High Closure Temperature Research Articles

Temperatures and cooling rates recorded in REE in coexisting pyroxenes in ophiolitic and abyssal peridotites

Mantle peridotites from ophiolites are commonly interpreted as having mid-ocean ridge (MOR) or supra-subduction zone (SSZ) affinity. Recently, an REE-in-two-pyroxene thermometer was developed (Liang et al., 2013) that has higher closure temperatures (designated as TREE) than major element based two-pyroxene thermometers for mafic and ultramafic rocks that experienced cooling. The REE-in-two-pyroxene thermometer has the potential to extract meaningful cooling rates from ophiolitic peridotites and thus shed new light on the thermal history of the different tectonic regimes. We calculated TREE for available literature data from abyssal peridotites, subcontinental (SC) peridotites, and ophiolites around the world (Alps, Coast Range, Corsica, New Caledonia, Oman, Othris, Puerto Rico, Russia, and Turkey), and augmented the data with new measurements for peridotites from the Trinity and Josephine ophiolites and the Mariana trench. TREE are compared to major element based thermometers, including the two-pyroxene thermometer of Brey and Köhler (1990) (TBKN). Samples with SC affinity have TREE and TBKN in good agreement. Samples with MOR and SSZ affinity have near-solidus TREE but TBKN hundreds of degrees lower. Closure temperatures for REE and Fe–Mg in pyroxenes were calculated to compare cooling rates among abyssal peridotites, MOR ophiolites, and SSZ ophiolites. Abyssal peridotites appear to cool more rapidly than peridotites from most ophiolites. On average, SSZ ophiolites have lower closure temperatures than abyssal peridotites and many ophiolites with MOR affinity. We propose that these lower temperatures can be attributed to the residence time in the cooling oceanic lithosphere prior to obduction. MOR ophiolites define a continuum spanning cooling rates from SSZ ophiolites to abyssal peridotites. Consistent high closure temperatures for abyssal peridotites and the Oman and Corsica ophiolites suggests hydrothermal circulation and/or rapid cooling events (e.g., normal faulting, unroofing) control the late thermal histories of peridotites from transform faults and slow and fast spreading centers with or without a crustal section.

Zircon multichronology: Fission‐track, U‐Pb, and combined fission‐track–U‐Pb studies

<scp>Z</scp>ircon multichronology: <scp>F</scp>ission‐track, <scp>U‐Pb</scp>, and combined fission‐track–<scp>U‐Pb</scp> studies

Geological framework and fission track dating of pseudotachylyte of the Atotsugawa Fault, Magawa area, central Japan

This paper describes newly discovered pseudotachylyte along the Atotsugawa Fault at the Magawa outcrop, where this fault divides Quaternary deposits in the SW from Triassic Hida granitic rocks to the NE. Within several meters of the fault surface, pseudotachylyte veins are found with a thickness of less than 10 cm, but are displaced by fault brecciation. Zircon fission track dating of pseudotachylyte samples yields ages of 48.6–50.2 Ma (sample AT-A), 55.1 Ma (AT-A'-1) and 60.9 Ma (AT-D-1); the latter is similar to the fission track ages of 56.1–60.1 Ma for granitic protoliths. The results of fission track length analyses in zircon suggest that pseudotachylytes (AT-A and AT-D-1) and protolith granite are mostly annealed. Consequently, the pseudotachylyte (AT-A) reached the highest temperature during 48.6–50.2 Ma, thereby resetting the fission track system totally in zircon during faulting. Another pseudotachylyte (AT-A'-1) and its wall rock granite contain shortened tracks within zircon grains suggesting partial annealing. The age distribution pattern of the former also contains decomposed age after the normality test (Shapiro–Wilk test) in which the major age yields 52.5 Ma. Accordingly, these pseudotachylytes yield a peak age of about 50 Ma, whereas the peak ages of one pseudotachylyte (AT-D-1) and the protolith Hida granitic rocks are about 60 Ma, representing the thermal effects not caused by frictional heating but by intrusions of Late Cretaceous to Paleogene granitoids that are probably concealed below the exposed Triassic Hida granitic rocks. Such thermal effects did not affect the K–Ar muscovite age (149 Ma) for the protolith granite because of the higher closure temperature of this system. Using the new geochronological data, we can elucidate the cooling history of the Hida granitic rocks, and constrain the timing of the main pulse of pseudotachylyte generation along the Atotsugawa Fault at about 50 Ma.

