The I-Xe chronometer

¹²⁹Xe, from the decay of the now-extinct 16.7 Ma¹²⁹I, accumulates in iodine-bearing sites and since most iodine host phases are secondary, the I-Xe system is typically a chronometer for post-formational processes. The validity of the I-Xe chronometer is confirmed by comparison with Pb-Pb ages on phosphate and feldspar separates from twelve meteorites. Phosphate separates are found to be concordant with Pb-Pb for all six samples in which useful I-Xe data were obtained. Feldspar is a better iodine host than apatite in H chondrites, typically providing good I-Xe isochrons. These too are concordant with the Pb-Pb ages of the corresponding phosphates for five out of six feldspar separates. The exception is Allegan whose feldspar yields one of the oldest I-Xe ages observed, similar to those for CI and CM magnetites. We attribute this to a more primary mineralization, predating the secondary phosphate from which the comparison Pb-Pb age was obtained. Absolute I-Xe ages, found using the reported Pb-Pb age of Acapulco phosphate provide an absolute I-Xe age of 4.566 ± 0.002 Ga for both Shallowater and Bjurböle irradiation standards. This allows relative I-Xe ages to be interpreted in the context of absolute ages.     

Iodine-xenon analysis of Chainpur (LL3.4) chondrules

We present I-Xe analyses of ten chondrules from Chainpur LL3.4 by IR laser-stepped heating. Five chondrules provided isochrons of varying quality, giving a range of ages from 0.5 Ma before Shallowater to 17.8 after Shallowater. This confirms the extended range of Chainpur chondrule ages determined by previous data. We discuss evidence for fluid alteration, shock, and thermal events in explaining the chondrule ages and suggest that chondrule remelting events, presumably from bombardment of the parent body surface, are responsible for resetting the I-Xe chronometer. Previous data show a negative correlation between ¹³²Xe/¹²⁹Xe of the trapped Xe component and ¹²⁷I/¹²⁹I of an initial iodine component. This behaviour that requires the presence of a component with trapped ¹²⁹Xe/¹³²Xe lower than the planetary value has been cited as evidence for closed system evolution of the I-Xe system. We find no evidence of an unambiguous trapped component lower than planetary and no evidence of a negative correlation in our data. Therefore we suggest that open system behaviour more suitably explains the I-Xe systematics of Chainpur chondrules.     

Trapped Xe and I-Xe ages in aqueously altered CV3 meteorites

Twenty-two dark inclusions (DIs) from Allende (18), Leoville (2), Vigarano (1) and Efremovka (1) were studied by the I-Xe method. All except two of these DIs (Vigarano 2226 and Leoville LV2) produce well-defined isochrons, and precise I-Xe ages. The Allende DIs formed a tight group about 1.6 Ma older than Shallowater (4.566 ± 0.002 Ga), about 5 Ma older than four previously studied Allende CAIs. Most of the dark inclusions require trapped Xe with less ¹²⁹Xe (or more ¹²⁸Xe) than conventional planetary Xe (well restricted in composition by Q-Xe or OC-Xe). Studies of an irradiated/unirradiated DI pair from Allende demonstrate that the ¹²⁸Xe/¹³²Xe ratio in trapped is normal planetary, so that a ¹²⁹Xe/¹³²Xe ratio below planetary seems to be required. Yet, this is not possible given constraints on ¹²⁹Xe evolution in the early solar system. Trends among all of the Allende DIs suggest that an intimate mixture of partially decayed iodine and Xe formed a pseudo trapped Xe component enriched in both ¹²⁹Xe and ¹²⁷I, and subsequently in ¹²⁸Xe after n-capture during reactor irradiation. Enrichment in radiogenic ¹²⁹Xe, but with a ¹²⁹Xe/¹²⁷I ratio less than that observed in the iodine host phase, places closure of this trapped mixture ≥13 Ma after precipitation of the major iodine-bearing phase. Because the I-Xe isochron is a mixing line between iodine-derived and trapped Xe (pseudo or not), I-Xe ages, given by the slope of this mixing line, are not compromised by the presence of pseudo trapped Xe, and the precision of the I-Xe ages is given by the statistics of the line fit.     

