Liquidus Conditions Research Articles

The development of internal pressure standards for in-house elastic wave velocity measurements in multi-anvil presses.

Ultrasonic systems are powerful tools to determine elastic wave velocities of minerals and materials at high pressure and temperature and have been extensively developed in recent decades. However, accurate measurement of sample length is required to convert travel times into wave velocities, limiting their use to synchrotron facilities or room temperature experiments in laboratories. We have made use of a close collaboration between the Bayerisches Geoinstiut and the P61B end-station beamline (PETRA III - DESY) to install ultrasonic systems and develop a novel dual travel time method for in situ pressure determination without the need for synchrotron radiation. Our method relies on the travel times of elastic waves through a reference material; it requires a thermocouple and is non-intrusive, with the reference material replacing the backing plate of the high-pressure assembly. Pressures obtained from this dual travel time method show excellent agreement with those obtained from x-ray diffraction using synchrotron radiation on standard materials. Our novel method enables in situ pressure determination at varying temperatures during in-house ultrasonic interferometry experiments. This allows us not only to determine the elastic behavior of minerals and materials but also to investigate phase diagrams, solidus, or liquidus conditions at varying pressures and temperatures during in-house experiments. During the installation of the pulse-echo ultrasonic system, we identified critical parameters for obtaining reliable data. While these requirements are well-known to experts, this study presents a comprehensive review of the different characteristics of ultrasonic systems, providing user-friendly guidelines for new users installing and operating such systems in high-pressure and high-temperature conditions.

The anorogenic late Ediacaran granite-rhyolite porphyries of Gabal Abu Durba, Sinai: termination of magmatism in the Neoproterozoic Arabian-Nubian Shield crust

ABSTRACT Anorogenic magmatism in the northern Arabian-Nubian Shield occurred during a long-lasting crustal extension event (<580 Ma), which succeeded the formation of the Pan-African orogenic belt in NE Africa and Arabia. Late Neoproterozoic anorogenic felsic volcanic-subvolcanic rocks, along with post-collisional granitoid suites, are exposed in the Gabal Abu Durba mountain range along the eastern flank of the Gulf of Suez in Sinai. These rocks include granite and rhyolite porphyries, metaluminous to weakly peralkaline with distinct potassic alkaline and ferroan affinities. Major and trace element characteristics such as K2O/MgO >16, total alkalis >8.5 wt.%, (K2O + Na2O)/CaO >10, agpaitic index (NK/A) >0.85 and Zr (>250 ppm), Nb (>20 ppm), Y (>80 ppm), Zn (>100 ppm) and Ce (>100 ppm), and 10,000 × Ga/Al >2.6 and [Zr + Nb + Y + Ce] >350 ppm are suggestive of an A-type granite genesis. Zircon U-Pb dating of two representative samples returned crystallization ages of 569 ± 2.6 Ma and 561.7 ± 3.2 Ma for granite and rhyolite porphyries, respectively. The melting temperatures estimated at 997–1020°C are consistent with high-temperature liquidus conditions. Fractional crystallization, coupled with less significant crustal assimilation, was likely the main mechanism of formation of these rocks from a common primitive asthenospheric mantle-derived trachytic magma in an anorogenic intra-plate setting. The A-type alkali granite and rhyolite porphyries of Gabal Abu Durba are manifestations of mantle upwelling and lower crust underplating in the late Ediacaran anorogenic stage. These felsic subvolcanic intrusions (≈ 569–562 Ma) along with their volcanic counterparts constitute a new unmatched rock unit that most probably represent the last chapter of the anorogenic magmatism in the Neoproterozoic Arabian-Nubian Shield crust before the beginning of Phanerozoic era.

Magmatic Processes at the Oman Ophiolite Paleoridge – Perspectives on the Role of Water