Discrimination of protolithic versus metamorphic zircon ages in eclogites: Constraints from the Erzgebirge metamorphic core complex (Germany)

Protolithic ages of meta-basites are crucial for tectonic reconstructions as they constrain the lifetime of oceans. However, in high-grade metamorphic regions most isotope systems record metamorphic ages due to the metamorphic overprint. The mineral zircon is the most suitable geochronometer for the determination of protolithic ages in such rocks due to its high closure temperature. Zircon from eclogites from the Eastern Erzgebirge metamorphic complex yielded both protolithic (~540Ma; 5 samples) and metamorphic (~340Ma; 4 samples) 207Pb/206Pb ages. Detailed investigations of zircon grains within thin rock sections show that the metamorphic ages are related to new zircon growth caused by water infiltration that triggered decomposition of omphacite into an amphibole–plagioclase symplectite. Rutile and apatite frequently grew together with metamorphic zircons indicating local mobilization of Zr, Nb, LREE and P. Eclogites with omphacite breakdown are very dark in colour. In difference, lighter coloured eclogites contain K-feldspar and quartz indicating minor melt infiltration that caused breakdown of garnet and transformation of amphibole into biotite. This supports additional new growth of zircon grains. In some samples idiomorphic zircon grains with a lot of micro-inclusions indicate a coupled dissolution–reprecipitation process. Additional rounded-resorbed and highly fractured zircon grains together with internal zircon textures imply multiple melt infiltration triggering multiple phases of zircon growth. Therefore, scattering zircon ages may reflect multiple growth stages or inheritance rather than prolonged periods of metamorphic growth. In contrast, protolithic zircon ages were found in eclogites where no water or melt infiltration occurred. This emphasizes the importance of water and/or melt infiltration for the new growth of zircon grains in eclogites as was already shown for granulites in various studies. In the Eastern Erzgebirge, most eclogites and amphibolites were derived from MORB-type or arc-related basalts while the plagioclase-rich amphibolites formed mainly from crustal sources. The protolith ages of meta-basites from the Eastern Erzgebirge (~540Ma) document a bimodal magmatic activity in a Cadomian back-arc basin.

Closure temperature of (U-Th)/He system in apatite obtained from natural drillhole samples in the Tarim basin and its geological significance

The apatite (U-Th)/He thermochronometry has been used to study the tectono-thermal evolution of mountains and sedimentary basins for over ten years. The closure temperature of helium is important for the apatite (U-Th)/He thermochronometry and has been widely studied by thermal simulation experiments. In this paper, the apatite He closure temperature was studied by establishing the evolutionary pattern between apatite He ages and apatite burial depth based on examined apatite He ages of natural samples obtained from drillholes in the Tarim basin, China. The study showed that the apatite He closure temperature of natural samples in the Tarim basin is approximately 88±5°C, higher than the result (∼75°C) obtained from the thermal simulation experiments. The high He closure temperature resulted from high effective uranium concentration, long-term radiation damage accumulation, and sufficient particle radii. This study is a reevaluation of the conventional apatite He closure temperature and has a great significance in studying the uplifting events in the late period of the basin-mountain tectonic evolution, of which the uplifting time and rates can be determined accurately.

Research on Calculating Methods of Jacking Force with High Temperature Closure for Continuous Rigid-Frame Bridge

When the continuous rigid frame bridge’s actual closure temperature is higher than the design closure temperature, the closure temperature’s difference will cause horizontal displacement at top of pier , and cause the structure to have the temperature intennal force. A horizontal jacking force is exerted to eliminate the adverse effect of closure temperature’s difference. One calculating method of jacking force is proposed in this paper: Eliminating the horizontal displacement at top of pier method. Taking the Longtan River Bridge as a practice example, the high closure temperature’s horizontal jacking force is calculated according to the method separately in this paper.

Decoupled crystallization and eruption histories of the rhyolite magmatic system at Tarawera volcano revealed by zircon ages and growth rates

We obtained U–Th disequilibrium age data on zircons from each of the four rhyolite eruptions that built Tarawera volcano in the last 22 ka within the Okataina Volcanic Center (OVC), caldera, New Zealand. Secondary ion mass spectrometry analyses on unpolished euhedral crystal faces that lack resorption features show that crystal growth variously terminated from near-eruption age to ~100 ka prior to eruption. Age-depth profiling of crystals reveals long periods of continuous (~34 ka) and discontinuous growth (~90 ka). Growth hiatuses of up to ~40 ka duration occur, but do not all relate to obvious resorption surfaces. Age differences up to similar magnitude are found on opposing faces of some crystals suggesting episodes of partial exposure to melts. These features are best explained by periodic, complete, or partial, sub-solidus storage and/or inclusion in larger crystal phases, followed by rapid liberation prior to eruption. This is supported by high abundances of U and Th (~500 − >2,000 ppm) in some zircons consistent with periods of high crystallinity (>70%) in the magmatic system, based on crystal/melt partitioning. Contemporaneous but contrasting rim-ward trends of these elements within crystals, even in the same lava hand sample, require synchronous growth in separate melt bodies and little connectivity within the system, but also significant crystal transport and mixing prior to eruption. Many crystals record continuity of growth through the preceding ~60 ka OVC caldera-collapse and subsequent eruptions from Tarawera. This demonstrates a decoupling between eruption triggers, such as shallow crustal extension and mafic intrusion, and the crystallization state of the OVC silicic magmatic system. The data highlights the need to distinguish between the time for accumulation of eruptible magma and the long-term magma residence time based on the age of crystals with high closure temperatures, when assessing the potential for catastrophic eruptions.