Ar-Ar and I-Xe ages and thermal histories of three unusual metal-rich meteorites

Portales Valley, Sombrerete, and Northwest Africa (NWA) 176 are three unrelated meteorites, which consist of silicate mixed with substantial amounts of metal and which likely formed at elevated temperatures as a consequence of early impacts on their parent bodies. Measured ³⁹Ar-⁴⁰Ar ages of these meteorites are 4477 ± 11 Ma and 4458 ± 16 Ma (two samples of Portales Valley), 4541 ± 12 Ma, and 4524 ± 13 Ma, respectively (Ma = million years; all one-sigma errors). The Ar-Ar data for Portales Valley show no evidence of later open system behavior suggested by some other chronometers. Measured ¹²⁹I-¹²⁹Xe ages of these three meteorites are 4559.9 ± 0.5 Ma, 4561.9 ± 1.0 Ma, and ∼4544 Ma, respectively (relative to Shallowater = 4562.3 ± 0.4 Ma). From stepwise temperature release data, we determined the diffusion characteristics for Ar and Xe in our samples and calculated approximate closure temperatures for the K-Ar and I-Xe chronometers. Adopting results and interpretations about these meteorites from some previous workers, we evaluated all these data against various thermal cooling models. We conclude that Portales Valley formed 4560 Ma ago, cooled quickly to below the I-Xe closure temperature, then cooled deep within the parent body at a rate of ∼4 °C/Ma through K-Ar closure. We conclude that Sombrerete formed 4562 Ma ago and cooled relatively quickly. NWA 176 likely formed and cooled quickly ∼4544 Ma ago, or later than formation times of most meteorite parent bodies. For all three meteorites, the Ar-Ar ages are in better agreement with I-Xe ages and preferred thermal models if we increase these Ar-Ar ages by ∼20 Ma. Such age corrections would be consistent with probable errors in ⁴⁰K decay parameters in current use, as suggested by others. The role of impact heating and possible disruption and partial reassembly of meteorite parent bodies to form some meteorites likely was an important process in the early solar system.     

Iodine-Xenon dating of chondrules from the Qingzhen and Kota Kota enstatite chondrites

Initial ¹²⁹I/¹²⁷I values (I-Xe ages) have been obtained for individual mineralogically characterized chondrules and interchondrule matrix from the enstatite chondrites Qingzhen (EH3) and Kota Kota (EH3). In view of the absence of aqueous alteration and the low-peak metamorphic temperatures experienced by these meteorites, we suggest that the I-Xe ages for the chondrules record the event in which they were formed. These ages are within the range recorded for chondrules from ordinary chondrites, demonstrating that chondrules formed during the same time interval in the source regions of both ordinary chondrites and enstatite chondrites. The timing of this chondrule-forming episode or episodes brackets the I-Xe closure age of planetesimal bodies such as the Shallowater aubrite parent body. Although chondrule formation need not have occurred close to planetesimals, the existence of planetesimals at the same time as chondrule formation provides constraints on models of this process. Whichever mechanisms are proposed to form and transport chondrules, they must be compatible with models of the protosolar nebula which predict the formation of differentiated bodies on the same timescale at the same heliocentric distance.     

