Short-lived radionuclides (e.g., (26)Al, (53)Mn, (60)Fe, (182)Hf) are widely used to refine the chronology of the early solar system. They provide chronological information, however, only if they were homogeneously distributed in the source region of the objects under scrutiny at the time of their formation. With the high level of precision now achieved on isotopic measurements, very short time intervals can in principle be resolved and a precise evaluation of the initial homogeneity degree becomes increasingly crucial. High-precision nickel isotope data for differentiated meteorites (angrites, ureilites) and chondritic (CB) components allow us to test the initial distribution of radioactive (60)Fe and stable Ni isotopes. Although these meteorites appear to have formed nearly contemporaneously, they yield variable initial (60)Fe/(56)Fe ratios. Besides, the CB metal nodules and ureilite silicates show nucleosynthetic anomalies. The new data presented here do not confirm the recently inferred late injection of (60)Fe into the protoplanetary disk. Instead, live (60)Fe was present, but heterogeneously distributed, from the start of the solar system, revealing an incomplete mixing of material from various nucleosynthetic sources and restricting the use of the (60)Fe-(60)Ni system as a chronometer.

The Jurassic (approximately 145 Ma) Nambija oxidized gold skarns are hosted by the Triassic volcanosedimentary Piuntza unit in the sub-Andean zone of southeastern Ecuador. The skarns consist dominantly of granditic garnet (Ad 20–98 ) with subordinate pyroxene (Di 46–92 Hd 17–42 Jo 0–19 ) and epidote and are spatially associated with porphyritic quartz-diorite to granodiorite intrusions. Endoskarn is developed at the intrusion margins and grades inwards into a potassic alteration zone. Exoskarn has an outer K- and Na-enriched zone in the volcanosedimentary unit. Gold mineralization is associated with the weakly developed retrograde alteration of the exoskarn and occurs mainly in sulfide-poor vugs and milky quartz veins and veinlets in association with hematite. Fluid inclusion data for the main part of the prograde stage indicate the coexistence of high-temperature (500°C to >600°C), high-salinity (up to 65 wt.% eq. NaCl), and moderate- to low-salinity aqueous-carbonic fluids interpreted to have been trapped at pressures around 100–120 MPa, corresponding to about 4-km depth. Lower-temperature (510–300°C) and moderate- to low-salinity (23–2 wt.% eq. NaCl) aqueous fluids are recorded in garnet and epidote of the end of the prograde stage. The microthermometric data (Th from 513°C to 318°C and salinity from 1.0 to 23 wt.% eq. NaCl) and δ 18 O values between 6.2‰ and 11.5‰ for gold-bearing milky quartz from the retrograde stage suggest that the ore-forming fluid was dominantly magmatic. Pressures during the early retrograde stage were in the range of 50–100 MPa, in line with the evidence for CO 2 effervescence and probable local boiling. The dominance of magmatic low-saline to moderately saline oxidizing fluids during the retrograde stage is consistent with the depth of the skarn system, which could have delayed the ingression of external fluids until relatively low temperatures were reached. The resulting low water-to-rock ratios explain the weak retrograde alteration and the compositional variability of chlorite, essentially controlled by host rock compositions. Gold was precipitated at this stage as a result of cooling and pH increase related to CO 2 effervescence, which both result in destabilization of gold-bearing chloride complexes. Significant ingression of external fluids took place after gold deposition only, as recorded by δ 18 O values of 0.4‰ to 6.2‰ for fluids depositing quartz (below 350°C) in sulfide-rich barren veins. Low-temperature (<300°C) meteoric fluids (δ 18 O water between −10.0‰ and −2.0‰) are responsible for the precipitation of late comb quartz and calcite in cavities and veins and indicate mixing with cooler fluids of higher salinities (about 100°C and 25 wt.% eq. NaCl). The latter are similar to low-temperature fluids (202–74.5°C) with δ 18 O values of −0.5‰ to 3.1‰ and salinities in the range of 21.1 to 17.3 wt.% eq. CaCl 2 , trapped in calcite of late veins and interpreted as basinal brines. Nambija represents a deep equivalent of the oxidized gold skarn class, the presence of CO 2 in the fluids being partly a consequence of the relatively deep setting at about 4-km depth. As in other Au-bearing skarn deposits, not only the prograde stage but also the gold-precipitating retrograde stage is dominated by fluids of magmatic origin.