Geochemical and U–Pb age constraints on the occurrence of polygenetic titanites in UHP metagranite in the Dabie orogen

Relict magmatic titanite was identified in the core of a few titanite grains with the overgrown rim of metamorphic titanite in UHP metagranite in the Dabie orogen. LA–ICPMS U–Pb dating gave Neoproterozoic ages for the magmatic titanite but Triassic ages for the metamorphic titanite. The magmatic and metamorphic titanites are clearly distinct by differences in petrological and geochemical compositions. The magmatic titanite occurs as residual cores that show bright BSE, the presence of allanite and quartz inclusions, low contents of CaO, Al2O3 and TiO2 but high contents of Fe2O3 and MgO. In trace elements, the magmatic titanite exhibits high REE and HFSE contents, distinctly negative Eu anomalies with flat MREE–HREE patterns, and high Th/U ratios. In contrast, the metamorphic titanite occurs as rims and grains of homogeneously dark BSE that contain inclusions of epidote, quartz, K-feldspar, rutile, biotite and phengite, and have relatively high contents of CaO, Al2O3 and TiO2, but low contents of Fe2O3 and MgO, and relatively low REE and HFSE contents, slightly negative Eu anomalies with HREE depletion relative to MREE, and low Th/U ratios. The Zr-in-titanite thermometry yields 727 to 877°C at 0.5 to 1.0GPa for the magmatic titanite, and 729 to 870°C at 1.5 to 2.0GPa for the metamorphic titanite. The Neoproterozoic U–Pb chronometric system of magmatic titanite survived the Triassic continental subduction-zone HP–UHP metamorphism. This suggests a relatively high closure temperature of >800°C for the titanite U–Pb system. The metamorphic titanite is principally a product of retrograde metamorphism during decompression exhumation at the transition from HP eclogite-facies to amphibolite-facies. Therefore, titanite holds a great potential to geochronological and petrogenetic studies of continental subduction-zone metamorphic rocks.

OXYGEN ISOTOPE COMPOSITION OF GARNET IN THE PENINSULA GRANITE, CAPE GRANITE SUITE, SOUTH AFRICA: CONSTRAINTS ON MELTING AND EMPLACEMENT MECHANISMS

Garnet is an accessory mineral in the Cape Granite Suite, and garnet δ 18 O values in the Peninsula Granite range in from 10.0 to 11.4‰ (mean 10.6 ± 0.6‰, n = 15). These values are consistent with the garnet being produced during incongruent melting of a metapelitic source that has a similar O-isotope composition to the Malmesbury Group. Peninsula Granite quartz δ 18 O values range from 13.2 to 14.0‰ (mean 13.6 ± 0.3‰, n = 17), at the high end of the range previously observed for the Cape Granite Suite. These high δ 18 O values are consistent with the source of the Peninsula Granite magma having a greater component of clay minerals, which have inherently high δ 18 O values. Garnet has a high closure temperature (>800 o C) to oxygen diffusion and its δ 18 O value should, therefore, correlate closely with that of the source. Quartz has a significantly lower closure temperature (~550 o C) than garnet, and sub-solidus oxygen isotope re-equilibration between quartz and feldspar during slow cooling ought to result in a greater variation in quartz δ 18 O values compared to that of garnet. That the reverse is the case suggests that granite magmas were derived from a moderately heterogeneous source, as expected for metasedimentary rocks. This source underwent melting to produce different batches of granitic magma containing entrained garnets of slightly different δ 18 O value. Magma batches were subsequently mixed and homogenized before and/or during the emplacement process, resulting in a narrower spread of quartz δ 18 O values.

Composition and Genesis of Depleted Mantle Peridotites from the Wadi Tayin Massif, Oman Ophiolite; Major and Trace Element Geochemistry, and Os Isotope and PGE Systematics

The Oman ophiolite consists of several massifs cropping out along a 500 km long band trending NW-SE along the coast of Oman; it is one of the best exposed sections of oceanic crust and mantle in the world. There is a gradient in igneous processes and composition in the ophiolite, with the northern massifs recording a polygenetic igneous history involving an increasingly important subduction component, whereas the southern massifs were formed primarily via a mid-ocean ridge basalt (MORB)-like, single-stage process at a submarine spreading ridge. In this study we use geochemical data from Wadi Tayin, which is one of the southern massifs of the Oman ophiolite, to constrain the composition and genesis of oceanic crust and upper mantle. The Wadi Tayin harzburgites are residues of partial melting that are as depleted as the most depleted mid-ocean ridge peridotites. They have low middle- to heavy rare earth element ratios, most probably reflecting melting close to and beyond the exhaustion of clinopyroxene. Like many abyssal peridotites, the Wadi Tayin samples show enrichment in highly incompatible elements, giving rise to U-shaped MORB-normalized trace element patterns. We favor the idea that this is caused by enrichment of highly incompatible elements along grain boundaries in peridotite, which may be the result of near-equilibrium partitioning between grain boundaries and grain interiors, rather than disequilibrium processes. Equilibrium partitioning between crystals and grain boundaries during melting and melt extraction is also our preferred explanation for ubiquitous high Pb contents relative to Ce and La in our samples as well as in most abyssal peridotites. The Wadi Tayin samples record substantial variability in terms of osmium isotopic composition and platinum group element (PGE) concentrations. The more radiogenic nature of the dunites and impregnated peridotites compared with the residual harzburgites may be due to relatively high Os-187/Os-188 in melt transported through the dunites. The presence of residual peridotites with whole-rock osmium isotopic compositions less radiogenic than MORB may be explained by melting of a veined (upwelling) mantle forming mixed melts in which much of the Os derives from veins with high Re/Os in a matrix of previously depleted peridotite. An important result of this study is that the shallowest samples are refertilized (i.e. anomalously enriched in incompatible elements), and record relatively high metamorphic closure temperatures. These observations suggest that migrating melt underwent crystallization during rapid cooling in the uppermost mantle during and immediately after ridge magmatism. A variety of geothermometers all yield the result that the stratigraphically highest harzburgites equilibrated at higher temperature than the deeper ones. The systematic decrease in closure temperature with increasing depth below the Moho transition zone probably reflects systematic variation in cooling rate as a function of depth in the mantle section. We hypothesize that the refertilization and higher closure temperatures recorded by the uppermost mantle samples are linked. More rapid cooling led to higher closure temperatures, and to partial crystallization of migrating melts in the shallowest part of the mantle section, yielding slightly elevated abundances of elements such as Ca and Na, and incompatible trace elements.