About Xe in meteoritic nanodiamonds

The analysis of excess ¹²⁹Xe in meteoritic nanodiamonds and the kinetics of its release during stepwise pyrolysis allow to suggest that (1) in the solar nebula ¹²⁹I atoms were adsorbed onto nanodiamond grains and (or) chemisorbed by forming covalent bonds with carbon atoms. Most ¹²⁹I atoms existed in a surface connected state, but a minor amount of them was in nanopores of the grains. At radioactive decay of ¹²⁹I the formed ¹²⁹Xe (¹²⁹Xe^∗) was trapped by diamond grains due to nuclear recoil. (2) During thermal metamorphism or aqueous alteration, the surface-sited ¹²⁹I atoms were basically lost. On the basis of these assumptions and calculated concentrations of ¹²⁹Xe^∗ in meteoritic nanodiamonds it is shown that the minimum closing time of the I-Xe system for meteorites of different chemical classes and low petrologic types may be about one million years relative to the minimally thermally metamorphized CO3 meteorite ALHA 77307. With increasing metamorphic grade the closing time of the I-Xe system increases and can range up to several ten millions years. This tendency is in agreement with an onion-shell model of structure and cooling history of meteorite parent bodies where the temperature increases in the direction from surface to center of the asteroids.     

The remarkable Re–Os chronometer in molybdenite: how and why it works

The Re–Os (rhenium–osmium) chronometer applied to molybdenite (MoS₂) is now demonstrated to be remarkably robust, surviving intense deformation and high‐grade thermal metamorphism. Successful dating of molybdenite is dependent on proper preparation of the mineral separate and analysis of a critical quantity of molybdenite, unique to each sample, such that recognized spatial decoupling of ¹⁸⁷Re parent and ¹⁸⁷Os daughter within individual molybdenite crystals is overcome. Highly precise, accurate and reproducible age results are derived through isotope dilution and negative thermal ion mass spectrometry (ID‐NTIMS). Spatial decoupling of parent–daughter precludes use of the laser ablation ICP‐MS microanalytical technique for Re–Os dating of molybdenite. The use of a reference or control sample is necessary to establish laboratory credibility and for interlaboratory comparisons. The Rb–Sr, K–Ar and ⁴⁰Ar/³⁹Ar chronometers are susceptible to chemical and thermal disturbance, particularly in terranes that have experienced subsequent episodes of hydrothermal/magmatic activity, and therefore should not be used as a basis for establishing accuracy in Re–Os dating of molybdenite, as has been done in the past. Re–Os ages for molybdenite are almost always in agreement with observed geological relationships and, when available, with zircon and titanite U–Pb ages. For terranes experiencing multiple episodes of metamorphism and deformation, molybdenite is not complicated by overgrowths as is common for some minerals used in U–Pb dating (e.g. zircon, monazite, xenotime), nor are Re and Os mobilized beyond the margins of individual crystals during solid‐state recrystallization. Moreover, inheritance of older molybdenite cores, incorporation of common Os, and radiogenic Os loss are exceedingly rare, whereas inheritance, common Pb and Pb loss are common complications in U–Pb dating techniques. Therefore, molybdenite ages may serve as point‐in‐time markers for age comparisons.     

Origin of iodine in volcanic fluids: I results from the Central American Volcanic Arc

The largest reservoir of crustal iodine is found in marine sediments, where it is closely associated with organic material. This presence, together with the existence of a long-lived, cosmogenic radioisotope ¹²⁹I (t_1/2 = 15.7 Ma), make this isotopic system well suited for the study of sediment recycling in subduction zones. Reported here are the results of ¹²⁹I/I ratios in volcanic fluids, collected during a comprehensive study of fluids and gases in the Central American Volcanic Arc. ¹²⁹I/I ratios, together with I, Br, and Cl concentrations, were determined in 79 samples from four geothermal centers and a number of crater lakes, fumaroles, hot springs, and surface waters in Costa Rica, Nicaragua, and El Salvador. Geothermal and volcanic fluids were found to have iodine concentrations substantially higher than values in seawater or meteoric waters. ¹²⁹I/I ratios in most of the geothermal fluids are below the preanthropogenic input ratio of 1500 × 10⁻¹⁵, demonstrating that recent anthropogenic additions are largely absent from the volcanic systems. The majority of the ¹²⁹I/I ratios are between 500 and 800 × 10⁻¹⁵. These ratios indicate minimum iodine ages between 25 and 15 Ma, in good agreement with the age of subducted sediments in this region. In all four geothermal systems, however, a few samples were found with iodine ages older than 40 Ma—that is, considerably below the expected age range for subducted sediments from the Cocos Plate. These samples probably reflect the presence of iodine derived from sediments in older accreted oceanic terraines. The iodine ages indicate that the magmatic end member for the volcanic fluids originates in the deeper parts of the subducted sediment column, with small additions from older iodine mobilized from the overlying crust. The high concentrations of iodine in geothermal fluids, combined with the observed iodine ages, demonstrate that remobilization in the main volcanic zone (and probably also in the forearc area) is an important part in the overall marine cycle of iodine and similar elements.     

