Volatile-bearing Minerals Research Articles

Unidirectional solidification textures and garnet layering in Y-enriched garnet-bearing aplite-pegmatites in the Cadomian Brno Batholith, Czech Republic

Layered, garnet bearing aplite pegmatite dykes of the Hlina granitic suite, ~2 to 50 m thick and up to ~200 m long with general NW SE orientation and dip 40 80 to NE or SW, cut granodiorites to granites at the SW part of the Brno Batholith, Brunovistulicum, Czech Republic. Aplite pegmatite bodies are characterized by alternation of two main textural units: (i) fine grained aplite unit with garnet stripes and (ii) coarse grained pegmatite unit with oriented Qtz, Plg and Kfs comb like crystals known as Unidirectional Solidification Textures (UST). Both units developed in zones with the thickness varying from several cm to ~1 2 m for aplite unit, and ~10 cm for the UST unit. All rock types are characterized by high contents of SiO2 (74.6 75.7 wt. %), K2O (4.61 4.94 wt. %), Na2O (3.82 4.21 wt. %), moderate concentration of CaO (0.94 1.11 wt. %), and low to very low concentrations of Fe2O3 T (0.62 0.93 wt. %), MgO (0.0 0.03 wt. %), and TiO2 ( 0.03 wt. %). Low K/Rb (212 241) and high K/Ba ratios (1034 2303) with deep Eu anomaly indicate high degree of fractionation. Both textural units consist of perthitic microcline, plagioclase An15 8 and quartz. Total amount of accessory minerals is typically very low, commonly < ~1 vol. % in aplite unit, and in the UST unit they are almost absent. Accessory minerals include relatively common Y rich garnet Sps42 38 Alm32 28 And15 7 Grs21 15 Prp2 1 (1.10 wt. % Y2O3, 0.53 wt. % Yb2O3 and 0.20 wt. % Er2O3) with oscillatory and sector zoning. Other minerals closely associated with garnet are extremely rare: magnetite, chloritized biotite, muscovite, Ta rich titanite I, Al, F rich titanite II, and ilmenite. Primary zircon, xenotime (Y), monazite (Nd), fersmite, ferrocolumbite, REE,Y rich pyrochlore are strongly altered. Geochemical and mineralogical features of the Hlina aplite pegmatites are characterized by (i) subaluminous chemistry with Nb >> Ta and HREE >> LREE, A type (NYF) affinity, indicated by elevated concentrations of Y, REE (especially HREE), Zr, U, Th, Nb, and Ta, (ii) occurrence of specific accessory minerals including Y rich garnet, and (iii) a remarkable, almost complete absence of micas and other minerals with volatiles (B, F, P and also H2O). The studied suite differs from all other granitic rocks with UST described to date, which are typically characterized by volatile bearing minerals and peraluminous signatures. Magmatic layering involving g

Tracing the evolution and distribution of F and Cl in plutonic systems from volatile-bearing minerals: a case study from the Liujiawa pluton (Dabie orogen, China)

Fluorine and chlorine play an important role in magmatic differentiation, hydrothermal alteration, and related mineralization processes, but tracing their evolution in magmatic and especially plutonic systems is not an easy task. The F and Cl in melts can be estimated from F and Cl concentrations in minerals, provided that partitioning between minerals and melts are constrained. Based on available partitioning models between mineral/melt, mineral/fluid, and melt/fluid, a set of equations has been derived to determine F and Cl concentrations in melts from the compositions of amphibole, biotite, and apatite. The new calculation procedure has been applied to a plutonic system, the Liujiawa pluton, eastern Dabie orogen (China). Cl and F concentrations in amphiboles, biotites, and apatites from different rock types (gabbronorite, two-pyroxene diorite, clinopyroxene diorite, and hornblende gabbro) have been determined by electron microprobe. Most amphiboles show a negative correlation between log(Cl/OH) and Mg-number and a positive correlation between log(F/OH) and A-site occupation. Biotites from the gabbronorite and two-pyroxene diorite show a slight positive correlation between log(Cl/OH) and Mg, which is however not the case for the clinopyroxene diorite. Apatites from all the samples are rich in F and show negative correlations between Cl and F concentrations. In our case study, we demonstrate that the Cl concentration in melt remains approximately constant at 1,000–2,000 ppm over the major crystallization interval, but decreases strongly at near solidus temperatures as a result of fluid exsolution. The F concentration in melt remains nearly constant at ca. 2,000–3,000 ppm at high temperatures as well as near solidus conditions, indicating that it is not largely affected by fluid exsolution because of its strongly preferred incorporation into melt. Interestingly, the evolution of Cl and F concentrations in melt with magmatic differentiation is similar to that determined in volcanic systems, suggesting that the evolution of Cl and F in melts during crystallization and late magmatic stages at depth (plutonic systems) is similar to that observed in volcanic systems during decompression and degassing.

