Incremental Heating Method Research Articles

Paleomagnetism and 40Ar/39Ar chronology of ignimbrites and lava flows, Central Volcanic Zone, Northern Chile

New 40Ar/39Ar ages and paleomagnetic data are presented from a wide range of ignimbrites and lava flows from the region east of the Atacama Desert in the northern Chilean part of Central Volcanic Zone of the Andes. Eight dacitic to rhyolitic ignimbrites, 12 lava flows and one shallow intrusion were dated by the 40Ar/39Ar incremental heating method, and are the first 40Ar/39Ar ages reported for these units. Ignimbrites range in age from 4.2 Ma to 0.5 Ma, with the ~100 km3 Puripicar ignimbrite dated at 4.18 ± 0.02 Ma, the ~2500 km3 Atana ignimbrite, one of the largest ignimbrites in the world, at 3.99 ± 0.02 Ma, and the Purico ignimbrite at 0.99 ± 0.03 Ma. The new 40Ar/39Ar ages fall within the uncertainty limits of published K-Ar dates for the Purico, Patao and Puripicar tuffs but are different at the 95% confidence level for Atana and Talabre tuffs. Isochron ages from the 12 lava flows range from 3.5 Ma to 0.1 Ma, with 11 of the flows younger than 2.5 Ma. One lava sample and the shallow intrusion are from the Plio-Pleistocene magnetite-rich El Laco volcanic complex, and give ages of 2.18 ± 0.03 Ma and 1.83 ± 0.09 Ma respectively. Paleomagnetic directions from 49 sites (23 ignimbrites and 26 lava flows) provide a mean direction of I = −43.8°, D = 352.7° with α95 = 10.0°, and a corresponding mean pole position of −81.1° and 179.1°. Normal directions are predominantly from lava flows and have steeper than expected inclinations (ΔI = 15.3°) whereas reverse sites are mostly ignimbrites and yield directions close to the geo-axial dipole field (ΔI = −3.6°). Paleosecular variation studies on a selection of 39 sites yield an angular standard deviation of site poles about the mean pole of 18.5°, considerably larger than predicted by current models of magnetic field variation with latitude.

The geologic history of Vesta inferred from combined 207Pb/206Pb and 40Ar/39Ar chronology of basaltic eucrites

Eucrites represent samples of mafic crust of the parent body, likely Vesta, and record a complex geologic history involving magmatism, metamorphism, impact processing, and metasomatism. For better understanding of the geologic history, we present combined 207Pb/206Pb and 40Ar/39Ar chronology of four basaltic eucrites showing distinct petrologic features: Agoult, an equilibrated eucrite with granulitic textures; Camel Donga, a brecciated equilibrated eucrite enriched in metallic iron; DaG 380, a partially equilibrated monomict eucrite; and NWA 049, a metasomatized polymict eucrite comprising mostly unequilibrated clasts. The 207Pb/206Pb dating in combination with the acid washing technique was applied to pyroxene-rich and plagioclase-rich fractions, while the 40Ar/39Ar dating was performed mainly on plagioclase grains using the laser incremental heating method. The Pb isotopic data for acid leaching residues are more radiogenic than those for acid washes, reflecting efficient removal of non-radiogenic Pb components by the acid washing. Among the residues, however, only those of the Agoult and Camel Donga plagioclase-rich fractions yielded isochrons, rather than errorchrons, with 207Pb/206Pb ages of 4532.39 ± 0.87 Ma and 4515.43 ± 0.42 Ma, respectively. The trace element systematics further suggest that these residues essentially represent contaminant oxide minerals such as ilmenite and chromite rather than plagioclase. On the other hand, the Ar isotopic data for Agoult and Camel Donga yielded plateau ages with weighted means of 4494 ± 9 Ma and 3749 ± 25 Ma, in contrast to those for DaG 380 and NWA 049 that did not return any plateau. The 207Pb/206Pb and 40Ar/39Ar ages of Agoult are interpreted to reflect the time of the U–Pb and K–Ar system closure during a prolonged thermal metamorphism. By combining the ages reported here and in the literature with the calculated closure temperatures, the cooling rate of Agoult is determined to be 11 ± 2 °C/Ma. This cooling rate corresponds to a burial depth of 10–20 km, indicating that granulitic eucrites were originally crystallized near the surface and were subsequently buried and metamorphosed. The ∼20 Ma younger 207Pb/206Pb ages of Camel Donga than Agoult are considered to reflect delayed burial metamorphism in the source region, whereas the plateau 40Ar/39Ar age would record the time of an impact causing brecciation and efficient Ar-degassing of the buried crust. Furthermore, the model 207Pb*/206Pb* dates calculated for acid-washed pyroxenes imply that the burial metamorphism of Agoult and DaG 380 were broadly contemporaneous and also that NWA 049 was metasomatized by fluids within the uppermost crust later than 4.4 Ga.