Application of the (U‐Th)/He Thermochronometry to the Tectono‐Thermal Evolution of Sedimentary Basin—A Case History of Well KQ1 in the Tarim Basin

AbstractThe (U‐Th)/He thermochronometry of apatite and zircon has been used as a new technique to study the tectonic uplift and thermal history of sedimentary basins in recent years. Based on the tested apatite and zircon helium ages data from well samples, an evolution model of apatite He ages with depth and/or temperature is built, which illustrates that the apatite helium closure temperature is about 85°C in the Tarim basin. However, the zircon He ages reveal that these samples hadn't undergone its higher closure temperature. The thermal history since Ordovician in Well KQ1 has been modeled by using the He ages, AFT and Ro data. The modeling result shows that the thermal gradient was about 35.5°C/km in the end of Ordovician, and 33.3~34.5°C/km during the period of Silurian to Devonian, and it decreased to 27.6°C/km in the end of Cretaceous. Therefore, the (U‐Th)/He ages may provide a new tool to rebuilt the thermal history of sedimentary basins, especially for the Low Paleozoic carbonate stratum in Tarim basin which lacks normal thermal indicators.

Hf–W thermochronometry: II. Accretion and thermal history of the acapulcoite–lodranite parent body

Acapulcoites and lodranites are highly metamorphosed to partially molten meteorites with mineral and bulk compositions similar to those of ordinary chondrites. These properties place the acapulcoites and lodranites between the unmelted chondrites and the differentiated meteorites and as such acapulcoites–lodranites are of special interest for understanding the initial stages of asteroid differentiation as well as the role of 26Al heating in the thermal history of asteroids. To constrain the accretion timescale and thermal history of the acapulcoite–lodranite parent body, and to compare these results to the thermal histories of other meteorite parent bodies, the Hf–W system was applied to several acapulcoites and lodranites. Acapulcoites Dhofar 125 and NWA 2775 and lodranite NWA 2627 have indistinguishable Hf–W ages of Δ t CAI = 5.2 ± 0.9 Ma and Δ t CAI = 5.7 ± 1.0 Ma, corresponding to absolute ages of 4563.1 ± 0.8 Ma and 4562.6 ± 0.9 Ma. Closure temperatures for the Hf–W system for acapulcoites and lodranites, estimated from numerical simulations of W diffusion in high-Ca pyroxene, are 975 ± 50 °C and 1025 ± 50 °C, respectively. Owing to these high closure temperatures, the Hf–W ages provide information on the earliest high-temperature evolution, and combined with thermal modeling indicate that the acapulcoite–lodranite parent body accreted ~ 1.5–2 Ma after CAI formation, was internally heated by 26Al decay, and reached its thermal peak ~ 3 Ma after CAI formation. Cooling rates for acapulcoites decreased from ~ 120 °C/Ma just below the thermal peak to ~ 50 °C/Ma at ~ 600 °C. Over the same temperature interval the cooling rate for lodranites decreased from ~ 100 °C/Ma to ~ 40 °C/Ma. These thermal histories may reflect cooling in the uppermost ~ 10 km of a parent body with a radius of ~ 35–100 km. Acapulcoites and lodranites evolved with a 180Hf/ 184W ratio of ~ 0.64, which is indistinguishable from that of H chondrites but significantly lower than 180Hf/ 184W~ 1.23 for carbonaceous chondrites. The low 180Hf/ 184W ratios of acapulcoites–lodranites were established before ~ 2 Ma and, hence, prior to partial melting in the parent body at ~ 3 Ma. Thus, they must reflect Hf–W fractionation of the precursor material by processes in the solar nebula. Combined with Hf–W ages of Δ t CAI < 1 Ma for differentiation of the parent bodies of magmatic iron meteorites and an Hf–W age of Δ t CAI ~ 2.5 Ma for the accretion of the H chondrite parent body, the Hf–W results for acapulcoites and lodranites reveal an inverse correlation between accretion age of asteroids and peak temperature in their interiors. The different thermal histories of most meteorite parent bodies, therefore, primarily reflect variations in their initial 26Al abundance, which is determined by their accretion time.

