Chondrite normalized rare earth element (REE) patterns of zircons generally have enriched Ce values relative to La and Pr, and depleted Eu values relative to Sm and Gd. High Ce contents in zircon may ...imply oxidizing conditions (Ce4+ is more compatible than Ce3+), whereas depleted Eu contents may imply reducing conditions (Eu2+ does not substitute into the zircon lattice). We report 41 experiments in which temperature, melt composition, and oxygen fugacity (fO2) were varied in order to explore the details of Ce and Eu incorporation into zircon. Crystals were synthesized in hydrous silicate melts at 10kbar and 800–1300°C. Synthetic rock mixes were doped with La+Ce+Pr (±P) or Sm+Eu+Gd and buffered at oxygen fugacities ranging from ∼IW (iron–wüstite) to >MH (magnetite–hematite); the run products were analyzed by electron microprobe to obtain crystal/melt partition coefficients. Cerium anomalies increase with higher oxygen fugacities and lower crystallization temperatures. In agreement with other experimental studies, peralkaline melts yield the largest zircon grains but show only modest Ce anomalies even at fO2s>MH. The same reason that zircons grown in peralkaline melts are easy to synthesize in the laboratory (these melts are capable of dissolving wt.% levels of Zr before zircon saturation due to high alkali content) makes the melt structure/composition atypical and not representative of most natural magmas. With this in mind, we synthesized zircons in a granitic melt with more modest alkali contents that require geologically plausible Zr contents for saturation. We obtained the following empirical relationship: lnCeCe∗D=(0.1156±0.0050)×ln(fO2)+13,860±708T(K)-6.125±0.484where (Ce/Ce∗)D is the Ce anomaly in zircon calculated from partition coefficients, and T is the zircon crystallization temperature in K. Europium anomalies from the same melt composition are more negative at lower oxygen fugacities, but with no resolvable temperature dependence, and can be described by the following empirical relationship: EuEu∗D=11+10-0.14±0.01×ΔNNO+0.47±0.04where (Eu/Eu∗)D is the Eu partitioning anomaly and ΔNNO is the difference in log units from the NNO buffer. If both Eu and Ce anomalies in zircons can be used as proxies for the oxidation state of Ce and Eu in the host melts, then it is clear that Eu2+ and Ce4+ can coexist in most zircon-saturated magmas. This implies that depletion of Eu melt contents by feldspar crystallization fractionation prior to (or during) zircon crystallization is not required to produce Eu anomalies. Thus, zircon Eu anomalies are a function of the oxygen fugacity and the Eu anomaly of the melt. Cerium anomalies of natural melts are not predicted to be as common because no major rock-forming phase depletes or enriches magmas in Ce compared to neighboring elements La and Pr. Thus, (Ce/Ce∗)D may be most readily applied to constrain the oxidation state of natural melts.
Magmatic outgassing of volatiles from Earth's interior probably played a critical part in determining the composition of the earliest atmosphere, more than 4,000 million years (Myr) ago. Given an ...elemental inventory of hydrogen, carbon, nitrogen, oxygen and sulphur, the identity of molecular species in gaseous volcanic emanations depends critically on the pressure (fugacity) of oxygen. Reduced melts having oxygen fugacities close to that defined by the iron-wüstite buffer would yield volatile species such as CH(4), H(2), H(2)S, NH(3) and CO, whereas melts close to the fayalite-magnetite-quartz buffer would be similar to present-day conditions and would be dominated by H(2)O, CO(2), SO(2) and N(2) (refs 1-4). Direct constraints on the oxidation state of terrestrial magmas before 3,850 Myr before present (that is, the Hadean eon) are tenuous because the rock record is sparse or absent. Samples from this earliest period of Earth's history are limited to igneous detrital zircons that pre-date the known rock record, with ages approaching ∼4,400 Myr (refs 5-8). Here we report a redox-sensitive calibration to determine the oxidation state of Hadean magmatic melts that is based on the incorporation of cerium into zircon crystals. We find that the melts have average oxygen fugacities that are consistent with an oxidation state defined by the fayalite-magnetite-quartz buffer, similar to present-day conditions. Moreover, selected Hadean zircons (having chemical characteristics consistent with crystallization specifically from mantle-derived melts) suggest oxygen fugacities similar to those of Archaean and present-day mantle-derived lavas as early as ∼4,350 Myr before present. These results suggest that outgassing of Earth's interior later than ∼200 Myr into the history of Solar System formation would not have resulted in a reducing atmosphere.
