Subsolidus Mineral Research Articles

When zircon drowns: Elusive geochronological record of water-fluxed orthogneiss melting in the Velay dome (Massif Central, France)

Zircon U-Pb geochronology is routinely performed to unravel the timing and duration of melting events within the continental crust. A comprehensive understanding of the zircon behavior during anatexis is therefore paramount to accurately assess the impact of crustal melting on orogenic processes. We report on an anatectic system from the southern part of the Velay dome (Variscan French Massif Central) encompassing late Carboniferous migmatites developed at the expense of peraluminous orthogneisses and associated with (leuco)granite bodies intrusive at higher structural levels. Our zircon U-Pb geochronological survey (494 analyzed grains) evidences a very scant record of the Variscan anatectic event. Indeed, oscillatory-zoned melt-precipitated zircon grains extracted from metatexites, leucosomes and (leuco)granites almost systematically yielded the late Neoproterozoic crystallization age of the felsic igneous protolith. Field and petrographic observations, whole-rock geochemical signatures, phase equilibrium and thermodynamically-constrained trace element modelling collectively indicate that partial melting of the orthogneisses took place at T ~ 700 °C and was fluxed by ingress of external water. The lack of new zircon growth is ascribed to a range of processes ultimately related to the low melting temperature including limited zircon solubility in the melt phase, occlusion of zircon in non-reacting source biotite and sluggish zircon dissolution kinetics. Peraluminous orthogneisses, yet highly fusible due to their “eutectic” subsolidus mineral assemblage, exhibit a very limited zircon record of low temperature water-fluxed melting. Thus, zircon has little potential to provide reliable chronological constraints on water-fluxed melting episodes affecting such lithologies.

Decoupled Ca and Fe + Mg content of S-type granites: An investigation on the factors that control the Ca budget of S-type granites

The compositional variation of S-type granites is thought to derive from source-controlled processes whereby magmas acquire their composition as a result of entrainment of variable proportions of a peritectic-rich residual mineral assemblage into the melt. A corollary of this model is that the entrained phases keep the stoichiometric proportions they have in the partially melted source. These proportions are controlled by the composition of the subsolidus minerals. Consequently, it is expected that the ratio between maficity (FeOt + MgO) and other compatible major elements in granites would be the same as those of the source. This is true for Ti but not for Ca which shows systematically higher Ca:maficity ratios. In order to explore the factors that cause this discrepancy, thermodynamic calculations were used to study the partial melting of a representative metapelite and a metagreywacke under a range of crustal pressure and temperature conditions. The modelling included segregation of melt or magma in pulses, disequilibrium melting of plagioclase (in which only a fraction of this phase participates in the melting system and produces two populations of plagioclase, the unreacted cores with the original, Na-rich composition and a second one with a Ca-rich composition) and the entrainment of up to 40 wt% minerals into the melt. This entrained assemblage consists of two fractions, one containing the minerals that are most likely to be surrounded by melt in the source and representing 80 wt% of the total assemblage, and another one consisting of the phases that are most likely to make up the solid framework, with the minerals within each fraction keeping stoichiometric proportions. The modelled magmas failed to reproduce the observed high Ca:maficity ratios. This indicates that there are other processes that increase the Ca content of granites. Several processes were evaluated, including the presence of apatite, enhanced availability of subsolidus plagioclase in the melting system, disequilibrium Ca-rich melts and enhanced entrainment of Ca-rich plagioclase. The last three processes were found to increase the Ca content but only enhanced entrainment of Ca-rich plagioclase increases the Ca:maficity ratio to values similar to those of the granites. A possible explanation for the preferential entrainment of Ca-rich plagioclase is that the density of this phase is different to that of garnet and ilmenite and similar to the density of the melt. Consequently, a significant fraction of the plagioclase in granites may actually be of residual origin. Other relevant findings of this study are that metagreywackes seem to be the main source of S-type granites, as previous investigations suggested, and that cordierite-rich granitoids represent mineral-rich magmas that are formed only from metapelites at pressures below 0.75 GPa. This investigation emphasizes the importance of source-controlled processes in the composition of S-type granites and indicates that entrainment of a mineral assemblage into the melt is not a simple and straightforward process as previously thought.