Dating young MORB of the Central Indian Ridge (19°S): Unspiked K-Ar technique limitations versus 40Ar/39Ar incremental heating method

Abstract We have applied the unspiked K-Ar and the 40 Ar/ 39 Ar methods to samples precisely collected and localised, on both Central Indian Ridge flanks, to test their effectiveness and reliability when applied to the dating of recent (i.e. less than 1 Ma) MORBs. Twenty six samples) from the sixty five samples collected every ∼500 m up to the Brunhes-Matuyama boundary on both ridge flanks, were selected based on their distance from the ridge axis. Therefore, we can evaluate whether the isotopic ages are a good indicator of the crystallisation age by considering their geographic position with respect to the ridge axis (zero age) and the B/M magnetic boundary. Direct comparison of the isotopic and model ages shows that only 9 out of 26 samples were successfully dated. The GIMNAUT – MORB's test case amply demonstrates that the unspiked K-Ar technique, when applied to submerged volcanic samples, is subject to potentially defective assumptions of trapped atmospheric argon, excess/fractionated argon and extremely sensitive to alteration. Although the unspiked K-Ar technique is theoretically capable to produce high precision ages, the comparison with the 40 Ar/ 39 Ar techniques reveals that only 15% (i.e. 4 samples out of 26) of the ages obtained here are geologically meaningful. Five of the seven 40 Ar/ 39 Ar incremental heating experiments provide meaningful ages. Because potential sources of systematic errors such as excess 40 Ar*, recoil of 39 Ar K and 37 Ar Ca can be identified and because effects of alteration are significantly reduced by the pre-heating of the samples up to 500–600°c, the 40 Ar/ 39 Ar incremental heating method appears to be the method of choice to date MORBs.

Age of Tertiary volcanic rocks on the West Greenland continental margin: volcanic evolution and event correlation to other parts of the North Atlantic Igneous Province

AbstractRadiometric ages for undated parts of the volcanic succession and intrusions in West Greenland were obtained by the40Ar–39Ar incremental heating method. Acceptable crystallization ages were obtained for 27 samples. Combined with published results the new data provide a volcanic stratigraphy correlatable throughout the Nuussuaq Basin. The thick onshore volcanic pile consists of four widespread formations: 62.5–61 Ma picrites (Vaigat Formation), 61–60 Ma depleted basalts (Maligât Formation and the Hellefisk-1 well), 60–58 Ma less-depleted basalts (Svartenhuk Formation) and 56–54 Ma enriched basalts (Naqerloq Formation). Two local successions comprise 53.5 Ma alkali basalts (Erqua Formation) and 38.7 Ma transitional basalts (Talerua Member). A central volcano developed on Ubekendt Ejland, leading to the Sarqâta qáqâ gabbro-granophyre intrusion at 57–55 Ma. Pre-break-up volcanism took place further south as early as 64–63 Ma. The offshore volcanic succession most probably comprises the known onshore succession plus some younger lavas. The change in spreading direction near the Paleocene–Eocene boundary took place west of the Nuussuaq Basinc.56.2 Ma. Some tectonomagmatic events are correlatable across the entire North Atlantic Igneous Province. A quiescent 58–56 Ma period correlates with similar periods in East Greenland and the Faroes, and the Naqerloq Formation is coeval with the Eocene basalts in East Greenland. The Paleocene and Eocene tholeiitic basalts are distinguishable chemically; in early Eocene time, mantle typical of the Iceland plume seems to have extended beneath the whole West Greenland margin as well as the central East Greenland margin.