AbstractThe Oman ophiolite is regarded as best proxy for accreted oceanic crust from typical fast‐spreading ridge systems on land. However, the Oman ophiolite is influenced by initial subduction zone initiation, and the nature of the details of the subduction zone setting is still under controversial debate. While a first magmatic phase shows features of magmatic accretion very similar to those known from the East Pacific Rise, except that the primary melts were slightly water‐enriched, a second type of magmatism is characterized by an apparent subduction‐zone related imprint, producing rocks like FAB basalts and boninites in the upper crust, as well as cross‐cutting gabbronorites and wehrlites in the deeper crust. In this paper, we apply diverse experimental studies in wet tholeiitic and peridotitic systems performed at lower pressures (100 to 500 MPa) in the experimental lab of the University Hannover, in order to constrain the details of the magmatic processes proceeded at the Oman ophiolite paleoridge during the Cretaceous, with special focus on the influence of water on the phase stabilities and phase relations. The experiments were performed in vertically oriented internally heated pressure vessels (IHPV) (see Berndt et al., 2002; Fig. 1). This facility uses as pressure medium mixtures of Ar and H2 in order to adjust the required fH2 in the vessel, enabling us to control the redox conditions. The fH2 prevailing in the IHPV at high P and T was measured with a Shaw‐membrane made of platinum. The overall variation in fO2 in all experimental series was in the range between ∼FMQ‐1 and ∼FMQ+3.2, thus covering the range of oxygen fugacities prevailing in natural MORB magmas (Bezos and Humler, 2005).For understanding the magmatic processes during the Oman ophiolite paleoridge accretion, transects through the lower (GT1) and middle (GT2) crust have been drilled in the frame of ICDP (International Continental Scientific Drilling Program). Drill sites have been selected in the Wadi Tayin massif, which is known that the influence of magmatic phase 2 characterized by subduction‐related primary melts is minimal. Details and progress obtained in the Oman Drilling Project (OmanDP) can be found here: (https://www.omandrilling.ac.uk/). Regarding the first magmatic phase of the processes at the Oman ophiolite paleoridge, a characteristic observation made during the description of the drilled cores GT1 (lower crust) and GT2 (mid‐curst) was that quite often layers in the layered gabbro series occur which show the presence of clinopyroxene joining olivine instead of plagioclase (under near liquidus conditions). In terms of lithologies this could be interpreted as presence of wehrlitic crystal mushes as early cumulates instead of troctolitic, which are the typical ones for primary magmatism at typical fast‐spreading ridges. This situation could be experimentally simulated by adding a moderate to high water activity to primitive MORB at pressures ≥ 200 MPa, resulting in a shift of the clinopyroxene‐in curve to higher temperatures above the plagioclase‐in curve (Feig et al. 2006; see Fig. 2).Regarding the second, late‐stage magmatic phase, the formation of typical Oman high‐Ca‐boninites could be experimentally simulated by water‐saturated partial melting of Oman harzburgite at 200 MPa and relatively low temperatures between 1100 and 1200°C. Depleted gabbronorites crosscutting layered gabbros of phase 1 magmatism can be regarded as cumulates formed in these boninitic melts. Late wehrlites crosscutting layered gabbro could be produced by accumulation of olivine and clinopyroxene at temperatures between 1040 and 1080°C in a hydrous gabbroic system at pressures &gt; 100 MPa with bulk water content of 2–3 wt%.

Temperature-pressure-composition model for melt viscosity in the Di-An-Ab system

A model for the viscosity (η) of melts in the system CaMgSi2O6-CaAl2Si2O8-NaAlSi3O8 (Di-An-Ab) based on data compiled from the literature is presented. The model is calibrated on 560 measurements of melt viscosity at 1 atm pressure on 40 individual melt compositions and 303 measurements obtained at high pressure on 18 melt compositions. The model is continuous and accounts for variations in melt composition (X), temperature (T) and pressure (P). At ambient pressure, the model spans 15 orders of magnitude of η, a T range of 933–2450 K, and reproduces the dataset well (RMSE = 0.26 log units). The high-P model is calibrated over a T range of 970–2470 K and pressures of 50 MPa to 13 GPa for viscosities of 10−1 to 1014.5 Pa s and reproduces the high-pressure dataset to within (larger) experimental uncertainties (RMSE = 0.35 log units). Used in conjunction with chemical proxies for melt structural organization (i.e. SM and NBO/T), the model illustrates the strong coupling between viscosity and changes in melt polymerization as a function of X and P. Di-An-Ab melts with SM values >25.34 have positive pressure coefficients and show a minimal to strong increase in viscosity with pressure. Model values of glass transition temperatures (Tg) (i.e. η ~ 1012 Pa s) range between 930 K and 1130 K at 1 atm; Tg values for highly polymerized melts are depressed by as much as 150 K at 5 GPa at elevated pressures but are higher for more depolymerized melts and higher P. Melt fragility (m) is only weakly affected by increased P as expressed by a slight decrease in m for the most fragile melts versus a slight increase for the least fragile melts. Activation energies (Ea) increase with rising P for depolymerized melts and decrease with rising P for polymerized melts with the crossover at SM = 25.34. Ea values increase at low T for all P conditions and the effects of P are most pronounced at lower T. Lastly, a comparison of the liquidus surface topology for the ternary system to the corresponding isokoms of viscosity shows the relative effects of composition and temperature on melt viscosity at liquidus conditions to be highly varied.