Phase equilibrium modelling and SHRIMP zircon U–Pb dating of medium-pressure pelitic granulites in the Helanshan complex of the Khondalite Belt, North China Craton, and their tectonic implications

The Helanshan complex in the Khondalite Belt, North China Craton (NCC) is characterized by the occurrence of both medium-pressure (MP) and high-pressure (HP) pelitic granulites. This paper presents new results from phase equilibrium modelling and SHRIMP zircon U–Pb dating on the MP garnet-sillimanite granulites, in order to better understand the relationship between the MP and HP pelitic granulites, the tectonic implications of the MP pelitic granulites for the formation of the Khondalite Belt, and their geodynamic implications for the Paleoproterozoic orogenesis. Based on petrographic observations, the metamorphic evolution can be divided into four stages (M1–M4). The M1–M3 stages are suprasolidus as reflected by the migmatitic structure on the outcrop. The peak metamorphic stage (M1) is represented by the rim of garnet porphyroblasts along with inclusions of sillimanite + quartz ± biotite, and rutile relicts + plagioclase + K-feldspar in the matrix; the M2 is defined by the ilmenite that have partially replaced rutile; and the M3 is characterized by the replacement of garnet by cordierite. The subsolidus mineral assemblage is garnet + biotite + plagioclase + K-feldspar + sillimanite + cordierite + ilmenite + quartz (M4). Combined with the phase equilibrium modelling in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O2 chemical system, the MP pelitic granulites record a clockwise P–T path, which is characterized by near-isothermal decompression (ITD) from ∼10.1 kbar at ∼830 °C to ∼5.8 kbar at ∼800 °C and subsequent close-to-isobaric cooling (IBC). SHRIMP zircon U–Pb dating yielded a metamorphic age of 1931 ± 21 Ma, interpreted as the time of the retrograde cooling. Integrated with previously reported results, we propose that slab break-off subsequent to Yinshan-Ordos collision at ca. 1.95 Ga led to uplift of the MP and HP granulites in the Khondalite to the middle crustal level in an extensional regime as indicated by their ITD evolution, which was followed by slow cooling at ca. 1.93 Ga as evidenced by their nearly IBC evolution.

Origin of alkaline carbonates in kimberlites of the Siberian craton: Evidence from melt inclusions in mantle olivine of the Udachnaya-East pipe

Alkaline carbonates hexagonal zemkorite (Na,K)2Ca(CO3)2 and orthorhombic shortite Na2Ca2(CO3)3 were found among groundmass minerals in kimberlites from some localities worldwide, including the unserpentinised units of the Udachnaya-East kimberlite. However, the source of alkalis and the origin of the unusual minerals in these kimberlites remain highly debatable. It is generally considered that they have hydrothermal or metasomatic origin while sodium may come from a crustal source. Orthorhombic nyerereite (Na,K)2Ca(CO3)2 and shortite were identified as daughter phases in secondary melt inclusions (MI) in olivine from the deepest mantle xenoliths (i.e., sheared peridotites) and in olivine xenocrysts derived from disintegrated mantle rocks from the Udachnaya-East pipe by Raman spectroscopy and SEM-EDS. The melt, hosted as the inclusions in olivine, was entrapped at a mantle depth. On the basis of similar mineralogy of MI to groundmass of the unserpentinised kimberlites, we suggest relation of MI to the Udachnaya kimberlite melts. The MI solidus temperature is as high as 500°С. Generally, MI nyerereite is considered as a magmatic mineral but experiments show it to be stable at relatively low temperatures (LT) T≤360°С. Thus, strictly speaking, it is a subsolidus mineral formed from high-temperature (HT) (T<800°С) hexagonal (Na,K)2Ca(CO3)2 carbonate. Shortite is also a subsolidus mineral, which may form by several subsolidus reactions in multicomponent systems, such as kimberlites, while breakdown of the HT hexagonal phase (Na,K)2Ca(CO3,SO4)2 into Na2Ca2(CO3)3 (shortite) and K3Na(SO4)2 (aphthitalite) is the basic mechanism. The solidus temperature for the Udachnaya-East kimberlite is about 300°С indicating that LT orthorhombic nyerereite may crystallise directly from the melt as well. Thus, (Na,K)2Ca(CO3)2 and Na2Ca2(CO3)3 carbonates in the groundmass of the unserpentinised Udachnaya-East kimberlites are of magmatic/subsolidus origin. This scenario for the origin of Na-K-Ca and Na-Ca carbonates in the Udachnaya-East kimberlites may have implications for other kimberlites elsewhere.