Noble gas retention chronologies for the St Séverin meteorite

⁴⁰Ar-³⁹Ar and ¹²⁹Xe-¹²⁸Xe analyses were performed on two lithologies (light and dark) of the St Séverin (LL6) chondrite. For the light and dark fractions, respectively, we obtained ⁴⁰Ar retention ages of 4.38 and 4.42 AE and ¹²⁹Xe retention ages of 8.4 and 15.2 myr after Bjurböle. The two methods give age differences of opposite sense, and by both methods the differences are significant. Both the ⁴⁰Ar and the ¹²⁹Xe ages are interpreted as dating relaxation of metamorphic conditions. These two chronometers are decoupled, however, and do not date the same events. ⁴⁰Ar-³⁹Ar reflect chondritic metamorphisrn on a 10⁸ yr time scale. The ¹²⁹Xe-¹²⁸Xe ages reflect isotopic closure at higher temperatures and earlier times.     

矿物之间的元素和同位素平衡:地质测温和等时线定年的热力学和动力学控制

在特定的地质事件过程中,矿物等时线放射体系是否达到并且保持了平衡是变质岩Sm-Nd和Rb-Sr同位素年代学中的一个重要问题。在这个问题上矿物对O同位素测温与矿物等时线定年相似,因此两者之间可以相互制约。在岩浆岩和变质岩中,矿物中Sm-Nd、Sr和O之间的扩散速率在无水的条件下一般具有可比性,因此矿物之间O同位素的平衡状态可以用来对Sm-Nd和Rb-Sr定年的有效性进行检验。对大别-苏鲁造山带超高压变质岩的Sm-Nd和Rb-Sr等时线矿物进行O同位素测温,得到Sm-Nd等时线有时给出三叠纪年龄,有时给出非三叠纪年龄;对应的矿物O同位素分馏分别处于平衡和不平衡状态。对于引起非三叠纪等时线年龄的原因,一方面可以是由于榴辉岩相变质过程中同位素体系没有达到平衡,另一方面则可能角闪岩相退变质作用打破了平衡。等时线矿物中初始同位素比值的均一化速率主要受慢扩散矿物的影响,而矿物等时线时钟的启动主要受高母/子比值矿物控制。因此在变质作用过程中,只有当高母/子比值矿物同时具有快的放射成因同位素扩散速率,才可能得到有效的矿物等时线来用于变质年龄的测定。根据不同矿物中不同元素在扩散速率上的差异,能够定量估计大陆碰撞过程中榴辉岩相变质的持续时间。应用增量方法和离子孔隙度经验模型,不仅分别能够从理论上准确计算所有固体矿物的氧同位素分馏系数和获得不同矿物中元素的扩散参数,而且分别能够定量预测热力学平衡条件下共生矿物之间的¹⁸O富集顺序和相同条件下矿物中元素扩散速率的相对快慢。     

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.     