Use of the extinct 182Hf-182W chronometer to constrain the timing of planetary accretion and differentiation rests on the assumption that the solar nebula had homogeneous tungsten isotopic composition. Here, we report deficiencies of ~0.1 part in 10,000 in the abundance of 184W in group IVB iron meteorites relative to the silicate Earth. These are most likely due to incomplete mixing at the planetesimal scale (2-4 km radius bodies) of the products of slow (s-) and rapid (r-) neutron-capture nucleosynthesis in the solar nebula. The correction that must be applied to the 182Hf-182W model age of core formation in IVB irons due to the presence of these nuclear anomalies is ~0.5 Myr.

Angrites are amongst the oldest basalts in the solar system and their origins are enigmatic, some even proposing the planet Mercury as the parent body (APB). Whatever their exact provenance their chronometry provides insights into early stages of planetary melting and differentiation. We present the first high-precision internal Hf-182-W-182 isochrons for such early differentiated objects. Angrites Sahara 99555, D'Orbigny, and Northwest Africa 2999 define ages of 5.1 +/- 1.3 Ma, 4.7 +/- 1.3 Ma and 9.5 +/- 3.3 Ma respectively after formation of calcium-aluminum-rich refractory inclusions (CAIs). These data are in good agreement with Al-26 - Mg-26, Mn-53 - Cr-53 and Most (207)pb - Pb-206 ages for other angrites and provide evidence for two texturally and temporally well-resolved groups. The quenched angrites (SAH 99555, D'Orbigny and five others) have a weighted mean age of 4562.1 +/- 0.4 Ma and are the products of igneous crystallization on the APB - 5 Ma after the formation of CAIs, whereas the more slowly cooled angrites (NWA 2999, Angra dos Reis, LEW 86010, average age: 4557.7 +/- 0.2 Ma) reflect metamorphic closure similar to 5 Ma later following second reheating process or a complex cooling history. The concordance obtained between various short-lived chronometers provides evidence that Al-26, Mn-53 and Hf-182 were homogeneously distributed in the solar nebula, although we cannot rule out the possibility of local small heterogeneities. Contrary to recent proposals, the data are also consistent with the previously determined age of the solar system based on Pb-207 - Pb-206 systematics of CAIs. The Hf-W data are discussed in the context of two endmember models for the early differentiation of the angrite parent body. In the first model, core formation occurred at 3-4 Ma after CAIs and both groups of angrites formed by two distinct partial melting events from the bulk mantle of the angrite parent body. in the second model, the angrite parent body underwent progressive core formation with an increasing degree of W-depletion over time. In this model, the two groups of angrites derive from distinct reservoirs. The heat sources responsible for such late melting and core formation are unclear. Quenched angtites are coeval with non-magmatic LAB iron meteorites and CB chondrules at similar to 4562 Ma. However, demonstration of a generic link between angrite melting and impact events must await the acquisition of still higher resolution chronometry. (c) 2007 Elsevier B.V. All rights reserved.

We report precise nickel isotopic compositions, measured with high-resolution MC-ICPMS, for calcium-aluminum-rich inclusions (CAIs) considered to be the earliest objects formed in the solar system. The CAIs display correlated anomalies of (60)Ni and (62)Ni, possibly coupled with effects on (96)Zr. This is not predicted from stellar production models unless the (60)Ni results from the decay of (60)Fe. The effects show the signature of neutron-burst nucleosynthesis in a massive star. Such a neutron burst is plausibly responsible for the abundance of many of the short-lived radioactivities present in the early solar system; therefore, at least some of the nucleosynthetic heterogeneity previously reported from CAIs, leached carbonaceous chondrites, and presolar grains is likely to have been generated by dust injection from a supernova at a time close to the beginning of the solar system, which is consistent with a trigger for solar nebula collapse, or generated directly by dust injection into the protoplanetary disk.

We report precise nickel isotopic compositions, measured with high-resolution MC-ICPMS, for calcium-aluminum-rich inclusions (CAIs) considered to be the earliest objects formed in the solar system. The CAIs display correlated anomalies of 60Ni and 62Ni, possibly coupled with effects on 96Zr. This is not predicted from stellar production models unless the 60Ni results from the decay of 60Fe. The effects show the signature of neutron-burst nucleosynthesis in a massive star. Such a neutron burst is plausibly responsible for the abundance of many of the short-lived radioactivities present in the early solar system; therefore, at least some of the nucleosynthetic heterogeneity previously reported from CAIs, leached carbonaceous chondrites, and presolar grains is likely to have been generated by dust injection from a supernova at a time close to the beginning of the solar system, which is consistent with a trigger for solar nebula collapse, or generated directly by dust injection into the protoplanetary disk.

