Piston cylinder experiments are used to investigate the effect of oxygen fugacity (ƒO
2
) on sulphur speciation and phase relations in arc magmas at 0.5–1.5 GPa and 840–950 °C. The experimental ...starting composition is a synthetic trachyandesite containing 6.0 wt% H
2
O, 2880 ppm S, 1500 ppm Cl and 3800 ppm C. Redox conditions ranging from 1.7 log units below the Ni–NiO buffer (NNO − 1.7) to NNO + 4.7 were imposed by solid-state buffers: Co–CoO, Ni–NiO, Re–ReO
2
and haematite–magnetite. All experiments are saturated with a COH fluid. Experiments produced crystal-bearing trachydacitic melts (SiO
2
from 60 to 69 wt%) for which major and volatile element concentrations were measured. Experimental results demonstrate a powerful effect of oxidation state on phase relations. For example, plagioclase was stable above NNO, but absent at more reduced conditions. Suppression of plagioclase stability produces higher Al
2
O
3
and CaO melts. The solid sulphur-bearing phases and sulphur speciation in the melt are strong functions of ƒO
2
, as expected, but also of pressure. At 0.5 GPa, the anhydrite stability field is intersected at NNO ≥ +2, but at 1.0 and 1.5 GPa, experiments at the same ƒO
2
produce sulphides and the stability field of sulphate moves towards higher ƒO
2
by ~1 log unit at 1.0 GPa and ~1.5 log units at 1.5 GPa. As a result, models that appeal to high oxidation state as an important control on the mobility of Cu (and other chalcophiles) during crustal differentiation must also consider the enhanced stability of sulphide in deep- to mid-crustal cumulates even for relatively oxidized (NNO + 2) magmas. Experimental glasses reproduce the commonly observed minimum in sulphur solubility between the S
2−
and S
6+
stability fields. The solubility minimum is not related to the Fe content (Fe
2+
/Fe
3+
or total) of the melt. Instead, we propose this minimum results from an unidentified, but relatively insoluble, S-species of intermediate oxidation state.
Interactions between crustal and mantle reservoirs dominate the surface inventory of volatile elements over geological time, moderating atmospheric composition and maintaining a life-supporting ...planet. While volcanoes expel volatile components into surface reservoirs, subduction of oceanic crust is responsible for replenishment of mantle reservoirs. Many natural, 'superdeep' diamonds originating in the deep upper mantle and transition zone host mineral inclusions, indicating an affinity to subducted oceanic crust. Here we show that the majority of slab geotherms will intersect a deep depression along the melting curve of carbonated oceanic crust at depths of approximately 300 to 700 kilometres, creating a barrier to direct carbonate recycling into the deep mantle. Low-degree partial melts are alkaline carbonatites that are highly reactive with reduced ambient mantle, producing diamond. Many inclusions in superdeep diamonds are best explained by carbonate melt-peridotite reaction. A deep carbon barrier may dominate the recycling of carbon in the mantle and contribute to chemical and isotopic heterogeneity of the mantle reservoir.
Many plutons preserve evidence of magma mixing between hydrous mafic magmas and resident felsic crystal-rich mushes. To investigate water transfer processes in such systems following thermal ...equilibration, we conducted 24 h experiments to establish the petrological evolution of a water-undersaturated (4 wt% H2O in the interstitial melt) quartz-bearing dacite crystal mush (0.5-0.8 in crystal fraction) intruded by a water-saturated (≥6 wt% H2O), initially crystal-free, andesite magma at 950 °C and 4 kbar (12 km depth). Our results show isothermal undercooling resulting from a change in liquidus temperatures of the interacting magmas due to their changing water content. Specifically, mafic samples dramatically crystallize during water escape into the felsic end-members and consequent increase in liquidus temperature. Conversely, the addition of water to the felsic mush reduces the liquidus temperature, leading to an increase in melt fraction. The experiments provide insights into how volatiles contribute to crystal mush rejuvenation (i.e., increase of melt fraction). However, H2O diffusion alone is not sufficient to promote melt extraction from short- and long-lived mushes in the Earth's crust.
Fluids liberated from subducting slabs are critical in global geochemical cycles. We investigate the behaviour of Mo during slab dehydration using two suites of exhumed fragments of subducted, ...oceanic lithosphere. Our samples display a positive correlation of δ
Mo
with Mo/Ce, from compositions close to typical mantle (-0.2‰ and 0.03, respectively) to very low values of both δ
Mo
(-1‰) and Mo/Ce (0.002). Together with new, experimental data, we show that molybdenum isotopic fractionation is driven by preference of heavier Mo isotopes for a fluid phase over rutile, the dominant mineral host of Mo in eclogites. Moreover, the strongly perturbed δ
Mo
and Mo/Ce of our samples requires that they experienced a large flux of oxidised fluid. This is consistent with channelised, reactive fluid flow through the subducted crust, following dehydration of the underlying, serpentinised slab mantle. The high δ
Mo
of some arc lavas is the complement to this process.
