Equilibration Pressures Research Articles

Eclogite and Garnet Pyroxenite Xenoliths from Kimberlites Emplaced Along the Southern Margin of the Kaapvaal Craton, Southern Africa: Mantle or Lower Crustal Fragments?

Abstract Eclogite xenoliths, together with garnet pyroxenites and some mafic garnet granulites, found in kimberlites located along the southern margin of the Kaapvaal craton in southern Africa have been analysed by electron microprobe and mass spectrometry techniques to determine their geochemical characteristics. The majority of eclogites are bimineralic with garnet and omphacitic clinopyroxene in subequal proportions, with rutile as the main accessory phase; a few contain kyanite. Based on K2O in clinopyroxene and Na2O in garnet, the eclogites can be classified as Group II eclogites, and the majority are high-Ca in character. Garnet pyroxenites comprise garnet clinopyroxenites and garnet websterites. Major and trace element concentrations and isotope ratios of reconstituted bulk rock compositions of the eclogites and garnet pyroxenites allow constraints to be placed on depth of origin and likely protolith history. Calculated Fe–Mg exchange equilibration temperatures for the eclogites range from 815 to 1000 °C, at pressures of 1·7 ± 0·4 GPa as determined by REE partitioning, indicating that they were sampled from depths of 50–55 km; i.e. within the lower crust of the Namaqua–Natal Belt. The garnet pyroxenites show slightly lower temperatures (686–835 °C) at similar pressures of equilibration. Initial 143Nd/144Nd and 87Sr/86Sr ratios (calculated to time of kimberlite emplacement) of both lithologies overlap the field for lower crustal samples from the Namaqua–Natal Belt. Further evidence for a crustal origin is found in the similar REE patterns shown by many of the associated garnet granulite xenoliths. Garnet pyroxenites are interpreted to have a similar origin as the associated eclogites but with the mafic protolith having insufficient Na (i.e. low modal plagioclase) to allow for development of omphacitic pyroxene. Metamorphism of the mafic protoliths to these eclogites and garnet pyroxenites is inferred to have occurred during crustal shortening and thickening associated with the collision of the Namaqua–Natal Belt with the Kaapvaal craton at 1–1·2 Ga.

A Palaeoproterozoic diamond-bearing lithospheric mantle root beneath the Archean Sask Craton, Canada

The recently recognised Sask Craton, a small terrane with Archean (3.3–2.5 Ga) crustal ages, is enclosed in the Paleoproterozoic (1.9–1.8 Ga) Trans Hudson Orogen (THO). Only limited research has been conducted on this craton, yet it hosts major diamond deposits within the Cretaceous (~106 to ~95 Ma) Fort à la Corne (FALC) Kimberlite Field. This study describes major, trace and platinum group element data, as well as osmium isotopic data from peridotitic mantle xenoliths (n = 26) from the Star and Orion South kimberlites.The garnet-bearing lithospheric mantle is dominated by moderately depleted lherzolite. Equilibration pressures and temperatures (2.7 to 5.5 GPa and 840 to 1250 °C) for these garnet peridotites define a cool geotherm indicative of a 210 km thick lithosphere, similar to other cratons worldwide. Many of the peridotite xenoliths show the major and trace element signatures of carbonatitic and kimberlitic melt metasomatism. The Re–Os isotopic data yield TRD (time of Re-depletion) model ages, which provide minimum estimates for the timing of melt depletion, ranging from 2.4 to 0.3 Ga, with a main mode spanning from 2.4 to 1.7 Ga. No Archean ages were recorded. This finding and the complex nature of events affecting this terrane from the Archean through the Palaeoproterozoic provide evidence that the majority of the lithospheric mantle was depleted and stabilised in the Palaeoproterozoic, significantly later than the Archean crust. The timing of the dominant lithosphere formation is linked to rifting (~2.2 Ga - 2.0 Ga), and subsequent collision (1.9–1.8 Ga) of the Superior and Hearne craton during the Wilson cycle of the Trans Hudson Orogen.

