65 research outputs found
Mechanisms of Ar release from Himalayan metamorphic hornblende
Changes in hornblende samples that occur during stepwise 39Ar/40Ar analysis were studied
experimentally and mineralogically. A complex succession of reactions was seen (hornblende
~ oxyhornblende ~ clinopyroxene-structured phase ~ fine-grained reaction products
~ glasses) in the temperature range 750-1300 °C. The release of Ar from hornblende
in the vacuum furnace appeared to occur by fundamentally different processes from those
resulting in Ar loss during metamorphism. Simple diffusional interpretations of the release patterns are, therefore, not capable of revealing the thermal history of samples. In principle, some useful information may be obtained if long-standing fractures andother defects control both natural Ar loss and mineral reactions within the laboratory
Diachroneity of the Clearwater west and Clearwater east impact structures supported by (U-Th)/He dating
Dendritic reidite from the Chesapeake Bay impact horizon, Ocean Drilling Program Site 1073 (offshore northeastern USA): A fingerprint of distal ejecta?
High-pressure minerals provide records of processes not normally preserved in Earthâs crust. Reidite, a quenchable polymorph of zircon, forms at pressures >20 GPa during shock compression. However, there is no broad consensus among empirical, experimental, and theoretical studies on the nature of the polymorphic transformation. Here we decipher a multistage history of reidite growth recorded in a zircon grain in distal impact ejecta (offshore northeastern United States) from the ca. 35 Ma Chesapeake Bay impact event which, remarkably, experienced near-complete conversion (89%) to reidite. The grain displays two distinctive reidite habits: (1) intersecting sets of planar lamellae that are dark in cathodoluminescence (CL); and (2) dendritic epitaxial overgrowths on the lamellae that are luminescent in CL. While the former is similar to that described in literature, the latter has not been previously reported. A two-stage growth model is proposed for reidite formation at >40 GPa in Chesapeake Bay impact ejecta: formation of lamellar reidite by shearing during shock compression, followed by dendrite growth, also at high pressure, via recrystallization. The dendritic reidite is interpreted to nucleate on lamellae and replace damaged zircon adjacent to lamellae, which may be amorphous ZrSiO4 or possibly an intermediate phase, all before quenching. These results provide new insights on the microstructural evolution of the highpressure polymorphic transformation over the microseconds-long interval of reidite stability during meteorite impact. Given the formation conditions, dendritic reidite may be a unique indicator of distal ejecta
Constraints on the tectonic and landscape evolution of the Bhutan Himalaya from thermochronometry
The observed geomorphology and calculated thermal histories of the Bhutan Himalaya provide
an excellent platform to test ideas regarding the influence of tectonics and climate on the evolution of a
convergentmountain range. However, little consensus has been reached regarding the late Cenozoic history of
the Bhutan Himalaya. Some researchers have argued that observed geologic relationships show slowing
deformation rates, such that the range is decaying from a geomorphic perspective, while others see the range
as growing and steepening. We suggest that a better understanding is possible through the integrated
interpretation of geomorphic and thermochronometric data from the comparison of predictions from models
of landscape evolution and thermal-kinematic models of orogenic systems. New thermochronometric data
throughout Bhutan aremost consistent with a significant decrease in erosion rates, from2 to 3 km/Ma down to
0.1â0.3 km/Ma, around 6â4Ma. We interpret this pattern as a decrease in rock uplift rates due to the activation
of contractional structures of the Shillong Plateau, an uplifted region approximately 100 km south of Bhutan.
