The Fe-C system at 5 GPa and implications for Earth’s core

Earth’s core may contain C, and it has been suggested that C in the core could stabilize the formation of a solid inner core composed of Fe₃C. We experimentally examined the Fe-C system at a pressure of 5 GPa and determined the Fe-C phase diagram at this pressure. In addition, we measured solid metal/liquid metal partition coefficients for 17 trace elements and examined the partitioning behavior between Fe₃C and liquid metal for 14 trace elements. Solid metal/liquid metal partition coefficients are similar to those found in one atmosphere studies, indicating that the effect of pressure to 5 GPa is negligible. All measured Fe₃C/liquid metal partition coefficients investigated are less than one, such that all trace elements prefer the C-rich liquid to Fe₃C. Fe₃C/liquid metal partition coefficients tend to decrease with decreasing atomic radii within a given period. Of particular interest, our 5 GPa Fe-C phase diagram does not show any evidence that the Fe-Fe₃C eutectic composition shifts to lower C contents with increasing pressure, which is central to the previous reasoning that the inner core may be composed of Fe₃C.     

The first absolute gravity measurements in Indonesia

For the purposes of the calibration of the superconducting gravimeter (SG) in Bandung and the establishment of the absolute gravity (AG) points, we carried out AG measurements for the first time in Indonesia in November 2002. The measurements in Bandung were conducted between November 15th and 20th by means of a FG5 (#210), and 14,520 effective drops were obtained. The gravity value newly determined at the AG point in Bandung is 977976701.2 μgal (1 μgal = 10⁻⁸ ms⁻²) and the scale factor for the SG is −52.22 μgal/V. We also established another AG point in Yogyakarta near Merapi volcano and carried out AG measurements in Yogyakarta between November 22nd and 26th. The gravity value determined for this station is 978203093.5 μgal.     

Platinum group element geochemistry of komatiites from the Alexo and Pyke Hill areas, Ontario, Canada

Thirty-three whole-rock drill core samples and thirteen olivine, chromite, and sulfide separates from three differentiated komatiite lava flows at Alexo and Pyke Hill, Canada, were analyzed for PGEs using the Carius tube digestion ID-ICP-MS technique. The emplaced lavas are Al-undepleted komatiites with ∼27% MgO derived by ∼50% partial melting of LILE-depleted Archean mantle. Major and minor element variations during and after emplacement were controlled by 30 to 50% fractionation of olivine Fo_93-94. The emplaced lavas are characterized by (Pd/Ir)_N = 4.0 to 4.6, (Os/Ir)_N = 1.07, and Os abundances of ∼2.3 ppb. Variations in PGE abundances within individual flows indicate that Os and Ir were compatible (bulk D_Os,Ir = 2.4-7.1) and that Pt and Pd were incompatible (bulk D_Pt,Pd < 0.2) during lava differentiation, whereas bulk D_Ru was close to unity. Analyses of cumulus olivine separates indicate that PGEs were incompatible in olivine (D_PGEs^Ol-Liq = 0.04-0.7). The bulk fractionation trends cannot be accounted for by fractionation of olivine alone, and require an unidentified Os-Ir-rich phase. The composition of the mantle source (Os = 3.9 ppb, Ir = 3.6 ppb, Ru = 5.4 ppb, Pt and Pd = 5.7 ppb) was constrained empirically for Ru, Pt, and Pd; the Os/Ir ratio was taken to be identical to that in the emplaced melt, and the Ru/Ir ratio was taken to be chondritic, so that the absolute IPGE abundances of the source were determined by Ru. This is the first estimate of the PGE composition of a mantle source derived from analyses of erupted lavas. The suprachondritic Pd/Ir and Os/Ir of the inferred Abitibi komatiite mantle source are similar to those in off-craton spinel lherzolites, orogenic massif lherzolites, and enstatite chondrites, and are considered to be an intrinsic mantle feature. Bulk partition coefficients for use in komatiite melting models derived from the source and emplaced melt compositions are: D_Os,Ir = 2.3, D_Ru = 1.0, D_Pt,Pd = 0.07. Ruthenium abundances are good indicators of absolute IPGE abundances in the mantle sources of komatiite melts with 26 to 29% MgO, as Ru fractionates very little during both high degrees of partial melting and lava differentiation.     

Highly siderophile element geochemistry of Os-enriched 2.8 Ga Kostomuksha komatiites, Baltic Shield