High‐grade metamorphism and partial melting of basic and intermediate rocks

AbstractRocks of basic and intermediate bulk composition occur in orogenic terranes from all geological time periods and are thought to represent significant petrological components of the middle and lower continental crust. However, the former lack of appropriate thermodynamic models for silicate melt, amphibole and clinopyroxene that can be applied to such lithologies at high temperature has inhibited effective phase equilibrium modelling of their petrological evolution during amphibolite‐ and granulite facies metamorphism. In this work, we present phase diagrams calculated in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O2(NCKFMASHTO) compositional system for a range of natural basic and intermediate bulk compositions for conditions of 2–12 kbar and 600–1050 ∘C using newly parameterized activity–composition relationships detailed in a companion paper by Green et al. in this issue. Particular attention is given to mid‐ocean ridge basalt (MORB) and diorite protolith bulk compositions. Calculated subsolidus mineral assemblages in all basic and intermediate rock types are modally dominated by hornblende and plagioclase, with variable proportions of epidote, clinopyroxene, garnet, biotite, muscovite, quartz, titanite or ilmenite present at different pressures. The H2O‐saturated (wet) solidus has a negativeP−Tslope and occurs between ∼620–690∘C at mid‐ to lower‐crustal pressures of 5–10 kbar. The lowest‐Tmelts generated close to the wet solidus are calculated to have granitic major‐element oxide compositions. Melting at higher temperature is attributed primarily to multivariate hydrate‐breakdown reactions involving biotite and/or hornblende. Partial melt compositions calculated at 800–1050∘C for MORB show good correlation with analysed compositions of experimental glasses produced via hydrate‐breakdown melting of natural and synthetic basic protoliths, with Niggli norms indicating that they would crystallize to trondhjemite or tonalite. Diorite is shown to be significantly more fertile than MORB and is calculated to produce high‐Tmelts (&gt;800 ∘C) of granodioritic composition. Subsolidus and suprasolidus mineral assemblages show no significant variation between different members of the basalt family, although theP−Tconditions at which orthopyroxene stabilizes, thus defining the prograde amphibolite–granulite transition, is strongly dependent on bulk‐rock oxidation state and water content. The petrological effects of open‐ and closed‐system processes on the mineral assemblages produced during prograde metamorphism and preserved during retrograde metamorphism are also examined via a case‐study analysis of a natural Archean amphibolite from the Lewisian Complex, northwest Scotland.

Earliest Paleoproterozoic high-grade metamorphism and orogenesis in the Gawler Craton, South Australia: The southern cousin in the Rae family?

Neoarchean–earliest Paleoproterozoic rocks of the Gawler Craton preserved within the Mulgathing Complex, South Australia, record metamorphism and deformation during an earliest Paleoproterozoic tectonothermal event known as the Sleafordian Orogeny. We present metamorphic data from pelitic gneisses of the Mulgathing Complex from three locations (Challenger, Mt Christie and Blackfellow Hill) in order to constrain the pressure–temperature–time (P–T–t) evolution of this orogeny. In situ U-Pb dating via laser ablation-inductively coupled plasma mass spectrometry, shows that monazites within garnet porphyroblasts have ages ca. 2470 Ma, providing an estimate of the timing peak P–T conditions. Monazite crystallisation from melt occurred at ca. 2460 Ma in the presence of peritectic garnet either late on the prograde path during biotite-dehydration melting, or immediately following peak-T. Lobate and patchy rims in leucosome-hosted monazite suggest a dissolution-reprecipitation process occurred during cooling across the elevated dry solidus at ca. 2440 Ma. Monazite younger than ca. 2430–2420 Ma that is located at grain boundaries between matrix minerals likely reflects monazite crystallisation during cooling across the dry solidus and/or subsolidus mineral breakdown during the waning stages of the thermal event. Phase diagram modelling of pelitic gneisses suggests peak metamorphic conditions at the three localities overlap at ∼7kbar and 850°C with apparent thermal gradients in the order of 125–135°C/kbar (∼35–45°C/km). Metamorphism involved a tight anticlockwise P–T–t path based on mineral assemblage paragenesis and indicates a slow, shallow decompressive-cooling path, sub-parallel to the apparent thermal gradient, in the order of few degrees per Myr. A protracted magmatic history involving bimodal magmatism associated with a thinned and underplated extensional basin in the Mulgathing Complex was likely a thermal precursor to the high thermal gradient metamorphism and convergent (transpressive) deformation. A significant period of post 2.4 Ga quiescence in the Gawler Craton subsequent to basin inversion suggests that this orogenesis may have occurred interior to a plate margin. The similarity in timing and style between the Sleafordian Orogeny in the Gawler Craton and the 2.5–2.28 Ga Arrowsmith Orogeny of the Rae Craton supports the notion that these terranes, together with the broader Gawler-Adélie Craton of the Mawson Continent, the North Australia Craton, elements of the North China, Sino-Korean and Dharwar cratons formed part of a family of Neoarchean terranes, described as the Rae family of cratons, with a distinctive tectono-metamorphic record as compared with other Archean cratons. We suggest the Mawson Continent is the southern member of this family, dominated as it is by terranes now present in the northern hemisphere.

