Younger Apparent Ages Research Articles

Minimizing the effects of Pb loss in detrital and igneous U–Pb zircon geochronology by CA-LA-ICP-MS

Abstract. Detrital zircon geochronology by laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) is a widely used tool for determining maximum depositional ages and sediment provenance, as well as reconstructing sediment routing pathways. Although the accuracy and precision of U–Pb geochronology measurements have improved over the past 2 decades, Pb loss continues to impact the ability to resolve zircon age populations by biasing affected zircon toward younger apparent ages. Chemical abrasion (CA) has been shown to reduce or eliminate the effects of Pb loss in zircon U–Pb geochronology but has yet to be widely applied to large-n detrital zircon analyses. Here, we assess the efficacy of the chemical abrasion treatment on zircon prior to analysis by LA-ICP-MS and discuss the advantages and limitations of this technique in relation to detrital zircon geochronology. We show that (i) CA does not systematically bias LA-ICP-MS U–Pb dates for 13 reference materials that span a wide variety of crystallization dates and U concentrations, (ii) CA-LA-ICP-MS U–Pb zircon geochronology can reduce or eliminate Pb loss in samples that have experienced significant radiation damage, and (iii) bulk CA prior to detrital zircon U–Pb geochronology by LA-ICP-MS improves the resolution of age populations defined by 206Pb/238U dates (Neoproterozoic and younger) and increases the percentage of concordant analyses in age populations defined by 207Pb/206Pb dates (Mesoproterozoic and older). The selective dissolution of zircon that has experienced high degrees of radiation damage suggests that some detrital zircon age populations could be destroyed or have their abundance significantly modified during this process. However, we did not identify this effect in either of the detrital zircon samples that were analyzed as part of this study. We conclude that pre-treatment of detrital zircon by bulk CA may be useful for applications that require increased resolution of detrital zircon populations and increased confidence that 206Pb/238U dates are unaffected by Pb loss.

Inference of young groundwater ages and modern groundwater proportions using chlorofluorocarbon and tritium/helium-3 tracers from West Hawai‘i Island

The geologically complex western aquifers of Hawai‘i Island serve as the primary reservoir of fresh potable water for residents, yet the area’s hydrogeologic characteristics are still not well understood. Groundwater age is a fundamental parameter used to better understand groundwater flow and storage, yet few studies have attempted to estimate apparent groundwater ages in Hawai‘i. This study implements a multi-tracer approach using chlorofluorocarbons, sulfur hexafluoride, and tritium/helium-3 to determine the apparent ages of young water masses from the West Hawai‘i aquifers. Chlorofluorocarbon and tritium/helium-3 tracer measurements suggest that the aquifers contain young groundwater with a median age of 53 years, but that a significant volume of older groundwater exists, which was recharged prior to the release of these anthropogenic tracers. Mixing models suggest that the groundwater consists of mixtures with an average 28% modern water (<70 years), with the remainder being recharged prior to 1953. Further, helium measurements demonstrate that the area’s groundwater is influenced by hydrothermal and magmatic processes from Hualālai volcano, which bias estimated groundwater ages if not properly considered. It is therefore beneficial to utilize a multi-tracer approach in an oceanic volcanic island setting, where groundwater ages are impacted by various external processes. The analysis of young apparent ages and proportions can aid in coastal aquifer characterization and subsequent groundwater quality assessment and is crucial for sustainable groundwater management.

Deciphering complex groundwater age distributions and recharge processes in a tropical and fractured volcanic multi‐aquifer system

AbstractGroundwater recharge in highly fractured volcanic aquifers in the humid tropics remains poorly understood. In this region, rapid demographic growth and unregulated land use change are resulting in extensive surface water pollution and a large dependency on groundwater extraction. Here we present a multi‐tracer approach including δ18Oδ2H, 3H/3He dating, and noble gases (NG) within the most prominent multi‐aquifer system of central Costa Rica, with the objective to assess dominant groundwater recharge mechanisms and age distributions. We sampled wells and large springs across an elevation gradient from 868 to 2421 m asl. Our results indicate relatively young apparent ages ranging from 0.0 ± 3.2 up to 43.5 ± 7.6 years within the unconfined aquifer system. Helium isotopes (R/Ra up to 5.4) indicate a dominant signal from the upper mantle and preclude 3H/3He dating in 50% of the samples. Potential recharge elevations (based on NG and δ18O) ranged from ~1350 to 2670 m asl. NG‐derived recharge temperatures ranged from 11.0°C to 19.4°C. Recharge estimates varied from 129 ± 78 to 1605 ± 196 mm/yr with a mean value of 642 ± 117 mm/yr, representing 20.1 ± 4.0% of the total mean annual rainfall as effective recharge. The shallow unconfined aquifer is characterized by young and rapidly infiltrating water, whereas the deeper aquifer units have relatively older water (&gt;60 years). These results are intended to guide the delineation and mapping of critical recharge areas in mountain headwaters to enhance water security and sustainability in the most important headwater dependent systems of Costa Rica.