39Ar/40Ar Ages from the Yozgat Batholith: Preliminary Data on the Timing of Late Cretaceous Extension in the Central Anatolian Crystalline Complex, Turkey

Isotopic dating of sheared and unsheared rocks can be important in understanding deformational processes in orogenic belts. This study examines 40Ar/39Ar dating of granitoids and mylonitic rocks to constrain intrusive and deformational events within the northern part of the central Anatolian crystalline complex (CACC). The Kerkenez granitoid within this complex, comprising primarily quartz monzonite and hornblende granite, contains discrete ductile shear zones. These zones are characterized by protomylonite and mylonite formations with metamorphism conditions that reach lower amphibolite facies, mylonitic foliations and lineations, and asymmetric kinematic indicators (e.g., asymmetric porphyroclasts, composite shear bands) with top-to-the-northwest shear senses. Considering the high closure temperatures (∼500°C for hornblende and ∼350°C for K-feldspar), both hornblende quartz monzonite and hornblende granite in the Kerkenez granitoid may have cooled rapidly, suggesting that hornblende quartz monzonite may have been emplaced at around 81.2 ± 0.5 Ma and that it is older than hornblende granite, which has a well-defined plateau age (72.6 ± 0.2 Ma). On the basis of intrusive relations and our 40Ar/39Ar age data, we can constrain the upper age limit (∼81 Ma) on the regional metamorphism in the northern part of the CACC. The 40Ar/39Ar dating of hornblendes in two mylonite samples from a ductile shear zone yields plateau ages of 71.6 ± 0.3 and 71.7 ± 0.2 Ma, respectively. K-feldspars in the same samples yield plateau ages of 71.6 ± 0.2 and 81.3 ± 0.2 Ma. Therefore, we adopt 71.6 ± 0.3 and 71.7 ± 0.2 Ma as the cooling ages of hornblende and K-feldspar, respectively, in the ductile shear zone. On the other hand, an age of 81.3 ± 0.2 Ma for deformed K-feldspar appears to reflect not the age of ductile deformation but rather the age of undeformed hornblende quartz monzonite. These age data suggest that the shear zones formed soon after the emplacement and cooling of hornblende granite. The cooling event of the shear zones is interpreted to be associated with the beginning of extension in the region. Furthermore, these data imply that metamorphism, emplacement, and cooling of the intrusives and ductile shearing of the intrusions were coeval in the region and occurred in the Late Cretaceous.

Hf–W thermochronometry: Closure temperature and constraints on the accretion and cooling history of the H chondrite parent body

We obtained Hf–W metal-silicate isochrons for several H chondrites of petrologic types 4, 5, and 6 to constrain the accretion and high-temperature thermal history of the H chondrite parent body. The silicate fractions have 180Hf/184W ratios up to ∼51 and 182W/184W ratios up to ∼33 ε units higher than the whole-rock. These high 180Hf/184W and radiogenic W isotope ratios result in highly precise Hf–W ages. The Hf–W ages of the H chondrites become younger with increasing metamorphic grade and range from ΔtCAI=1.7±0.7 Ma for the H4 chondrite Ste. Marguerite to ΔtCAI=9.6±1.0 Ma for the H6 chondrites Kernouvé and Estacado. Closure temperatures for the Hf–W system in H chondrites were estimated from numerical simulations of W diffusion in high-Ca pyroxene, the major host of radiogenic 182W in H chondrites, and range from 800±50 °C for H4 chondrites to 875±75 °C for H6 chondrites. Owing to these high closure temperatures, the Hf–W system closed early and dates processes associated with the earliest evolution of the H chondrite parent body. Consequently, the high-temperature interval of ∼8 Ma as defined by the Hf–W ages is much shorter than intervals obtained from Rb–Sr and Pb–Pb dating. For H4 chondrites, heating on the parent body probably was insufficient to cause W diffusion in high-Ca pyroxene, such that the Hf–W age of ΔtCAI=1.7±0.7 Ma for Ste. Marguerite was not reset and most likely dates chondrule formation. This is consistent with Al–Mg ages of ∼2 Ma for L and LL chondrules and indicates that chondrules from all ordinary chondrites formed contemporaneously. The Hf–W ages for H5 and H6 chondrites of ΔtCAI=5.9±0.9 Ma and ΔtCAI=9.6±1.0 Ma correspond closely to the time of the thermal peak within the H chondrite parent body. Combined with previously published chronological data the Hf–W ages reveal an inverse correlation of cooling rate and metamorphic grade: shortly after their thermal peak H6 chondrites cooled at ∼10 °C/Ma, H5 chondrites at ∼30 °C/Ma and H4 chondrites at ∼55 °C/Ma. These Hf–W age constraints are most consistent with an onion-shell structure of the H chondrite parent body that was heated internally by energy released from 26Al decay. Parent body accretion started after chondrule formation at 1.7±0.7 Ma and probably ended before 5.9±0.9 Ma, when parts of the H chondrite parent body already had cooled from their thermal peak. The well-preserved cooling curves for the H chondrites studied here indicate that these samples derive from a part of the H chondrite parent body that remained largely unaffected by impact disruption and reassembly but such processes might have been important in other areas. The H chondrite parent body has a 180Hf/184W ratio of 0.63±0.20, distinctly lower than the 180Hf/184W=1.21±0.06 of carbonaceous chondrite parent bodies. This difference reflects Hf–W fractionation within the first ∼2 Ma of the solar system, presumably related to processes in the solar nebula.