Mineral Chemistry Perspective of Nain Ophiolite Mélange, Central Iran

The present study documents detailed mineral chemi-stry perspective of Nain ophiolite melange (NOM) of Central Iran with an aim of deciphering the mineral systematics and understanding geothermobarometric equilibration. The NOM covers ~600 km2 and is located at the northwest margin of Central Iranian Microcontinental block. NOM is represented by a sheared, tectonized and serpentinized peridotite in-truded by coarse-grained pegmatitic gabbroic dykes, layered gabbro, sheeted dolerite dykes (with typical rodingite alteration) and pillow basalts. Plagioclase in pillow basalt is albitic and indicates its spilitic affinity, while pyroxene is typically quad pyroxene (augite to diopside). Amphiboles belong to calcic group and range from actinolite to magnesio hornblende. Ilme-nite is the characteristic opaque phase. Clinopyroxene thermometry records a temperature span of 1100–1300C, while amphibole thermometry records 979–1145C. Two-feldspar thermometry also records a similar thermometric range. Amphibole barometry shows higher pressure of equilibration for mantle pegmatite in general and a very low equilibration pressure for sheeted dyke. Pyroxene compositions typically indicate a calc-alkaline basaltic (orogenic) parentage. NOM signifies lherzolite ophiolite type in a chromite-free environment and it is analogous to an idealized ophiolite succession, but has been emplaced in the form of discrete tectonic melange.

Insights into the mantle geochemistry of scandium from a meta-analysis of garnet data

The meta-analysis of about 13,000 analyses of scandium content in garnet grains shows that, below the spinel–garnet transition, this phase carries about three-quarters of the Sc budget of the mantle, indicating its control on Sc mobility. The Sc content of garnets in mafic rocks is low, due to a dilution effect resulting from their high modal content in garnet. Garnets from ultramafic rocks exhibit a wider range of Sc concentrations. We assess the relative influence of thermobarometry, crystal chemistry and fluid-related events on the distribution of Sc in garnet from such rocks to improve the tracking of geochemical processes in the mantle. Pressure and temperature of equilibration in the mantle are second-order factors influencing the Sc content of garnet, while crystal chemistry, in particular CrCr+Al and CaCa+Mg, is the main parameter controlling the compatibility of Sc. Scandium is incorporated in both X and Y sites of Cr-Ca-rich garnets, resulting in a behaviour intermediate between rare-earth elements, incorporated in the X site, and trivalent transition elements, occupying the Y site. This affinity for both sites results in a mild compatibility of Sc in the garnet stability field of the mantle; hence Sc concentration in garnet increases with melt extraction and can be reduced by silicate-melt metasomatism. In contrast, metasomatism by volatile-rich fluids increases the Sc concentration in garnet. The control of garnet on the compatibility of Sc in deep lithospheric rocks demonstrates the potential of using Sc to track the conditions of formation of magmas and their residual rocks, as well as the origin and nature of metasomatic fluids.

Lower crustal hydrothermal circulation at slow-spreading ridges: evidence from chlorine in Arctic and South Atlantic basalt glasses and melt inclusions

Hydrothermal circulation at slow-spreading ridges is important for cooling the newly formed lithosphere, but the depth to which it occurs is uncertain. Magmas which stagnate and partially crystallize during their rise from the mantle provide a means to constrain the depth of circulation because assimilation of hydrothermal fluids or hydrothermally altered country rock will raise their chlorine (Cl) contents. Here we present Cl concentrations in combination with chemical thermobarometry data on glassy basaltic rocks and melt inclusions from the Southern Mid-Atlantic Ridge (SMAR; ~ 3 cm year−1 full spreading rate) and the Gakkel Ridge (max. 1.5 cm year−1 full spreading rate) in order to define the depth and extent of chlorine contamination. Basaltic glasses show Cl-contents ranging from ca. 50–430 ppm and ca. 40–700 ppm for the SMAR and Gakkel Ridge, respectively, whereas SMAR melt inclusions contain between 20 and 460 ppm Cl. Compared to elements of similar mantle incompatibility (e.g. K, Nb), Cl-excess (Cl/Nb or Cl/K higher than normal mantle values) of up to 250 ppm in glasses and melt inclusions are found in 75% of the samples from both ridges. Cl-excess is interpreted to indicate assimilation of hydrothermal brines (as opposed to bulk altered rock or seawater) based on the large range of Cl/K ratios in samples showing a limited spread in H2O contents. Resorption and disequilibrium textures of olivine, plagioclase and clinopyroxene phenocrysts and an abundance of xenocrysts and gabbroic fragments in the SMAR lavas suggest multiple generations of crystallization and assimilation of hydrothermally altered rocks that contain these brines. Calculated pressures of last equilibration based on the major element compositions of melts cannot provide reliable estimates of the depths at which this crystallization/assimilation occurred as the assimilation negates the assumption of crystallization under equilibrium conditions implicit in such calculations. Clinopyroxene–melt thermobarometry on rare clinopyroxene phenocrysts present in the SMAR magmas yield lower crustal crystallization/assimilation depths (10–13 km in the segment containing clinopyroxene). The Cl-excesses in SMAR melt inclusions indicate that assimilation occurred before crystallization, while also homogeneous Cl in melts from Gakkel Ridge indicate Cl addition during magma chamber processes. Combined, these observations imply that hydrothermal circulation reaches the lower crust at slow-spreading ridges, and thereby promotes cooling of the lower crust. The generally lower Cl-excess at slow-spreading ridges (compared to fast-spreading ridges) is probably related to them having few if any permanent magma chambers. Magmas therefore do not fractionate as extensively in the crust, providing less heat for assimilation (on average, slow-spreading ridge magmas have higher Mg#), and hydrothermal systems are ephemeral, leading to lower total degrees of crustal alteration and more variation in the amount of Cl contamination. Hydrothermal plumes and vent fields have samples in close vicinity that display Cl-excess, mostly of > 25 ppm, which thus can aid as a guide for the exploration of (active or extinct) hydrothermal vent fields on the axis.

