64 research outputs found
The magmatic to hydrothermal evolution of the intrusive Mont Saint-Hilaire Complex: Insights into the late-stage evolution of peralkaline rocks
The Cretaceous Mont Saint-Hilaire complex (Quebec, Canada) comprises
three major rock units that were emplaced in the following sequence: (I)
gabbros; (II) diorites; (III) diverse partly agpaitic foid syenites. The
major element compositions of the rock-forming minerals, age-corrected
Nd and oxygen isotope data for mineral separates and trace element data
of Fe-Mg silicates from the various lithologies imply a common source
for all units. The distribution of the rare earth elements in
clinopyroxene from the gabbros indicates an ocean island basalt type
composition for the parental magma. Gabbros record temperatures of 1200
to 800 degrees C, variable silica activities between 0 center dot 7 and
0 center dot 3, and f(O2) values between -0 center dot 5 and +0 center
dot 7 (log delta FMQ, where FMQ is fayalite-magnetite-quartz). The
diorites crystallized under uniform a(SiO2) (a(SiO2) = 0 center dot 4-0
center dot 5) and more reduced f(O2) conditions (log delta FMQ similar
to-1) between similar to 1100 and similar to 800 degrees C. Phase
equilibria in various foid syenites indicate that silica activities
decrease from 0 center dot 6-0 center dot 3 at similar to 1000 degrees C
to < 0 center dot 3 at similar to 550 degrees C. Release of an aqueous
fluid during the transition to the hydrothermal stage caused a(SiO2) to
drop to very low values, which results from reduced SiO(2) solubilities
in aqueous fluids compared with silicate melts. During the hydrothermal
stage, high water activities stabilized zeolite-group minerals. Fluid
inclusions record a complex post-magmatic history, which includes
trapping of an aqueous fluid that unmixed from the restitic foid
syenitic magma. Cogenetic aqueous and carbonic fluid inclusions reflect
heterogeneous trapping of coexisting immiscible external fluids in the
latest evolutionary stage. The O and C isotope characteristics of
fluid-inclusion hosted CO(2) and late-stage carbonates imply that the
surrounding limestones were the source of the external fluids. The
mineral-rich syenitic rocks at Mont Saint-Hilaire evolved as follows:
first, alkalis, high field strength and large ion lithophile elements
were pre-enriched in the (late) magmatic and subsequent hydrothermal
stages; second, percolation of external fluids in equilibrium with the
carbonate host-rocks and mixing processes with internal fluids as well
as fluid-rock interaction governed dissolution of pre-existing minerals,
element transport and precipitation of mineral assemblages determined by
locally variable parameters. It is this hydrothermal interplay between
internal and external fluids that is responsible for the mineral wealth
found at Mont Saint-Hilaire
Life histories and distribution of ostracods with depth in western Lake Geneva (Petit-Lac), Switzerland: A reconnaissance study
Because environmental conditions within a given basin are different for each season and at different water depth, knowledge of the life history and depth distribution of target species is important for environmental and palaeoenvironmental interpretations based on ostracod species assemblages and/or the geochemical compositions of their valves. In order to determine the distribution of species with depth as well as the life history of species from Lake Geneva, a one year sampling campaign of living ostracods was conducted at five sites (2, 5, 13, 33 and 70 m water depth) on a monthly basis in the Petit-Lac (western basin of Lake Geneva, Switzerland). Based on the results, the different species can be classified into three groups. Littoral taxa are found at 2 and 5 m water depth and include, in decreasing numbers of individuals, Cypridopsis vidua (O. F.Müller, 1776), Pseudocandona compressa (Koch, 1838), Limnocythere inopinata (Baird, 1843), Herpetocypris reptans (Baird, 1835), Potamocypris smaragdina (Vávra, 1891), Potamocypris similis (G. W. Müller, 1912), Plesiocypridopsis newtoni (Brady & Robertson, 1870), Prionocypris zenkeri (Chyzer & Toth, 1858) and Ilyocypris sp. Brady & Norman, 1889. Sublittoral species are found in a majority at 13 m water depth and to a lesser extend at 33 m water depth and include, in decreasing numbers of individuals, Fabaeformiscandona caudata (Kaufmann, 1900), Limnocytherina sanctipatricii, Candona candida (O. F. Müller, 1776) and Isocypris beauchampi (Paris, 1920). Profundal species are found equally at 13, 33 and 70 m water depth and includes, in decreasing numbers of individuals, Cytherissa lacustris (Sars, 1863), Candona neglecta Sars, 1887 and Cypria lacustris Lilljeborg, 1890. The occurrence of Limnocytherina sanctipatricii (Brady & Robertson, 1869) is restricted from late winter to late spring when temperatures are low, while C. vidua, L. inopinata, P. smaragdina, P. similis, P. newtoni and Ilyocypris sp. occur predominantly from