Chiral Proportions of Nepheline Originating from Low-Viscosity Alkaline Melts. A Pilot Study

Chromatographic interaction between infiltrating solutions of racemic mixtures of enantiomers and enantiomorphic minerals with chiral excess has been proposed as a scenario for the emergence of biomolecular homochirality. Enantiomer separation is supposed to be produced by different partition coefficients of both enantiomers with regard to crystal faces or walls of capillary tubes in the enantiomorphic mineral. Besides quartz, nepheline is the only common magmatic mineral with enantiomorphic symmetry. It crystallizes from SiO2-undersaturated melts with low viscosity and is a promising candidate for chiral enrichment by autocatalytic secondary nucleation. Under liquidus conditions, the dynamic viscosity of silicate melts is mainly a function of polymerization. Melts with low concentrations of SiO2 (&lt;55 wt%) and rather high concentrations of Na2O (&gt;7 wt%) are only slightly polymerized and hence are characterized by low viscosities. Such melts can ascend, intrude or extrude by turbulent flow. Fourteen volcanic and subvolcanic samples from alkaline provinces in Africa and Sweden were chemically analyzed. Polished thin sections containing fresh nepheline phenocrysts were etched with 1% hydrofluoric acid at 20 °C for 15 to 25 min. Nepheline crystals suitable for a statistical evaluation of their etch figures were found in four samples. Crystals with chiral etch figures are mainly not twinned. Their chiral proportions in grain percentages of single crystals are close to parity in three samples. Only one sample shows a slight chiral excess (41.67% L-type vs. 58.33% D-type) but at a low level of significance (15 vs. 21 crystals, respectively).

Phase Equilibria of Pantelleria Trachytes (Italy): Constraints on Pre-eruptive Conditions and on the Metaluminous to Peralkaline Transition in Silicic Magmas

The relationships between trachytes and peralkaline rhyolites (i.e. pantellerites and comendites), which occur in many continental rift systems, oceanic islands and continental intraplate settings, is unclear. To fill this gap, we have performed phase equilibrium experiments on two representative metaluminous trachytes from Pantelleria to determine both their pre-eruptive equilibration conditions (pressure, temperature, H2O content and redox state) and liquid lines of descent. Experiments were performed in the temperature range 750-950 °C, pressure 0·5-1·5 kbar and fluid saturation conditions with XH2O [=H2O/(H2O+CO2)] ranging between zero and unity. Redox conditions were fixed below the nickel-nickel oxide buffer (NNO). The results show that at 950 °C and melt water contents (H2Omelt) close to saturation, trachytes are at liquidus conditions at all pressures. Clinopyroxene is the liquidus phase, being followed by iron-rich olivine and alkali feldspar. Comparison of experimental and natural phases (abundances and compositions) yields the following pre-eruptive conditions: P=160·5 kbar, T=925±25 °C, H2Omelt = 261 wt %, and fO2 between NNO- 0·5 and NNO- 2. A decrease in temperature from 950 °C to 750 °C, as well as of H2Omelt, promotes a massive crystallization of alkali feldspar to over 80 wt %. Iron-bearing minerals show gradual iron enrichment when T and fO2 decrease, trending towards the compositions of the phenocrysts of natural pantellerites. Despite the metaluminous character of the bulk-rock compositions, residual glasses obtained after 80 wt % crystallization evolve toward comenditic compositions, owing to profuse alkali feldspar crystallization, which decreases the Al2O3 of the melt, leading to a consequent increase in the peralkalinity index [PI = molar (Na2O+K2 O)/Al2 O3 ]. This is the first experimental demonstration that peralkaline felsic derivatives can be produced by low-pressure fractional crystallization of metaluminous mafic magmas. Our results show that the pantelleritic magmas of basalt-trachyte-rhyolite igneous suites require at least 95 wt % of parental basalt crystallization, consistent with trace element evidence. Redox conditions, through their effect on Fe-Ti oxide stabilities, control the final iron content of the evolving melt.

Mineral chemistry and thermobarometry of Eocene monzogabbroic stocks from the Bafra (Samsun) area in Turkey: implications for disequilibrium crystallization and emplacement conditions

Monzogabbro stocks including felsic enclaves (monzosyenite) around the Bafra (Samsun) area at the western edge of the Eastern Pontides cut Eocene-aged volcanic and sedimentary units. The monzogabbros contain plagioclase, alkali feldspar, clinopyroxene, olivine, hornblende, biotite, apatite, and iron-titanium oxides, whereas the felsic enclaves contain alkali feldspar, plagioclase, hornblende, biotite, clinopyroxene, and iron-titanium oxides. Mineral chemistry data suggest that magmas experienced hydrous and anhydrous crystallization in deep and shallow crustal magma chambers. Several thermobarometers were used to estimate temperatures of crystallization and emplacement for the mafic and felsic magmas. Clinopyroxene thermobarometry yielded 1100–1232 C and 5.9–8.1 kbar for monzogabbros, and 931–1109 C and 1.8–6.9 kbar for felsic enclaves. Hornblende thermobarometry and oxygen fugacity estimates reveal 739–971°C, 7.0–9.2 kbar and 10−9.71 for monzogabbros and 681–928°C, 3.0–6.1 kbar and 10−11.34 for felsic enclaves. Biotite thermobarometry shows elevated oxygen fugacity varying from 10−18.9–10−11.07 at 632–904°C and 1.29–1.89 kbar for monzogabbros, to 10−15.99 –10−11.82 at 719–873°C and 1.41–1.77 kbar for felsic enclaves. The estimated zircon and apatite saturation temperatures are 504–590°C and 693–730°C for monzogabbros and 765–775°C and 641–690°C for felsic enclaves, respectively. These data imply that several phases in the gabbroic and syenitic magmas did not necessarily crystallize simultaneously and further indicate that the mineral compositions may register intervals of disequilibrium crystallization. Besides, thermobarometry contrasts between monzogabbro and felsic enclave may be partly a consequence of extended interactions between the mafic and felsic magmas by mixing/mingling and diffusion. Additionally, the hot felsic magma was close to liquidus conditions (crystallinity < 30%) when injected into cooler mafic magma (crystallinity > 50%), and thus, the monzogabbro stocks reflect hybrid products from the mingling and incomplete mixing of these two magmas.