K–Ar ages of meteorites: Clues to parent-body thermal histories

Whereas most radiometric chronometers give formation ages of individual meteorites >4.5 Ga ago, the K–Ar chronometer rarely gives times of meteorite formation. Instead, K–Ar ages obtained by the ³⁹Ar–⁴⁰Ar technique span the entire age of the solar system and typically measure the diverse thermal histories of meteorites or their parent objects, as produced by internal parent body metamorphism or impact heating. This paper briefly explains the Ar–Ar dating technique. It then reviews Ar–Ar ages of several different types of meteorites, representing at least 16 different parent bodies, and discusses the likely thermal histories these ages represent. Ar–Ar ages of ordinary (H, L, and LL) chondrites, R chondrites, and enstatite meteorites yield cooling times following internal parent body metamorphism extending over ∼200 Ma after parent body formation, consistent with parent bodies of ∼100 km diameter. For a suite of H-chondrites, Ar–Ar and U–Pb ages anti-correlate with the degree of metamorphism, consistent with increasing metamorphic temperatures and longer cooling times at greater depths within the parent body. In contrast, acapulcoites–lodranites, although metamorphosed to higher temperatures than chondrites, give Ar–Ar ages which cluster tightly at ∼4.51 Ga. Ar–Ar ages of silicate from IAB iron meteorites give a continual distribution across ∼4.53–4.32 Ga, whereas silicate from IIE iron meteorites give Ar–Ar ages of either ∼4.5 Ga or ∼3.7 Ga. Both of these parent bodies suffered early, intense collisional heating and mixing. Comparison of Ar–Ar and I–Xe ages for silicate from three other iron meteorites also suggests very early collisional heating and mixing. Most mesosiderites show Ar–Ar ages of ∼3.9 Ga, and their significantly sloped age spectra and Ar diffusion properties, as well as Ni diffusion profiles in metal, indicate very deep burial after collisional mixing and cooling at a very slow rate of ∼0.2 °C/Ma. Ar–Ar ages of a large number of brecciated eucrites range over ∼3.4–4.1 Ga, similar to ages of many lunar highland rocks. These ages on both bodies were reset by large impact heating events, possibly initiated by movements of the giant planets. Many impact-heated chondrites show impact-reset Ar–Ar ages of either >3.5 Ga or <1.0 Ga, and generally only chondrites show these younger ages. The younger ages may represent orbital evolution times in the asteroid belt prior to ejection into Earth-crossing orbits. Among martian meteorites, Ar–Ar ages of nakhlites are similar to ages obtained from other radiometric chronometers, but apparent Ar–Ar ages of younger shergottites are almost always older than igneous crystallization ages, because of the presence of excess (parentless) ⁴⁰Ar. This excess ⁴⁰Ar derives from shock-implanted martian atmosphere or from radiogenic ⁴⁰Ar inherited from the melt. Differences between meteorite ages obtained from other chronometers (e.g., I–Xe and U–Pb) and the oldest measured Ar–Ar ages are consistent with previous suggestions that the ⁴⁰K decay parameters in common use are incorrect and that the K–Ar age of a 4500 Ma meteorite should be possibly increased, but by no more than ∼20 Ma.     

Iodine-xenon analysis of ordinary chondrite halide: implications for early solar system water

We report the results of iodine-xenon analyses of irradiated halide grains extracted from the H-chondrite Monahans (1998) and compare them with those from Zag (Whitby et al., 2000) to address the timing of aqueous processing on the H-chondrite parent body. Xe isotopic analyses were carried out using the RELAX mass spectrometer with laser stepped heating. The initial ¹²⁹I/¹²⁷I ratio in the Monahans halide was determined to be (9.37 ± 0.06) × 10⁻⁵ with an iodine concentration of ∼400 ppb. Significant scatter, especially in the Zag data, indicates that a simple interpretation as a formation age is unreliable. Instead we propose a model whereby halide minerals in both meteorites formed ∼5 Ma after the enstatite achondrite Shallowater (at an absolute age of 4559 Ma). This age is in agreement with the timing of aqueous alteration on the carbonaceous chondrite parent bodies and ordinary chondrite metamorphism and is consistent with the decay of ²⁶Al as a heat source for heating and mobilisation of brines on the H-chondrite parent body. Post accretion surface impact events may have also contributed to the heat source.     