Magmatic iron meteorites are considered to be remnants of the metallic cores of differentiated asteroids, and may be used as analogues of planetary core formation. The Fe isotope compositions (delta Fe-57/54) of metal fractions separated from magmatic and non-magmatic iron meteorites span a total range of 0.39 parts per thousand, with the delta(57/54) Fe values of metal fractions separated from the IIAB irons (delta(57/) Fe-54 0.12 to 0.32 parts per thousand) being significantly heavier than those from the IIIAB (delta(57/14) Fe 0.01 to 0.15 parts per thousand), IVA (delta Fe-57/54-0.07 to 0.17 parts per thousand) and IVB groups (delta Fe-57/54 0.06 to 0.14 parts per thousand). The delta Fe-57/54 values of troilites (FeS) separated from magmatic and non-magmatic irons range from -0.60 to -0.12 parts per thousand, and are isotopically lighter than coexisting metal phases. No systematic relationships exist between metal-sulphide fractionation factor (Delta Fe-57/54(M-FeS) = delta Fe-57/54(metal)-delta Fe-57/54(FeS)) metal composition or meteorite group, however the greatest Delta Fe-57/54(M-FeS) values recorded for each group are strikingly similar: 0.79, 0.63, 0.76 and 0.74 parts per thousand for the IIAB, IIIAB, IAB and IIICD irons, respectively. Delta(57/54) FeM-FeS values display a positive correlation with kamacite bandwidth, i.e. the most slowly-cooled meteorites, which should be closest to diffusive equilibrium, have the greatest Delta Fe-57/54(M-FeS) values. These observations provide suggestive evidence that Fe isotopic fractionation between metal and troilite is dominated by equilibrium processes and that the maximum Delta Fe-57/54(M-FeS) value recorded (0.79 +/- 0.09 parts per thousand) is the best estimate of the equilibrium metal-sulphide Fe isotope fractionation factor. Mass balance models using this fractionation factor in conjunction with metal delta(57/54) Fe values and published Fe isotope data for pallasites can explain the relatively heavy delta(57/54) Fe values of IIAB metals as a function of large amounts of S in the core of the IIAB parent body, in agreement with published experimental work. However, sequestering of isotopically light Fe into the S-bearing parts of planetary cores cannot explain published differences in the average delta Fe-57/54 values of mafic rocks and meteorites derived from the Earth, Moon and Mars and 4-Vesta. The heavy delta Fe-57/54 value of the Earth's mantle relative to that of Mars and 4-Vesta may reflect isotopic fractionation due to disproportionation of ferrous iron present in the proto-Earth mantle into isotopically heavy ferric iron hosted in perovskite, which is released into the magma ocean, and isotopically light native iron, which partitions into the core. This process cannot take place at significant levels on smaller planets, such as Mars, as perovskite is only stable at pressures > 23 GPa. Interestingly, the average delta(57/54) Fe values of mafic terrestrial and lunar samples are very similar if the High-Ti mare basalts are excluded from the latter.If the Moon's mantle is largely derived from the impactor planet then the isotopically heavy signature of the Moon's mantle requires that the impacting planet also had a mantle with a delta Fe-57/54 value heavier than that of Mars or 4-Vesta, which then implies that the impactor plane must have been greater in size than Mars. (c) 2006 Elsevier B.V. All rights reserved.

Core formation in planetesimals can, in principle, be dated using the short-lived Hf-182-W-182 chronometer. However, it has been predicted that burnout and production of W isotopes by nuclear reactions can substantially modify the compositions in iron meteorites when exposed for several hundred million years (Myr) to galactic cosmic-rays. This may severely limit the use of the Hf-W system for determining chronologies of metal segregation as recorded in iron meteorites. Here we present the first experimental evidence of cosmogenic effects on W isotopes in two magmatic iron meteorites, Carbo (IID) and Grant (IIIAB). The W-182/W-184 ratio near the pre-atmospheric centre of the meteorites is lower by similar to 0.5 epsilon compared to values near the pre-atmospheric surface. The W-182/(184) W ratio displays excellent correlations with the 3 He concentration, which in turn provides a proxy for the fluence of the relevant cosmic-ray particles. Using new nuclear physics parameters, a first order correction for cosmic-ray effects on W isotopes is proposed based on the 3 He concentration and on an independent exposure age of the meteorite. This method is then applied to other magmatic iron meteorites (Negrillos, Cape of Good Hope, Navajo and Arispe). When corrected for cosmogenic effects the W isotopic compositions of Carbo, Grant, Negrillos, Cape of Good Hope, and Navajo are similar to the initial composition recorded in Allende CAIs. This shows that at least some magmatic meteorites from groups IIAB, IID, IIIAB, and IVB segregated within less than 1.2 Myr of the Hf-W system closure in Allende CAIs. The isotopic variations among the samples indicate that the iron meteorites studied in this paper segregated over a time interval of similar to 4 Myr, Carbo, being the oldest iron meteorites of this study. (c) 2006 Elsevier B.V. All rights reserved.