Petrochemical evidence of magma mingling and mixing in the Tertiary monzogabbroic stocks around the Bafra (Samsun) area in Turkey: implications of coeval mafic and felsic magma interactions

Miocene aged calc-alkaline mafic host stocks (monzogabbro) and felsic microgranular enclaves (monzosyenite) around the Bafra (Samsun) area within Tertiary volcanic and sedimentary units of the Eastern Pontides, Northeast Turkey are described for the first time in this paper. The felsic enclaves are medium to fine grained, and occur in various shapes such as, elongated, spherical to ellipsoidal, flame and/or rounded. Most enclaves show sharp and gradational contacts with the host monzogabbro, and also show distinct chilled margins in the small enclaves, indicating rapid cooling. In the host rocks, disequilibrium textures indicating mingling or mixing of coeval mafic and felsic magmas are common, such as, poikilitic and antirapakivi textures in feldspar phenocrysts, sieve textured-patchy-rounded and corroded plagioclases, clinopyroxene megacrysts mantled by bladed biotites, clinopyroxene rimmed by green hornblendes, dissolution in clinopyroxene, bladed biotite, and acicular apatite. The petrographical and geochemical contrasts between the felsic enclaves and host monzogabbros may partly be due to a consequence of extended interaction between coeval felsic and mafic magmas by mixing/mingling and diffusion. Whole-rock and Sr-Nd isotopic data suggests that the mafic host rocks and felsic enclaves are products of modified mantle-derived magmas. Moreover, the felsic magma was at near liquidus conditions when injected into the mafic host magma, and that the mafic intrusion reflects a hybrid product formed due to the mingling and partial (incomplete) mixing of these two magmas.

Solidification of trapped liquid in rocks and crystals

Trapped liquid in an igneous cumulate solidifies over a range of time and temperature that can be retrieved by use of the lever rule in binary solutions applied to plagioclase using the range in the An content found for the individual rock studied. The An range, so called, when measured in sufficient detail, defines the solidification history. The instantaneous solid composition along the solidus defines the zoning of the plagioclase as it follows the trapped liquid on the liquidus. The reference bulk composition of the trapped liquid is given by an intercept on the initial solid-liquid lever, defined by the fraction of plagioclase in the trapped parent magma times the residual porosity. The mafic fraction is assumed to solidify by reaction independently of the plagioclase zoning. The residual porosity is estimated from the content of evolved components in the trapped liquid and can also be calculated from the An range itself when that is calibrated to a value independently determined from the evolved components. Examples from a recent treatment of residual porosity are given for the solidification of selected rock compositions from the Kiglapait and Skaergaard intrusions. The same principles apply to the solidification of melt inclusions, with the difference that the latter tends to sample an evolved sheath by capture, rather than a parent magma trapped by closure of a cumulate, because only the cumulate has had time to exchange the evolved rejected solute owing to its slow solidification. Experimental examples of melt inclusions trapped at liquidus conditions demonstrate the highly evolved composition of the sheath compared to the bulk glass composition. The application of the solidification principle to presumed liquid immiscibility in the Skaergaard intrusion suggests that the melt inclusions are evolved during the capture process and then solidified as given by the An range in the plagioclase. The hypothesis of liquid immiscibility is not needed in this analysis. The very different end-stage feldspar histories of the two intrusions is attributed to the sequestering of low-melting feldspar components in the trapped liquids of the Skaergaard cumulates, compared to the lesser amounts trapped in the Kiglapait cumulates.