spring to early autumn when temperatures are high. Individuals of C. neglecta, C. candida, F. caudata, P. compressa, C. lacustris, H. reptans and Cp. lacustris occur throughout the year with juveniles and adults occurring during the same period (C. neglecta at 70 m, C. lacustris at 13, 33 and 70 m, and H. reptans at 2, 5 and 13 m water depth) or with juveniles occurring during a different period of the year than adults (C. neglecta at 13 and 33 m and C. candida, F. caudata and P. compressa at their respective depth of occurrence). Among the environmental parameters investigated, an estimate of the relationship between ostracod autoecology and environmental parameters suggests that in the Petit-Lac: (i) water temperature and substrate characteristics are important factors controlling the distribution of species with depth, (ii) water temperature is also important for determining the timing of species development and, hence, its specific life history, and (iii) water oxygen and sedimentary organic matter content is less important compared to the other environmental parameter monitored
Anatomy of contaminated aquifers of an industrial site: insights from the stable isotope compositions of waters and dissolved inorganic carbon
The hydrogen and oxygen isotopes of water and the carbon isotope
composition of dissolved inorganic carbon (DIC) from different aquifers
at an industrial site, highly contaminated by organic pollutants
representing residues of the former gas production, have been used as
natural tracers to characterize the hydrologic system. On the basis of
their stable isotope compositions as well as the seasonal variations,
different groups of waters (precipitation, surface waters, groundwaters
and mineral waters) as well as seasonably variable processes of mixing
between these waters can clearly be distinguished. In addition,
reservoir effects and infiltration rates can be estimated. In the
northern part of the site an influence of uprising mineral waters within
the Quaternary aquifers, presumably along a fault zone, can be
recognized. Marginal infiltration from the Neckar River in the cast and
surface water infiltration adjacent to a steep hill on the western edge
of the site with an infiltration rate of about one month can also be
resolved through the seasonal variation. Quaternary aquifers closer to
the centre of the site show no seasonal variations, except for one
borehole close to a former mill channel and another borehole adjacent to
a rain water channel. Distinct carbon isotope compositions and
concentrations of DIC for these different groups of waters reflect
variable influence of different components of the natural carbon cycle:
dissolution of marine carbonates in the mineral waters, biogenic,
soil-derived CO2 in ground- and surface waters, as well as additional
influence of atmospheric CO2 for the surface waters. Many Quaternary
aquifer waters have, however, distinctly lower delta(13)C(DIC) values
and higher DIC concentrations compared to those expected for natural
waters. Given the location of contaminated groundwaters at this site but
also in the industrially well-developed valley outside of this site, the
most likely source for the low C-13(DIC) values is a biodegradation of
anthropogenic organic substances, in particular the tar oils at the
site
Early diagenesis of bone and tooth apatite in fluvial and marine settings: Constraints from combined oxygen isotope, nitrogen and REE analysis
Fossil bones and teeth of Late Pleistocene terrestrial mammals from
Rhine River gravels (RS) and the North Sea (NS), that have been exposed
to chemically and isotopically distinct diagenetic fluids (fresh water
versus seawater), were investigated to study the effects of early
diagenesis on biogenic apatite. Changes in phosphate oxygen isotopic
composition (delta O-18(PO4)), nitrogen content (wt.% N) and rare earth
element (REE) concentrations were measured along profiles within bones
that have not been completely fossilized, and in skeletal tissues (bone,
dentine, enamel) with different susceptibilities to diagenetic
alteration. Early diagenetic changes of elemental and isotopic
compositions of apatite in fossil bone are related to the loss of the
stabilizing collagen matrix. The REE concentration is negatively
correlated with the nitrogen content, and therefore the amount of
collagen provides a sensitive proxy for early diagenetic alteration. REE
patterns of RS and NS bones indicate initial fossilization in a fresh
water fluid with similar REE compositions. Bones from both settings have
nearly collagen-free, REE-, U-, F- and Sr-enriched altered outer rims,
while the collagen-bearing bone compacta in the central part often
display early diagenetic pyrite void-fillings. However, NS bones exposed
to Holocene seawater have outer rim delta O-18(PO4) values that are 1.1
to 2.6 parts per thousand higher compared to the central part of the
same bones (delta O-18(PO4) = 18.2 +/- 0.9 parts per thousand, n = 19).