Eocene to Miocene igneous activity in NE Greenland: northward younging of magmatism along the East Greenland margin

Radiometric dating by the 40 Ar– 39 Ar incremental heating method was carried out on lavas, sills, dykes, and a central intrusion from NE Greenland. Eighteen samples gave acceptable crystallization ages. Lavas of both Lower and Upper Plateau Lava Series gave ages in the range 55.5–53.5 Ma and cannot be constrained to better than 56–53 Ma. Sills and dykes from Traill Ø to Shannon, with compositions fairly similar to those of the lavas, gave ages of 55.1–51.3 Ma, contemporaneous with and slightly younger than the lavas. Alkaline lavas on inland nunataks have ages of 53–50 Ma, and the Kap Broer Ruys intrusive centre has an age of 48.7 ± 0.5 Ma. An alkaline sill on Hvalrosø is much younger at 20.3 ± 0.1 Ma. There are no pre-breakup lavas onshore NE Greenland. We surmise that the hot mantle of the Iceland plume arrived and melted extensively beneath the northern basins only at the time of breakup around 55 Ma. Post-breakup intrusive events in NE Greenland coincided with plate-tectonic events such as reorganization, uplift and opening in the north. The Hvalrosø sill represents a local small melting event that may be related to coeval opening of the Lena Trough. Supplementary materials: Details of the source data, results, and the compositions and locations of the dated samples, are available at www.geolsoc.org.uk/SUP18738 .

Fuerteventura – Assessment of a calibration site for cosmogenic 3He exposure dating with the 40Ar/39Ar incremental heating method

In situ Terrestrial Cosmogenic Nuclides (hereafter TCNs) are increasingly important for absolutely dating terrestrial events and processes. This study aimed at improving our knowledge of the production rate of Terrestrial Cosmogenic 3He formed in situ in rock surfaces at low latitude and sea level as well as re-evaluation of the Canary Islands as a calibration site for TCNs. For this purpose, we sampled basaltic lava flows from some of the youngest and yet undated volcanic sites and used the 40Ar/39Ar incremental heating method on groundmass samples and in situ cosmogenic 3He on olivine and clinopyroxene phenocrysts. 40Ar/39Ar analysis was done on a Hiden HAL Series 1000 triple filter quadrupole mass spectrometer with extraction furnace. Incremental heating data shows ages in the Late Pleistocene from 52.7 ± 21.6 ka to 398.6 ± 27.6 ka.We measured cosmogenic 3He concentrations in olivine and clinopyroxene phenocrysts from flow top samples on a MAP 215-50 sector mass spectrometer with a crushing device and a diode laser extraction system. Exposure age calculations yielded ages in the range 38.9 ± 4.0 ka to 62.3 ± 6.7 ka for the youngest lava flow and the data series is in broad agreement with the argon data up to 250 ka and reveals a more continuous time line of volcanism during the late Pleistocene on the island. However, the dataset was not sufficient for calculation of production rates for in situ Terrestrial Cosmogenic 3He as many samples showed signs of erosion. Calculated erosion rates range from none to as high as 7.3 mm/kyr assuming a rock density of 2.9 g/cm2. This finding puts a constraint on the use of Fuerteventura as a calibration site for exposure histories older than 50–100 ka. A comparison with cosmogenic 36Cl data supports these findings and indicates substantial weathering.

Stratigraphy and age of the Eocene Igtertivâ Formation basalts, alkaline pebbles and sediments of the Kap Dalton Group in the graben at Kap Dalton, East Greenland