Magma mingling and hybridization in the Kuh-e Dom pluton, Central Iran

The upper Eocene Kuh-e Dom granitoids in Central Iran comprise mafic microgranular enclaves ranging from a few centimeters to meters in size. The spherical to ellipsoidal enclaves consist of diorite, quartz-diorite, monzodiorite and quartz-monzodiorite whereas the more felsic host intrusions mainly comprise monzogranite and granodiorite. Most enclaves show sharp contacts with the host granites and distinct chilled margins in the small enclaves indicate rapid cooling after entering the felsic host magma. Disequilibrium textures include: (1) poikilitic and antirapakivi textures in feldspar megacrysts, (2) ocelli quartz rimmed by fine-grained mineral aggregates, (3) rounded and corroded plagioclase, (4) patchy plagioclase, (5) mafic clots, (6) blade biotite, (7) small lath-shaped plagioclase in larger tabular plagioclase, (8) acicular apatite, and (9) crenulated and cuspate contacts between enclaves and host rocks. These textures indicate mingling/mixing of two different magmas. Microprobe data show variable plagioclase compositions ranging between oligoclase and andesine in granodiorite, and between andesine to labradorite in the enclaves with normal zoning and oscillatory zoning. Geochemically, the enclaves are Na-rich whereas the host rocks are K-rich. In addition, the enclaves are depleted in LREE and LILE and enriched in HREE and Ti, P, Nb and Y, relative to their host granodiorites. Average initial Nd–Sr isotopic compositions for the host rocks are εNd (47Ma)=−5.09 and Sri=0.7068, compared to enclave Sri=0.7060 and εNd (47Ma)=−2.89. Fractionation of both enclaves and host rocks from the same melt cannot explain these petrological, geochemical and isotopic differences. Instead, enclaves and host rocks more likely originated from two separate magmas, which subsequently mingled and mixed. The mafic enclaves are modified mantle-derived melts while the granodioritic host rocks are partial melts of lower continental crust. The mafic enclave magma was at near liquidus conditions when injected into the granitic host melt, and the Kuh-e Dom intrusion reflects a hybrid product from mingling and partial (incomplete) mixing of these two melts.

Evidence for low viscosity garnet-rich layers in the upper mantle

Abstract The rheological properties of upper mantle rocks play an important role in controlling the dynamics of the lithosphere and mantle convection. Experimental studies and microstructures in naturally deformed mantle rocks usually imply that olivine controls the upper mantle rheology. Here we show for the first time evidence from the geometry of folded compositional layers in mantle rocks from Western Norway that garnet-rich rocks can have lower solid-state viscosities than olivine-rich rocks. Modeling of melt-free and dry rheology of garnet and olivine confirms that the reversed viscosity contrast between garnet-rich and olivine-rich layers for this folding event can be achieved over a relatively wide range of temperatures at low stress conditions when the fine-grained garnet deforms by diffusion creep while the coarse-grained olivine deforms by dislocation creep and/or diffusion creep. In general, modeling of the fold viscosity contrast shows that in the stable subcontinental lithospheric mantle or convecting mantle such a reversed viscosity contrast can be formed due to diffusion creep processes in fine-grained garnets in a dry mantle environment or at conditions where the garnet-pyroxene layer is partially molten, i.e. close to solidus–liquidus conditions in the upper mantle. Alternatively in cold plate tectonic settings, e.g. in subduction zones, some water-weakening is a feasible mechanism to create the reversed viscosity contrast between garnet and olivine.

On the Skaergaard intrusion and forward modeling of its liquid line of descent: A reply to “Principles of applied experimental igneous petrology” by Morse, 2008, Lithos 105, pp. 395−399

Forward modeling based on an experimental investigation successfully duplicated main features of the gabbros in the Skaergaard layered series [Thy, P., Lesher, C.E., Nielsen, T.F.D., and Brooks, C.K. 2006. Experimental constraints on the Skaergaard liquid line of descent. Lithos 92, 154–180.]. The foundation for the modeling was equilibrium melting experiments that were controlled by temperature and oxygen fugacity at low-pressure conditions. The experimental techniques and methods were chosen to represent a reasonable approximation to the inferred emplacement and crystallization conditions of the Skaergaard intrusion. The dike rocks used as starting materials define a strong differentiation trend that represents the Skaergaard liquid line of descent. This suite of dikes allowed liquidus conditions to be defined for a range of composition and temperature. The initial redox conditions were chosen based on measured and calculated estimates for the gabbros of interest. The melting experiments defined liquidus and subliquidus conditions that were used to understand crystallization of the lower and middle zones of the Skaergaard layered series, and can be extrapolated to the upper zone gabbros assuming perfect fractional crystallization. The forward modeling reproduces the cryptic variation seen in the main gabbro minerals (olivine, augite, plagioclase) well into the upper zone and provides reasonable liquidus temperatures and compositions. It can be shown that based on the assumption of Fe–Ti oxide modes in the middle and upper zones, a range of oxygen fugacity trends can be obtained. We repeat our previous conclusion that iron depletion and strong reduction in oxygen fugacity in the upper zone are only feasible for very high Fe–Ti oxide modes that exceed the experimental evidence as well as the observations from the gabbros. A strong drop in oxygen fugacity in the upper zone requires a significant sink for Fe–Ti oxides that so far has not been identified. We thus reject Morse's [Morse, S.A., 2008. Principles of applied experimental igneous petrology: a comment on “Experimental Constraints on the Skaergaard liquid line of descent” by Thy, Lesher, Nielsen, and Brooks, 2006, Lithos 92: 154–180. Lithos 105, pp. 395−399.] contention that we violated in our original study established principles of applied experimental igneous petrology. Such principles dictate that experimental and forward models are carefully tested against field observations before petrologic processes can be verified.