P–T–t evolution of pelitic gneiss from the basement underlying the Northwestern Ordos Basin, North China Craton, and the tectonic implications

Using petrography, phase equilibria modeling and in situ (U–Th)–Pb monazite geochronology, we show that pelitic gneiss from close to the bottom of the Qitan1 borehole in the northwest of the Ordos Basin records a four stage metamorphic evolution. The M1–M3 stages, which were suprasolidus, are represented by: rutile relicts in the matrix and biotite+K-feldspar+plagioclase+sillimanite+quartz inclusions in the cores of garnet porphyroblasts (M1); inclusion-free mantles on garnet with ilmenite (partially replacing rutile) and biotite+K-feldspar+plagioclase+sillimanite+quartz in the matrix (M2); and, the appearance of cordierite in the assemblage biotite+K-feldspar+plagioclase+sillimanite+ilmenite+quartz (M3). The final subsolidus mineral assemblage is garnet+biotite+K-feldspar+plagioclase+sillimanite+cordierite+ilmenite+quartz (M4). The P–T conditions for the mineral assemblage evolution (M1→M4) are constrained by a P–T pseudosection constructed in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O2 chemical system. The P–T–t path is clockwise involving slight heating during near isothermal decompression from ∼8.8kbar at ∼769°C to 5.5kbar at 785°C followed by close-to-isobaric cooling across the solidus. Two cores of monazite and three unzoned grains that have similar populations of dates yield robust ages of 1.96–1.94Ga, interpreted to date the timing of late prograde-to-peak metamorphism (M1–M2). In addition, one rim and two unzoned monazite grains with similar populations of dates yield robust ages of 1.90–1.88Ga, interpreted to date the timing of post decompression slow cooling across the solidus (M3→M4). On the basis of these results, the pelitic gneiss is interpreted to have formed by crustal thickening (M1) during collision between the Yinshan and Ordos Terranes, the latter with the Khondalite Belt protoliths along its northern margin (present co-ordinates). This collision culminated at ca. 1.95Ga and was followed by decompression (M2–M3) and slow cooling (M4) to ca. 1.89Ga during post-collisional extension. The similarity between the P–T–t path determined in this study and those from HP/MP granulites in the Khondalite belt confirms that they experienced a common tectono-metamorphic history. The P–T–t path records an evolution from an apparent thermal gradient of ∼75°C/kbar at peak pressure to ∼150°C/kbar after decompression. Together with the occurrence of mid-Paleoproterozoic magmatism in the basement of the Ordos Basin, these features indicate that the Ordos Terrane was the upper plate at a convergent margin with the Khondalite Belt representing the forearc sediments along its northern edge. Collision with the Yinshan Terrane, part of the subducting plate, led to shallow slab breakoff and the development of a two-sided hot collision and formation of a wide plateau-like hot orogen, similar to those in the numerical models of Sizova et al. (2014).

Melting and Phase Relations of Carbonated Eclogite at 9-21 GPa and the Petrogenesis of Alkali-Rich Melts in the Deep Mantle

The melting and phase relations of carbonated MORB eclogite have been investigated using the multi-anvil technique at 9–21 GPa and 1100–1900°C. The starting compositions were two synthetic mixes, GA1 and Volga, with the CO2 component added as CaCO3 (cc): GA1 + 10%cc (GA1cc) models altered oceanic crust recycled into the convecting mantle via subduction, and Volga + 10%cc (Volga-cc) models subducted oceanic crust that has lost some of its siliceous component in the sub-arc regime (GA1 minus 6·5 wt % SiO2). The subsolidus mineral assemblage at 9 and 13 GPa includes garnet, clinopyroxene, magnesite, aragonite, a high-pressure polymorph of TiO2 (only at 9 GPa) and stishovite (only at 13 GPa). At 17–21 GPa clinopyroxene is no longer stable; the mineral assemblage consists predominantly of garnet with subordinate magnesite (only at 17 GPa), Na-rich aragonite, stishovite, Ca-perovskite (mostly at 21 GPa), and K-hollandite (mostly at 17 GPa). Na-carbonate with an inferred composition (Na,K)2(Ca,Mg,Fe)(CO3)2 was present in Volga-cc at 21 GPa and 1200°C. Diamond (or graphite) crystallized in most runs in the GA1cc composition, but it was absent in experiments with the Volga-cc composition. In Volga-cc, the solidus temperatures are nearly constant between 1200 and 1300°C over the entire pressure range investigated. In GA1cc, the solidus is located at similar temperatures at 9–13 GPa, but at higher temperatures of 1300–1500°C at 17–21 GPa. The difference in solidi between the GA1cc and Volga-cc compositions can be explained by a change in Na compatibility between 13 and 17 GPa as omphacitic clinopyroxene disappears, resulting in the formation of Na-carbonate or Na-rich melt in Volga-cc. The solidus temperature in GA1cc also increases with increasing pressure as a consequence of carbonate reduction and diamond precipitation, possibly brought on either via progressive Fe2+–Fe3+ transition in garnet at higher pressures or by a decrease of the activity of the diopside component in clinopyroxene. The low-degree melts are highly alkalic (K-rich at 9–13 GPa and Na-rich at 17–21 GPa) carbonatites, changing towards SiO2-rich melts with increasing temperature at constant pressure. The solidi of both compositions remain higher than typical subduction pressure–temperature (P–T) profiles at 5–10 GPa; however, at higher pressures the flat solidus curve of carbonated eclogite may intersect the subduction P–T profile in the Transition Zone, where carbonated eclogite can produce alkali- and carbonate-rich melts. Such subduction-related alkali-rich melts can be potential analogues of kimberlite and carbonatite melt compositions and important agents of mantle metasomatism and diamond formation in the Transition Zone and in cratonic roots. Melting of carbonated eclogite produces a garnet-bearing refractory residue, which could be stored in the Transition Zone or lower mantle.