A NE–SW-trending graben at Kap Dalton on the Blosseville Kyst contains an at least 600 m thick succession of Eocene basalt lavas and sediments. The succession has been investigated by new field work, geochemical analysis and radiometric dating by the 40Ar-39Ar incremental heating method. The results show that the volcanic succession comprises about 220 m of the uppermost plateau basalt formation, the Skrænterne Formation. This is separated from the overlying lava flows of the Igtertivâ Formation by 7 m of sediments that represent a period of around six million years. The two formations can be distinguished by different trace element ratios. The Igtertivâ Formation comprises an at least 300 m thick main succession of flows dated to 49.09 ± 0.48 Ma, overlain by sediments of the Bopladsdalen Formation. A basal conglomerate in the sediments contains pebbles of alkaline igneous rocks of which three were dated at 49.17 ± 0.35 Ma, 47.60 ± 0.25 Ma, and 46.98 ± 0.24 Ma. The sediments are thus younger than 47 Ma. Above 30 m of sediments occur two Igtertivâ Formation lava flows dated to 43.77 ± 1.08 Ma. The overlying sediments of the Bopladsdalen and Krabbedalen Formations are therefore not older than about 44 Ma and palynological evidence shows that they are also not much younger than this. Use of the Geological Time Scale 2012 has resulted in good agreement between radiometric and palynological ages. The Igtertivâ Formation lava flows were fed from a regional coast-parallel dyke swarm indicating a new rifting episode at 49–44 Ma. This coincides with a major mid-Eocene plate reorganisation event in the North Atlantic and the start of northward-propagation of the Reykjanes Ridge through the continent. The Igtertivâ rift may have been directly instrumental for the initiation of this process.

Errata: 40Ar/39Ar ages of granitoids from the Truong Son fold belt and Kontum massif in Laos

Geochemical and geochronological studies were conducted on granitoids in central-southern Laos. S-type ilmenite-series granitoids associated with Sn mineralization are distributed in the Truong Son fold belt of central Laos. Plateau ages determined by 40Ar/39Ar incremental heating method for biotite and muscovite of the granitoids range from 253 Ma to 247 Ma and from 244 Ma to 199 Ma in the southwest and northeast of the fold belt, respectively. During this period, the fractionated granites in the early stage are related to mineralization of medium-large Sn deposits. The granitoids of the Truong Son fold belt formed in the Indosinian orogeny are contemporaneous with the Late Permian-Early Jurassic Sn-bearing granites in the Southeast Asian Sn Belt. In contrast, I-type magnetite-series granitoids are distributed in the Kontum massif of southeasternmost Laos. Plateau ages for biotite and previously reported mineral ages of the magnetite-series granitoids range widely from 414 Ma to 252 Ma. The Carboniferous-Permian granitoids represents adakitic characteristics and the Permian adakitic granodiorites are associated with Cu ± Au mineralized quartz veins.

40Ar/39Ar ages of granitoids from the Truong Son fold belt and Kontum massif in Laos

Geochemical and geochronological studies were conducted on granitoids in central-southern Laos. S-type ilmenite-series granitoids associated with Sn mineralization are distributed in the Truong Son fold belt of central Laos. Plateau ages determined by 40Ar/39Ar incremental heating method for biotite and muscovite of the granitoids range from 253 Ma to 247 Ma and from 244 Ma to 199 Ma in the southwest and northeast of the fold belt, respectively. During this period, the fractionated granites in the early stage are related to mineralization of medium-large Sn deposits. The granitoids of the Truong Son fold belt formed in the Indosinian orogeny are contemporaneous with the Late Permian-Early Jurassic Sn-bearing granites in the Southeast Asian Sn Belt. In contrast, I-type magnetite-series granitoids are distributed in the Kontum massif of southeasternmost Laos. Plateau ages for biotite and previously reported mineral ages of the magnetite-series granitoids range widely from 414 Ma to 252 Ma. The Carboniferous-Permian granitoids represents adakitic characteristics and the Permian adakitic granodiorites are associated with Cu ± Au mineralized quartz veins.

