Eocene animal trace fossils in 1.7-billion-year-old metaquartzites

2021 ◽  
Vol 118 (40) ◽  
pp. e2105707118
Author(s):  
Stefan Bengtson ◽  
Birger Rasmussen ◽  
Jian-Wei Zi ◽  
Ian R. Fletcher ◽  
James G. Gehling ◽  
...  
Keyword(s):  
Host Rock ◽  
Pore Space ◽  
Trace Fossils ◽  
Metamorphic Event ◽  
Early Age ◽  
Metasedimentary Rocks ◽  
Southwestern Australia ◽  
Quartz Cement

The Paleoproterozoic (1.7 Ga [billion years ago]) metasedimentary rocks of the Mount Barren Group in southwestern Australia contain burrows indistinguishable from ichnogenera Thalassinoides, Ophiomorpha, Teichichnus, and Taenidium, known from firmgrounds and softgrounds. The metamorphic fabric in the host rock is largely retained, and because the most resilient rocks in the sequence, the metaquartzites, are too hard for animal burrowing, the trace fossils have been interpreted as predating the last metamorphic event in the region. Since this event is dated at 1.2 Ga, this would bestow advanced animals an anomalously early age. We have studied the field relationships, petrographic fabric, and geochronology of the rocks and demonstrate that the burrowing took place during an Eocene transgression over a weathered regolith. At this time, the metaquartzites of the inundated surface had been weathered to friable sandstones or loose sands (arenized), allowing for animal burrowing. Subsequent to this event, there was a resilicification of the quartzites, filling the pore space with syntaxial quartz cement forming silcretes. Where the sand grains had not been dislocated during weathering, the metamorphic fabric was seemingly restored, and the rocks again assumed the appearance of hard metaquartzites impenetrable to animal burrowing.

Alteration of magmatic monazite in granitoids from the Ryoke belt (SW Japan): Processes and consequences

2020 ◽  
Vol 105 (4) ◽  
pp. 538-554
Author(s):  
Etienne Skrzypek ◽  
Shuhei Sakata ◽  
Dominik Sorger
Keyword(s):  
Secondary Recrystallization ◽  
Weighted Average ◽  
Aqueous Fluid ◽  
Biotite Granite ◽  
Metamorphic Event ◽  
Metasedimentary Rocks ◽  
Migmatitic Gneiss ◽  
Granite Samples ◽  
Greenschist Facies Metamorphism ◽  
Sw Japan

Abstract The alteration of magmatic monazite and its consequences for monazite geochronology are explored in granitoids from the western part of the Ryoke belt (Iwakuni-Yanai area, SW Japan). Biotite-granite samples were collected in two plutons emplaced slightly before the main tectono-metamorphic event: the first one, a massive granite (Shimokuhara) adjoins schistose rocks affected by greenschist facies metamorphism; and the second, a gneissose granite (Namera) adjoins migmatitic gneiss that experienced upper-amphibolite facies conditions. Despite contrasting textures, the granite samples have similar mineral modes and compositions. Monazite in the massive granite is dominated by primary domains with limited secondary recrystallization along cracks and veinlets. It is variably replaced by allanite+apatite±xenotime±Th-U-rich phases. The outermost rims of primary domains yield a weighted average 206Pb/238U date of 102 ± 2 Ma while the Th-U phases show Th-U-Pb dates of 58 ± 5 and 15 to 14 ± 2–3 Ma. Monazite in the gneissose granite preserves sector- or oscillatory-zoned primary domains cross-cut by secondary domains enriched in Ca, Y, U, P, and containing numerous inclusions. The secondary domains preserve concordant 206Pb/238U dates spreading from 102 ± 3 to 91 ± 2 Ma while primary domain analyses are commonly discordant and range from 116 to 101 Ma. Monazite alteration textures in the two granites chiefly reflect differences in their post-magmatic histories. In the massive granite, monazite replacement occurred via a nearly stoichiometrically balanced reaction reflecting interaction with an aqueous fluid enriched in Ca+Al+Si±F during hydrothermal alteration of the granitic assemblage, likely below 500 °C. In the gneissose granite, a small amount of anatectic melt, probably derived from the neighboring metasedimentary rocks, was responsible for pseudomorphic recrystallization of monazite by dissolution-reprecipitation above 600 °C. Regardless of whether monazite underwent replacement or recrystallization, primary monazite domains preserve the age of magmatic crystallization for both plutons (102 ± 2 and 106 ± 5 Ma). Conversely, the age of monazite alteration is not easily resolved. Monazite replacement in the massive granite might be constrained using the Th-U-rich alteration products; with due caution and despite probable radiogenic Pb loss, the oldest date of 58 ± 5 Ma could be ascribed to chloritization during final exhumation of the granite. The spread in apparently concordant 206Pb/238U dates for secondary domains in the gneissose granite is attributed to incomplete isotopic resetting during dissolution-reprecipitation, and the youngest date of 91 ± 2 Ma is considered as the age of monazite recrystallization during a suprasolidus metamorphic event. These results reveal a diachronous, ca. 10 Ma-long high-temperature (HT) history and an overall duration of about 15 Ma for the metamorphic evolution of the western part of the Ryoke belt.