“Planetary” noble gas components and the nucleosynthetic history of solar system material

Models capable of explaining the differences between the isotopic compositions of the planetary noble gas components “Q” and “P3” (widespread in primitive meteorites) and average solar system material as sampled by the solar wind are presented, and their implications discussed.Small, variable amounts of known presolar components and ¹²⁹Xe from ¹²⁹I decay are present in Q gases alongside a solar composition that has been mass fractionated. These most likely arise either from mixing during parent body processing or co-release from poorly retentive phases during analysis. Thus the heavy noble gas budget of primitive meteorites is dominated by a component derived from material with the average composition of the solar system.In contrast, P3 seems best explained as a presolar component, consistent with isolation from bulk material that subsequently evolved to the solar composition as newly synthesised material was added. Examination of Kr-P3 identifies the addition as having had the signature of the weak s-process, and demonstrates that a second process that contributes the isotopes of krypton not produced in the s-process (residual isotopes) must also have added material to the reservoir as it evolved to the solar composition. Total s-process contributions required of ¹³²Xe and ⁸⁴Kr are at least ∼1% of the present budget, as is that of the residual krypton “component”.While concentrations of P3 vary with extents of parent body processing, concentrations of ¹²⁹Xe excess from ¹²⁹I decay associated with P3 are roughly constant in those least processed meteorites that retain a P3 signature (apart from ALH77307). This has a natural explanation in the different chemical behaviours of iodine and xenon if ¹²⁹I was alive in P3 in the early solar system. The presence of live ¹²⁹I in P3 in the early solar system imposes a loose constraint that the P3 component was isolated from a parent reservoir less than ∼100 Myr before the formation of the solar system.The model of evolution from P3 gases to the solar composition requires that residual krypton isotopes are not products of a conventional r-process that also synthesises ¹²⁹I. Separate sites for synthesis of residual isotopes of krypton and the heavy element r-process are consistent with observations of metal poor stars, but do not correspond to the two r-processes invoked to account for variations between the ¹⁸²Hf and ¹²⁹I systems. Since the weak s-process is implicated as the source of the s-process material contributed as the P3 reservoir evolved to a solar composition, the massive stars that host it may also host the process that synthesises residual krypton isotopes. The recent presence of nearby massive stars is consistent with an emerging picture of the environment of solar system formation.     

I-Xe and 40Ar-39Ar dating of silicate from Weekeroo Station and Netschaëvo IIE iron meteorites

Silicate inclusions from two IIE iron meteorites were dated by the I-Xe and ⁴⁰Ar-³⁹Ar techniques. Weekeroo Station, a ‘normal’ IIE iron, shows no loss of radiogenic ⁴⁰Ar at low temperature, and the well-defined ⁴⁰Ar-³⁹Ar plateau yields an age of 4.54 ± 0.03 Byr. The xenon data define a good I-Xe correlation with an age of +10.9 ± 0.5 Myr relative to Bjurböle [the monitor error (±2.5 Myr) is not included].^Despite its relatively young age, Weekeroo Station's $\text{(}{}^{129}\text{Xe}{}^{132}\text{Xe)}{}_{\mathrm{trappad}}$ ratio (= 0.84 ± 0.05) lies significantly below the solar value. Netschaëvo silicate has a chondritic composition, unlike ‘normal’ IIE silicate which is more differentiated. Nevertheless Netschaëvo gives a ⁴⁰Ar-³⁹Ar plateau-age of only 3.79 ± 0.03 Byr, with the xenon data failing to define an I-Xe isochron.Only irons from the IAB and IIE groups contain silicate inclusions, but these two groups differ in many other respects, mostly suggesting that IAB meteorites are more primitive. The I-Xe chronology supports this suggestion inasmuch as Weekeroo Station formed well after (8–15 Myr) IAB silicates. In terms of Scott and Wasson's (1976) model, ages for Weekeroo Station date the shock event which formed ‘normal’ IIE irons by mixing the low-melting fraction of the parent silicate with shock-liquefied metal. Scott and Wasson's suggestion that Netschaëvo represents IIE parent material, however, is contradicted by Netschaëvo's 3.8 Byr age.The four silicate-bearing IIE irons which have now been dated can be subdivided into distinct pairs: Weekeroo Station and Colomera formed near the beginning of the solar system, while Netschaëvo and Kodaikanal both formed only 3.8 Byr ago. A review of other properties of these meteorites generally support this subdivision.This work underscores the complexity of the history of IIE meteorites; in particular, an adequate model must account for the formation of two IIE irons at 3.8 Byr without disturbing rare gases in Weekeroo Station. All formation models are quite speculative, but the one which seems best to fit the available evidence postulates two parent bodies: the 3.8 Byr old silicate formed on one parent body, all other IIE material resided in a separate body, and subsequent collision(s) mixed the young silicate with IIE metal.     