Constraints on the development of Proterozoic basins in central India from 40Ar/39Ar analysis of authigenic glauconitic minerals

Ages of some key stratigraphic sequences in central Indian Proterozoic basins are based predominantly on lithostratigraphic relationships that have been constrained by only a few radioisotopic dates. To help improve age constraints, single grains of glauconitic minerals taken from sandstone and limestone in two Proterozoic sequences in the Pranhita-Godavari Valley and the Chattisgarh basin were analyzed by the 40 Ar/ 39 Ar incremental heating method. Analysis of the age spectra distinguishes between ages that are interpreted to reflect the time of glauconite formation, and anomalous ages that result from inherited argon or postcrystallization heating. The analyses indicate an age of 1686 ± 6 Ma for the Pandikunta Limestone and 1566 ± 6 Ma for the Ramgundam Sandstone, two units in the western belt of Proterozoic sequences in Pranhita-Godavari Valley. Glauconite from the Chanda Limestone, in the upper part of this sequence, contains inherited 40 Ar but is interpreted to reflect an age of ca. 1200 Ma. Glauconite from the Somanpalli Group in the eastern belt of the Pranhita-Godavari Valley gives an age of 1620 ± 6 Ma. In the Chattisgarh basin, glauconite from two units gives disturbed ages that suggest a period of regional heating in the Chattisgarh basin at ca. 960–1000 Ma. These new ages indicate that these sequences are 200–400 m.y. older than previously recognized, which has important implications for geochemical studies of Mesoproterozoic ocean redox conditions in addition to providing important constraints on regional tectonics and lithostratigraphy.

Paleomagnetic field properties at high southern latitude

Statistical analyses of paleomagnetic data from lava flows are used to study geomagnetic field behavior on million year timescales. Previous paleomagnetic studies have lacked high‐latitude measurements necessary to investigate the persistence of geomagnetic anomalies observed in the recent and historical field and replicated in some numerical geodynamo simulations. These simulations suggest that reduced convective flow inside the tangent cylinder may affect the magnetic field at high latitude, whereas lower‐latitude observations are expressions of columnar/helical flow outside the tangent cylinder. This paper presents new paleointensity and paleodirectional data from 100 volcanic sites in the Erebus Volcanic Province (EVP), Antarctica, and 21 new age determinations by the 40Ar/39Ar incremental heating method. The new EVP data are combined with previously published paleomagnetic and geochronological results, providing 133 sites, 91 having radioisotopic dates. Modified Thellier‐Thellier paleointensity estimates are reported for 47 sites (37 have dates). Ages for the combined data set span 0.03 to 13.42 Ma. The 125 high‐quality EVP directional data selected from the merged data set have a non‐Fisherian distribution and a mean direction with an inclination anomaly of ∼3°, but 95% confidence limits include the prediction from a geocentric axial dipole. Virtual geomagnetic pole (VGP) dispersions for Brunhes, Matuyama, and the combined 0–5 Ma data set are consistently high compared with values from middle‐ to low‐latitude regions regardless of the criterion used to determine transitional fields. With VGP latitude cut off at 45°, the dispersion (23.9 ± 2.1°) for the combined 0–5 Ma EVP data set is consistent with earlier high‐latitude data and paleosecular variation (PSV) in Model G but not with some more recent statistical PSV models. Mean EVP paleointensity of 31.5 ± 2.4 μT, derived from 41 high‐quality sites, is about half the current value at McMurdo (∼63 μT). The result is essentially independent of data selection criteria. High VGP dispersion and low‐intensity values support the global observation of anticorrelation between directional variability and field strength. Simulations of time‐varying dipole strength show that uneven temporal sampling may bias the mean EVP intensity estimate, but the possibility of persistently anomalous field behavior at high latitude cannot be excluded.

Multiple Brunhes Chron excursions recorded in the West Eifel (Germany) volcanics: Support for long-held mantle control over the non-axial dipole field