New Ni and Co metal-silicate partitioning data and their relevance for an early terrestrial magma ocean

It is generally assumed that the abundances of the siderophile (metal-loving) elements in the Earth's mantle are the result of core formation in the early Earth. In the most popular model the low contents of siderophile elements in the Earth's mantle are the result of metal-silicate equilibration followed by metal accumulation at the base of a magma ocean. This model is primarily based on the pressure dependence of Ni and Co metal-silicate partition coefficients first determined by Li and Agee [Li, J., Agee, C.B., 1996. Geochemistry of mantle–core differentiation at high pressures. Nature 381, 686–689]. We have undertaken a further test of this hypothesis by performing a large number of additional partition experiments with the two key elements Ni and Co at a variety of temperatures and a pressure range extending from ambient pressure to 25 GPa (~750 km depth in the Earth). The unexpected result of the new experiments is the recognition of two pressure regimes in the temperature and pressure dependences of Ni–Fe and to a lesser extent of Co–Fe exchange metal-silicate partition coefficients, KDNi–Fe and KDCo–Fe. The strong pressure and temperature dependences of KD⁎Ni–Fe and KD⁎Co–Fe at atmospheric pressure change below about 5 GPa to much weaker dependences at higher pressures. As a consequence the pressure dependences of KDNi–Fe and KDCo–Fe cannot be regressed with a single fit over the entire pressure range. Fits based on partition data obtained in this work and from the literature between 5 GPa and 25 GPa were extrapolated to pressures and temperatures to lower mantle liquidus conditions. The Co–Fe partition coefficients required to satisfy the present mantle composition is reached at about 35 GPa, whereas the proper Ni–Fe ratio requires 45 to 50 GPa. At 55 GPa Ni and Co partition coefficients would be identical but the Ni- and Co-contents imposed on the Earth's mantle by metal-silicate equilibration would be too low. This would exclude single stage magma ocean models for producing approximately chondritic Ni/Co ratio in the Earth's mantle. However, the accuracy of the experimental data obtained here and from the literature is not sufficient to unambiguously exclude single stage equilibration. The results of this and other recent studies exclude that Fe-, Ni-, and Co-contents of the present upper mantle were established by metal-silicate equilibration at upper mantle conditions (<25 GPa).

Experimental constraints on the origin of Martian meteorites and the composition of the Martian mantle

The major element compositions of partial melts of a chondritic (Homestead L5) model Mars mantle have been determined at 4.7–5.0 GPa in a multi-anvil device over a temperature interval that ranged from near solidus to near liquidus conditions. The purpose of these experiments is to determine if the major element abundances of Martian basalts (shergottite meteorites) or their parent magmas can be derived from a model Martian mantle composition at high pressure. Partial melting of our model composition at 4.7–5.0 GPa produces liquids with super-chondritic CaO/Al 2O 3 similar to those of proposed Martian basalt parent magmas. The concentrations of CaO and Al 2O 3 in the high-pressure experimental liquids are, however, lower than in Martian basalt parent magmas. We conclude that matches for both ratios and concentrations involving CaO and Al 2O 3 between current model Mars compositions and proposed Martian basalt parent magmas would require at least two stages of magmatic differentiation. For example, partial melting at 5 GPa (425 km depth in Mars) produces a magma having super-chondritic CaO/Al 2O 3, but subsequent, lower pressure differentiation of olivine (±low-Ca pyroxene) is needed to increase the CaO and Al 2O 3 concentrations to those of calculated Martian basalt parent magmas. This two-stage polybaric differentiation would be consistent with either a magma ocean or mantle plume-melting scenario. On the other hand, these multi-stage differentiation scenarios cannot reconcile the significant mismatch between the FeO content of our experimental liquids and Martian basalt parent magmas. A remedy for this apparent inconsistency might require a bulk Martian mantle or shergottite parent source region with a composition and Mg# closer to that of H-chondrites, but still much less magnesian than are terrestrial upper-mantle basalt source regions.

An experimental study of rare earth element partitioning between a shergottite melt and pigeonite: implications for the oxygen fugacity of the mart ian interior