However, low-relief, fluvial landscapes throughout the Bhutanese hinterland record a late pulse of surface uplift
likely due to a recent increase in rock uplift rates. Constraints from our youngest thermochronometers suggest
that this increase in rock uplift and surface uplift occurred within the last 1.75Ma. These results imply that
the dynamics of the Bhutan Himalaya and Shillong Plateau have been linked during the late Cenozoic, with
structural elements of both regions active in variable ways and times over that interval
Refining lunar impact chronology through high spatial resolution 40Ar/39Ar dating of impact melts
Quantitative constraints on the ages of melt-forming impact events on the Moon are based primarily on isotope
geochronology of returned samples. However, interpreting the results of such studies can often be difficult
because the provenance region of any sample returned from the lunar surface may have experienced
multiple impact events over the course of billions of years of bombardment. We illustrate this problem with
new laser microprobe 40Ar/39Ar data for two Apollo 17 impact melt breccias. Whereas one sample yields a
straightforward result, indicating a single melt-forming event at ca. 3.83 Ga, data from the other sample document
multiple impact meltâforming events between ca. 3.81 Ga and at least as young as ca. 3.27 Ga. Notably,
published zircon U/Pb data indicate the existence of even older melt products in the same sample. The revelation
of multiple impact events through 40Ar/39Ar geochronology is likely not to have been possible using
standard incremental heating methods alone, demonstrating the complementarity of the laser microprobe
technique. Evidence for 3.83 Ga to 3.81 Ga melt components in these samples reinforces emerging interpretations
that Apollo 17 impact breccia samples include a significant component of ejecta from the Imbrium basin
impact. Collectively, our results underscore the need to quantitatively resolve the ages of different melt generations
from multiple samples to improve our current understanding of the lunar impact record, and to establish
the absolute ages of important impact structures encountered during future exploration missions in the
inner Solar System
New Age and Geochemical Data from the Southern Colville and Kermadec Ridges, SW Pacific: Insights into the recent geological history and petrogenesis of the Proto-Kermadec (Vitiaz) Arc
Highlights
âą Age and petrogenesis of the Miocene-Pleistocene proto Kermadec arc: the Kermadec and Colville Ridge
âą Complex interplay between element flux from the subducting Pacific Plate and heterogenous mantle wedge
âą New insights into the recent tectonic history of the Kermadec arc system
Abstract
The intra-oceanic Kermadec arc system extends ~1300âŻkm between New Zealand and Fiji and comprises at least 30 arc front volcanoes, the Havre Trough back-arc and the remnant Colville and Kermadec Ridges. To date, most research has focussed on the Kermadec arc front volcanoes leaving the Colville and Kermadec Ridges virtually unexplored. Here, we present seven 40Ar/39Ar ages together with a comprehensive major and trace element and Sr-, Nd-, and Pb-isotope dataset from the Colville and Kermadec Ridges to better understand the evolution, petrogenesis and splitting of the former proto-Kermadec (Vitiaz) Arc to form these two remnant arc ridges. Our 40Ar/39Ar ages range from ~7.5â2.6âŻMa, which suggests that arc volcanism at the Colville Ridge occurred continuously and longer than previously thought. Recovered Colville and Kermadec Ridge lavas range from mafic picro-basalts (MgOâŻ=âŻ~8âŻwt%) to dacites. The lavas have arc-type normalised incompatible element patterns and Sr and Pb isotopic compositions intermediate between Pacific MORB and subducted lithosphere (including sediments, altered oceanic crust and serpentinised uppermost mantle). Geochemically diverse lavas, including ocean island basalt-like and potassic lavas with high Ce/Yb, Th/Zr, intermediate 206Pb/204Pb and low 143Nd/144Nd ratios were recovered from the Oligocene South Fiji Basin (and Eocene Three Kings Ridge) located west of the Colville Ridge. If largely trench-perpendicular mantle flow was operating during the Miocene, this geochemical heterogeneity was likely preserved in the Colville and Kermadec sub arc mantle. The Colville and Kermadec Ridge data therefore highlight the complex interplay between pre-existing mantle heterogeneities and material fluxes from the subducting Pacific Plate. The new data allow us to present a holistic (yet simplified) picture of the tectonic evolution of the late Vitiaz Arc and northern Zealandia since the Miocene and how this tectonism influences volcanic activity along the Kermadec arc at the present
The (U-Th)/He isotope system applied to the dating of distal ejecta from the Chesapeake Bay impact structure
Argon behaviour in an inverted Barrovian sequence, Sikkim Himalaya: the consequences of temperature and timescale on <sup>40</sup>Ar/<sup>39</sup>Ar mica geochronology
40Ar/39Ar dating of metamorphic rocks sometimes yields complicated datasets which are difficult to interpret in terms of timescales of the metamorphic cycle. Single-grain fusion and step-heating data were obtained for rocks sampled through a major thrust-sense shear zone (the Main Central Thrust) and the associated inverted metamorphic zone in the Sikkim region of the eastern Himalaya. This transect provides a natural laboratory to explore factors influencing apparent 40Ar/39Ar ages in similar lithologies at a variety of metamorphic pressure and temperature (PâT) conditions.