Textural, geochronological and chemical constraints from polygenetic titanite and monogenetic apatite from a mid-crustal shear zone: An integrated EPMA, SIMS, and TIMS study

Titanites extracted from a 520 ± 9 Ma syn-orogenic syenite at Otjimbingwe (Damara orogen, Namibia) show shape, colour and U–Pb systematics that vary with position in the syenite relative to a major shear zone. Most titanite fractions from syenite cropping out outside the shear zone are characterized by a medium brown anhedral core overgrown by a pale brown subhedral rim. EMPA and SIMS analyses show that the cores are enriched in Al, Fe and sometimes Na, Mg and Mn but also in REE, Nb and Zr relative to the rim. TIMS U–Pb ages from these titanites gave relatively imprecise 207Pb/ 206Pb ages between 680 ± 21 Ma and 695 ± 20 Ma as well as 570 ± 8 Ma to 604 ± 9 Ma mainly due to their low 206Pb/ 204Pb ratios. Combined Th–U–Pb relationships suggest that these titanites contain a component similar to ca. 715–750 Ma-old titanite populations observed so far only in Neoproterozoic syenites from the northern part of the orogen. Titanites from syenites within the shear zone are dark brown, mostly euhedral to subhedral and display only occasionally a pale brown rim. These titanites show decreasing concentrations of Na, Mg, Al and Fe and increasing concentrations of Mn from core to rim, interpreted to reflect growth zonation. Abundances of MREE, HREE, Y, Nb and Zr are higher in the rim than in the core. 207Pb/ 206Pb ages obtained on these titanites range from 470 ± 2 Ma to 538 ± 3 Ma for most samples. Only the samples with the highest 206Pb/ 204Pb ratios (between 350 and 230) have U–Pb ages similar to the previously published U–Pb zircon age of 520 ± 9 Ma from the same complex. Other titanite fractions with lower 206Pb/ 204Pb ratios have slightly older ages than ca. 520 Ma, indicating some inheritance. Inherited titanite survived multiple metamorphic episodes in the Damara orogen between ca. 750 Ma and 470 Ma and the igneous event that lead to the intrusion of the Otjimbingwe syenite at 520 Ma; these data imply a high closure temperature of U–Pb in titanite of 650–> 700 °C consistent with previous studies. Coeval apatite has no apparent inheritance, low 206Pb/ 204Pb ratios between 55 and 170 and U–Pb ages similar to, or only slightly younger than the U–Pb zircon or titanite age obtained on the titanite fractions with the highest 206Pb/ 204Pb ratios. Assuming a closure temperature for U–Pb of ≥ 900 °C for zircon, ∼ 700 °C for titanite and ∼ 450 °C for apatite, initial fast cooling of the complex can be inferred. Our data demonstrate that titanite can contain multiple age domains, preserving a chronological and chemical record of previous igneous events.

Early Palaeozoic cooling of the southern Prince Charles Mountains, East Antarctica: Synchronous cooling of three stratigraphic levels

Dating of orogenic events on the East Antarctic Shield has generally involved mineral isotopoe systems with high closure temperatures (i.e. U-Pb). This has lead to the recognition of Archaean (c. 3.2-2.5 Ga), Meso-Neoproterozoic (c. 1.3-0.9 Ga) and early Palaeozoic (c. 550?450 Ma) orogenic events. Geochronological studies from the southern Prince Charles Mountains indicate both Neoproterozoic (Grenville) and early Palaeozoic (Pan-African) orogenesis has affected the Archaean to Neoproterozoic bedrock (Fig. 1a). Understanding the complex geological evolution of this region is crucial for constraining the geodynamics of Neoproterozoic East Antarctic/Indian continental reconstructions (Fig. 1b-c). 40Ar/39Ar data from the southern Prince Charles Mountains indicates that regional cooling occurred during early Palaeozoic orogenesis. Apparent ages obtained from this study indicate: (1) cooling of mica occurred between c. 500 and c. 480 Ma (Tc 350 ? 300°C), and (2) reset of hornblende occurred at c. 520 Ma (Tc 500°C). Samples targeted for argon thermochronology were from a temporally constrained vertical crustal section. Apparent age data from individual lithostratigraphic units indicate regional cooling occurred across all three stratigraphic levels in the early Palaeozoic. Affects of this event were: (1) total reset of biotite and mica from the Archaean- Mesoproterozoic strata, (2) partial reset of hornblende from the Archaean strata and (2) crystallisation of fine- grained sericite in the upper Neoproterozoic level. We attribute this synchronous cooling to reflect thermal resetting of the individual units that were already at similar vertical crustal levels. Coupled with structural and metamorphic observations we suggest the ?Pan-African? development of the region was related to large-scale reactivation and thin-skinned deformation. This attributes the gross crustal architecture of the southern Prince Charles Mountains to pre- `Pan-African' events (> c. 550 Ma). This period is coincident with the formation and break-up of Rodinia.