Surprisingly, even the collagen-rich bone compacta with low REE contents
and apatite crystallinity seems altered, as NS tooth enamel (delta
O-18(PO4) =15.0 +/- 0.3 parts per thousand, n=4) has about 3%. lower
delta O-18(PO4) values, values that are also similar to those of enamel
from RS teeth. Therefore, REE concentration, N content and apatite
crystallinity are in this case only poor proxies for the alteration of
delta O-18(PO4) values. Seawater exposure of a few years up to 8 kyr can
change the delta O-18(PO4) values of the bone apatite by > 3 parts per
thousand. Therefore, bones fossilized in marine settings must be treated
with caution for palaeoclimatic reconstructions. However, enamel seems
to preserve pristine delta O-18(PO4) values on this time scale. Using
species-specific calibrations for modern mammals, a mean delta O-18(H2O)
value can be reconstructed for Late Pleistocene mammalian drinking water
of around -9.2 +/- 0.5 parts per thousand, which is similar to that of
Late Pleistocene groundwater from central Europe. (c) 2008 Elsevier B.V.
All rights reserved
Modelling changes in stable isotope compositions of minerals during net transfer reactions in a contact aureole: Wollastonite growth at the northern Hunter Mountain Batholith (Death Valley National Park, USA)
One of the world's largest wollastonite deposits was formed at the
contact of the northern Hunter Mountain Batholith (California, USA) in
Paleozoic sediments. Wollastonite occurs as zones of variable thickness
surrounding layers or nodules of quartzite in limestones. A minimum
formation temperature of 650 degrees C is estimated from isolated
periclase-bearing lenses in that area. Contact metamorphism of siliceous
carbonates has produced mineral assemblages that are consistent with
heterogeneous, and partly limited infiltration of water-rich fluids,
compatible with O-18/O-16 and C-13/C-12 isotopic patterns recorded in
carbonates. Oxygen isotope compositions of wollastonites in the study
area may also not require infiltration of large quantities of
externally-derived fluids that were out of equilibrium with the rocks.
8180 values of wollastonite are high (14.8 parts per thousand to 25.0
parts per thousand; median: 19.7 parts per thousand) and close to those
of the host limestone (19.7 parts per thousand to 28 parts per thousand;
median: 24.9 parts per thousand) and quartz (18.0 parts per thousand. to
29.1 parts per thousand; median: 22.6 parts per thousand). Isotopic
disequilibrium exists at quartz/wollastonite and wollastonite/calcite
boundaries. Therefore, classical batch/Rayleigh fractionation models
based on reactant and product equilibrium are not applicable to the
wollastonite rims. An approach that relies on local instantaneous mass
balance for the reactants, based on the wollastonite-forming reaction is
suggested as an alternative way to model wollastonite reaction rims.