Resistance of zircons to U–Pb resetting in a prograde metamorphic sequence of Caledonian age in East Greenland

1985 ◽  
Vol 22 (3) ◽  
pp. 330-338 ◽  
Author(s):  
J. J. Peucat ◽  
D. Tisserant ◽  
R. Caby ◽  
N. Clauer
Keyword(s):  
Source Area ◽  
Detrital Zircons ◽  
Metamorphic Event ◽  
Great Resistance ◽  
East Greenland ◽  
Geological Interpretation ◽  
Source Areas ◽  
Metasedimentary Rocks ◽  
Sedimentary Source

In the Alpefjord area, Caledonian metamorphism from the chlorite zone to the sillimanite zone is seen to cut across the sedimentary pile of the lower Eleonore Bay Group. Zircons have been collected from quartzite layers enriched in heavy minerals.U–Pb zircon dating in the chlorite and the sillimanite zones does not reveal the Caledonian event but, instead, previous episodes at 1100 and 2500 Ma ago. The Caledonian event can be recognized in anatectic gneisses where detrital zircons are surrounded by overgrowths, K–Ar and Rb–Sr methods yield 1030–410 Ma ages on micas, with a positive correlation between the degree of apparent reselling of mica ages and the grade of the Caledonian metamorphism.The following geological interpretation of the age data is proposed. (1) A major metamorphic event occurred around 1100 Ma ago in the source area for the lower Eleonore Bay Group sediments. During this Grenvillian event, Archaean detrital zircons were affected by an episodic lead loss and a muscovite phase recorded the cooling and uplift of a basement source area. (2) Erosion of this source area occurred after 1100 Ma, followed by sedimentation of the lower Eleonore Bay Group [Formula: see text]. The 2500–1100 Ma U–Pb system remained nearly a closed system during Caledonian metamorphism up to and including sillimanite-zone conditions.This example shows the great resistance or inherited zircons to an important secondary Pb loss during Caledonian metamorphism and consequently shows that the lower-intercept ages of zircons from metasedimentary rocks do not always record the last metamorphic event observed in situ, but retain memories of previous geological events in the sedimentary source areas. By contrast, zircons separated from quartzitic xenoliths in migmatitic gneisses have recorded a disturbance in their U–Pb systems that corresponds to Caledonian partial melting.

scholarly journals Albite-chlorite and talc-chlorite deposits in metasedimantary and granitoid rocks of central Sardinia (Italy)

2002 ◽  
Vol 50 ◽  
Author(s):  
MADDALENA FIORI ◽  
SILVANA MARIA GRILLO
Keyword(s):  
Metamorphic Event ◽  
Metasedimentary Rocks ◽  
Granitoid Rocks ◽  
Central Eastern

The albite and talc-chlorite deposits occurring in central-eastern Sardinia are associated with metasedimentary rocks with intrusions of Hercynian granitoids which contain abundant roof pendants of Palaeozoic metasedimentary rocks. The talc-chlorite bodies are hosted by the roof pendants of mica schists and variably recrystallised metalimestones and metadolostones, whereas deposits of albite generally occur in the granitoids, and to a lesser extent, in the roof pendants represented by mica schists. The talc- and Mg-chlorite deposits are considered to have been formed during the metamorphic event coeval with the emplacement of the granitoid rocks, whereas the albitic bodies were formed during later, post-magmatic, metasomatic processes.