Gas permeability measurements from pressure pulse decay laboratory data using pseudo-pressure and pseudo-time transformations

This paper presents a pseudo-pressure and pseudo-time straight-line approach to interpret laboratory pulse decay data in order to estimate rock core permeability using gas as the pore fluid. The implementation of the straight-line approach provides a practical method to estimate gas permeability from experimental data, as long as changes in gas viscosity and compressibility are negligible. On the other hand, pseudo-pressure and pseudo-time allow the transformation of the compressible flow equation from its highly nonlinear form to a quasi-linear partial differential equation, where changes in gas viscosity, gas compressibility and compressibility factor are accounted for. The purpose of this work is to combine both, pseudo-functions and the straight-line approach to estimate gas permeability from pressure pulse laboratory data with a more rigorous treatment of gas properties. Five pulse decay experiments were performed in Marcellus shale cores at pore pressures ranging from 130 to 700 psi in a triaxial cell to estimate permeability and porosity of ultra-low permeability cores. The experiments were made in an increasing order of equilibration pressure starting from 130 until 700 psi, and vertical and radial stresses kept constant at 1500 psi. Permeability estimates were compared against the P2-approach to show the validity of the proposed method at low gas pressures.

Shallow melting of MORB‐like mantle under hot continental lithosphere, Central Anatolia

AbstractWidespread mafic volcanism, elevated crustal temperatures, and plateau‐type topography in Central Anatolia, Turkey, could collectively be the result of lithospheric delamination, mantle upwelling, and tectonic escape. We use results from 40Ar/39Ar geochronology, basalt geochemistry, and a passive‐source broadband seismic experiment obtained in a collaborative international effort (Continental Dynamics‐Central Anatolia Tectonics) to investigate the upper mantle structure and evolution of melting conditions over an ∼2400 km2 area south and west of Hasan volcano. New 40Ar/39Ar dates for the basalts mostly cluster between 0.2 and 0.6 Ma, but some scoria cones are as old as 2.5 Ma. Basalts are dominantly Mg‐rich (Mg# = 62–71), moderately alkaline (normative Ne < 5 wt %), and, based on major and trace element signatures, derived from a peridotitic source. Covariations between radiogenic isotope and trace element signatures reveal contributions from a subduction‐related component and intraplate‐like mantle asthenosphere, as well as from ambient upper mantle. Central Anatolian basalts reflect maximum mantle potential temperatures of <1350°C and an average pressure of melt equilibration of 1.4 GPa, which are cooler and shallower than for basalts from Eastern and Western Anatolia. When considered in light of regionally slow upper mantle shear wave velocities, the mantle lithosphere may be thin and infiltrated by melts, or largely absent. An absence of secular changes in melting conditions suggests little to no lithospheric thinning over the past ∼1 Ma, despite evidence for lithospheric extension. Hasan basalts appear to be generated by decompression melting in response to the rollback of the Cyprean slab.