Petrogenesis of the Barcroft pluton, northern White-Inyo Mountains, east-central California

The White-Inyo Range lies within the regional transition from Paleozoic-Precambrian North American continental basement to outboard Mesozoic and younger accreted terranes and a superimposed Andean-type arc. In the central White Mountains, the metaluminous Barcroft granodiorite invaded a major NE-striking, SE-dipping high-angle reverse fault—the Barcroft break. Because it is a relatively isolated igneous body and is well exposed over an elevation range of 1,500–4,000 m, its thermal history and that of the surrounding superjacent section are clearer than those of nearly coeval, crowded plutons emplaced in the hotter Sierra Nevada belt. The Barcroft pluton was emplaced as a compositionally heterogeneous series of areally scattered melt pulses episodically injected over the approximate interval 167–161 Ma. The oldest dated rocks are relatively quartzofeldspathic, whereas the youngest is more ferromagnesian, suggesting progressive partial fusion of a relatively mafic protolith. Heavy rare earth-enriched zircons indicate that Barcroft melts were derived at mid-crustal depths from a previously emplaced metabasaltic protolith containing plagioclase but lacking garnet. Granodioritic magma genesis involved the possible mixing of mafic and felsic melts, as well as very minor assimilation of country rocks, but mainly by fractional fusion and crystallization. Bulk chemical, rare earth, and isotopic data suggest that analyzed Barcroft rocks are members of a single suite. Granodioritic rocks are slightly more magnetite-rich at higher elevations on the NE, nearer the roof of the pluton. Earlier thermobarometry chronicled cooling and re-equilibration of the Barcroft pluton from its margins inward, as well as from mid-crustal generation depths of ~25 km through ascent and stalling at ~10–12 km. Refractory phase assemblages crystallized along the pluton margins, whereas subsolidus minerals in the interior of the of body continued to exchange with upper crustal deuteric and surficial aqueous fluids during exhumation and cooling.

Nyerereite from calcite carbonatite at the Kerimasi Volcano, Northern Tanzania

The extinct Quaternary Kerimasi volcano located in the southern part of the Gregory Rift, northern Tanzania, contains both intrusive and extrusive calciocarbonatites. One carbonate mineral with a high content of Na and Ca has been found in a sample of volcanic carbonatite, which is probably a cumulate rock. On the basis of Raman spectroscopy and SEM/EDS, this mineral was identified as nyerereite, ideally Na2Ca(CO3)2. It occurs as solid inclusions up to 300 × 200 μm in size in magnetite and contains (wt. %) 25.4–27.4 Na2O, 26.0–26.8 CaO, 1.6–1.9 K2O, 0.6–1.8 FeO, 0.3–0.6 SrO, <0.4 BaO, 1.4–2.3 SO3, and 0.6–0.9 P2O5. The average mineral formula is (Na1.84K0.08)Σ1.92(Ca1.00Fe0.03Sr0.01)Σ1.04[(CO3)1.91(SO4)0.05(PO4)0.02]Σ1.98. A few inclusions in magnetite also contain calcite, which is considered here to be a late-stage, subsolidus mineral. The occurrence of nyerereite in carbonatite supports Hay’s (1983) idea that some of the extrusive carbonatites at the Kerimasi volcano were originally alkaline rich and contained both calcite and nyerereite as primary minerals.

NYEREREITE - PIRSSONITE - CALCITE - SHORTITE RELATIONSHIPS IN ALTERED NATROCARBONATITES, OLDOINYO LENGAI, TANZANIA

Samples of natrocarbonatite showing various degrees of alteration are preserved in an older carbonatite platform that existed before and until the 1917 explosive eruption at Oldoinyo Lengai volcano, in Tanzania. The carbonatite samples contain two mineral assemblages: the first consists of relict primary magmatic minerals: nyerereite, fluorite, apatite-(CaF), barite, magnetite and khanneshite. The second consist of low-temperature subsolidus minerals: pirssonite, calcite, shortite, nahcolite, trona, thermonatrite, jacobsite and barytocalcite. Observed relationships between key primary and subsolidus minerals show the following sequence of formation: nyerereite → pirssonite → calcite → shortite. Three major processes, mineral dissolution, metasomatic replacement and crystallization from solution, alter the natrocarbonatite to pirssonite carbonatite, shortite carbonatite and calcite carbonatite. Nyerereite – pirssonite – calcite relationships depend on variations in P(H 2 O) and P(CO 2 ).