New analyses of highly siderophile elements (HSE; Re, Os, Ir, Ru, Pt, and Pd) obtained by Carius tube digestion isotope dilution inductively coupled plasma mass-spectrometry (ID-ICPMS) technique are reported for ¹⁸⁷Os-enriched 2.8 Ga komatiites from the Kostomuksha greenstone belt. As a result of a significant improvement in the yield over our previous digestions by the NiS fire-assay technique, these komatiites have now been shown to contain 22 to 25% more Os, Ir, and Pt and 34% more Ru. The emplaced komatiite lavas at Kostomuksha thus had siderophile element abundances comparable to those of the Abitibi belt. The discrepancies observed between the two techniques are interpreted to be the result of incomplete digestion of HSE carriers (particularly chromite) during the NiS fire-assay procedure. Our results for UB-N peridotite reference material agree well with those obtained by the high-pressure ashing digestion ID-ICPMS technique reported in the literature. Two types of komatiite lavas have been distinguished in this study based on the IPGE (Os, Ir, and Ru) behavior during lava differentiation. The Kostomuksha type is unique and is characterized by an incompatible behavior of IPGEs, with bulk solid-liquid partition coefficients for IPGEs being close to those for olivine. Cumulate zones in this type of komatiite lava occupy <20% of the total thickness of the flows. The Munro type exhibits a compatible behavior of IPGEs during lava differentiation. The cumulate zone in this type of komatiite occupies >20% of the total thickness of the flows. The calculated bulk partition coefficients indicate that, as with the other Munro-type komatiite lavas, the bulk cumulate contained an IPGE-rich minor phase(s) in addition to olivine. The non-CI chondritic HSE pattern for the source of the Kostomuksha komatiites calculated here is similar to that of Abitibi komatiites and to average depleted spinel lherzolite (ADSL) and supports the hypothesis of a non-CI chondritic HSE composition of the Earth’s mantle. The absolute HSE abundances in the source of the Kostomuksha komatiite have been demonstrated to be comparable to those of the source of Abitibi komatiites, even though the two komatiites contrast in their Os isotopic compositions. This supports the earlier hypothesis that if core-mantle interaction produced the ¹⁸⁷Os/¹⁸⁸Os radiogenic signature in the Kostomuksha source, it must have occurred in the form of isotope exchange at the core-mantle boundary. Other explanations of the radiogenic Os signature are similarly constrained to conserve the elemental abundance pattern in the mantle source of Kostomuksha komatiites.     

Compositions of group IVB iron meteorites and their parent melt

The concentrations of P, V, Cr, Fe, Co, Ni, Cu, Ga, Ge, As, Mo, Ru, Rh, Pd, W, Re, Os, Ir, Pt, and Au in the group IVB iron meteorites Cape of Good Hope, Hoba, Skookum, Santa Clara, Tawallah Valley, Tlacotepec, and Warburton Range have been measured by laser ablation inductively coupled plasma mass spectrometry. The data were fitted to a model of fractional crystallization of the IVB parent body core, from which the composition of the parent melt and metal/melt distribution coefficients for each element in the system were determined, for a chosen value of D(Ni). Relative to Ni and chondritic abundances, the parent melt was enriched in refractory siderophiles, with greatest enrichment of 5× chondritic in the most refractory elements, and was strongly volatile-depleted, down to 0.00014× chondritic in Ge. Comparison to an equilibrium condensation sequence from a gas of solar composition indicates that no single temperature satisfactorily explains the volatility trend in the IVB parent melt; a small (<1%) complement of ultrarefractory components added to metal that is volatile-depleted but otherwise has nearly chondritic abundances (for Fe, Co and Ni) best explains the volatility trend. In addition to this volatility processing, which probably occurred in a nebular setting, there was substantial oxidation of the metal in the IVB parent body, leading to loss of Fe and other moderately siderophile elements such as Cr, Ga, and W, and producing the high Ni contents that are observed in the IVB irons. By assuming that the entire IVB parent body underwent a similar chemical history as its core, the composition of the silicate that is complementary to the IVB parent melt was also estimated, and appears to be similar to that of the angrite parent.     

Unique achondrite Northwest Africa 11042: Exploring the melting and breakup of the L chondrite parent body

Northwest Africa (NWA) 11042 is a heavily shocked achondrite with medium‐grained cumulate textures. Its olivine and pyroxene compositions, oxygen isotopic composition, and chromium isotopic composition are consistent with L chondrites. Sm‐Nd dating of its primary phases shows a crystallization age of 4100 ± 160 Ma. Ar‐Ar dating of its shocked mineral maskelynite reveals an age of 484.0 ± 1.5 Ma. This age coincides roughly with the breakup event of the L chondrite parent body evident in the shock ages of many L chondrites and the terrestrial record of fossil L chondritic chromite. NWA 11042 shows large depletions in siderophile elements (<0.01×CI) suggestive of a complex igneous history involving extraction of a Fe‐Ni‐S liquid on the L chondrite parent body. Due to its relatively young crystallization age, the heat source for such an igneous process is most likely impact. Because its mineralogy, petrology, and O isotopes are similar to the ungrouped achondrite NWA 4284 (this work), the two meteorites are likely paired and derived from the same parent body.     

ANALYSIS OF SURFACE AIR TEMPERATURE RECORDED IN LAHORE AND MIANWALI OVER THE PAST THREE DECADES

Surface air temperatures recorded over the past three decades at the weather stations located in Lahore (anindustrialized and densely populated city) and Mianwali (a small and sparsely populated city) were analyzed in order tostudy their climatic trend.Lahore,where meteorological data are recorded at two weather stations (city station and air-port station) indicates a cooling trend,of about 0.5℃ per record period of 1953—1992,for the airport station (31°31′N,74°24′E) and a slight warming trend,of about 0.2℃,for the city station (31°33′N,74°20′E) for the record period of 1950—1992.The Mianwali weather station (32°33′N,71°31′E) also shows a slight cooling trend,of about 0.4℃ per recordperiod of 1959—1992.The climatic variability at these stations was studied by computing seasonal and annual tempera-ture anomalies.The results are explained in terms of the local environmental conditions.     