Thermal evolution of the Lesser Himalaya, central Nepal: Insights from K-white micas compositional variation

The Lesser Himalayan low- to medium-grade metamorphic rocks in central Nepal are rich in K-white micas occurring as porphyroclasts and in matrix defining S 1 and S 2. Porphyroclasts are usually zoned with celadonite-poor cores and celadonite-rich rims. The cores are the relics of igneous or high grade metamorphic muscovites, and the rims were re-equilibrated or overgrown under lower T metamorphic conditions. The matrix K-white micas defining S 1, pre-dating the Main Central Thrust activity, are generally celadonite-rich. They show heterogeneous compositional zoning with celadonite-rich cores and celadonite-poor rims. They were recrystallized at lower T condition prior to the Main Central Thrust activity, most probably prior to the India–Asia collision (pre-Himalayan metamorphism). The matrix K-white micas along S 2, synchronous to the Main Central Thrust activity (Neohimalayan metamorphism), are relatively celadonite-poor and were recrystallized under relatively higher T condition. K-white micas defining S 1 also were partially re-equilibrated during the Neohimalayan metamorphism. The average compositions of recrystallized K-white micas defining both S 1 and S 2 become gradually poor in (Fe + Mg)- and Si-contents and rich in Al- and Ti-contents from south to north showing an increase of metamorphic grade from structurally lower to higher parts in the Lesser Himalaya. This shows that the metamorphism is inverted throughout the inner Lesser Himalaya. The tectono-metamorphic significance of the published K–Ar and 40Ar / 39Ar K-white micas ages from the Lesser Himalaya need re-evaluation in the context of observed intrasample compositional variation and zoning, and possible higher closure temperature (∼500 °C) for K–Ar system.

Cretaceous isochron ages from K–Ar and 40Ar / 39Ar dating of eclogitic rocks in the Tso Morari Complex, western Himalaya, India

The Tso Morari Complex, which is thought to be originally the margin of the Indian continent, is composed of pelitic gneisses and schists including mafic rock lenses (eclogites and basic schists). Eclogites studied here have the mineral assemblage Grt + Omp + Ca-Amp + Zo + Phn + Pg + Qtz + Rt. They also have coesite pseudomorph in garnet and quartz rods in omphacite, suggesting a record of ultrahigh-pressure metamorphism. They occur only in the cores of meter-scale mafic rock lenses intercalated with the pelitic schists. Small mafic lenses and the rim parts of large lenses have been strongly deformed to form the foliation parallel to that of the pelitic schists and show the mineral assemblages of upper greenschist to amphibolite facies metamorphism. The garnet–omphacite thermometry and the univariant reaction relations for jadeite formation give 13–21 kbar at 600 °C and 16–18 kbar at 750 °C for the eclogite formation using the jadeite content of clinopyroxene ( X Jd = 0.48). Phengites in pelitic schists show variable Si / Al and Na / K ratios among grains as well as within single grains, and give K–Ar ages of 50–87 Ma. The pelitic schist with paragonite and phengite yielded K–Ar ages of 83.5 Ma (K = 4.9 wt.%) for paragonite–phengite mixture and 85.3 Ma (K = 7.8 wt.%) for phengite and an isochron age of 91 ± 13 Ma from the two dataset. The eclogite gives a plateau age of 132 Ma in Ar/Ar step-heating analyses using single phengite grain and an inverse isochron age of 130 ± 39 Ma with an initial 40Ar / 36Ar ratio of 434 ± 90 in Ar/Ar spot analyses of phengites and paragonites. The Cretaceous isochron ages are interpreted to represent the timing of early stage of exhumation of the eclogitic rocks assuming revised high closure temperature (500 °C) for phengite K–Ar system. The phengites in pelitic schists have experienced retrograde reaction which modified their chemistry during intense deformation associated with the exhumation of these rocks with the release of significant radiogenic 40Ar from the crystals. The argon release took place in the schists that experienced the retrogression to upper greenschist facies metamorphisms from the eclogite facies conditions.

LA-MC-ICPMS Pb–Pb dating of rutile from slowly cooled granulites: Confirmation of the high closure temperature for Pb diffusion in rutile