The archaeomagnetic field recorded in ancient kiln walls in Si Satchanalai, Sukhothai

Archaeological dating is crucial in archaeology as it is a key to understand human history. However, traditional dating methods used by archaeologists such as potassium-argon dating and luminescence dating can provide ambiguous age results, e.g., argon loss during the dating returns young apparent ages. Therefore, I plan to establish an archaeomagnetic secular variation (ASV) curve to resolve this problem and use the ASV curve as an alternative tool to date archaeological artefacts. However, archaeomagnetic data in Thailand are absent from literature. Therefore, the ASV curve cannot be constructed from the archaeomagnetic data for this locality. To provide archaeomagnetic data to construct the ASV curve, the directions of the Earth’s magnetic field recorded in kiln walls from Ban Ko Noi (KN123, age 1,370 ± 100 A.D.), Si Satchanalai were measured. The mean declination and inclination of 49.6° and 32.6° with 95% confidence limit of 5.4° were determined from 10 samples from kiln KN123. Mean directions from this study were also compared with the directions of the Earth’s magnetic field in Thailand during 1,370 A.D. from the global archaeomagnetic field model ARCH3k.1. Declination and inclination from this study show significant departure from the field predicted by the ARCH3k.1 model.

Age of magmatism and alteration of basaltic rocks cored at the base of IODP Site U1513, Naturaliste Plateau, southwestern Australia

An 82.2 m-thick sequence of basaltic rocks was recovered from a deep-sea core on the eastern flank of the Naturaliste Plateau, offshore southwestern Australia during International Ocean Discovery Program (IODP) Expedition 369. The basaltic rocks were cored at the base of IODP Site U1513 and represent the acoustic basement of the Mentelle Basin. The recovered materials consist of subaerial to shallow-water basalt flows interbedded with volcaniclastic beds and cross-cutting dolerite dykes, all of which have been altered to some degree. Existing paleomagnetic data obtained from the overlying sedimentary sequences indicate that the basaltic sequence cored at Site U1513 was deposited before chron M10Nn.1n (which could be as young as 130.9 Ma or as old as 134.2 Ma, depending on the time scale used). Here, we present the results of 40Ar/39Ar step-heating experiments conducted on whole-rock samples of two basalt flows and six dolerite dykes to provide greater certainty of the timing of magmatism and subsequent alteration. Each sample was also characterised using optical and scanning electron microscopy together with X-ray fluorescence (XRF) and X-ray diffraction (XRD) analyses. The dolerite dykes are the only rocks that were sampled that record a remnant estimate of the primary cooling age of the magmatic event at ca 134 Ma. This magmatism was contemporaneous with rifting that preceded the break-up of Greater India from Australia/Antarctica and is potentially associated with the Greater Kerguelen large igneous province. The basalt flows record younger apparent ages (between 118 and 110 Ma) that cannot represent extrusive ages given their stratigraphic context, as well as older ages obtained from cross-cutting dykes. We interpret the younger apparent ages to reflect a phase of hydrothermal alteration that reset the argon isotopic system. A tectonic reconstruction, which shows the distribution of magmatism associated with the Greater Kerguelen large igneous province, indicates that the earliest phases of magmatism occurred within an ellipse with a NW–SE-oriented long axis. The reconstruction also shows that the Naturaliste Plateau was approximately 500 km from a known source of magmatism at ca 115 Ma. This distance is within the radius of a mantle plume head and indicates that hydrothermal and/or magmatic activity responsible for resetting the argon isotopic system of the dated basalt flows may have been associated with a mantle plume or was otherwise driven by a local source of magmatism that remained active during the Aptian (126.3–113.0 Ma). KEY POINTS 40Ar/39Ar dating indicates Naturaliste Plateau magmatism occurred between 141.3 and 130.9 Ma. The basaltic rocks potentially record a phase of hydrothermal alteration at ca 118–110 Ma. Greater Kerguelen large igneous province magmatism occurred within an ellipse with a NW–SE oriented long axis.