Petit-spot as definitive evidence for partial melting in the asthenosphere caused by CO2

The deep carbon cycle plays an important role on the chemical differentiation and physical properties of the Earth’s mantle. Especially in the asthenosphere, seismic low-velocity and high electrical conductivity due to carbon dioxide (CO2)-induced partial melting are expected but not directly observed. Here we discuss the experimental results relevant to the genesis of primitive CO2-rich alkali magma forming petit-spot volcanoes at the deformation front of the outer rise of the northwestern Pacific plate. The results suggest that primitive melt last equilibrated with depleted peridotite at 1.8–2.1 GPa and 1,280–1,290 °C. Although the equilibration pressure corresponds to the pressure of the lower lithosphere, by considering an equilibration temperature higher than the solidus in the volatile–peridotite system along with the temperature of the lower lithosphere, we conclude that CO2-rich silicate melt is always produced in the asthenosphere. The melt subsequently ascends into and equilibrates with the lower lithosphere before eruption.

Revisiting the electron microprobe method of spinel-olivine-orthopyroxene oxybarometry applied to spinel peridotitesk

Natural peridotite samples containing olivine, orthopyroxene, and spinel can be used to assess the oxygen fugacity f O 2 of the upper mantle. The calculation requires accurate and precise quantification of spinel Fe 3+ /ΣFe ratios. Wood and Virgo (1989) presented a correction procedure for electron microprobe (EPMA) measurements of spinel Fe 3+ /ΣFe ratios that relies on a reported correlation between the difference in Fe 3+ /ΣFe ratio by Mossbauer spectroscopy and by electron microprobe (ΔFe 3+ /ΣFe Moss-EPMA ) and the Cr# [Cr/(Al+Cr)] of spinel. This procedure has not been universally adopted, in part, because of debate as to the necessity and effectiveness of the correction. We have performed a series of replicate EPMA analyses of several spinels, previously characterized by Mossbauer spectroscopy, to test the accuracy and precision of the Wood and Virgo correction. While we do not consistently observe a correlation between Cr# and ΔFe 3+ /ΣFe Moss-EPMA in measurements of the correction standards, we nonetheless find that accuracy of Fe 3+ /ΣFe ratios determined for spinel samples treated as unknowns improves when the correction is applied. Uncorrected measurements have a mean ΔFe 3+ /ΣFe Moss-EPMA = 0.031 and corrected measurements have a mean ΔFe 3+ /ΣFe Moss-EPMA = −0.004. We explain how the reliance of the correction on a global correlation between Cr# and MgO concentration in peridotitic spinels improves the accuracy of Fe 3+ /ΣFe ratios despite the absence of a correlation between ΔFe 3+ /ΣFe Moss-EPMA and Cr# in some analytical sessions. Precision of corrected Fe 3+ /ΣFe ratios depends on the total concentration of Fe, and varies from ±0.012 to ±0.032 (1σ) in the samples analyzed; precision of uncorrected analyses is poorer by approximately a factor of two. We also present an examination of the uncertainties in the calculation contributed by the other variables used to derive f O 2 . Because there is a logarithmic relationship between the activity of magnetite and log f O 2 , the uncertainty in f O 2 relative to the QFM buffer contributed by the electron microprobe analysis of spinel is asymmetrical and larger at low ferric Fe concentrations (+0.3/−0.4 log units, 1σ, at Fe 3+ /ΣFe = 0.10) than at higher ferric Fe concentrations (±0.1 log units, 1σ, at Fe 3+ /ΣFe = 0.40). Electron microprobe analysis of olivine and orthopyroxene together contribute another ±0.1 to ±0.2 log units of uncertainty (1σ). Uncertainty in the temperature and pressure of equilibration introduce additional errors on the order of tenths of log units to the calculation of relative f O 2 . We also document and correct errors that appear in the literature when formulating f O 2 that, combined, could yield errors in absolute f O 2 of greater than 0.75 log units—even with perfectly accurate Fe 3+ /ΣFe ratios. Finally, we propose a strategy for calculating the activity of magnetite in spinel that preserves information gained during analysis about the ferric iron content of the spinel. This study demonstrates the superior accuracy and precision of corrected EPMA measurements of spinel Fe 3+ /ΣFe ratios compared to uncorrected measurements. It also provides an objective method for quantifying uncertainties in the calculation of f O 2 from spinel peridotite mineral compositions.