Dissociation of CAS phase in the uppermost lower mantle

The high-pressure stability limit of calcium aluminosilicate (CAS) phase has been examined in its end-member CaAl4Si2O11 composition at 18–39 GPa and 1,670–2,300 K in a laser-heated diamond-anvil cell (LHDAC). The in-situ synchrotron X-ray diffraction measurements revealed that the CAS phase decomposes into three-phase assemblage of cubic Al-bearing CaSiO3 perovskite, Al2O3 corundum, and SiO2 stishovite above 30 GPa and 2,000 K with a positive pressure–temperature slope. Present results have important implications for the subsolidus mineral assemblage of subducted sediment and the melting phase relation of basalt in the lower mantle.

Hematite in porous red-clouded feldspars: Evidence of large-scale crustal fluid–rock interaction

Transmission electron microscopy shows that red-clouded alkali feldspars from granitic rocks contain numerous open pores up to several hundred nanometres in cross-section, and that almost every pore contains rosettes or needles of crystalline hematite. In the samples described here, the hematite forms at the reaction interfaces at which plagioclase feldspar is replaced by alkali feldspar. These observations, together with the fact that the origin of the porosity can be explained by the mechanism of sub-solidus mineral–fluid replacement processes, indicate that hematite is a direct product of fluid–rock interaction and is not a solid state exsolution product from the feldspar.

Mineralogical and chemical transformation of Oldoinyo Lengai natrocarbonatites, Tanzania

The minerals of Oldoinyo Lengai natrocarbonatite lavas are unstable under atmospheric conditions. Subsolidus mineral assemblages in natrocarbonatites were studied in 105 samples from contemporary eruptions ranging from present day to about 100 years old. The subsolidus minerals in natrocarbonatites were formed (i) along cracks on the lava surface from hot gases escaping during cooling, (ii) as atmospheric alteration by solution of water-soluble minerals, in particular halides and gregoryite, and by hydration of nyerereite under the influence of meteoric water and (iii) by reaction with fumarole gases. After solidification, the lavas were cut by a network of thin cracks, the edges of which are covered by polymineralic encrustations. Samples collected 2–24 h after eruption contain nahcolite, trona, sylvite, and halite with accessory kalicinite and villiaumite. Atmospheric humidity results immediately (≥ 2 h after eruption) in alteration of black lavas that is marked by the appearance of white powdery thermonatrite with nahcolite on the lava surface. Subsequent reaction (weeks, months, years) of natrocarbonatite with meteoric water and the atmosphere results in the formation of pirssonite, gaylussite, shortite, trona, thermonatrite, nahcolite and calcite. Generally, the first important step is the formation of pirssonite and the end-members are calcite carbonate rocks or loose aggregates. Fumarolic activity is common for the active northern crater of the volcano. Reaction of hot (54–141 °C) fumarolic gases with natrocarbonatite leads to the formation of sulphur, gypsum, calcite, anhydrite, monohydrocalcite, barite and celestine. Changes in mineralogy of the natrocarbonatite lead to substantial chemical transformation. The most obvious chemical changes in this process are the loss of Na, K, Cl and S, combined with an increase in H 2O, Ca, Sr, Ba, F and Mn. The oxygen and carbon isotopic composition of altered natrocarbonatites shows a significant shift from the primary “Lengai Box” to high values of δ 18O and δ 13C. Calcite exhibits δ 13C values between − 2‰ and − 4‰ PDB and δ 18O values of + 23‰ to + 26‰ SMOW. The observed assemblages of secondary minerals formed by reaction with atmosphere and meteoric water, the changes in chemical composition of the natrocarbonatite and field observations suggest that alteration of natrocarbonatite is an open-system low-temperature process. It takes place at temperatures between 8 and 43 °C with the addition of H 2O to the system and the removal of Na, K, Cl and S from the carbonatites. Low-temperature thermodynamic models developed for alkali carbonate systems can be used for the interpretation of Oldoinyo Lengai subsolidus mineralization.

Anomalous U–Pb systematics in mantle-derived baddeleyite xenocrysts from Île Bizard: evidence for high temperature radon diffusion?