Multi-tracer (δ18O, δD, 3H, CFCs and SF6) investigation of groundwater recharge and apparent age at the Bamenda Highlands along the Cameroon volcanic line

The Bamenda Highlands contains numerous dilute and soda springs of unknown hydrological provenance. These dilute springs are a vital source of drinking water to the inhabitants. As a contribution to water management, this study investigated the spring water recharge, residence time and sub-surface circulation using δ18O, δD, 3H, CFCs, SF6 and TDS. A plot of both water types along the local meteoric water line indicated the meteoric origin and rapid recharge following precipitation. Dilute springs with a δ18O altitude effect of 0.27/100 m suggested recharge at different elevations. The CFC-12, CFC-11 and CFC-113 dating (complemented by 3H) showed reasonable young apparent ages of dilute springs ranging from 21 to > 52 (mean/median of 25) years and old soda springs (> 52 years). SF6 concentrations in dilute springs revealed exceptionally young ages relative to CFCs modelled ages suggesting a terrigenous enrichment of the former. Thus, groundwater dating with SF6 is unreliable in the area. Besides the old apparent ages of soda springs, their highly depleted δ18O and high TDS (mean of 1396 mg/l) indicated palaeo-recharge and high water–rock interaction, respectively. In contrast, the low TDS of dilute springs (< 140 mg/l) indicated low-water interaction. Most dilute springs showed exponential mixing models indicating heterogeneous recharge under unconfined aquifer conditions. Conversely, the old soda springs displayed piston flow and binary mixing models. The meteoric recharge and similar residence time of the shallow dilute springs suggested natural resilience to short-term changes in climatic conditions. Thus, low shallow groundwater abstraction is sustainable at the Bamenda Highlands.

40Ar/39Ar thermochronological constraints on the retrogression and exhumation of ultra-high pressure (UHP) metamorphic rocks from Xitieshan terrane, North Qaidam, China

Paleozoic ultra-high pressure (UHP) eclogite-bearing mafic boudins are exposed in gneiss in the Xitieshan terrane, North Qaidam. Petrological and thermobarometric studies show that garnet-amphibolite and amphibolite rocks record upper amphibolite-facies and lower amphibolite-facies retrograde metamorphic conditions of 8.1–9.1kbar, 630–670°C and 5.0–7.2kbar, 530–630°C, respectively. To better constrain the timing of retrogression and exhumation, 40Ar/39Ar stepwise heating analyses have been performed on minerals that were extracted from these UHP mafic rocks and their country rocks. Amphibole yields disturbed release spectra marked by anomalously old apparent ages in low temperature steps, suggesting the presence of extraneous 40Ar. Isochron ages of ~425Ma and ~419Ma are obtained for amphibole from the garnet-amphibolite and amphibolite, respectively, which are interpreted as the best estimate for the age of amphibolite-facies retrogression during exhumation. Muscovite from the gneiss yields consistent plateau and isochron ages of ~409Ma. K-feldspar from the same gneiss yields rising staircase-shaped age spectra, which are characterized by younger apparent ages in the first steps (263–281Ma) and higher plateau ages in the last segment (294–321Ma), indicating partial diffusive loss of argon from the K-feldspar samples. Biotite from schist yields flat age spectra with a plateau age of ~397Ma that is interpreted as a cooling age. Our new 40Ar/39Ar data, together with published LA-ICPMS ages of zircon from a coesite-bearing amphibolite indicate that the Xitieshan UHP metamorphic rocks may have experienced two stages of exhumation. During the first stage, from ~95–31km depth, the UHP metamorphic rocks were exhumed with a relatively rapid velocity of ~4.0km/Myr, resulting from buoyancy forces. Subsequently, the rate of exhumation decreased gradually from ~1.1 to 0.1km/Myr between ~31–7km depth. This later gentle exhumation was probably governed by extension and/or erosion.