The depth of sub-lithospheric diamond formation and the redistribution of carbon in the deep mantle

Most diamonds form in the Earth's lithosphere but a small proportion contain Si-rich majoritic garnet inclusions that indicate formation in the deeper mantle. The compositions of syngenetic garnet inclusions can potential yield information on both the depth and mantle lithology in which the diamonds formed. Pressure dependent changes in garnet compositions have been calibrated using the results of experiments conducted in a multi-anvil apparatus at pressures between 6 and 16 GPa and temperatures of 1000 to 1400 °C. Using the results of these experiments a barometer was formulated based on an empirical parameterisation of the two major majoritic substitutions, referred to as majorite (Maj; Al3+=Mg2++Si4+), and Na-majorite (Na-Maj; Mg2++Al3+=Na++Si4+). Moreover, previously published experimental garnet compositions from basaltic, kimberlite, komatiite and peridotite bulk compositions were included in the calibration, which consequently covers pressures from 6 to 20 GPa and temperatures from 900 to 2100 °C. Experimental pressures are reproduced over these conditions with a standard deviation of 0.86 GPa.The barometer is used to determine equilibration pressures of approximately 500 reported garnet inclusions in diamonds from a range of localities. As the majority of these inclusions are proposed to be syngenetic this allows a detailed picture of diamond formation depths and associated source rocks to be established using inclusion chemistry. Geographic differences in diamond source rocks are mapped within the sub-lithospheric mantle to over 500 km depth. Continuous diamond formation occurs over this depth range within lithologies with eclogitic affinities but also in lithologies that appear transitional between eclogitic and peridotitic bulk compositions, with an affinity to pyroxenites. The geographic differences between eclogitic and pyroxenitic diamond source rocks are rationalised in terms of diamond formation within downwelling and upwelling regimes respectively. Macroscopic diamond formation in rocks with pyroxenite compositions are likely facilitated in the deep mantle by higher average oxidation states and low mineral H2O solubility compared to the surrounding mantle, which aid the mobility of C–O–H volatile species. The apparent lack of inclusions with a peridotite affinity may result from generally low oxygen fugacities in such lithologies, which reduces carbon mobility, and the lack of a suitable oxidising agent to allow diamonds to form from CH4. This glimpse of deep carbon cycle processes implies that heterogeneities in the carbon content, redox state and chemical composition of the mantle may be strongly coupled.

Reconciling mantle wedge thermal structure with arc lava thermobarometric determinations in oceanic subduction zones

AbstractSubduction zone mantle wedge temperatures impact plate interaction, melt generation, and chemical recycling. However, it has been challenging to reconcile geophysical and geochemical constraints on wedge thermal structure. Here we chemically determine the equilibration pressures and temperatures of primitive arc lavas from worldwide intraoceanic subduction zones and compare them to kinematically driven thermal wedge models. We find that equilibration pressures are typically located in the lithosphere, starting just below the Moho, and spanning a wide depth range of ∼25 km. Equilibration temperatures are high for these depths, averaging ∼1300°C. We test for correlations with subduction parameters and find that equilibration pressures correlate with upper plate age, indicating overriding lithosphere thickness plays a role in magma equilibration. We suggest that most, if not all, thermobarometric pressure and temperature conditions reflect magmatic reequilibration at a mechanical boundary, rather than reflecting the conditions of major melt generation. The magma reequilibration conditions are difficult to reconcile, to a first order, with any of the conditions predicted by our dynamic models, with the exception of subduction zones with very young, thin upper plates. For most zones, a mechanism for substantially thinning the overriding plate is required. Most likely thinning is localized below the arc, as kinematic thinning above the wedge corner would lead to a hot fore arc, incompatible with fore‐arc surface heat flow and seismic properties. Localized subarc thermal erosion is consistent with seismic imaging and exhumed arc structures. Furthermore, such thermal erosion can serve as a weakness zone and affect subsequent plate evolution.