Volcanic records of excursional geomagnetic field behavior, in particular paleointensity estimates, are fragmentary for the Pleistocene. The West Eifel volcanic field is unique in that 12 of 66 measured lava flow sites record Virtual Geomagnetic Poles (VGPs) clustered between 34–45°N latitude and 30–50°E longitude over the Middle East and Southeast Asia. Paleointensity determinations reveal that 13 of 37 lavas erupted during times when the ambient field was weak. Specifically, results on nine transitionally magnetized and four normally magnetized lavas indicate paleofield intensities less than 30 μT associated with virtual dipole moments (VDMs) <4 × 10 22 Am 2 [Schnepp, E., Hradetzky, H., 1994. Combined paleointensity and 40Ar/ 39Ar age spectrum data from volcanic rocks of the West Eifel field (Germany): evidence for an early Brunhes geomagnetic excursion. J. Geophys. Res 99, 9061–9076]. Until now, the age of these lava flows has been known only from imprecise 40Ar/ 39Ar dating that implied acquisition of magnetic remanence between about 570 and 450 ka. Thus, they have been interpreted to record a single, but poorly defined, excursion in the lower Brunhes Chron. To incorporate these paleofield data into the global high-resolution Geomagnetic Instability Time Scale (GITS), we have determined precise ages of groundmass from 12 excursional and 7 normally magnetized lava flows using the 40Ar/ 39Ar incremental heating method. Unspiked K–Ar age determinations from two excursional samples are indistinguishable from their 40Ar/ 39Ar results. Ages of the excursional lava flows fall into five groups at 722 ± 38, 626 ± 24, 578 ± 8, 555 ± 4, and 528 ± 16 ka (±2 σ analytical uncertainties, relative to 1.194 Ma Alder Creek sanidine). The group of three excursional flows with an age of 578 ± 8 ka correspond temporally with lava flow sequences on Tahiti and La Palma that record the Big Lost excursion at 579 ± 6 ka. Eight other excursional lavas erupted from 722 to 528 ka correlate with less well-defined excursions or paleointensity minima differentially expressed in marine sediment records. Our findings that at least four, and perhaps five, temporally distinct excursions are recorded between 722 and 528 ka further suggest that each weakening of the dynamo during this period was associated with demise of the axial dipole. The VGPs produced during these excursions are found to be dispersed longitudinally across Eurasia and are remarkably similar to those associated with the calculated south VGPs at West Eifel for the non-axial dipole (NAD) field during the 20th century. A simple model in which the axial dipole repeatedly collapses leaving behind a residual NAD field that is largely held by stationary sources, probably in the lowermost mantle, provides an explanation for the recurrent excursional behavior during this ∼200 kyr period.

Cretaceous alkaline intra-plate magmatism in the Ecuadorian Oriente Basin: Geochemical, geochronological and tectonic evidence

Small volumes of Cretaceous alkaline basaltic magmas have been identified in the sedimentary infill of the Ecuadorian Oriente foreland basin. They are characterized by a restricted range of compositional variation, low LILE/HFSE ratios and Sr–Nd isotope values within the range of oceanic island basalts (OIB). Reflection seismic data show that a pre-existing NNE–SSW Triassic and Jurassic rift controls the location and occurrence of these alkaline eruptive sites. Radiometric ages ( 40Ar– 39Ar, incremental heating method) and the biostratigraphic record of their surrounding sediments indicate a NNE–SSW systematic age variation for the emplacement of this alkaline volcanism: from Albian (110 ± 5.2 Ma) in the northern part of the Oriente Basin, to Campanian (82.2 ± 2.0 Ma) in the west-central part. The geochemical, geochronological and tectonic evidences suggest that asthenospheric mantle has upwelled and migrated to the SSW, into the region underlying the pre-existing Triassic and Jurassic rift (thin-spot?). We propose that subduction was abandoned, subsequent to the accretion of allochthonous terranes onto the Ecuadorian and Colombian margin in the latest Jurassic–earliest Cretaceous, causing the relict slab material, corresponding to the eastwards-directed leading plate, to roll-back. Unmodified asthenospheric mantle migrated into the region previously occupied by the slab. This resulted in partial melting and the release of magmatic material to the surface in the northern part of the Oriente Basin since at least Aptian times. Then, magmatism migrated along the SSW-trending Central Wrench Corridor of the Oriente Basin during the Upper Cretaceous, probably as a consequence of the lateral propagation of the transpressive inversion of the Triassic–Jurassic rift. Eventually, the Late Cretaceous east-dipping Andean subduction system was renewed farther west, and the development of the compressional retro-foreland Oriente Basin system halted the Cretaceous alkaline magmatic activity.