MIS 5e at San Giovanni di Sinis (Sardinia, Italy): Stratigraphy, U/Th dating and “eustatic” inferences

The most representative Late Pleistocene (“Tyrrhenian”) outcrop in Sardinia (San Giovanni di Sinis) was re-visited based on detailed sedimentological and stratigraphical analysis supported by U-series dating of fossil corals. The stratigraphy shows shoreface–backshore sandstones overlying an erosional surface cut on vertebrate-bearing layers. Facies analysis and sequence-stratigraphic approaches suggest “eustatic” changes during the overall depositional interval of the marine sequence. Disconformities distinguish two coastal units suggesting a lateral shifts of the depositional environments driven by millennial-scale frequency sea level fluctuations, never exceeding a maximum height +1 to +3 m and by a subsequent rise to +5 ÷ +5.5 m asl.These deposits have historically been assigned to MIS 5, on the basis of i) their palaeontological content (warm “Senegalese” fauna), ii) lithostratigraphic interpretations and iii) amino-acid, geochronological data and generally attributed to the high sea-level episode of MIS 5e sub-stage, through sequential interpretation. U-series measurements in colonies of Cladocora caespitosa from the main outcrop of San Giovanni di Sinis and other minor deposits from the eastern side of the isthmus and from Capo San Marco, yielded 230Th-ages confirming an assignment of the embedding sediments to MIS 5e.Similar measurements in a pelecypod valve (Cerastoderma sp.) and a vermetid colony at San Marco yielded much younger apparent ages, tentatively attributed to late “diagenetic” U-uptake processes. Both the first minor pulses and the most important “eustatic” pulse recorded at San Giovanni di Sinis cannot be assigned unequivocally to one of the MIS 5e sea-level oscillations proposed in the recent literature, due to dating inaccuracies and uncertainties, but it seems probable that it occurred during an early part of the interval.

Strontium isotope zoning in garnet: implications for metamorphic matrix equilibration, geochronology and phase equilibrium modelling

AbstractIn principle, garnet growth rates may be calculated from 87Rb/86Sr and 87Sr/86Sr measurements in garnet subsamples and the surrounding rock matrix. Because of low Rb/Sr, garnet should passively record the matrix decay of 87Rb to 87Sr as a progressive increase in 87Sr/86Sr from core to rim. This concept was tested by collecting Rb‐Sr data for five garnet grains from four major orogenic belts: eastern Vermont (c. 380 Ma), western New Hampshire (c. 320 Ma), southern Chile (c. 75 Ma) and northwestern Italy (c. 35 Ma). Both normal Sr isotope zoning (increasing 87Sr/86Sr from core to rim) and inverse Sr zoning (decreasing 87Sr/86Sr from core to rim) were observed. Garnet and matrix isotope data commonly yielded grossly inaccurate model ages. Incomplete Rb and Sr equilibration among matrix minerals is invoked to explain the deviations between theoretical v. measured zoning patterns and the age disparities. Initially, the reactive matrix is dominated by rapidly equilibrating Rb‐rich mica, which imparts high 87Sr/86Sr values in garnet cores. Progressive participation of slower equilibrating Sr‐rich plagioclase buffers or even reduces 87Sr/86Sr, possibly leading to flat or decreasing 87Sr/86Sr from garnet cores to rims. Unusually high 87Sr/86Sr in garnet in combination with bulk matrix compositions causes erroneously young apparent ages, so metamorphic ages, growth rates, and associated heating and loading rates are likely suspect. Although Rb‐Sr may be the most susceptible because of the profound disparities between mica and feldspar, zircon reactivity might influence the Lu‐Hf system by up to a few per cent. The Sm‐Nd system seems generally immune to these effects. Pseudosection analysis and conventional garnet geochronology, which presume complete matrix equilibration during metamorphism, may require modification to account for differences between whole‐rock v. reactive matrix compositions.

Isotopic and geochemical indicators for groundwater flow and multi-component mixing near disturbed salt anticlines

In the present study, a set of environmental tracers was applied to establish aquifer fingerprints and chemical equilibria in groundwater of highly saline environments in contact with the Zechstein formation. With this approach it was possible to resolve prevailing multi-component mixing processes in the groundwater which were found to occur on three time scales: A modern groundwater component could be identified and quantified by the presence of 3H and 3Hetri in the samples which indicates young apparent groundwater ages (0–50a). The presence of 14C in the samples could be used for a classification between the modern groundwater component, and an old groundwater component. Contact between meteoric water and Zechstein salts (naturally or anthropogenically induced) could be identified by the presence or absence of Permian crystallization water from the dissolution of e.g. Carnallite and/or Kieserite. Using this, localized interactions between water-bearing strata and saline formations were identified. Noble gas values (He, Ne, 3He/4He) showed strong variations and unusually high excess air values. Qualitatively, these could be explained by the impact of the highly saline environment on noble gas diffusion. Implications relating to potentially on-going salt dissolution in areas of the thickest (Stassfurt) salt series induced by groundwater flow are discussed, as the investigated area is heavily affected by ongoing subrosion.