An experimental study of amphibole stability in low-pressure granitic magmas and a revised Al-in-hornblende geobarometer

We report new experimental data on the composition of magmatic amphiboles synthesised from a variety of granite (sensu lato) bulk compositions at near-solidus temperatures and pressures of 0.8–10 kbar. The total aluminium content (Al^(tot)) of the synthetic calcic amphiboles varies systematically with pressure (P), although the relationship is nonlinear at low pressures (<2.5 kbar). At higher pressures, the relationship resembles that of other experimental studies, which suggests of a general relationship between Al^(tot) and P that is relatively insensitive to bulk composition. We have developed a new Al-in-hornblende geobarometer that is applicable to granitic rocks with the low-variance mineral assemblage: amphibole + plagioclase (An_(15–80)) + biotite + quartz + alkali feldspar + ilmenite/titanite + magnetite + apatite. Amphibole analyses should be taken from the rims of grains, in contact with plagioclase and in apparent textural equilibrium with the rest of the mineral assemblage at temperatures close to the haplogranite solidus (725 ± 75 °C), as determined from amphibole–plagioclase thermometry. Mean amphibole rim compositions that meet these criteria can then be used to calculate P (in kbar) from Al^(tot) (in atoms per formula unit, apfu) according to the expression: P (kbar)=0.5+0.331(8)×Al^(tot) +0.995(4)×(Al^(tot))^2. This expression recovers equilibration pressures of our calibrant dataset, comprising both new and published experimental and natural data, to within ±16 % relative uncertainty. An uncertainty of 10 % relative for a typical Al^(tot) value of 1.5 apfu translates to an uncertainty in pressure estimate of 0.5 kbar, or 15 % relative. Thus the accuracy of the barometer expression is comparable to the precision with which near-solidus amphibole rim composition can be characterised.

The making of an underplate: Pyroxenites from the Ethiopian lithosphere

Magmatic underplates, which are characterized by high seismic velocity and anomalously high density in comparison to normal lower crust, are among the most significant compositional heterogeneities identified in the rift lithosphere. The interaction of a magma with the lithospheric mantle to form pyroxenites is considered a viable mechanism for the creation of an underplate, though the details of such interaction are difficult to constrain with geophysical techniques alone. Here we present a petrographic and geochemical study of pyroxenite xenoliths erupted on the western Ethiopian plateau from along the Yerer Tullul Wellel Volcano-Tectonic lineament (YTVL). The equilibration pressures recorded by the xenoliths (0.88–1.49GPa) are consistent with an origin within a seismically and gravitationally defined underplate that exists beneath the western Ethiopian plateau. Three groups of pyroxenite have been identified: Group I – characterized by a relative enrichment of orthopyroxene, and clinopyroxene with unfractionated heavy rare elements (HREE; TbN/YbN=1.2–1.6); Group II – characterized by a relative enrichment in clinopyroxene, which have fractionated HREE (TbN/YbN=1.6–2.0); Group III – transitional xenoliths that contain pyroxene characteristic of Group I but also contain clinopyroxene characteristic of Group II. The magma responsible for the formation of Group I xenoliths was likely of sub-alkaline composition and derived from a spinel peridotite on the basis of relatively unfractionated REE profiles. This magma may have been the Oligocene Low Titanium (LT)-type flood basalts associated with the Afar plume, or subduction-related magmatism associated with island arc activity during the construction of the regional lithosphere during the Neoproterozoic Pan African Orogeny. The magma responsible for the formation of Group II xenoliths was likely of alkaline composition and derived from a source with some garnet on the basis of fractionated HREE. This magma may have been the Oligocene High Titanium (HT)-type flood basalts associated with the Afar plume or Miocene-recent magmas associated with the reactivation of the YTVL during rift development. Our results show that the lithospheric mantle beneath the YTVL was impacted by at least two metasomatic events and exhibits characteristics more similar to the xenolith suites from the rift valley than elsewhere on the western Ethiopian plateau. We propose that the magmatic underplate beneath the western Ethiopian plateau was formed by multiple episodes of magma-lithosphere interaction, likely developing over time rather than in a single event.

An experimental investigation of the stability of majoritic garnet in the Earth’s mantle and an improved majorite geobarometer