The Fortuna oxidized gold skarn deposit is located in the northern part of the Nambija gold district, southern Ecuador. It has been subdivided into four mineralized sites, covering a distance of 1 km, which are named from north to south: Cuerpo 3, Mine 1, Mine 2, and Southern Sector. Massive skarn bodies occur in K–Na metasomatized volcanic and volcaniclastic rocks of the Triassic Piuntza unit. They appear to result from selective replacement of volcaniclastic rocks. Very minor presence of bioclast relicts suggests the presence of subordinate limestone. Endoskarn type alteration with development of Na-rich plagioclase, K-feldspar, epidote, actinolite, anhedral pyroxene, and titanite affects a quartz–diorite porphyritic intrusion which crops out below the skarn bodies in Mine 2 and the Southern Sector. Endoskarn alteration in the intrusion grades into a K-feldspar ± biotite ± magnetite assemblage (K-alteration), suggesting that skarn formation is directly related to the quartz–diorite porphyritic intrusion, the latter being probably emplaced between 141 and 146 Ma. The massive skarn bodies were subdivided into a dominant brown garnet skarn, a distal green pyroxene–epidote skarn, and two quartz-rich varieties, a blue-green garnet skarn and light green pyroxene–garnet skarn, which occur as patches and small bodies within the former skarn types. The proximal massive brown garnet skarn zone is centered on two 060° trending faults in Mine 2, where the highest gold grades (5–10 g/t) were observed. It grades into a distal green pyroxene–epidote skarn zone to the North (Cuerpo 3). Granditic garnet shows iron enrichment from the proximal to the distal zone. Diopsidic pyroxene exhibits iron and manganese enrichment from proximal to distal zones. The retrograde stage is weakly developed and consists mainly of mineral phases filling centimeter-wide veins, vugs, and interstices between garnet and pyroxene grains. The main filling mineral is quartz, followed by K-feldspar, epidote, calcite, and chlorite, with minor sericite, apatite, titanite, hematite, pyrite, chalcopyrite, and gold. Metal and sulfur contents are low at Fortuna, and the highest gold grades coincide with high hematite abundance, which suggests that retrograde stage and gold deposition took place under oxidizing conditions. Fluid inclusions from pyroxene indicate precipitation from high temperature—high to moderate salinity fluids (400 to 460°C and 54- to 13-wt% eq. NaCl), which result probably from boiling of a moderately saline (∼8-wt% eq. NaCl) magmatic fluid. Later cooler (180 to 475°C) and moderate to low saline fluids (1- to 20-wt% eq. NaCl) were trapped in garnet, epidote, and quartz, and are interpreted to be responsible for gold deposition. Chlorite analysis indicates temperature of formation between 300 and 340°C in accordance with fluid inclusion data. It appears, thus, that gold was transported as chloride complexes under oxidizing conditions and was deposited at temperatures around 300°C when transport of chloride complexes as gold carriers is not efficient.

High-precision W isotopic compositions are presented for 35 iron meteorites from 7 magmatic groups (IC, IIAB, IID, IIIAB, IIIF, IVA, and IVB) and 3 non-magmatic groups (IAB, IIICD, and IIE). Small but resolvable isotopic variations are present both within and between iron meteorite groups. Variations in the 182W/184W ratio reflect either time intervals of metal–silicate differentiation, or result from the burnout of W isotopes caused by a prolonged exposure to galactic cosmic rays. Calculated apparent time spans for some groups of magmatic iron meteorites correspond to 8.5±2.1 My (IID), 5.1±2.3 My (IIAB), and 5.3±1.3 My (IVB). These time intervals are significantly longer than those predicated from models of planetesimal accretion. It is shown that cosmogenic effects can account for a large part of the W isotopic variation. No simple relationship exists with exposure ages, compromising any reliable method of correction. After allowance for maximum possible cosmogenic effects, it is found that there is no evidence that any of the magmatic iron meteorites studied here have initial W isotopic compositions that differ from those of Allende CAIs [ε 182W=−3.47±0.20; [T. Kleine, K. Mezger, H. Palme, E. Scherer and C. Münker, Early core formation in asteroids and late accretion of chondrite parent bodies: evidence from 182Hf–182W in CAIs, metal-rich chondrites and iron meteorites, Geochim. Cosmochim. Acta (in press)]. Cosmogenic corrections cannot yet be made with sufficient accuracy to obtain highly precise ages for iron meteorites. Some of the corrected ages nevertheless require extremely early metal–silicate segregation no later than 1 My after formation of CAIs. Therefore, magmatic iron meteorites appear to provide the best examples yet identified of material derived from the first planetesimals that grew by runaway growth, as modelled in dynamic simulations. Non-magmatic iron meteorites have a more radiogenic W isotopic composition than magmatic ones, even without cosmogenic corrections. This indicates that most of the IAB irons formed between 5±3 and 11±6 My after Allende CAIs. Similarly, the IIE irons formed between 9±4 and 14±5 My after the start of the solar system. Unlike IABs and IIEs, IIICDs do not show any resolvable W isotopic differences relative to Allende CAIs.