The mantle and crustal evolution of two garnet peridotite suites from the Western Gneiss Region, Norwegian Caledonides: An isotopic investigation

A compilation of published and unpublished geochronological and isotopic data from garnet-bearing orogenic peridotites in the HP/UHP Western Gneiss Region (WGR) of the Norwegian Caledonides indicate a common origin for all WGR peridotites, followed by different, though related, Proterozoic and Phanerozoic histories for those in the northwestern WGR (NW peridotites) compared to those in the central and western WGR (CW peridotites). All peridotites are refractory fragments of the subcontinental lithosphere generated by Archean melt extraction, which produced strongly depleted dunites and harzburgites with relict orthopyroxene and majoritic garnet megacrysts (M 1NW) within the NW peridotites. The Archean history is preserved by Re–Os sulfide and whole-rock ages from several WGR bodies and by Sm–Nd ages from the M 1NW megacrysts. Subsequently the CW peridotites were re-fertilized within the lithospheric mantle by mid-Proterozoic or older silicate melts that generated M 2CW garnet pyroxenites and adjacent garnet peridotites. Clinopyroxenes from these bodies show large variation in 143Nd/ 144Nd, but nearly constant 87Sr/ 86Sr, suggesting autometasomatism of depleted mantle by LREE-enriched, Rb-poor melts derived from equally depleted mantle. NW peridotites lack mid-Proterozoic garnet pyroxenite intrusions, but M 2NW garnet-rich assemblages that exsolved from relict M 1 megacrysts may have equilibrated at the same time as the M 2CW refertilization. Sm–Nd and Lu–Hf mineral apparent isochron ages from both suites range from 1.75 to ca. 0.87 Ga. The age spectrum suggests continuous diffusion among M 2 minerals that formed ≥ 1.75 Ga ago punctuated by partial re-equilibration during a 1.0 Ga thermal event. Much later the NW peridotites were transferred from the mantle wedge into the crust as the WGR was subducted into the mantle during the ca 400 Ma Scandian Orogeny. Further subduction heterogeneously metasomatized and recrystallized the NW peridotites to form M 3NW garnet, clinopyroxene and, where metasomatism was pervasive, new M 3NW radiogenic ( 87Sr/ 86Sr > 0.715), LIL-enriched minerals (phlogopite, amphibole) and microdiamond consistent with invasion by hydrous fluids from the enclosing Proterozoic gneisses. Nine young apparent ages (672 to 424 Ma), all from exsolved or recrystallized garnets within NW peridotites, represent mixed (M 2NW and M 3NW) apparent ages. The three youngest ages (weighted mean of 429.5 ± 3.1 Ma; 2 σ) may date M 3NW prograde re-equilibration during earliest Scandian subduction. The CW peridotites show no evidence of prograde M 3 re-equilibration, suggesting derivation from a different part of the Laurentian mantle wedge during the exhumation of the WGR from the mantle.

High-resolution calibration of Eocene strata: 40Ar/39Ar geochronology of biotite in the Green River Formation

The Sixth tuff of the Wilkins Peak Member of the Green River Formation is an important potential calibration point for magnetic stratigraphy, mammalian evolution, and lacustrine processes. However, two recent 40 Ar/ 39 Ar ages based on laser fusion of biotite from this important bed are not in agreement with one other. Most large, euhedral biotite from the basal 1 cm give concordant age spectra when incrementally heated. However, some crystals yield discordant age spectra and have plateau and integrated ages that are scattered toward both older and younger apparent ages. Electron-probe microanalysis indicates that intergrown alteration phases occur in a small fraction of Sixth tuff biotite crystals, and likely contributed to open-system processes that led to the observed discordance and age scatter. A new age of 49.62 ± 0.10 Ma is obtained from 22 concordant incremental-heating experiments. This age agrees with a new 49.78 ± 0.08 Ma age for the sanidine from the Layered tuff, which is stratigraphically 24 m below the Sixth tuff.