I-Xe dating: intercomparisons of neutron irradiations and reproducibility of the Bjurböle standard

Parameters for a number of neutron irradiations are examined and results intercompared for the Bjurböle meteorite; data for the 1967 Valecitos-1 irradiation are presented. Apparent I-Xe ‘formation’ ages are reproducible for three different samples of Bjurböle, suggesting isotopic homogeneity for initial iodine in the bulk material. The systematics of neutron capture in ¹³⁵Xe (produced from ²³⁵U neutron fission) are examined and verified in irradiated BCR-1.     

U–Pb isotopic dating of titanite microstructures: potential implications for the chronology and identification of large impact structures

Identifying and dating large impact structures is challenging, as many of the traditional shock indicator phases can be modified by post-impact processes. Refractory accessory phases, such as zircon, while faithful recorders of shock wave passage, commonly respond with partial U–Pb age resetting during impact events. Titanite is an accessory phase with lower Pb closure temperature than many other robust chronometers, but its potential as indicator and chronometer of impact-related processes remains poorly constrained. In this study, we examined titanite grains from the Sudbury (Ontario, Canada) and Vredefort (South Africa) impact structures, combining quantitative microstructural and U–Pb dating techniques. Titanite grains from both craters host planar microstructures and microtwins that show a common twin–host disorientation relationship of 74° about <102>. In the Vredefort impact structure, the microtwins deformed internally and developed high- and low-angle grain boundaries that resulted in the growth of neoblastic crystallites. U–Pb isotopic dating of magmatic titanite grains with deformation microtwins from the Sudbury impact structure yielded a ²⁰⁷Pb/²⁰⁶Pb age of 1851?±?12 Ma that records either the shock heating or the crater modification stage of the impact event. The titanite grains from the Vredefort impact structure yielded primarily pre-impact ages recording the cooling of the ultra-high-temperature Ventersdorp event, but domains with microtwins or planar microstructures show evidence of U–Pb isotopic disturbance. Despite that the identified microtwins are not diagnostic of shock-metamorphic processes, our contribution demonstrates that titanite has great potential to inform studies of the terrestrial impact crater record.     

Dating of Dissolved Iodine in Pore Waters from the Gas Hydrate Occurrence Offshore Shimokita Peninsula,Japan: 129I Results from the D/V Chikyu Shakedown Cruise