A series of experiments on a synthetic, pigeonite-saturated, basaltic shergottite were carried out to constrain the variation of D(Eu/Gd) pigeonite/melt and D(Eu/Sm) pigeonite/melt with oxygen fugacity ( f O2). The experiments have been run under both dry and hydrous conditions. The shergottite was doped with 0.1, 0.5, and 1.0 wt.% Eu, Gd, and Sm oxides in different experiments and was equilibrated at liquidus conditions for 24 hours. D(Eu/Gd) pigeonite/melt in dry melts ranges from 0.156 ± 0.014 ( f O2 = IW − 1) to 0.630 ± 0.102 (IW + 3.5). D(Eu/Sm) pigeonite/melt in dry melts ranges from 0.279 ± 0.021 (IW − 1) to 1.114 ± 0.072 (IW + 3.5). Due to difficulties with low- f O2 experiments, hydrous distribution coefficients were measured, but were not used in the calibration of the Eu-oxybarometers. These two Eu-oxybarometers provide an accurate way to measure f O2 recorded during pigeonite crystallization, thereby yielding a record of f O2 during the earliest period of Martian meteorite parent magma crystallization history. Using this new calibration, Martian meteorite pigeonite cores record f O2 values of IW − 0.6 (±0.3) (QUE94201) to IW + 1.9 (±0.6) (Shergotty). These new values differ in magnitude, but not trend, from previously published data. The pigeonite Eu-oxybarometer yields an f O2 range in the basaltic shergottites of 2 to 3 orders of magnitude. Several processes have been proposed to explain the origin of this f O2 range, the majority of which rely on assimilation of an oxidized source. A potential correlation between this new pigeonite data and recent Fe-Ti oxide data, however, is consistent with intrinsic f O2 differences in the magma source region being responsible for the measured f O2 variations. This implies that the Martian meteorite source region, the mantle or lithosphere, may be heterogeneous in nature. However, the process of assimilation cannot be completely ruled out in that an assimilation event that took place before crystallization commenced would result in the overprinting of the source region f O2 signature.

Generation and differentiation of group II kimberlites: constraints from a high-pressure experimental study to 10 GPa

Experiments have been performed in the multicomponent (natural) bulk system to constrain the conditions of generation and differentiation of a K-rich group II kimberlite (now also referred to as orangeite). The group II composition examined was saturated in olivine, orthopyroxene, and garnet at near liquidus conditions in the pressure range 4 to 10 GPa. In the range 2 to 3 GPa, the liquidus phase was olivine only. The potassic nature of the melts in the bulk compositions studied was ensured by the absence of any K-bearing phase in the residual assemblage at P > 4 GPa. Phlogopite is destabilized toward higher pressures by a carbonation reaction of the type phlogopite + CO 2 = enstatite + garnet + K 2CO 3 (liquid) + H 2O leading to alkalic, carbonatitic liquids coexisting with a garnet–peridotite (harzburgite or lherzolite) residue over a wide pressure–temperature space at pressures in excess of 4 GPa. Evidently, CO 2-bearing systems do not favor the stability of phlogopite and/or K-richterite amphibole at pressures in excess of 4 to 5 GPa, and it is suggested that the carbonate-bearing and potassic character of any mantle melt originating from this depth is most likely the product of a two-stage process: either a carbonate-bearing protolith is invaded by a potassic melt or fluid (probably supercritical), or a potassic protolith (after metasomatism) has been invaded by a carbonatite melt.

Fluid inclusion microthermometry in Be–Ta–(Li–Sn)-bearing pegmatites from the Borborema Province, Northeast Brazil

Fluid inclusions in Be–Ta–Li–Sn-bearing pegmatites have been reported in the literature as hypersaline aqueous brines or low-salinity aqueous or aqueous–carbonic fluids. The evolution paths of volatile fluids in equilibrium with the different crystallizing phases of rare-element pegmatites do not seem to be unique. The aim of this work is to contribute to this discussion, on the basis of fluid inclusion information from the Borborema Pegmatite Province (BPP) in Northeast Brazil. Some well-known heterogeneous Be–Ta–(Li–Sn) mineralized pegmatites of this province were systematically sampled along cross sections with the purpose of studying the fluid inclusions in the different pegmatite zones (I, muscovite-rich contact zone; II, homogeneous or graphic pegmatite; III, blocky feldspar zone; and IV, quartz core). The observed fluid inclusions (mainly in quartz) were almost always secondary or pseudosecondary, grouped along healed fractures with random orientation. Even some trails paralleling quartz grain limits were not undoubtedly primary. Almost all these fluid inclusions were filled with low-salinity aqueous or aqueous–carbonic fluids. Primary inclusions were recognized only in some well-formed crystals from druses with erratic distribution in the pegmatites. Microthermometric data from these samples confirm that the various crystallizing phases of the pegmatites were in equilibrium with low-salinity aqueous or aqueous–carbonic fluids (with insignificant N 2 or CH 4 contents) with frequent accidentally trapped solids. The various types of primary inclusions found in these crystals compare well with the types of secondary inclusions of the samples from the systematic cross sections. The relative age of the crystals hosting primary fluid inclusions, the relative age of primary inclusion types in successive crystal-growth zones, the relative age of secondary fluid inclusion trails and the corresponding microthermometric data allowed to establish the most probable evolution of the fluid characteristics during pegmatite crystallization. An early low-salinity aqueous–carbonic fluid with CO 2 contents of 20 mol% was present during the crystallization of early minerals, such as garnet and tourmaline of the border zone and graphic quartz, beryl, manganotantalite, and blue tourmaline of zones II and III. The end of zone III crystallization occurred in the presence of a low-salinity, aqueous fluid with very low CO 2 contents, as indicated by inclusions in the smoky core of quartz crystals at the limit between zones III and IV. Later aqueous fluids barren in CO 2 and with salinities increasing from 5% to 20% coexisted with the formation of the pegmatite nucleus and replacement bodies, as indicated by inclusions in the border of these crystals and in euclase. The estimated trapping conditions of 3.5 kbar and 580°C for the earliest inclusion types agree with the regional metamorphic conditions and with the known liquidus conditions of rare-element-bearing pegmatite crystallization. The trapping conditions of inclusions in euclase and coexisting quartz crystal rims in the pegmatitic core, at 3.5 kbar and 400°C are slightly below the conditions admitted for the solidus of rare-element pegmatites and indicate an isobaric cooling during pegmatite crystallization.