40Ar/39Ar and K–Ar age constraints on the timing of regional deformation, south coast of New South Wales, Lachlan Fold Belt: Problems and implications

Four slate samples from subduction complex rocks exposed on the south coast of New South Wales, south of Batemans Bay, were analysed by K–Ar and 40Ar/39Ar step‐heating methods. One sample contains relatively abundant detrital muscovite flakes that are locally oblique to the regional cleavage in the rock, whereas the remaining samples appear to contain sparse detrital muscovite. Separates of detrital muscovite yielded plateau ages of 505 ± 3 Ma and 513 ± 3 Ma indicating that inheritance has not been eliminated by metamorphism and recrystallisation. Step‐heating analyses of whole‐rock chips from all four slate samples produced discordant apparent age spectra with ‘saddle shapes’ following young apparent ages at the lowest temperature increments. Elevated apparent ages associated with the highest temperature steps are attributed to the presence of variable quantities of detrital muscovite (<1–5%). Two whole‐rock slate samples yielded similar 40Ar/39Ar integrated ages of ca 455 Ma, which are some 15–30 million years older than K–Ar ages for the same samples. These discrepancies suggest that the slates have also been affected by recoil loss/redistribution of 39Ar, leading to anomalously old 40Ar/39Ar ages. Two other samples, from slaty tectonic mélange and intensely cleaved slate, yielded average 40Ar/39Ar integrated ages of ca 424 Ma, which are closer to associated mean K–Ar ages of 423 ± 4 Ma and 409 ± 16 Ma, respectively. Taking into account the potential influences of recoil loss/redistribution of 39Ar and inheritance, the results from the latter samples suggest a maximum age of ca 440 Ma for deformation/metamorphism. The current results indicate that recoil and inheritance problems may also have affected whole‐rock 40Ar/39Ar data reported from other regions of the Lachlan Fold Belt. Therefore, until these effects are adequately quantified, models for the evolution of the Lachlan Fold Belt, that are based on such whole‐rock 40Ar/39Ar data, should be treated with caution.

A short duration of chondrule formation in the solar nebula: evidence from 26Al in Semarkona ferromagnesian chondrules

The 26Al– 26Mg systems of five ferromagnesian chondrules from the least metamorphosed ordinary chondrite Semarkona (LL3.0) were studied using a secondary ion mass spectrometer. Their glass or plagioclase portions contain excesses of 26Mg, and in two chondrules the 26Mg excesses are well correlated with 27Al/ 24Mg, which demonstrates the in-situ decay of 26Al. The initial 26Al/ 27Al ratios in these chondrules obtained from the slope of isochrons show a narrow range of between 6 × 10 −6 and 9 × 10 −6, indicating their short formation duration of less than 1 My. If the solar nebula was initially homogeneous in Al isotopes, the chondrule formation ages are ∼2 My younger than those of CAIs. Our results based on the study of the least metamorphosed UOC are consistent with the previous studies on Al-rich chondrules that the chondrule formation started at least 2 My after CAIs. Alternatively, the older records before 2 My were erased by chondrule recycling process. It further suggests that the young apparent ages (3 to >5 My after CAIs) for chondrules in type 3.4 UOCs are due to the disturbance of the 26Al– 26Mg system, possibly during parent body metamorphism. The result is not consistent with the extended nebular time scale of >5 My and the chondrule formation by planetary processes. The Ni isotopic analysis of the FeO-rich olivines in a type II chondrule in Semarkona did not show any detectable excess 60Ni in spite of their high Fe/Ni ratios. The upper limit of the initial 60Fe/ 56Fe ratio of the solar system was estimated to be 3.4 × 10 −7, which is consistent with the previous estimate (0.2–1.9 × 10 −7) from eucrites. This result confirms that the 60Ni excess previously observed from CAIs was not due to the decay of the short-lived nuclide 60Fe, but a Ni isotopic anomaly of nucleosynthetic origin.

Cooling Flow Star Formation and the Apparent Stellar Ages of Elliptical Galaxies

Simple theoretical arguments indicate that cooled interstellar gas in bright elliptical galaxies forms into a young stellar population having a bottom-heavy but optically luminous initial mass function extending to approximately 2 M middle dot in circle. When the colors and spectral features of this young population are combined with those of the underlying old stellar population, the apparent ages are significantly reduced, similar to the relatively young apparent ages observed in many elliptical galaxies. Galactic mergers are not required to resupply young stars. The sensitivity of continuous star formation to LB and LX&solm0;LB is likely to account for the observed spread in apparent ages among elliptical galaxies. Local star formation is accompanied by enhanced stellar Hbeta equivalent widths, stronger optical emission lines, more thermal X-ray emission, and lower apparent temperatures in the hot gas. The young stars should cause M&solm0;L to vary with galactic radius, perturbing the fundamental plane of the old stars alone.