Mantle baddeleyite inclusions in titanomagnetite megacrysts from the Île Bizard alnöite intrusion display both an unusual morphology and anomalous U–Pb systematics. These baddeleyite crystals have a cubic to octahedral form, possibly indicating crystallization with tetragonal symmetry under upper mantle conditions. The U–Pb results show a very consistent discordance pattern that is independent of grain size, grain shape, or geochemistry. Compared to the intrusion age of 139.9±2.6 Ma, based on U–Pb groundmass perovskite, the weighted mean 207Pb/ 235U age of 140.7±0.2 Ma is interpreted as the best estimate for the time of baddeleyite crystallization. The 206Pb/ 238U age of 138.7±0.3 Ma is interpreted as the time when these baddeleyite crystals remained closed to high temperature diffusion of the intermediate daughter product 222Rn. The association of titanomagnetite–baddeleyite–kimzeyite–zirconolite in the main Île Bizard intrusion breccia could be linked to a precursor carbonatite event that occurred at least 2 million years prior to formation of the Île Bizard alnöite magma. A significantly younger, less precise U–Pb age of 109 Ma was obtained for melanite, which could reflect a more complicated post-crystallization and subsolidus mineral growth history.

SIMS determination of trace element partition coefficients between garnet, clinopyroxene and hydrous basaltic liquids at 2–7.5 GPa and 1080–1200°C

Trace element partition coefficients ( D's) for up to 13 REE, Nb, Ta, Zr, Hf, Sr and Y have been determined by SIMS analysis of seven garnets, four clinopyroxenes, one orthopyroxene and one phlogopite crystallized from an undoped basanite and a lightly doped (200 ppm Nb, Ta and Hf) quartz tholeiite. Experiments were conducted at 2–7.5 GPa, achieving near-liquidus crystallization at relatively low temperatures of 1080–1200°C under strongly hydrous conditions (5–27 wt.% added water). Garnet and pyroxene D REE show a parabolic pattern when plotted against ionic radius, and conform closely to the lattice strain model of Blundy and Wood (Blundy, J.D., Wood, B.J., 1994. Prediction of crystal–melt partition coefficients from elastic moduli. Nature 372, 452–454). Comparison, at constant pressure, between hydrous and anhydrous values of the strain-free partition coefficient ( D 0) for the large cation sites of garnet and clinopyroxene reveals the relative importance of temperature and melt water content on partitioning. In the case of garnet, the effect of lower temperature, which serves to increase D 0, and higher water content, which serves to decrease D 0, counteract each other to the extent that water has little effect on garnet–melt D 0 values. In contrast, the effect of water on clinopyroxene–melt D 0 overwhelms the effect of temperature, such that D 0 is significantly lower under hydrous conditions. For both minerals, however, the lower temperature of the hydrous experiments tends to tighten the partitioning parabolas, increasing fractionation of light from heavy REE compared to anhydrous experiments. Three sets of near-liquidus clinopyroxene–garnet two-mineral D values increase the range of published experimental determinations, but show significant differences from natural two-mineral D's determined for subsolidus mineral pairs. Similar behaviour is observed for the first experimental data for orthopyroxene–clinopyroxene two-mineral D's when compared with natural data. These differences are in large part of a consequence of the subsolidus equilibration temperatures and compositions of natural mineral pairs. Great care should therefore be taken when using natural mineral–mineral partition coefficients to interpret magmatic processes. The new data for strongly hydrous compositions suggest that fractionation of Zr–Hf–Sm by garnet decreases with increasing depth. Thus, melts leaving a garnet-dominated residuum at depths of about 200 km or greater may preserve source Zr/Hf and Hf/Sm. This contrasts with melting at shallower depths where both garnet and clinopyroxene will cause Zr–Hf–Sm fractionation. Also, at shallower depths, clinopyroxene-dominated fractionation may produce a positive Sr spike in melts from spinel lherzolite, but for garnet lherzolite melting, no Sr spike will result. Conversely, clinopyroxene megacrysts with negative Sr spikes may crystallize from magmas without anomalous Sr contents when plotted on mantle compatibility diagrams. Because the characteristics of strongly hydrous silicate melt and solute-rich aqueous fluid converge at high pressure, the hydrous data presented here are particularly pertinent to modelling processes in subduction zones, where aqueous fluids may have an important metasomatic role.

Biotite dehydration, partial melting, and fluid composition; experiments in the system KAlO2-FeO-MgO-SiO2-H2O-CO2

Biotite solid-solutions are a significant H 2 O reservoir in the lithosphere, and the assemblage Bt + Opx + Kfs + Qtz is commonly used to estimate a H₂O in high-grade metamorphic and magmatic rocks. Here we report experimental constraints on subsolidus mineral equilibria involving biotite and orthopyroxene in the system KAlO 2 -MgO-FeO-SiO 2 -H 2 O-CO 2 . Our experiments address the question of stability of biotite of a given XFe in the assemblage Bt + Qtz ± Sa, or the stability of the assemblage Opx + Sa ± Qtz. Clemens (1993) and Clemens et al. (1997) concluded that CO 2 has no effect other than to lower a H₂O and thereby raise the solidus T. Our data at X Fl H₂O &lt; 1 extend these conclusions to encompass Fe-bearing systems more similar to natural rocks. From a comparison of experimental data and calculated isopleths of biotite composition in the divariant assemblage Bt + Opx + Kfs + Qtz + fluid, it appears that phlogopite-annite solid-solutions must be significantly non-ideal (at T , 800 °C) or that enstatite-ferrosilite solid-solutions must have negative values for their Margules-type parameters. Ignoring these factors would result in any calculated a H₂O values being too low. Although various models allow us to estimate X H₂O in H 2 O-CO 2 fluids, we are still unable to use biotite equilibria to estimate a H₂O accurately during high-grade metamorphism and magma crystallization. We also consider qualitatively the effects Fe-Mg biotite solid-solution on partial melting equilibria in fluid-poor (rock-dominated) systems in which hydration-dehydration reactions control the fluid composition.