This model reproduces many of the measured delta O-18 values of
wollastonite reaction rims of the current study to within +/- 1 parts
per thousand, even though the wollastonite compositions vary by almost
10 parts per thousand. (C) 2011 Elsevier B.V. All rights reserved
Controls on ostracod valve geochemistry, Part 1: Variations of environmental parameters in ostracod (micro-)habitats
The variations of environmental conditions (T°, pH, δ13CDIC, [DIC], δ18O, Mg/Ca, and Sr/Ca) of ostracod habitats were examined to determine the controls of environmental parameters on the chemical and isotopic composition of ostracod valves. Results of a one-year monitoring of environmental parameters at five sites, with depths of between 2 and 70 m, in Lake Geneva indicate that in littoral to sub-littoral zones (2, 5, and 13 m), the chemical composition of bottom water varies seasonally in concert with changes in temperature and photosynthetic activity. An increase of temperature and photosynthetic activity leads to an increase in δ13C values of DIC and to precipitation of authigenic calcite, which results in a concomitant increase of Mg/Ca and Sr/Ca ratios of water. In deeper sites (33 and 70 m), the composition of bottom water remains constant throughout the year and isotopic values and trace element contents are similar to those of deep water within the lake. The chemical composition of interstitial pore water also does not reflect seasonal variations but is controlled by calcite dissolution, aerobic respiration, anaerobic respiration with reduction of sulphate and/or nitrate, and methanogenesis that may occur in the sediment pores. Relative influence of each of these factors on the pore water geochemistry depends on sediment thickness and texture, oxygen content in bottom as well as pore water. Variations of chemical compositions of the ostracod valves of this study vary according to the specific ecology of the ostracod species analysed, that is its life-cycle and its (micro-)habitat. Littoral species have compositions that are related to the seasonal variations of temperature, δ13C values of DIC, and of Mg/Ca and Sr/Ca ratios of water. In contrast, the compositions of profundal species are largely controlled by variations of pore fluids along sediment depth profiles according to the specific depth preference of the species. The control on the geochemistry of sub-littoral species is a combination of controls for the littoral and profundal species as well as the specific ecology of the species
Stable isotope compositions of mammoth teeth from Niederweningen, Switzerland: Implications for the Late Pleistocene climate, environment, and diet
Oxygen and carbon isotope compositions of well-preserved mammoth teeth
from the Middle Wurmian (40-70 ka) peat layer of Niederweningen, the
most important mammoth site in Switzerland, were analysed to reconstruct
Late Pleistocene palaeoclimatic and palaeoenvironmental conditions.
Drinking water (delta(18)O values of approximately -12.3 +/- 0.9 parts
per thousand were calculated front oxygen isotope compositions of
mammoth tooth enamel apatite using a species-specific calibration for
modern elephants. These delta(18)O(H2O) values reflect the mean oxygen
isotope composition of the palaeo-precipitation and are similar to those
directly measured for fate Pleistocene groundwater from aquifers in
northern Switzerland and southern Germany. Using a present-day
delta(18)O(H2)o-precipitation-air temperature relation for Switzerland,
a mean annual air temperature (MAT) of around 4.3 +/- 2.1 degrees C can
be calculated for the Middle Wurmian at this site. This MAT is in good
agreement with palaeotemperature estimates on the basis of Middle
Wurmian groundwater recharge temperatures and beetle assemblages. Hence,
the climatic conditions in this region were around 4 degrees C cooler
during the Middle Wurmian interstadial phase, around 45-50ka BP, than
they are today.
During this period the mammoths from Niederweningen lived in an open
tundra-like, C(3) plant-dominated environment as indicated by enamel
(delta(13)C values of -11.5 +/- 0.3 parts per thousand and pollen and
macroplant fossils found in the embedding peat. The low variability of
enamel delta(13)C and delta(18)O values from different mammoth teeth
reflects similar environmental conditions and supports a relatively
small time frame for the fossil assemblage. (C) 2006 Elsevier Ltd and
INQUA. All rights reserved
Stable isotope composition of smectite in suevites at the Ries crater, Germany: Implications for hydrous alteration of impactites
The 24-km diameter Ries crater, Germany, exhibits well-preserved crater
filling and surficial melt-rich breccia deposits that are believed to
have been altered by post-impact hydrothermal fluids. The alteration
mineralogy of the crater filling breccias is characterized by clay
(smectite, chlorite) and a zeolite assemblage, and secondary clay phases
(smectite, minor halloysite) in surficial melt-bearing breccia deposits.