New time constraints on dyke swarms related to the Paraná-Etendeka magmatic province, and subsequent South Atlantic opening, southeastern Brazil

40Ar 39Ar incremental heating analyses were carried out on tholeiitic dyke swarms of Santa Catalina (Florianópolis) and Rio de Janeiro and alkaline dykes from the proximity of Rio de Janeiro City, southeastern Brazil. Most of the plagioclases from the tholeiitic dykes display disturbed age spectra, with younger apparent ages most often clearly correlated with alteration phases. However, a calculated weighted mean age from the less disturbed released fractions of the NE-oriented Santa Catarina dyke swarm gives an age of 129.4 ± 0.3 Ma which is contemporaneous with the Ponta Grossa Dyke Swarm (PGDS) and at least the northern Paraná Continental Flood Basalts (CFB). The Rio de Janeiro tholeiitic dykes display age spectra affected by alteration and probably excess argon, but with low-temperature ‘mini-plateau ages’ between 130.6 and 129.8 Ma, which are concordant with previous data obtained on the PGDS and Rio de Janeiro dyke swarms by Renne et al. (1996a) and Turner et al. (1994). These new geochronological data when added to the previous ones show that the initiation of Gondwana's breakup in the region was characterized by a volcano-tectonic system with a geometry more complicated than the rift-rift-rift system suggested by many authors. The different rift arms of this structural feature were operating during a brief time period, mainly from 131.5 to 129 Ma, and were contemporaneous at least with the northern part of the Paraná-Etendeka Province (PEP). Single grain and bulk sample biotite from the alkaline, mainly coast-parallel dyke swarm near Rio de Janeiro City display plateau ages around 80 Ma (81.8 ± 1.8 and 78.8 ± 2.8 Ma) and 70 Ma (72.5 ± 0.5 and 69.7 ± 0.2 Ma). These magmatic intrusions refer to Late Cretaceous flexuring and vertical movements and rift basin formations (Almeida, 1976) due the northward progressing rifting motions separating South America and Africa during South Atlantic Ocean formation.

Disturbed 40Ar 39Ar systematics in hydrothermal biotite and hornblende at the Scotia gold mine, Western Australia: Evidence for argon loss associated with post-mineralisation fluid movement

Homblende and biotite that formed during gold mineralisation at the Scotia mine, Western Australia, have erratic 40Ar 39Ar release spectra and total gas ages that are ∼200–900 million year younger than the ca. 2600–2620 Ma minimum age of gold mineralisation, as given by 40Ar 39Ar plateau (muscovite) ages of crosscutting pegmatite dykes. Analysed hornblendes are dominated by magnesio hornblende but also contain small domains of ferro-actinolitic hornblende, actinolitic hornblende, and actinolite. Biotite also appears to be substantially altered to chlorite along cleavage planes. Relatively young apparent ages and high K/Ca ratios of argon released from hornblendes at temperatures less than ∼1000°C are interpreted to be the result of degassing of contaminant biotite. However, this cannot totally explain the young ages of hornblendes. Gas fractions released at furnace temperatures above 1000 C, where the effect of biotite degassing is demonstrably negligible, still have apparent ages that are ∼200–900 million years younger than the age of muscovite from post-gold pegmatite dykes. The close proximity of disturbed hydrothermal hornblende samples to apparently undisturbed pegmatite muscovite samples (less than a few metres in some cases) is difficult to reconcile with argon loss in hydrothermal hornblende being the product of thermally-driven volume diffusion. Given a suitable thermal history, argon loss could occur preferentially in hornblendes if (1) the closure (for slow cooling) and blocking (for reheating) temperatures of hydrothermal hornblendes were lower than published estimates, as has been observed in structurally complex metamorphic hornblendes and/or (2) the closure and blocking temperature of pegmatite muscovite were higher than commonly estimated. However, neither of these interpretations can easily explain the large variation in hornblende ages. It is instead suggested that argon loss occurred during mineral-fluid interaction during movement of a retrograde fluid along the mineralised lode structures and that this occurred at ambient temperatures below the blocking temperature of pegmatite muscovite. There is abundant geological evidence for the passage of such a fluid at the Scotia mine, including the presence of numerous late brittle fractures containing retrogressive low-temperature mineral assemblages. Late fluid movement is probably related to Proterozoic orogenic activity along the nearby southeastern margin of the Yilgarn Craton. The difference in argon systematics between hydrothermal minerals and pegmatite muscovite is largely ascribed to the relatively low permeability of the more massive pegmatite dykes (with respect to ore zones) preventing fluid egress to muscovite samples. The variations in ages of hydrothermal minerals are probably related to the extent of fluid/mineral interaction as this is a function of parameters, such as fluid/rock ratio, fluid P-T-X conditions, permeability, and mineral microstructures that may vary on short time and length scales. Recognition of possible argon loss in hornblende via fluid interaction is important for the interpretation of 40Ar- 39Ar systematics in environments, such as many hydrothermal ore deposits, where minerals may be exposed to fluids after crystallisation.