The stability of the majorite component in garnet has been experimentally investigated at high pressure and high temperature, focusing on the effect of bulk composition and temperature. High-pressure experiments were performed in a multi-anvil apparatus, at pressures ranging from 6 to 14.5 GPa, and temperatures between 1400 and 1700 °C. Experiments were performed in a range of bulk compositions in the system SiO2–Al2O3–Cr2O3–CaO–MgO with varying Cr/(Cr + Al) ratios. The majorite content of garnet gradually increases with pressure, and the composition of the garnet, specifically the Cr/(Cr + Al) ratio, exerts a significant effect on the majorite substitution. We found no significant effect of temperature. We use the experimental results in combination with the literature data to derive two empirical geobarometers, which can be used to determine the equilibration pressure of natural majoritic garnets of peridotitic and eclogitic bulk compositions. The barometer for peridotitic compositions is $${\text{P}} = - 77.1 + 27.6 \times {\text{Si}} + 1.67 \times {\text{Cr}}$$ And the barometer for eclogitic compositions is $${\text{P}} = - 29.6 + 11.8 \times {\text{Si}} + 7.81 \times {\text{Na}} + 4.49 \times {\text{Ca}}.$$

Thermal structure and melting conditions in the mantle beneath the Basin and Range province from seismology and petrology

AbstractTo better constrain the temperature structure in the upper mantle, we jointly invert seismic surface wave velocities and basalt thermobarometry. New measurements of the water concentration (1.0–3.5 wt %) and oxygen fugacity (FMQ + 0.5 to + 1.5) of basalts from seven recently active volcanic fields in the Basin and Range province (Cima, Pisgah, Amboy, Big Pine, Black Rock, Snow Canyon, W. Grand Canyon) enable more accurate equilibration pressure (P) and temperature (T) estimates of the mantle melts. We developed a revised thermobarometer that more precisely predicts the results of laboratory experiments on melts equilibrated with olivine and orthopyroxene and accounts for the effects of water and CO2. Applying these methods to basalts from the Basin and Range we find that most equilibrated near the dry solidus in P‐T space and at depths in the vicinity of the lithosphere‐asthenosphere boundary (LAB) inferred from receiver function analysis and Rayleigh surface wave tomography. The wet basalts should have begun melting well below the dry solidus, so the depths of equilibration probably reflect ponding of rising melts beneath the nominally dry lithosphere. A two‐parameter thermal model is sufficient to simultaneously satisfy both the seismological and petrological constraints. In the model, the depth to the dry solidus defines the bottom boundary of the conductive lid, while the potential temperature (Tp) controls the asthenosphere and LAB thermal structure. The optimum estimates of Tp range from &lt;1300 to &gt;1500°C, and depths to the LAB range from ∼55 to 75 km, with uncertainties on the order of ±50°C and ±10 km. In contrast to standard tomographic images or basalt thermobarometry, the output of the joint inversion is a geotherm that can be tested quantitatively against other observations.

Impact-induced melting during accretion of the Earth

Because of the high energies involved, giant impacts that occur during planetary accretion cause large degrees of melting. The depth of melting in the target body after each collision determines the pressure and temperature conditions of metal-silicate equilibration and thus geochemical fractionation that results from core-mantle differentiation. The accretional collisions involved in forming the terrestrial planets of the inner Solar System have been calculated by previous studies using N-body accretion simulations. Here we use the output from such simulations to determine the volumes of melt produced and thus the pressure and temperature conditions of metal-silicate equilibration, after each impact, as Earth-like planets accrete. For these calculations a parametrised melting model is used that takes impact velocity, impact angle and the respective masses of the impacting bodies into account. The evolution of metal-silicate equilibration pressures (as defined by evolving magma ocean depths) during Earth's accretion depends strongly on the lifetime of impact-generated magma oceans compared to the time interval between large impacts. In addition, such results depend on starting parameters in the N-body simulations, such as the number and initial mass of embryos. Thus, there is the potential for combining the results, such as those presented here, with multistage core formation models to better constrain the accretional history of the Earth.

Microfluidic Bubble Point Measurement Using Thermal Nucleation

We present a highly miniaturized sensor for performing accurate on-chip detection of gas equilibration (bubble point) pressures in liquids, which is manufactured using low-cost batch microelectromechanical system technology. The measurement of gas equilibration (bubble point) pressures in liquids has a diverse spectrum of applications ranging from the oil industry, where the bubble point of a crude oil is important for making informed decisions on production and exploration, to the fisheries industry, where gas saturation needs to be controlled to ensure animal health. Oilfield measurements of bubble point pressures often require acquisition of large sample volumes that are subsequently investigated off-line in laboratory facilities, with important consequences on both cost and operational response times. The present work demonstrates the potential of microelectromechanical system technology to respond to such diverse industry needs by providing an economical solution able to perform in situ real-time monitoring of gas equilibration pressures with significantly reduced equipment size and analysis time. By implementing rapid on-chip local heating using an integrated platinum electrode and subsequently observing the fluid behavior—either microscopically or with embedded thermal conductivity sensors—we show that we can effectively overcome the nucleation barrier and perform highly accurate, repeatable measurements of the bubble point pressure.