scholarly journals Biologically–induced elemental variations in Antarctic sandstones: a potential test for Martian micro-organisms

2004 ◽  
Vol 3 (2) ◽  
pp. 97-106 ◽  
Author(s):  
Rebecca L. Blackhurst ◽  
Kym Jarvis ◽  
Monica M. Grady
Keyword(s):  
Host Rock ◽  
Inductively Coupled Plasma ◽  
Pore Space ◽  
Atomic Emission ◽  
Life Forms ◽  
Dry Valleys ◽  
Inductively Coupled ◽  
Icp Ms ◽  
Trace Element Study ◽  
Micro Organisms

Lichen-dominated cryptoendolithic communities from the Dry Valleys of Antarctica have been the subject of much research over recent years owing to their potential as analogues of Martian life forms. Their ability to mobilize iron compounds and organize themselves into distinct coloured biotic zones suggests that they may alter the chemistry of their host rock. By conducting a major, minor and trace element study utilizing inductively coupled plasma atomic emission spectroscopy (ICP-AES) and mass spectrometry (ICP-MS) techniques, we have been investigating the relationship between the microbes and the chemistry of the sandstones. Different layers within a suite of sandstones collected from six localities in the Dry Valleys have been analysed to establish if or how the microbes influence or directly affect the chemical composition of the rocks. Background petrographic studies have shown significant differences in mineralogical maturity between rocks colonized by the communities and those that are not, and the chemistry results have shown significant elemental disparity between colonized and uncolonized rocks. By obtaining accurate percentages of the minerals present in each sample the differences in elemental concentrations could be construed to be caused by the differences in mineralogy between samples. The nature and extent of the concentration differences has led to the conclusion that either the cryptoendolith communities are able to alter their host rock by the solubilization and mobilization of elements that are then subsequently removed or that the organisms are simple opportunists that can only successfully colonize rocks that provide the ideal substrate, being mineralogically mature with ample pore space and less concentrated in the elements tested for.

scholarly journals Rock Acoustics of Diagenesis and Cementation

2021 ◽  
Author(s):  
José M. Carcione ◽  
Davide Gei ◽  
Stefano Picotti ◽  
Ayman Qadrouh ◽  
Mamdoh Alajmi ◽  
...  
Keyword(s):  
Pore Space ◽  
Simple Procedure ◽  
Pressure Effects ◽  
Temperature History ◽  
Burial Depth ◽  
Order Kinetic ◽  
Seismic Velocities ◽  
Quartz Cement ◽  
Temperature And Pressure ◽  
Constant Heating

We simulate the effects of diagenesis, cementation and compaction on the elastic properties of shales and sandstones with four different petro-elastical theories and a basin-evolution model, based on constant heating and sedimentation rates. We consider shales composed of clay minerals, mainly smectite and illite, depending on the burial depth, and the pore space is assumed to be saturated with water at hydrostatic conditions. Diagenesis in shale (smectite/illite transformation here) as a function of depth is described by a 5th-order kinetic equation, based on an Arrhenius reaction rate. On the other hand, quartz cementation in sandstones is based on a model that estimates the volume of precipitated quartz cement and the resulting porosity loss from the temperature history, using an equation relating the precipitation rate to temperature. Effective pressure effects (additional compaction) are accounted for by using Athy equation and the Hertz-Mindlin model. The petro-elastic models yield similar seismic velocities, despite the different level of complexity and physics approaches, with increasing density and seismic velocities as a function of depth. The methodology provides a simple procedure to obtain the velocity of shales and sandstones versus temperature and pressure due to the diagenesis-cementation-compaction process.

Ultrastructure and geochemistry of endolithic microorganisms in limestone of the Niagara Escarpment

1997 ◽  
Vol 43 (3) ◽  
pp. 211-219 ◽  
Author(s):  
F. G. Ferris ◽  
E. A. Lowson
Keyword(s):  
Electron Microscopy ◽  
Host Rock ◽  
Heterotrophic Bacteria ◽  
Dissolved Inorganic Carbon ◽  
Organic Phosphorus ◽  
Pore Space ◽  
Fluorescent Microscopy ◽  
Polymeric Materials ◽  
Dry Weight ◽  
Niagara Escarpment