Insights into alpine moraine development from cosmogenic 36Cl buildup dating

We have used cosmogenic 36Cl surface exposure dating to determine apparent construction ages of late Pleistocene moraines in the Sierra Nevada, the White Mountains, and the Wind River Range, all in the western United States. The inferred glacial chronologies from the various localities all exhibit certain characteristics: (1) Local records are fragmentary and deposits of some glacial advances are always missing; no location has deposits of all glaciations and no glacial advance is recorded at all locations. This is due either to unfavorable conditions for glacier development at some times or to obliteration of earlier deposits by later, more extensive glaciers. (2) Most moraines have young apparent exposure ages, with only a few older than the last glacial cycle. This may be due to young true ages of these deposits, erosion of moraine surfaces, or obliterative overlap and covering of older deposits by younger ones. (3) Many deposits that were previously correlated (e.g., based on their stratigraphic positions) are not correlative; they may represent different stades and, sometimes, even different glaciations. Similarly, some previously uncorrelated deposits have the same exposure ages and may be correlative. (4) Clusters of several distinct moraines of nearly the same age are present at most locations. These clusters suggest that alpine moraines are formed during short deposition episodes that last between several hundred and several thousand years.

Ion Exchange and Dehydration Effects on Potassium and Argon Contents of Clinoptilolite

AbstractZeolite-rich Miocene tuffs are an important part of the principal hydrochemical barrier to water-borne radionuclide transport from a potential high-level nuclear waste repository at Yucca Mountain, Nevada. The timing of zeolitization is an issue that relates to paleohydrology, permeability, zeolite stability, and unsaturated-zone geochemical processes. Exploratory K/Ar dating of clinoptilolite, the most abundant and widespread zeolite, shows a striking and consistent pattern of increasing apparent ages (2–13 Ma) with depth. Only the isotopic ages from the saturated zone are compatible with geologic evidence suggesting an age &gt;10 Ma for most of the zeolites.Factors that may be responsible for the young apparent ages in the unsaturated zone were investigated. Cation exchange with recharge water and Ar diffusion under unsaturated conditions (processes that may be characteristic of the unsaturated zone) were evaluated experimentally for their effects on K/Ar systematics. Cation exchanging a natural clinoptilolite with Ca-, Cs-, K-, and Na-chloride solutions showed minimal effects on radiogenic Ar content. However, clinoptilolite heated at 2007deg;C for 16 hours in air lost a significant amount of its radiogenic Ar compared with minimal losses from clinoptilolite heated in water at 100°C for over 5 months. The preliminary results indicate that Ar loss from incompletely hydrated clinoptilolite may be a major factor contributing to the young apparent ages of clinoptilolite in the unsaturated zone at Yucca Mountain.

Middle tertiary hydrothermal activity and uplift of the northern flinders ranges, South Australia: Insights from apatite fission‐track thermochronology

Apatite fission‐track thermochronology reveals that hydrothermal activity was pervasive during mid‐Tertiary time within the Paralana Fault zone and adjacent country rocks of the northern Flinders Ranges, South Australia. Apatite fission‐track ages from the Mt Painter block, west of the Paralana Fault zone, range between ∼ 175 and 260 Ma. The mean lengths of confined fission‐tracks for these samples are ∼ 12 μm with s.d. of the track length distributions of ∼ 2 μm. These data suggest that rocks exposed in the Mt Painter block cooled slowly though the apatite partial annealing zone (∼ 110–50°C) starting before 200 Ma and continuing into the Tertiary. In contrast, apatite samples from the Paralana Fault zone yield apparent ages as young as 25–30 Ma and mean track lengths mostly < 12 μm, with s.d. ∼ 3 μm. These young apparent ages and relatively short track lengths require mid‐Tertiary palaeotemperatures of 90–105°C for the fault zone, assuming a heating time in the order of 1 Ma. This heating is interpreted to be the result of hydrothermal activity within the fault zone when it was buried beneath Mesozoic and Tertiary sedimentary strata. At least one stage of uplift of the northern Flinders Ranges during Miocene time elevated these rocks above the basin sediments, resulting in cooling due to a significant reduction in hot springs activity.