volatile elements in the Moon (e.g., Na, K, Rb, Cs, and the volatile siderophile elements such as Cu, Ge, etc.), which failed to condense from the Earthand Impactor-derived materials; this component constitutes about 4% of the Moon. It is argued that Mo may behave as a volatile element under the relatively oxidising conditions necessary for the condensation of the proto-Moon. The late stage veneer may also be the reducing agent responsible for forming a small lunar core, which depletes the primitive silicate mantle of the Moon in the more siderophile elements. This core is presumed to have formed in equilibrium with the lunar mantle and, therefore, has Ni ( Ni + Fe) = 0.45 ± 0.1 ; a mass balance of Ni in the Moon constrains the lunar core to ∼ 1% of the Moon's mass. Metal separation to the core at an assumed temperature of 1300°C takes place at an oxygen fugacity 0.8 log-bar units below the iron-wüstite (IW) oxygen buffer. These conditions may then be used to calculate the siderophile trace element concentrations in the core-forming metal and thus the siderophile element abundances of the bulk Moon (core plus primitive mantle). The model accounts satisfactorily for most of the siderophile elements, including Fe, Ni, Co, W, P, and Cu. The relatively well-constrained lunar abundances of V, Cr, and Mn are also accounted for; their depletion in the Moon is inherited from the Earth's mantle. The consequences of the model for the composition of the Earth's mantle are addressed in O'Neill (1991).

Mantle heterogeneities: Isotopic and trace element characterisation of a major OIB source component interpreted as representative of oceanic crust segments recycled into the mantle through its historical evolution

We have analyzed tin together with other trace elements in a variety of mantle-derived rocks by isotope dilution-spark source mass spectrometry. Tin concentrations in oceanic basalts (mid-ocean ridge basalts, or MORB; oceanic island basalts, or OIB; island arc volcanics, or IAV) vary from 0.4 to 6 ppm. Tin is a moderately siderophile element which behaves like the moderately incompatible and lithophile element Sm during igneous processes in the mantle. The SnSmratio is therefore similar in various reservoirs of the silicate portion of the Earth at the present day.To investigate possible secular variations of siderophile/chalcophile element abundances, we have also analyzed komatiites and basalts with ages ranging from Archean to Tertiary. Rocks having ages up to 3.4 Ga yield SnSm ratios comparable to those of the oceanic basalts. These results indicate that core growth during the last 3.4 Ga was negligibly small. In addition, the SnSm ratios in different mantle reservoirs are identical within the uncertainty of the data scatter. This indicates that tin was homogenized within the mantle after core formation. We conclude that the value of SnSm = 0.32found in mantlederived rocks is representative of the primitive mantle because the estimated continental-crustal abundances of Sn and Sm yield a similar SnSm ratio. This results in Sn = 0.12 ± 0.01 ppm for the primitive mantle abundance, in agreement with that determined from fertile peridotite xenoliths (0.15 ± 0.02 ppm).To determine an absolute depletion factor for the volatile siderophile element tin in the primitive mantle, we have also analyzed four carbonaceous chondrites, including the CI chondrites Orgueil and Ivuna. We obtain a CI-value of tin of 1.62 ± 0.03 ppm. Tin is depleted in the Earth's primitive mantle by a factor of 33 ± 3; the volatility corrected depletion factor is similar to other moderately siderophile elements (Newsom, 1990).

The metal phase in unequilibrated ordinary chondrites and its implications for calculated accretion temperatures

The abundance, composition and grain size of the metal available to volatile siderophile elements strongly affect the condensation of these elements. These parameters are redefined on the basis of published chemical analyses and new mechanical analyses of the unequilibrated ordinary chondrites. The results suggest that previous workers have seriously overestimated the amount of metal present and available during condensation, and seriously underestimated the heat of solution of Bi in chondritic metal. Correction of these parameters results in nominal accretion temperatures for Bi which are substantially (95–110°K) lower than those calculated earlier, and which are discordant with the temperatures inferred for chalcophile trace elements.