Radiometric ages for basement rocks from the Emperor Seamounts, ODP Leg 197

The Hawaiian‐Emperor seamount chain is the “type” example of an age‐progressive, hot spot‐generated intraplate volcanic lineament. However, our current knowledge of the age distribution within this province is based largely on radiometric ages determined several decades ago. Improvements in instrumentation, sample preparation methods, and new material obtained by recent drilling warrant a reexamination of the age relations among the older Hawaiian volcanoes. We report new age determinations (40Ar‐39Ar incremental heating method) on whole rocks and feldspar separates from Detroit (Sites 1203 and 1204), Nintoku (Site 1205), and Koko (Site 1206) Seamounts (Ocean Drilling Program (ODP) Leg 197) and Meiji Seamount (Deep Sea Drilling Project (DSDP) Leg 19, Site 192). Plateaus in incremental heating age spectra for Site 1203 lava flows give a mean age of 75.8 ± 0.6 (2σ) Ma, which is consistent with the normal magnetic polarity directions observed and biostratigraphic age assignments. Site 1204 lavas produced discordant spectra, indicating Ar loss by reheating and K mobilization. Six plateau ages from lava flows at Site 1205 give a mean age of 55.6 ± 0.2 Ma, corresponding to Chron 24r. Drilling at Site 1206 intersected a N‐R‐N magnetic polarity sequence of lava flows, from which six plateau ages give a mean age of 49.1 ± 0.2 Ma, corresponding to the Chron 21n‐22r‐22n sequence. Plateau ages from two feldspar separates and one lava from DSDP Site 192 range from 34 to 41 Ma, significantly younger than the Cretaceous age of overlying sediments, which we relate to postcrystallization K mobilization. Combined with new dating results from Suiko Seamount (DSDP Site 433) and volcanoes near the prominent bend in the lineament [Sharp and Clague, 2002], the overall trend is increasing volcano age from south to north along the Emperor Seamounts, consistent with the hot spot model. However, there appear to be important departures from the earlier modeled simple linear age progression, which we relate to changes in Pacific plate motion and the rate of southward motion of the Hawaiian hot spot.

A Complex History for the Caribbean Oceanic Plateau: Petrology, Geochemistry and Geochronology of the Beata Ridge

The Caribbean province is an area where the thickness of oceanic crust is between 8 and more than 20 kin, well in excess that of 'normal' oceanic crust. This has led to the conclusion, supported by geochemical studies and tectonic reconstructions, that much of the Caribbean plate formed as an oceanic plateau in the eastern Pacific. Most authors accept that the Caribbean plateau formed through melting of the Galapagos plume head. Through eastward movement of the Farallon plate, part of this plateau was entrained between the two American plates while the southern part collided with northern South America. At least two major phases have been recognized in the plateau building. The first, volumetrically most important event was around 90-88 Ma, and the second around 76 Ma. New petrological, geochemical and geochronological data from the Beata topographic ridge, south of Hispaniola, highlight the complexi ty of the Caribbean plateau construction and may require a revision of this model. The Beata ridge is an area of particularly thick crust (up to 20 km) between the Dominican and Haitian sub-basins. The ridge is 2000 to 5000 m deep and is bounded to the west by a normal fault. In this study we focus on the magmatic samples recovered by submersible during the Nautica-Beata cruise. The ridge is mainly composed by gabbros and dolerites, with minor diorite and rare pillow basalt. The gabbros and dolerites contain euhedral plagioclase (An54_62) and clinopyroxene (Wo37_4o En44_47 Fs13 19) grains with a variable but minor amount of olivine and/or orthopyroxene relicts, ilmenite and acicular apatite. The presence of orthopyroxene and the early crystallisation of plagioclase indicate a tholeiitic affinity. Textures that vary significantly, from coarse-grained gabbros to very fine-grained dolerites, reflect differences in cooling rates and suggest a subsurface, hypabyssal environment. The rare basaltic flows contain phenocrysts of weathered olivine, plagioclase (Anso-61) and clinopyroxene (Wo44-47 En40-44 Fs12-16) in a matrix of plagioclase microlites (An5o-55), clinopyroxene microcrysts (Wo45-46 En37-4o Fs15-16) and a glassy, vesicular mesostasis. The early crystallisation of olivine, an absence of orthopyroxene and the late crystallisation of plagioclase suggest an alkaline affinity. The major element compositions of gabbros and dolerites plot on simple trends that correspond to fractional crystallisation of olivine, clinopyroxene and plagioclase (MgO 5-11%; A1203 12-17%; CaO 8-15%). Trace-element ratios are close to chondritic [(Nb/Zr)N 0.85-1.1 ] and rare-earth-element patterns are almost flat [(La/Yb)N 0.63-1.02]. The source was isotopically depleted [~Na +7.4 to +9.5]. The basalts have higher trace-element ratios and enriched rare earth element patterns [(Nb/Zr)N 3.45; (La/Yb)N 28-30]. Their source was less depleted than that of the gabbros and dolerites [eNa +5]. Several samples were dated by the 39Ar/4~ incremental heating method, either on whole rocks or separated plagioclases. Six samples have ages between 84 to 70 Ma and these correspond with previous dates within the province. But others are surprisingly young, with ages at 62, 56 and 50 Ma. There is no apparent order to the ages: all are found in different rock types and from one end of the ridge to the other. Within the younger age group, consistent results were obtained for both whole rocks and plagioclases. In contrast, within the older age group, whole-rocks and plagioclase gave conflicting ages. For this reason, we consider that the young ages are not due to thermal resetting but probably represent a true magmatic event. In contrast, the ages for older