The Fe–Mg exchange coefficient between olivine (ol) and melt (m), defined as
Kd
Fe
T
-
Mg
= (Fe
ol
/Fe
m
)·(Mg
m
/Mg
ol
), with all Fe
T
expressed as Fe
2+
, is one of the most widely used ...parameters in petrology. We explore the effect of redox conditions on
Kd
Fe
T
-
Mg
using experimental, olivine-saturated basaltic glasses with variable H
2
O (≤ 7 wt%) over a wide range of
f
O
2
(iron-wüstite buffer to air), pressure (≤ 1.7 GPa), temperature (1025–1425 °C) and melt composition. The ratio of Fe
3+
to total Fe (Fe
3+
/∑Fe), as determined by Fe K-edge µXANES and/or Synchrotron Mössbauer Source (SMS) spectroscopy, lies in the range 0–0.84. Measured Fe
3+
/∑Fe is consistent (± 0.05) with published algorithms and appears insensitive to dissolved H
2
O. Combining our new data with published experimental data having measured glass Fe
3+
/∑Fe, we show that for Fo
65–98
olivine in equilibrium with basaltic and basaltic andesite melts,
Kd
Fe
T
-
Mg
decreases linearly with Fe
3+
/∑Fe with a slope and intercept of 0.3135 ± 0.0011. After accounting for non-ideal mixing of forsterite and fayalite in olivine, using a symmetrical regular solution model, the slope and intercept become 0.3642 ± 0.0011. This is the value at Fo
50
olivine; at higher and lower Fo the value will be reduced by an amount related to olivine non-ideality. Our approach provides a straightforward means to determine Fe
3+
/∑Fe in olivine-bearing experimental melts, from which
f
O
2
can be calculated. In contrast to
Kd
Fe
T
-
Mg
, the Mn–Mg exchange coefficient,
Kd
Mn
-
Mg
, is relatively constant over a wide range of P–T–
f
O
2
conditions. We present an expression for
Kd
Mn
-
Mg
that incorporates the effects of temperature and olivine composition using the lattice strain model. By applying our experimentally-calibrated expressions for
Kd
Fe
T
-
Mg
and
Kd
Mn
-
Mg
to olivine-hosted melt inclusions analysed by electron microprobe it is possible to correct simultaneously for post-entrapment crystallisation (or dissolution) and calculate melt Fe
3+
/∑Fe to a precision of ≤ 0.04.
Mantle-derived, hydrous mafic magmas are often invoked as a mechanism to transfer heat, mass and volatiles to felsic plutons in the Earth’s crust. Field observations suggest that mafic, water-rich ...magmas often intrude viscous felsic crystal-rich mushes. This scenario can advect water from the crystallising mafic magma to the felsic magma, leading to an increase in melt fraction in the felsic mush and subsequent mobilisation, at the same time as the mafic magma becomes quenched through a combination of cooling and water loss. To investigate such a scenario, we conducted experiments on a water-undersaturated (4 wt% H
2
O in the interstitial melt) dacitic crystal mush (50–80 vol% quartz crystals) subject to volatile supply from a water-saturated (≥6 wt% H
2
O) andesite magma at 950 °C and 4 kbar. Our experimental run products show unidirectional solidification textures (i.e. comb layering) as crystals nucleate at the mafic–felsic interface and grow into the mafic end-member. This process is driven by isothermal and isobaric undercooling resulting from a change in liquidus temperature as water migrates from the mafic to the felsic magma. We refer to this process as “chemical quenching” and suggest that some textures associated with natural mafic–felsic interactions are not simply cooling-driven in origin, but can be caused by exsolution of volatiles adjacent to an interface, whether a water-undersaturated felsic magma (as in our experiments) or a fracture.
The majority of basaltic magmas stall in the Earth's crust as a result of the rheological evolution caused by crystallization during transport. However, the relationships between crystallinity, ...rheology and eruptibility remain uncertain because it is difficult to observe dynamic magma crystallization in real time. Here, we present in-situ 4D data for crystal growth kinetics and the textural evolution of pyroxene during crystallization of trachybasaltic magmas in high-temperature experiments under water-saturated conditions at crustal pressures. We observe dendritic growth of pyroxene on initially euhedral cores, and a surprisingly rapid increase in crystal fraction and aspect ratio at undercooling ≥30 °C. Rapid dendritic crystallization favours a rheological transition from Newtonian to non-Newtonian behaviour within minutes. We use a numerical model to quantify the impact of rapid dendritic crystallization on basaltic dike propagation, and demonstrate its dramatic effect on magma mobility and eruptibility. Our results provide insights into the processes that control whether intrusions lead to eruption or not.
The Ethiopian Rift Valley hosts the longest record of human co-existence with volcanoes on Earth, however, current understanding of the magnitude and timing of large explosive eruptions in this ...region is poor. Detailed records of volcanism are essential for interpreting the palaeoenvironments occupied by our hominin ancestors; and also for evaluating the volcanic hazards posed to the 10 million people currently living within this active rift zone. Here we use new geochronological evidence to suggest that a 200 km-long segment of rift experienced a major pulse of explosive volcanic activity between 320 and 170 ka. During this period, at least four distinct volcanic centres underwent large-volume (>10 km
) caldera-forming eruptions, and eruptive fluxes were elevated five times above the average eruption rate for the past 700 ka. We propose that such pulses of episodic silicic volcanism would have drastically remodelled landscapes and ecosystems occupied by early hominin populations.
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