Quartz and rutile were synthesized from silica-saturated aqueous fluids between 5 and 20 kbar and from 700 to 940°C in a piston-cylinder apparatus to explore the potential pressure effect on Ti ...solubility in quartz. A systematic decrease in Ti-in-quartz solubility occurs between 5 and 20 kbar. Titanium K-edge X-ray absorption near-edge structure (XANES) measurements demonstrate that Ti
4+
substitutes for Si
4+
on fourfold tetrahedral sites in quartz at all conditions studied. Molecular dynamic simulations support XANES measurements and demonstrate that Ti incorporation onto fourfold sites is favored over interstitial solubility mechanisms. To account for the
P
–
T
dependence of Ti-in-quartz solubility, a least-squares method was used to fit Ti concentrations in quartz from all experiments to the simple expression
where
R
is the gas constant 8.3145 J/K,
T
is temperature in Kelvin,
is the mole fraction of TiO
2
in quartz and
is the activity of TiO
2
in the system. The
P
–
T
dependencies of Ti-in-quartz solubility can be used as a thermobarometer when used in combination with another thermobarometer in a coexisting mineral, an independent
P
or
T
estimate of quartz crystallization, or well-constrained phase equilibria. If temperature can be constrained within ±25°C, pressure can be constrained to approximately ±1.2 kbar. Alternatively, if pressure can be constrained to within ±1 kbar, then temperature can be constrained to approximately ±20°C.
Silica glass has been shown in numerous studies to possess significant capacity for permanent densification under pressure at different temperatures to form high density amorphous (HDA) silica. ...However, it is unknown to what extent the processes leading to irreversible densification of silica glass in cold-compression at room temperature and in hot-compression (e.g., near glass transition temperature) are common in nature. In this work, a hot-compression technique was used to quench silica glass from high temperature (1100 °C) and high pressure (up to 8 GPa) conditions, which leads to density increase of ~25% and Young's modulus increase of ~71% relative to that of pristine silica glass at ambient conditions. Our experiments and molecular dynamics (MD) simulations provide solid evidences that the intermediate-range order of the hot-compressed HDA silica is distinct from that of the counterpart cold-compressed at room temperature. This explains the much higher thermal and mechanical stability of the former than the latter upon heating and compression as revealed in our in-situ Brillouin light scattering (BLS) experiments. Our studies demonstrate the limitation of the resulting density as a structural indicator of polyamorphism, and point out the importance of temperature during compression in order to fundamentally understand HDA silica.
Bubbles grow in decompressing magmas by simple expansion and by diffusive supply of volatiles to the bubble/melt interface. The latter phenomenon is of significant geochemical interest because ...diffusion can fractionate elements and isotopes (or isotopologues) of dissolved components. This raises the possibility that the character of volatile components in bubbles may not reflect that of volatiles dissolved in the host melt over the lifetime of a bubble—even in the absence of equilibrium vapor/melt isotopic fractionation. Recent experiments have confirmed the existence of an isotope mass effect on diffusion of the volatile element Cl in silicate melt Fortin et al. (Isotopic fractionation of chlorine during chemical diffusion in a dacitic melt and its implications for isotope behavior during bubble growth (abstract), 2016 Fall AGU Meeting,
2016
), so there is a clear need to understand the efficacy of diffusive fractionation during bubble growth. In this study, numerical models of diffusion and mass redistribution during bubble growth were implemented for both “passive” volatiles—those whose concentrations are generally well below saturation levels—and “active” volatiles such as CO
2
and H
2
O, whose elevated concentrations and limited solubilities are the cause of bubble nucleation and growth. Both diffusive and convective bubble-growth scenarios were explored. The magnitude of the isotope mass effect on passive volatiles partitioned into bubbles growing at a constant rate
R
in a static system depends upon
R
/
D
L
,
K
d
and
D
H
/
D
L
(
K
d
= bubble/melt partition coefficient;
D
H
/
D
L
= diffusivity ratio of the heavy and light isotopes). During convective bubble growth, the presence of a discrete (physical) melt boundary layer against the growing bubble (of width
x
BL
) simplifies outcomes because it leads to the quick onset of steady-state fractionation during growth, the magnitude of which depends mainly upon
R∙x
BL
/
D
L
and
D
H
/
D
L
(bubble/melt fractionation is maximized at
R∙x
BL
/
D
L
≈0.1). Constant
R
is unrealistic for most real systems, so other scenarios were explored by including the solubility and EOS of an “active” volatile (e.g., CO
2
) in the numerical simulations. For plausible decompression paths,
R
increases exponentially with time—leading, potentially, to larger isotopic fractionation of species partitioned into the growing bubble. For volatile species whose isotope mass effects on diffusion have been measured (Cl, Li), predicted isotope fractionation in the exsolved vapor can be as large as −4‰ for Cl and −25‰ for Li.