Abstract The Nambija gold district, southeastern Ecuador, consists of oxidized skarns developed mainly in volcaniclastic rocks of the Triassic Piuntza unit, which occurs as a 20-km-long, north-trending, contact-metamorphosed lens within the Jurassic Zamora batholith. High gold grades (10–30 g/t) are accompanied in most mines by very low Fe, Cu, Zn, and Pb sulfide contents. The skarn is constituted dominantly by massive brown garnet (mean Ad38). Subordinate pyroxene-epidote skarn developed mainly at the margins of brown garnet skarn bodies. Mostly idiomorphic and more andraditic garnet (mean Ad45) occurs in blue-green skarn formed as a later phase, in places with high porosity, at the transition with vugs and discontinuous dilational type I veins. The last garnet generations are mainly andraditic and occur largely as honey-yellow to red-brown clusters and cross-cutting bands (mean Ad84). As typical for other skarns developed in volcaniclastic rocks, mineral zoning is poorly defined. The retrograde overprint is weakly developed, commonly fails to alter the prograde minerals, and is mainly recognized in mineral infilling of structurally controlled (N10°–60°E) vugs and up to several-centimeter-wide type I veins, as well as interstices in blue-green skarn. Retrograde minerals are milky quartz, K-feldspar, calcite, chlorite, and hematite, ±plagioclase, ±muscovite, plus minor amounts of pyrite, chalcopyrite, hematite, sphalerite, and gold. Vugs and type I veins are cut by thin (1–2-mm) throughgoing type II veins that show similar orientations and mineralogy. Native gold is associated with retrograde alteration, mainly in the irregular vugs and type I veins, and subordinately in interstitial spaces and throughgoing type II veins. It is not observed in sulfide-rich type III veins, which cut the previous vein generations. High-temperature (up to 500°C) and high-salinity (up to 60 wt % NaCl equiv) inclusions in pyroxene represent the best approximation of the fluid responsible for a significant part of the prograde skarn stage. Such a highly saline fluid is interpreted as the result of boiling of a moderately saline (~8–10 wt % NaCl equiv) magmatic fluid at temperatures of ~500°C. Moderate-to low-salinity fluid inclusions (20−2 wt % NaCl equiv) in paragenetically later garnet as well as in epidote and quartz from vugs and type I veins may represent later, slightly lower temperature (420° −350°C) trapping of similar moderately saline fluids with or without some degree of boiling and mixing. The similarity of salinities and homogenization temperatures in late garnet, epidote, and quartz fluid inclusions is consistent with the apparent continuum between the prograde and retrograde skarn stages, as illustrated by the general lack of prograde mineral alteration, even at the contacts with retrograde fillings. Gold deposition, together with that of small amounts of hematite, chalcopyrite, and pyrite, took place during fluid cooling in the retrograde skarn stages but not during the last retrograde alteration, as indicated by the absence of gold in the sulfide-rich type III veins. The abundance of gold-bearing samples with high hematite/sulfide ratios and generally low total sulfide contents suggests high oxygen fugacities during gold deposition. The northeast structural control of vugs and type I veins, compatible with regional northeast-striking structures, in part with a dilational character, suggests that skarn formation, including gold deposition in the retrograde stage, took place under conditions of tectonic stress. Minimum Re-Os ages of 145.92 ± 0.46 and 145.58 ± 0.45 Ma for molybdenite from type III veins are compatible with skarn formation and gold mineralization during Late Jurassic magmatism. A genetic relationship with felsic porphyry intrusions that cut the Jurassic Zamora batholith and crop out near several gold skarns is suggested by a published hornblende K-Ar age of 141 ± 5 Ma for a felsic porphyry in the northern part of the Nambija district. Furthermore, the minimum Re-Os ages of ~146 Ma are just slightly younger than the published K-Ar ages (154 ± 5, 157 ± 5 Ma) for the Pangui porphyry copper belt about 70 km north of Nambija.