Exsolution and shock microstructures of igneous pyroxene clasts in the Northwest Africa 7533 Martian meteorite

Northwest Africa (NWA) 7533 is a Martian regolith breccia. This meteorite (and its pairings) offers a good opportunity to study (near‐) surface processes that occurred on early Mars. Here, we have conducted a transmission electron microscope study of medium‐ and coarse‐grained (a few tens to hundreds of micrometers) Ca‐rich pyroxene clasts in order to define their thermal and shock histories. The pyroxene grains have a high‐temperature (magmatic) origin as revealed by the well‐developed pigeonite–augite exsolution microstructure. Exsolution lamella characteristics (composition, thickness, and spacing) indicate a moderately slow cooling. Some of the pyroxene clasts display evidence for local decomposition into magnetite and silica at the submicron scale. This phase decomposition may have occurred at high temperature and occurred at high oxygen fugacity at least 2–3 log units above the QFM buffer, after the formation of the exsolution lamellae. This corresponds to oxidizing conditions well above typical Martian magmatic conditions. These oxidizing conditions seem to have prevailed early and throughout most of the history of NWA 7533. The shock microstructure consists of (100) mechanical twins which have accommodated plastic deformation. Other pyroxene shock indicators are absent. Compared with SNC meteorites that all suffered significant shock metamorphism, NWA 7533 appears only mildly shocked. The twin microstructure is similar from one clast to another, suggesting that the impact which generated the (100) twins involved the compacted breccia and that the pyroxene clasts were unshocked when they were incorporated into the NWA 7533 breccia.     

Potassium isotopic composition of Australasian tektites

Abstract— We have analyzed the potassium isotopic composition of four tektites from the Australasian strewn field, spanning a wide diversity of thermal histories, inferred from textures and volatile element contents. Our results indicate no isotopic differences between tektites and terrestrial crustal rocks, placing stringent limits of ≤2% loss of potassium during the brief duration of high temperature heating experienced by these samples. This confirms that the chemical composition of tektites is entirely a reflection of source rock composition and has not been modified by the tektiteforming process for elements less volatile than potassium. Losses of more volatile components, e.g., the halogens and water, are not precluded by the present data. Coupling a radiative cooling temperature‐time path with potassium vapor pressure data indicates that tektite melt drops are not likely to develop bulk elemental fractionation during the brief heating episodes of tektites for peak temperatures <2273 K. The extent of K isotopic fractionation is independent of droplet size but dependent on peak heating temperature. The exact peak temperature depends on the choice of vapor pressure data used for K, which need to be better constrained.     

The Fe/Mn ratio in MORB and OIB determined by ICP-MS

Variations in the abundance of iron in the mantle may have important consequences for mantle dynamics and geochemistry. The abundance of iron in lavas derived from mantle source regions varies during partial melting and subsequent fractionation, so that source heterogeneities are not easily resolved in iron abundances alone. However, manganese is a geochemically similar element, so that the planetary Fe/Mn ratio is approximately constant. Here, we report new Fe/Mn results for mid-oceanic ridge basalts (MORBs), oceanic island basalts (OIBs) and komatiites using a precise inductively coupled plasma mass spectrometric (ICP-MS) method to measure Fe/Mn to better than 0.5% (2σ). As a measure of reproducibility of Fe/Mn, five olivine and five orthopyroxene grains from a Kilbourne Hole peridotite xenolith yielded Fe/Mn 69.8 ± 0.4 and 44.3 ± 0.2, respectively. To avoid ubiquitous secondary Fe-Mn oxides, Fe/Mn ratios in Pacific, Atlantic, and Indian MORBs were determined by Laser Ablation ICP-MS. MORB Fe/Mn (53-56) corrected for crystal fractionation yielded a value of 54.0 ± 1.2 (1σ). Icelandic basalts and picrites (MgO 10-29%) had Fe/Mn ratios 56-61, with a single exception. Six relatively fresh komatiites from Belingwe (MgO 20-29%) yielded Fe/Mn values of 58.3 ± 0.2 (1σ). Basalts from Tahiti and Reunion exhibited high Fe/Mn (>65), like Hawaii. This implies that the mantle source regions of Tahiti and Reunion lavas may have been enriched in Fe relative to other mantle reservoirs (e.g., MORBs, Iceland, Belingwe). Combined with previous results for Hawaii, we now find that Fe/Mn > 65 is characteristic of at least two plumes from the Pacific Superswell. It is conceivable that this is evidence for excess Fe due to core-mantle interaction in these mantle plumes, although partial melting of secondary pyroxenites may cause similar variations in Fe/Mn. Heterogeneity of Fe/Mn in mantle-derived lavas is now clearly documented.