Climate change vulnerability assessment of species Foden, Wendy B.; Young, Bruce E.; Akçakaya, H. Resit ...
Wiley interdisciplinary reviews. Climate change,
January/February 2019, Letnik:
10, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Assessing species' vulnerability to climate change is a prerequisite for developing effective strategies to conserve them. The last three decades have seen exponential growth in the number of studies ...evaluating how, how much, why, when, and where species will be impacted by climate change. We provide an overview of the rapidly developing field of climate change vulnerability assessment (CCVA) and describe key concepts, terms, steps and considerations. We stress the importance of identifying the full range of pressures, impacts and their associated mechanisms that species face and using this as a basis for selecting the appropriate assessment approaches for quantifying vulnerability. We outline four CCVA assessment approaches, namely trait‐based, correlative, mechanistic and combined approaches and discuss their use. Since any assessment can deliver unreliable or even misleading results when incorrect data and parameters are applied, we discuss finding, selecting, and applying input data and provide examples of open‐access resources. Because rare, small‐range, and declining‐range species are often of particular conservation concern while also posing significant challenges for CCVA, we describe alternative ways to assess them. We also describe how CCVAs can be used to inform IUCN Red List assessments of extinction risk. Finally, we suggest future directions in this field and propose areas where research efforts may be particularly valuable.
This article is categorized under:
Climate, Ecology, and Conservation > Extinction Risk
Assessing species' vulnerability to climate change is becoming a prerequisite for conservation planning, but choosing approaches, methods and data can be challenging. Key to informing such choices is consideration of the full range of climate change pressures and their likely mechanisms of impact on individuals, subpopulations and species. Navigate a sound path through do's and don'ts, and explore resources and future perspectives in this exciting field.
The chemical diffusivities of 25 trace elements (Sc, V, Rb, Sr, Y, Zr, Nb, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, Hf, Ta, Th, and U) in basaltic melt were measured in diffusion ...couple experiments performed at 1 GPa pressure and temperatures from 1250 to 1500 °C. Trace element concentration gradients developed in the glasses were simultaneously characterized using laser ablation ICP/MS to create an internally consistent data set. A ratio-fitting technique was employed to accurately determine the relative diffusivities of the rare earth elements (REE). All diffusion coefficients conform to the expected Arrhenius relation
D
=
D
0
exp(−
E
a
/RT
), where the constant log(
D
0
, m
2
/s) ranges from −3.81 to −5.11 and
E
a
ranges from 161.73 to 223.81 kJ/mol. The slowest diffusivities are obtained for the high-field-strength elements; the fastest diffusivities are obtained for the low-field-strength elements. Trace element diffusion in MORB follows the compensation law, where log
D
0
is linearly correlated with
E
a
. Arrhenius parameters for diffusion of trivalent REE monotonically increase from La to Lu and are near-linear functions of bond strength (the variation in Arrhenius parameters means that the diffusivities decrease monotonically from La to Lu at a given
T
). The new data for trace element diffusion in basaltic melt can be used to explore the potential for diffusive fractionation of trace elements using kinetic models. Concentrations of the slower-diffusing heavy REE may be altered relative to those of the faster-diffusing light REE as a diffusive boundary layer develops in melt–melt and crystal–melt systems. The results indicate that diffusion in basalt can be an effective mechanism to fractionate trace elements from one another.