The 40Ar/39Ar dataset records progressively younger apparent age populations and a decrease in within-sample dispersion with increasing temperature through the sequence. The white mica populations span ~ 2â9 Ma within each sample in the structurally lower levels (garnet grade) but only ~ 0â3 Ma at structurally higher levels (kyanite-sillimanite grade). Mean white mica single-grain fusion population ages vary from 16.2 ± 3.9 Ma (2Ï) to 13.2 ± 1.3 Ma (2Ï) from lowest to highest levels. White mica step-heating data from the same samples yields plateau ages from 14.27 ± 0.13 Ma to 12.96 ± 0.05 Ma. Biotite yield older apparent age populations with mean single-grain fusion dates varying from 74.7 ± 11.8 Ma (2Ï) at the lowest structural levels to 18.6 ± 4.7 Ma (2Ï) at the highest structural levels; the step-heating plateaux are commonly disturbed.
Temperatures > 600 °C at pressures of 0.4â0.8 GPa sustained over > 5 Ma, appear to be required for white mica and biotite ages to be consistent with diffusive, open-system cooling. At lower temperatures, and/or over shorter metamorphic timescales, more 40Ar is retained than results from simple diffusion models suggest. Diffusion modelling of Ar in white mica from the highest structural levels suggests that the high-temperature rocks cooled at a rate of ~ 50â80 °C Maâ 1, consistent with rapid thrusting, extrusion and exhumation along the Main Central Thrust during the mid-Miocene
(U-Th)/He zircon dating of Chesapeake Bay distal impact ejecta from ODP site 1073
Single crystal (UâTh)/He dating has been undertaken on 21 detrital zircon grains extracted from a core sample from Ocean Drilling Project (ODP) site 1073, which is located ~390 km northeast of the center of the Chesapeake Bay impact structure. Optical and electron imaging in combination with energy dispersive Xâray microanalysis (EDS) of zircon grains from this late Eocene sediment shows clear evidence of shock metamorphism in some zircon grains, which suggests that these shocked zircon crystals are distal ejecta from the formation of the ~40 km diameter Chesapeake Bay impact structure. (UâTh/He) dates for zircon crystals from this sediment range from 33.49 ± 0.94 to 305.1 ± 8.6 Ma (2Ï), implying crystalâtoâcrystal variability in the degree of impactârelated resetting of (UâTh)/He systematics and a range of different possible sources. The two youngest zircon grains yield an inverseâvariance weighted mean (UâTh)/He age of 33.99 ± 0.71 Ma (2Ï uncertainties n = 2; mean square weighted deviation = 2.6; probability [P] = 11%), which is interpreted to be the (UâTh)/He age of formation of the Chesapeake Bay impact structure. This age is in agreement with K/Ar, 40Ar/39Ar, and fission track dates for tektites from the North American strewn field, which have been interpreted as associated with the Chesapeake Bay impact event
Long-term evolution of an Oligocene/Miocene maar lake from Otago, New Zealand
Foulden Maar is a highly resolved maar lake deposit from the South Island of New Zealand comprising laminated diatomite punctuated by numerous diatomaceous turbidites. Basaltic clasts found in debris flow deposits near the base of the cored sedimentary sequence yielded two new âŽâ°Ar/ÂłâčAr dates of 24.51â±â0.24 and 23.38â±â0.24 Ma (2Ï). The younger date agrees within error with a previously published âŽâ°Ar/ÂłâčAr date of 23.17â±â0.19 Ma from a basaltic dyke adjacent to the maar crater. The diatomite is inferred to have been deposited over several tens of thousands of years in the latest Oligocene/earliest Miocene, and may have been coeval with the period of rapid glaciation and subsequent deglaciation of Antarctica known as the Mi-1 event. Sediment magnetic properties and SEM measurements indicate that the magnetic signal is dominated by pseudo-single domain pyrrhotite. The most likely source of detrital pyrrhotite is schist country rock fragments from the inferred tephra ring created by the phreatomagmatic eruption that formed the maar. Variations in magnetic mineral concentration indicate a decrease in erosional input throughout the depositional period, suggesting long-term (tens of thousands of years) environmental change in New Zealand in the latest Oligocene/earliest Miocene
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