Endolithic microbial communities dominated by photosynthetic cyanobacteria occur at a depth of 0.5 – 3.0 mm inside dolomitic limestone that forms the vertical cliffs of the Niagara Escarpment in Ontario, Canada. Coccoid Gloeocapsa was the most prominent cyanobacterial species in colonized rock samples examined by differential interference contrast and episcopic fluorescent microscopy. Filamentous varieties of cyanobacteria were less common than coccoid forms but could be easily distinguished in all slide mount preparations. Scanning electron microscopy showed that the endoliths occurred in communal populations situated within the open pore spaces of the limestone. Heterotrophic bacteria were also observed growing as epiphytes on dead and living cyanobacteria, and in epilithic biofilms on pore space walls. In thin-sectioned specimens examined by transmission electron microscopy, these adherent bacteria commonly exhibited extensive accumulations of fibrous extracellular polymeric materials external to their cell walls. Biomass concentrations, estimated from the organic phosphorus content of the samples, generally paralleled rock porosity and ranged from 3.3 to 17.1% dry weight. Carbon isotope (14C) measurements indicated that atmospheric carbon dioxide is used by the endolithic cyanobacteria, rather than dissolved inorganic carbon from the weathering of carbonate minerals in the limestone host rock. In addition, whole rock multielement analyses revealed an enrichment of some elements (e.g., phosphorus, barium, lead, and zinc) in the endolith zone over the host rock, while other elements (e.g., magnesium, calcium, iron, and copper) were depleted. The implication is that the endolithic microorganisms play an active role in the biogeochemical cycling of nutrient and trace elements in cliff face ecosystems of the Niagara Escarpment.Key words: limestone, endolith, biofilm, cyanobacteria, geochemistry.

Correlation of Pore Structure and Permeability by SEM-Image Analysis

1990 ◽  
Vol 48 (1) ◽  
pp. 428-429
Author(s):  
C. A. Callender ◽  
Wm. C. Dawson ◽  
J. J. Funk
Keyword(s):  
Image Analysis ◽  
Pore Structure ◽  
Imaging System ◽  
Pore Space ◽  
Simulation Models ◽  
Image Analyzer ◽  
Imaging Data ◽  
Sem Images ◽  
Thin Sections ◽  
Rock Types

The geometric structure of pore space in some carbonate rocks can be correlated with petrophysical measurements by quantitatively analyzing binaries generated from SEM images. Reservoirs with similar porosities can have markedly different permeabilities. Image analysis identifies which characteristics of a rock are responsible for the permeability differences. Imaging data can explain unusual fluid flow patterns which, in turn, can improve production simulation models.Analytical SchemeOur sample suite consists of 30 Middle East carbonates having porosities ranging from 21 to 28% and permeabilities from 92 to 2153 md. Engineering tests reveal the lack of a consistent (predictable) relationship between porosity and permeability (Fig. 1). Finely polished thin sections were studied petrographically to determine rock texture. The studied thin sections represent four petrographically distinct carbonate rock types ranging from compacted, poorly-sorted, dolomitized, intraclastic grainstones to well-sorted, foraminiferal,ooid, peloidal grainstones. The samples were analyzed for pore structure by a Tracor Northern 5500 IPP 5B/80 image analyzer and a 80386 microprocessor-based imaging system. Between 30 and 50 SEM-generated backscattered electron images (frames) were collected per thin section. Binaries were created from the gray level that represents the pore space. Calculated values were averaged and the data analyzed to determine which geological pore structure characteristics actually affect permeability.

A Community Based High Risk Register for Hearing Loss

1982 ◽  
Vol 47 (4) ◽  
pp. 373-375 ◽  
Author(s):  
James L. Fitch ◽  
Thomas F. Williams ◽  
Josephine E. Etienne
Keyword(s):  
Hearing Loss ◽  
System Identification ◽  
High Risk ◽  
Language Learning ◽  
Education Program ◽  
Communication System ◽  
Auditory Signal ◽  
Early Age ◽  
Community Based ◽  
Children With Hearing Loss

The critical need to identify children with hearing loss and provide treatment at the earliest possible age has become increasingly apparent in recent years (Northern & Downs, 1978). Reduction of the auditory signal during the critical language-learning period can severely limit the child's potential for developing a complete, effective communication system. Identification and treatment of children having handicapping conditions at an early age has gained impetus through the Handicapped Children's Early Education Program (HCEEP) projects funded by the Bureau of Education for the Handicapped (BEH).

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