Iodine concentration and radioisotopic composition (¹²⁹I/I) were measured in the pore waters from the gas hydrate occurrence in the forearc basin offshore Shimokita Peninsula, north-eastern Japan, to determine the source formation of I and accompanying hydrocarbons. Iodine concentrations correlate well with the alkalinity and SO₄ patterns, reflecting degradation stages of I-rich buried organic matter, rapidly increasing in the sulfate reduction interval, and becoming constant below 250 meters below the seafloor with an upwelling flux of 1.5 × 10⁻¹¹ µmol cm⁻² year⁻¹. The ¹²⁹I/I ratios of 300 × 10⁻¹⁵–400 × 10⁻¹⁵ in deep pore waters suggest ages for iodine and hydrocarbon sources as old as 40 Ma. These ages correlate well with the coaly source formations of the Eocene age thought to be responsible for the conventional natural gas deposits underlying the gas hydrate stability zone. Similar profiles are observed in ¹²⁹I/I ratios of pore waters in the gas hydrate stability zone from the forearc basin in the eastern Nankai Trough, offshore central Japan, where pore waters are enriched in I and reach ages as old as ∼50 Ma through the sediment column. At the outer ridge site along the trough, on the other hand, relatively younger I are more frequently delivered probably through thrusts/faults associated with subduction. The nature of source formations of I and hydrocarbons in the offshore Shimokita Peninsula has a more terrestrial contribution compared with those in the Nankai Trough, but these formations are also considerably older than the host sediments, suggesting long-term transport of I and hydrocarbons for the accumulation of gas hydrates in both locations.     

I-Xe and 40Ar-39Ar analyses of silicate from the Eagle Station pallasite and the anomalous iron meteorite Enon

Silicate from two unusual iron-rich meteorites were analyzed by the I-Xe and ⁴⁰Ar-³⁹Ar techniques, Enon, an anomalous iron meteorite with chondritic silicate, shows no loss of radiogenic ⁴⁰Ar at low temperature, and gives a plateau age of 4.59 ± 0.03 Ga. Although the Xe data fail to define an I-Xe correlation (possibly due to a very low iodine content), the inferred $\text{Pu}\text{U}$ ratio is more than 2σ above the chondritic value, and the Pu abundance derived from the concentration of Pu-fission Xe is 6 times greater than the abundance inferred for Cl meteorites. These findings for Enon, coupled with data for IAB iron meteorites, suggest that presence of chondritic silicate in an iron-rich meteorite is diagnostic of an old radiometric age with little subsequent thermal disturbance. The Eagle Station pallasite, the most ¹⁶O-rich meteorite known, gives a complex ⁴⁰Ar-³⁹Ar age pattern which suggests a recent (?0.85 Ga) severe thermal disturbance. The absence of excess ¹²⁹Xe, and the low trapped Ar and Xe contents, are consistent with this interpretation. The similarity between ⁴⁰Ar-³⁹Ar data for Eagle Station and for the olivine-rich meteorite Chassigny lends credence to the previous suggestion of a connection between Chassigny and pallasites, in the sense that similar processes operating at similar times on different parent bodies may have been involved in the formation of olivine in both types of meteorites.     

Iodine as a tracer of organic material: I results from gas hydrate systems and fore arc fluids

The strong association of iodine with organic material and the presence of the cosmogenic radioisotope ¹²⁹I make the iodine isotopic system useful in tracing and dating organic materials and their derivatives. We present here results from two new applications of this system, investigations of gas hydrates from the Peru Margin (ODP Leg 201, Site 1230) and of fluids collected from the fore arc region of the North Island, New Zealand. Pore fluids from Site 1230 are strongly enriched in iodine and show a distinct decrease in ¹²⁹I/I ratios from 920 × 10^− 15 close to the surface to 140 × 10^− 15 at a depth of 200 mbsf, suggesting the presence of a shallow, young source and deep, old source of fluids. The fore arc fluids from New Zealand are also enriched in iodine and show a similar range in ¹²⁹I/I ratios. In both cases minimum ages are calculated to be between 40 and 60 Ma for these fluids. The results are in good agreement with earlier investigations of gas hydrate systems at Blake Ridge and Nankai Trough and of fore arc fluids from Central America and Japan, but are not compatible with derivation from subducting sediments in active margins. The results indicate that continental margins contain large amounts of old iodine, reflecting the presence of large quantities of organic material stored in these regions. Results for gas hydrate systems and fore arc fluids show similar characteristics, but differ strongly from those obtained for fluids collected from the main zones of volcanic activity associated with active margins.