Rapid crystal growth can lead to disequilibrium uptake of growth-medium components whose diffusivities limit their dispersal near an advancing crystal interface. The recent documentation of an ...isotope mass effect on diffusion raises the possibility that even isotope ratios in crystals may be subject to this effect. Building upon existing 1-dimensional treatments, we describe a numerical modeling approach in which a spherical grain grows at the center of an infinite spherical medium of predetermined composition. Local equilibrium at the interface between the crystal and the growth medium is assumed, but the concentration of the species of interest in the growth medium is allowed to vary near the interface as a consequence of slow diffusion combined with rejection from (or incorporation within) the growing crystal. The disequilibrium uptake of elements and isotopes depends upon the ratio of crystal growth rate (
R) to diffusivity in the growth medium (
D). Conditions of fast mineral growth in a viscous magma—e.g., in lava lakes or small igneous bodies—result in accumulation of elements with
K
<<
1 (or depletion of elements with
K
>>
1) near the growing mineral interface, forming a compositional boundary layer in the growth medium. In a static system, the magnitude of this compositional perturbation depends critically upon the diffusivity of the element or isotope of interest in the growth medium. If the system is dynamic—i.e., experiencing free or forced convection—then the vigor of convection also affects behavior. Significant fractionation of elements and isotopes is predicted to occur within the boundary layer during progressive crystal growth because diffusion rates of individual elements vary with size and charge and those of isotopes of the same element depend on their masses. Local equilibrium at the interface between the crystal and its growth medium means that a fast-growing crystal will record this fractionation in its resulting radial concentration profile. If the boundary-layer thickness,
BL, is small (say, <
100 μm) and the equilibrium partition coefficient,
K, is <
0.5, then a first-order estimate of the steady-state isotopic fractionation in a growing crystal is given by
δ
(
‰
)
=
1000
⋅
(
1
−
D
A
D
B
)
⋅
(
R
⋅
B
L
D
A
)
⋅
(
1
−
K
)
,
where
D
A
and
D
B
are the diffusivities of the faster and slower species in the growth medium and δ is the deviation from the equilibrium isotope ratio in parts per thousand. For isotopes of a single element,
D
A
and
D
B
will generally differ by <
1%, but plausible
R/
D ratios can nevertheless lead to deviations from equilibrium between the crystal and the growth medium of up to ~
3‰. The model may bear on disequilibrium crystal-growth phenomena in a variety of geologic settings—including element- and isotopic profiles in crystals of both igneous and metamorphic rocks. It is suggested that compositional core to rim profile of a crystal may be a proxy for the near surface composition of the growth medium during crystal growth. Isotopic effects are discussed in detail because these have not been addressed previously; igneous systems are emphasized because higher crystal growth rates are more conducive to disequilibrium (including in the compositions of melt inclusions).
Several studies have reported the
P
–
T
dependencies of Ti-in-quartz solubility, and there is close agreement among three of the four experimental calibrations. New experiments were conducted in the ...present study to identify potential experimental disequilibrium, and to determine which Ti-in-quartz solubility calibration is most accurate. Crystals of quartz, rutile and zircon were grown from SiO
2
-, TiO
2
-, and ZrSiO
4
-saturated aqueous fluids in an initial synthesis experiment at 925 °C and 10 kbar in a piston-cylinder apparatus. A range of quartz crystal sizes was produced in this experiment; both large and small examples were analyzed by electron microprobe to determine whether Ti concentrations are correlated with crystal size. Cathodoluminescence images and EPMA measurements show that intercrystalline and intracrystalline variations in Ti concentrations are remarkably small regardless of crystal size. The average Ti-in-quartz concentration from the synthesis experiment is 392 ± 1 ppmw Ti, which is within 95 % confidence interval of data from the 10 kbar isobar of Wark and Watson (Contrib Mineral Petrol 152:743–754,
2006
) and Thomas et al. (Contrib Mineral Petrol 160:743–759,
2010
). As a cross-check on the Ti-in-quartz calibration, we also measured the concentration of Zr in rutile from the synthesis experiment. The average Zr-in-rutile concentration is 4337 ± 32 ppmw Zr, which is also within the 95 % confidence interval of the Zr-in-rutile solubility calibration of Ferry and Watson (Contrib Mineral Petrol 154:429–437,
2007
). The
P
–
T
dependencies of Ti solubility in quartz and Zr solubility in rutile were applied as a thermobarometer to the experimental sample. The average Ti-in-quartz isopleth calculated from the calibration of Thomas et al. (Contrib Mineral Petrol 160:743–759,
2010
) and the average Zr-in-rutile isopleth calculated from the calibration of Tomkins et al. (J Metamorph Geol 25:703–713,
2007
) cross at 9.5 kbar and 920 °C, which is in excellent agreement with the
P
–
T
conditions of the synthesis experiment. Separates of the high-Ti quartz from the initial synthesis experiment described above were used as starting material in subsequent experiments at 20 kbar, at which pressure the solubility of Ti in quartz is expected to be significantly lower in the recrystallized quartz. These recrystallization experiments were conducted under wet and dry conditions at 925 °C, and under wet conditions at 850 °C. Both wet and dry recrystallization experiments produced polycrystalline quartzites. Rutile occurs as inclusions in quartz, and as individual crystals dispersed along quartz grain boundaries. Quartz that grew during the recrystallization experiments has dark cathodoluminescence indicating substantially lower Ti concentrations. The average Ti concentrations in quartz from the recrystallization experiments are within the 95 % confidence interval of a linear fit to the 20 kbar data of Thomas et al. (Contrib Mineral Petrol 160:743–759,
2010
). Collectively, the results from the synthesis and recrystallization experiments confirm that the Ti-in-quartz concentrations used to calibrate the
P
–
T
dependencies of Ti-in-quartz solubility in Thomas et al.’s (Contrib Mineral Petrol 160:743–759,
2010
) calibration represent the equilibrium concentrations of Ti in quartz.
PDF