A short interval of Jurassic continental flood basalt volcanism in Antarctica as demonstrated by [formula omitted] geochronology

A continental flood basalt province, the Ferrar Group (Kirkpatrick Basalt and Ferrar Dolerite), crops out along 3000 km of the Transantarctic Mountains in Antarctica and is temporally related to the break-up of Gondwanaland. Although a wide range of dates, between 90 and 193 Ma, have been published for the Kirkpatrick Basalt, it is now recognized that the young dates reflect non-ideal behavior of Ar in the matrix. In order to refine the geochronology, feldspar separates have been analyzed by the 40 Ar 39 Ar incremental heating method. The main objectives are to constrain the duration of extrusive activity and the timing of volcanism along the outcrop belt. Basalt samples have been studied from the three principal outcrop areas, yielding the following apparent ages: central Transantarctic Mountains 176.8 ± 0.5 Ma; south Victoria Land 176.4 ± 0.4 Ma; north Victoria Land 176.6 ± 0.7 Ma. Ages from different stratigraphic levels within each area and from the three different areas are not analytically distinct. The data imply that the eruptive activity which produced the Kirkpatrick Basalt occurred within a short interval of less than about 1 m.y. at 176.6 ± 1.8 Ma, over an area which included more than 1200 km of the Transantarctic Mountains. The Jurassic volcanism in Antarctica represents a short episode of magmatism, comparable in duration with other well dated continental flood basalt provinces. The linearly extensive outcrop of the Ferrar Province and the rapid eruption of the lavas suggests that lithospheric stretching exerted a major control on magmatism. The poorly constrained age of the Bajocian-Bathonian boundary makes the previously suggested connection between Ferrar volcanism and an extinction event at that boundary uncertain.

40Ar/ 39Ar incremental heating plateaus for biotites with excess argon

The 40Ar/ 39Ar incremental-heating dating method has been applied to metamorphic biotite containing excess 40Ar. Samples from two localities, the Appalachian Piedmont of northern Delaware and the Rocky Mountains of southwestern Montana, U.S.A., which were previously shown to yield anomalous old K-Ar dates with no chronological significance, have been studied. The 40Ar/ 39Ar release spectra show generally well-defined plateaus with apparent ages within analytical uncertainty of those previously determined by conventional K-Ar. Similar results for biotite from other metamorphic localities have been reported. Thus, excess 40Ar-bearing biotite from regional metamorphic rocks appears to yield generally plateau-type incremental gas release patterns. Biotite with excess argon can show the same release for the radiogenic and incorporated 40Ar components which results from a uniform distribution of 40Ar relative to K or possibly from the method of argon release during vacuum heating. Biotite cooling ages and anomalously old dates may be indistinguishable by the incremental-heating method. For biotites, particularly those from metamorphic or slowly cooled rocks, well-defined 40Ar/ 39Ar plateaus cannot be unambiguously interpreted without a priori geochronological or geological information.