Using stable isotope analysis of secondary smectitic clay fractions,
evidence of significant hydrous alteration of impactites at large
water/rock ratios was found. The estimated fluid temperatures, using
data derived by delta(18)O and delta D fractionation, suggest smectite
precipitation in surficial breccias in equilibrium with meteoric fluids
at temperatures 16 +/- 5 degrees C in agreement with the long-term
variation of modern precipitation in the area. The stable isotope
composition of smectite in crater-fill breccia, however, suggests a
trend of monotonously increasing temperatures from 43 to 112 degrees C.
with increasing depth through the breccia sequence. This demonstrates a
different origin of alteration and temperature distribution for the
surficial and crater filling melt-bearing impact breccias in the Ries
crater. Our results suggest that the inverted structure of hydrothermal
systems observed in some terrestrial impact craters, including the Ries
crater, could indicate the initial configuration of a thermal anomaly in
the crater filling sequence, but which is replaced with a normal
hydrothermal convection in crater proper, during the course of
post-impact cooling. (C) 2010 Elsevier B.V. All rights reserved
Nd and Sr isotope compositions in modern and fossil bones - Proxies for vertebrate provenance and taphonomy
Rare earth elements (REE), while not essential for the physiologic
functions of animals, are ingested and incorporated in ppb
concentrations in bones and teeth. Nd isotope compositions of modern
bones of animals from isotopically distinct habitats demonstrate that
the (143)Nd/(144)Nd of the apatite can be used as a fingerprint for
bedrock geology or ambient water mass. This potentially allows the
provenance and migration of extant vertebrates to be traced, similar to
the use of Sr isotopes. Although REE may be enriched by up to 5 orders
of magnitude during diagenesis and recrystallization of bone apatite, in
vivo (143)Nd/(144)Nd may be preserved in the inner cortex of fossil
bones or enamel. However, tracking the provenance of ancient or extinct
vertebrates is possible only for well-preserved archeological and
paleontological skeletal remains with in vivo-like Nd contents at the
ppb-level. Intra-bone and -tooth REE analysis can be used to screen for
appropriate areas. Large intra-bone Nd concentration gradients of
10(1)-10(3) are often measured. Nd concentrations in the inner bone
cortex increase over timescales of millions of years, while bone rims
may be enriched over millenial timescales. Nevertheless, epsilon(Nd)
values are often similar within one epsilon(Nd) unit within a single
bone. Larger intra-bone differences in specimens may either reflect a
partial preservation of in vivo values or changing epsilon(Nd) values of
the diagenetic fluid during fossilization. However, most fossil
specimens and the outer rims of bones will record taphonomic
(143)Nd/(144)Nd incorporated post mortem during diagenesis. Unlike REE
patterns, (143)Nd/(144)Nd are not biased by fractionation processes
during REE-uptake into the apatite crystal lattice, hence the
epsilon(Nd) value is an important tracer for taphonomy and reworking.
Bones and teeth from autochthonous fossil assemblages have small
variations of +/- 1 epsilon(Nd) unit only. In contrast, fossil bones and
teeth from over 20 different marine and terrestrial fossil sites have a
total range of epsilon(Nd) values from -13.0 to 4.9 (n = 80), often
matching the composition of the embedding sediment. This implies that
the surrounding sediment is the source of Nd in the fossil bones and
that the specimens of this study seem not to have been reworked.
Differences in epsilon(Nd) values between skeletal remains and embedding
sediment may either indicate reworking of fossils and/or a REE-uptake
from a diagenetic fluid with non-sediment derived epsilon(Nd) values.
The latter often applies to fossil shark teeth, which may preserve
paleo-seawater values. Complementary to epsilon(Nd) values,
(87)Sr/(86)Sr can help to further constrain the fossil provenance and
reworking. (C) 2011 Elsevier Ltd. All rights reserved
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