Phase equilibrium constraints on the viscosity of silicic magmas II: implications for mafic–silicic mixing processes

Isobaric crystallisation paths obtained from phase equilibrium experiments show that, whereas in rhyolitic compositions melt fraction trends are distinctly eutectic, dacitic and more mafic compositions have their crystallinities linearly correlated with temperatures. As a consequence, the viscosities of the latter continuously increase on cooling, whereas for the former they remain constant or even decrease during 80% of the crystallisation interval, which opens new perspectives for the fluid dynamical modelling of felsic magma chambers. Given the typical dyke widths observed for basaltic magmas, results of analogue modelling predict that injection of mafic magmas into crystallising intermediate to silicic plutons under pre-eruption conditions cannot yield homogeneous composition. Homogenisation can occur, however, if injection takes place in the early stages of magmatic evolution (i.e. at near liquidus conditions) but only in magmas of dacitic or more mafic composition. More generally, the potential for efficient mixing between silicic and mafic magmas sharing large interfaces at upper crustal levels is greater for dry basalts than for wet ones. At the other extreme, small mafic enclaves found in many granitoids behave essentially as rigid objects during a substantial part of the crystallisation interval of the host magmas, which implies that finite strain analyses carried out on such markers can give only a minimum estimate of the total amount of strain experienced by the host pluton. Mafic enclaves carried by granitic magmas behave as passive markers only at near solidus conditions, typically when the host granitic magma shows near-solid behaviour. Thus they cannot be used as fossil indicators of direction of magmatic flow.

Viscosity of Himalayan leucogranites: Implications for mechanisms of granitic magma ascent

The viscosity of the Gangotri Himalayan leucogranite has been experimentally determined between 800° and 1100°C, 300 and 800 MPa, for meltwater contents of 3.98 and 6.66 wt %. The melt viscosity is independent of pressure and shows an Arrhenian behavior relative to temperature within the range of conditions investigated. We present an empirical relation that can be used to determine leucogranite magma viscosities knowing their meltwater content and temperature. This relation together with phase equilibria experiments constrain the viscosity of the Himalayan leucogranites to be around 104.5 Pa s during their emplacement. These viscosities and the widths of dikes belonging to the feeder system are consistent with the theoretical relationship relating these two parameters and show that the precursor magma of the leucogranite was at near liquidus conditions when emplaced within host rocks with preintrusion temperatures around 350°C. Calculated terminal ascent rates for the magma in the dikes are around 1 m/s. Magma chamber assembly time is, on this basis, estimated to be less than 100 years (for a volume of 150 km3). In addition, the dynamical regime of the magma flow in the dikes was essentially laminar, thus allowing preservation of any chemical heterogeneity acquired in the source. These results constrain the viscosity of melts formed during the first steps of crustal anatexis, those involving muscovite breakdown, to be also around 104.5 Pa s. Thus compaction may not be the only mechanism of melt segregation in partially melted crustal rocks in view of the very short timescale inferred for magma ascent and emplacement.

Compressibility of molten high-Ti mare glass: Evidence for crystal-liquid density inversions in the lunar mantle

Static compression density measurements of molten Apollo 14 black glass have been performed at liquidus conditions in the pressure range 1.0–11.5 GPa. A range of values for the Birch-Murnaghan isothermal bulk modulus ( K 2173 K = 13–23 GPa) and the pressure derivative ( dK dP = 7−3 , respectively) fit the data within the uncertainties; K 2173 K = 15.5 GPa and dK dP = 5.2 provides the best fit to the four measured pressure-density pairs corrected to isothermal conditions. The ultramafic high-Ti mare glass is significantly more compressible than other ultrabasic and basic silicate liquids previously studied. The compressibility appears to be related to TiO 2 content. The Apollo 14 black glass sample has the highest TiO 2 content of the pristine mare glasses, which are thought to be primary melts from a differentiated and strongly heterogeneous lunar mantle. Our density measurements confirm the prediction that molten black glass will be denser than equilibrium olivine and orthopyroxene at pressures greater than ∼2.0 and ∼0.5 GPa, respectively. These density inversions correspond to depths greater than approximately 410 and 100 km in the lunar mantle. Since orthopyroxene replaces olivine at ∼1.5 GPa (300 km) on the liquidus, our results constrain the maximum depth of buoyancy for high-Ti magmas at <400 km in the lunar mantle.