Formation of diamondiferous kyanite–eclogite in a subduction mélange

Diamond- and kyanite-bearing eclogites from the Lace kimberlite on the Kaapvaal craton have common picritic to gabbroic oceanic protoliths with bimineralic eclogites, lying on arrays of Eu∗ and ∑REE that are consistent with accumulation and fractionation of plagioclase and olivine. However, they also show significant compositional differences, such as more grossular-rich garnet and aluminous clinopyroxene (cpx), which require the operation of additional processes. Their nature is elucidated using mineral major- and trace-element compositions, as well as Sr isotope ratios determined by in situ techniques.Highly variable major-element compositions across the co-genetic eclogite suites exert a strong effect on the trace-element distribution between garnet and cpx, whereby Sc, Ge, Sr, Y, Cd, REE, Th and U partition more strongly into garnet with increasing grossular-content. Thus, significant differences between the trace-element compositions of garnet can ensue from crystal-chemical effects alone, making their use as petrogenetic indicators potentially ambiguous. After correcting for these compositional effects, garnet in kyanite-/diamond eclogites, and in eclogites devoid of accessory minerals but with similar signatures, shows depletion (or dilution) in Sc, Ge, Y, In, Zr, Hf and the HREE, and enrichment in the LREE and Th compared to garnet in bimineralic eclogites. This is interpreted as the signature of a pelite-derived melt, which was transferred by addition of aluminous cpx that later exsolved kyanite and garnet, as observed in other aluminous eclogite suites. Continental input can explain initial (at 2.9Ga) 87Sr/86Sr⩾0.714 measured in cpx in eleven samples with low 87Rb/86Sr (<0.01). The association of diamond with kyanite suggests that diamond formation is also linked to this event, possibly due to diamond formation by oxidation of reduced carbon, such as methane, and attendant reduction of Fe3+ in garnet.This model of sediment melt–oceanic crust interaction reconciles evidence for both low- and high-pressure igneous processes in some aluminous eclogites. We suggest that a subduction mélange is a favourable setting for the transfer of a sediment-derived signature into oceanic crust, leading to formation of diamondiferous kyanite–eclogites from bimineralic eclogites. Diapirism, fluxed by the presence of partial melt, may have facilitated dispersal of the eclogites in the lithosphere column, consistent with their widely varying equilibration pressures ranging from ∼5 to 8GPa.

Erratum to: Melting the hydrous, subarc mantle: the origin of primitive andesites

An excel spreadsheet is included in online supplemental materials to aid with use of the model presented. The spreadsheet calculates the first three equations in Table 6 using two methods: it calculates (1) P (GPa) and T (°C) of last equilibration for known H2O contents of last equilibration or (2) H2O (wt%) and T (°C) of last equilibration when the pressure (GPa) of last equilibration is known. To use the spreadsheet, enter your compositions into the ‘Input’ sheet as well as your known H2O or pressure for each composition. The sheet goes through the following steps:

Experimental calibration of a garnet–clinopyroxene geobarometer for mantle eclogites

Thermodynamic parameters have been calibrated for a geobarometer suitable for use on eclogitic mantle xenoliths. The barometer is based on the incorporation of tetrahedrally coordinated aluminum in clinopyroxene coexisting with garnet and has been calibrated using the results of piston cylinder and multi-anvil experiments performed between pressures of 3 and 7 GPa and temperatures from 1,200 to 1,550 °C. Starting materials were hydrous and anhydrous synthetic mixtures of basaltic bulk compositions that yielded homogeneous bimineralic garnet–clinopyroxene phase assemblages. The experimental data set was expanded by employing results from previous experimental studies conducted in eclogitic systems, which widened the range of applicable conditions and compositions. The calibration reproduces experimental pressures of bimineralic eclogite assemblages, in addition to SiO2-saturated and kyanite-bearing eclogites, to within 0.4 GPa at the 95 % confidence interval. The barometer was then used to examine equilibration pressures recorded by natural mantle eclogites from various xenolith locations covering a wide pressure, temperature, and compositional range.