Rapid Pb–Pb dating of natural rutile crystals by laser ablation multiple-collector inductively coupled plasma mass spectrometry (LA-MC-ICPMS) is investigated as a tool for constraining geological temperature–time histories. LA-MC-ICPMS was used to analyse Pb isotopes in rutile from granulite-facies rocks from the Reynolds Range, Northern Territory, Australia. The resultant ages were compared with previous U–Pb zircon and monazite age determinations and new mica (muscovite, phlogopite, and biotite) Rb–Sr ages from the same metamorphic terrane. Rutile crystals ranging in size from 3.5 to 0.05 mm with ⩽20 ppm Pb were ablated with a 300–25 μm diameter laser beam. Crystals larger than 0.5 mm yielded sufficiently precise 206Pb/ 204Pb and 207Pb/ 204Pb ratios to correct for the presence of common Pb, and individual rutile crystals often exhibited sufficient Pb isotopic heterogeneity to allow isochron calculations to be performed on replicate analyses of a single crystal. The mean of 12 isochron ages is 1544 ± 8 Ma (2 SD), with isochron ages for single crystals having uncertainties as low as ±1.3 Myr (2 SD). The 207Pb– 206Pb ages calculated without correction for common Pb are typically <0.5% higher than the common-Pb-corrected isochron ages reflecting the very minor amounts of common Pb present in the rutile. The LA-MC-ICPMS method described samples only the outer 0.1–0.2 mm of the rutile crystals, resulting in a grain size-independent apparent closure temperature ( T c) for Pb diffusion in rutile that is less than the T c of monazite ⩽0.1 mm in diameter, but significantly higher than the Rb–Sr system in muscovite (550 °C), phlogopite (435 °C) and biotite (400 °C). Even small rutile crystals are extremely resistant to isotopic resetting. For the established slow cooling rate of ca. 3 °C/Myr, the T c for Pb diffusion in the analysed rutile is ca. 630 °C. This is in excellent agreement with recent experimental results that indicate that rutile has a higher T c than previously thought (ca. 600–640 °C for rutile 0.1–0.2 mm diameter cooled at 3 °C/Myr; near 600 °C [Cherniak D.J., 2000. Pb diffusion in rutile. Contrib. Mineral. Petrol. 139, 198-207], versus 400 °C [Mezger, K., Hanson G.N., Bohlen S.R., 1989a. High precision U–Pb ages of metamorphic rutile: applications to the cooling history of high-grade terranes. Earth Planet. Sci. Lett. 96, 106–118.] for 1 °C/Myr), and with current T c estimates for monazite and other high temperature geochronometers, which have been revised upwards in recent years. The new rutile ages, together with the other geochronological data from the region, support the interpretation that the Reynolds Range underwent prolonged slow cooling on a conductive geotherm, under nearly steady-state conditions. Slow cooling at ca. 3 °C/Myr persisted for at least 40 Myr followed the peak of high- T/low- P metamorphism to granulite-facies conditions, and probably continued at ca. 2–3 °C/Myr for ca. 200 Myr overall.

U-Pb and 207Pb- 206Pb ages of zircons from basaltic eucrites: Implications for early basaltic volcanism on the eucrite parent body

We have undertaken petrologic and SHRIMP U-Th-Pb isotopic studies on zircons from basaltic eucrites (Yamato [Y]-75011, Y-792510, Asuka [A]-881388, A-881467 and Padvarninkai) with different thermal and shock histories. Eucritic zircons are associated with ilmenite in most cases and have subhedral shapes in unmetamorphosed and metamorphosed eucrites. Some zircons in highly metamorphosed eucrites with granulitic texture occur alone in pyroxene, and typically have rounded to subrounded shapes due to recrystallization. Superchondritic Zr/Hf ratios of eucritic zircons indicate that they crystallized from incompatible element-rich melts after crystallization of ilmenite. Concentrations of uranium and thorium in zircons in the unmetamorphosed eucrite Y-75011 are higher than those in metamorphosed eucrites. The U-Pb systems of eucritic zircons are almost concordant but some zircon grains show reverse discordance. Radiogenic lead-loss up to 48% from zircons is observed in the shock-melted eucrite Padvarninkai. The 207Pb- 206Pb ages of zircon in Y-75011 (4550 ± 9 Ma, n = 5) are nearly identical, within analytical uncertainty, to the ages of zircons from the metamorphosed eucrite Y-792510 (4545 ± 15 Ma, n = 13), the highly metamorphosed eucrites A-881388 (4555 ± 54 Ma, n = 5) and A-881467 (4558 ± 13 Ma, n = 8), and the shock-melted eucrite Padvarninkai (4555 ± 13 Ma, n = 18). The averaged 207Pb- 206Pb age of zircon from five eucrites analyzed in this study is 4554 ± 7 Ma (95% confidence limits, n = 49), indistinguishable from the averaged U-Pb age (4552 ± 9 Ma) of the same samples. Because of the high closure temperature of lead in zircon ( T closure = ∼1050°C with a cooling rate of 0.2°C/yr), the 207Pb- 206Pb ages of eucritic zircon do not represent metamorphic ages but crystallization ages of extrusive lavas. This fact strongly suggests that volcanism of the eucrite parent body occurred at a very early stage of the Solar System history, 7–20 Ma after CAI formation (4567.2 ± 0.6 Ma), thus basaltic eucrites crystallized from parental magmas within a short interval following the differentiation of their parent body. The U-Pb ages of eucritic zircons are older than the U-Pb, Sm-Nd and Rb-Sr ages of some basaltic eucrites, which is consistent with differences in closure temperatures of each isotopic system, and suggests that thermal and shock metamorphism affected the isotopic systems of pyroxene, plagioclase and phosphates.