Melting study of an alkali basalt JB-1 up to 12.5 GPa: behavior of potassium in the deep mantle

In order to study the role of alkali elements in global mantle circulation, we studied melting phase relations of an alkali basalt JB-1 up to 12.5 GPa under dry conditions. Compatibility of Na and K in clinopyroxene solid solution changes dramatically in the studied pressure range. The liquidus clinopyroxene/meltpartition coefficient for Na2O increases from 0.08 at 1 atm to 2.0 at 10 GPa and that for K2O increases discontinuously at around 7 GPa. Subsolidus clinopyroxene of JB-1 contains over 1 wt.% K2O at pressures above 7 GPa, whereas it contains less than 0.1% at pressures below 5 GPa. Accordingly, subsolidus assemblage of the alkali basalt contains small amount of potassium feldspar up to 6 GPa but contains only clinopyroxene, garnet and coesite at pressures above 7 GPa. There is a huge temperature increase in the JB-1 solidus at pressures between 6 and 7.5 GPa where the subsolidus mineral assemblage changes. The geometry of the solidus curve indicates that partial melting of the alkali basaltic materials must occur at 6–7 GPa in adiabatically ascending mantle flow with broad range of potential mantle temperatures (PMT≥1200°C). Origin of the occurrence of ultrapotassic magmas in continental alkali basalt provinces and that for mildly potassium rich magmas in ocean island provinces are discussed in the light of present experiments.

Oxygen-isotope exchange and mineral alteration in gabbros of the Lower Layered Series, Kap Edvard Holm Complex, East Greenland

Multiple intrusions of gabbros, mafic dikes, and syenites in the Kap Edvard Holm Complex gave rise to prolonged circulation of meteoric hydrothermal solutions and extreme isotope exchange and mineral alteration in the 3600-m-thick Lower Layered Series gabbros. In the Lower Layered Series, δ18O of plagioclase varies from +0.3‰ to -5.8‰, and it decreases with an increase in the volume of secondary talc, chlorite, and actinolite. In the same gabbros, pyroxenes have a more restricted range in δ18O, from 5.0‰ to 3.8‰ and values of δ18Opyroxeneare independent of the abundance of secondary minerals, which ranges from 14% to 30%. These relations indicate that large amounts of water continued to flow through the rocks at temperatures of <500-600°C, altering the gabbros to assemblages of talc + chlorite + actinolite ± epidote ±albite and causing significant oxygen-isotope exchange in plagioclase, but not in pyroxene. The extensive low-temperature secondary mineralization and 18O depletion of plagioclase in the Lower Layered Series are associated with the later emplacement of dikes and gabbros and syenites, which created new fracture systems and provided heat sources for hydrothermal fluid circulation. This produced subsolidus mineral alteration and isotope exchange in the Lower Layered Series that are distinct from those in the Skaergaard and Cuillin gabbros of the North Atlantic Tertiary province, but are similar to those observed in some oceanic gabbros.

Mineralogy and petrology of alkaline granophyric xenoliths from the Thorsmörk ignimbrite, southern Iceland

The Thorsmörk ignimbrite, southern Iceland, contains a suite of granophyre xenoliths displaying magmatic or high-temperature sub-solidus mineral assemblages. These granophyres are consanguineous with the erupting comenditic magma. Four types of mineral assemblages are distinguished: 1. (A) oligoclase, edenitic hornblende, salitic pyroxene, magnesian biotite, magnetite and sphene; 2. (B) oligoclase, manganoan to sodic ferro-augite, fayalite, richterite, ilmenite and magnetite; 3. (C) anorthoclase, ferrohedenbergite to aegirine hedenbergite, ilmenite, magnetite and (riebeckite); 4. (D) cryptoperthite, aegirine hedenbergite to (aegirine), aenigmatite, arfvedsonite, ilmenite and magnetite. Geothermometry shows that the xenoliths have crystallized between 900°C and 500°C at moderate oxygen fugacities, just above the FMQ buffer. It is further demonstrated that a hot vapour phase heavily charged with sodium and halogens, played a major role in the late sub-solidus crystallization of the different types.