Evidence of water degassing during emplacement and crystallization of 2.7 Ga komatiites from the Agnew-Wiluna greenstone belt, Western Australia

Komatiites are ancient volcanic rocks, mostly over 2.7 billion years old, which formed through >30% partial melting of the mantle. This study addresses the crucial relationship between volcanology and physical manifestation of primary magmatic water content in komatiites of the Agnew-Wiluna greenstone belt, Western Australia, and documents the degassing processes that occurred during the emplacement and crystallization of these magmas. The Agnew-Wiluna greenstone belt of Western Australia contains three co-genetic komatiite units that (1) display laterally variable volcanological features, including thick cumulates and spinifex-textured units, and (2) were emplaced as both lava flows and intrusions at various locations. Komatiite sills up to 500 m thick contain widespread occurrence of hydromagmatic amphibole in orthocumulate- and mesocumulate-textured rocks, which contain ca. 40–50 wt% MgO and <3 wt% TiO2. Conversely, komatiite flows do not contain any volatile-bearing mineral phases: ~150-m-thick flows only contain vesicles, amygdales and segregation structures, whereas <5–10-m-thick flows lack any textural and petrographic evidence of primary volatile contents. The main results of this study demonstrate that komatiites from the Agnew-Wiluna greenstone belt, irrespective of their initial water content, have degassed upon emplacement, flow and crystallization. More importantly, data show that komatiite flows most likely degassed more water than komatiite intrusions. Komatiite degassing may have indirectly influenced numerous physical and chemical parameters of the water from the primordial oceans and hence indirectly contributed to the creation of a complex zonation at the interface between water and seafloor.

Metasomatism of garnet peridotite from Jiangzhuang, southern Sulu UHP belt: constraints on the interactions between crust and mantle rocks during subduction of continental lithosphere

The Jiangzhuang ultrahigh-pressure (UHP) metamorphic peridotite from south Sulu, eastern China occurs as a layer within gneiss with eclogite blocks, and consists of coarse-grained garnet porphyroblasts and a fine-grained matrix assemblage of garnet + forsterite + enstatite + diopside ± phlogopite ± Ti-clinohumite ± magnesite. Both types of garnet are characterized by high MgO content and depletion of light rare earth element (LREE) and enrichment of heavy rare earth element, but the matrix garnet has lower MgO, TiO2 and higher Cr2O3 and REE contents. Diopside displays LREE enrichment, and has low but variable large-ion lithophile element (LILE) contents. Phlogopite is a major carrier of LILE. Ti-clinohumite contains high Nb, Ta, Cr, Ni, V and Co contents. The P–T conditions of 4.5–6.0 GPa and 850–950 °C were estimated for matrix mineral assemblages. Most peridotites are depleted in Al2O3, CaO and TiO2, and enriched in SiO2, K2O, REE and LILE. In contrast to phlogopite-free peridotites, the phlogopite-bearing peridotites have higher K2O, Zr, REE and LILE contents. Zircon occurs only in the phlogopite-bearing peridotites, shows no zoning, with low REE contents and Th/U ratios, and yields tight U–Pb ages of 225–220 Ma, indicating the peridotites experienced consistent Triassic UHP metamorphism with subducted supercrustal rocks. These data demonstrate that the Jiangzhuang peridotites were derived from the depleted mantle wedge of the North China Craton, and experienced various degrees of metasomatism. The phlogopite-free peridotites may have been subjected to an early cryptic metasomatism at UHP conditions of the mantle wedge, whereas the phlogopite-bearing peridotites were subjected to a subsequent strong metasomatism, characterized by distinctly enrichment in LILE, LREE, Zr and K as well as the growth of zircon and volatile-bearing minerals at UHP subduction conditions. The related metasomatism may have resulted from the filtration of fluids sourced mainly from deeply subducted supracrustal rocks.

The three partners of metamorphic petrology

Study of the evolution of the Earth’s crust advanced greatly in the last half-century through the dynamic collaboration of three equally contributing partners: field observations, together with microanalysis of mineral assemblages, experimental petrology, and stability calculations based on systematically measured and compiled thermodynamic properties of minerals. In addition to providing the initial inspiration for experimental and theoretical studies of petrogenesis, quantitative field observations have had an important feedback relationship with the indoor approaches. It has been demonstrated that physico-chemical properties of mineral systems can, in favorable circumstances, be inferred from purely geologic criteria, and, in some cases, field criteria have been used to challenge the validity of existing experimental or thermodynamic data, leading to reinvestigations and revisions. The “geo-experimental” method may be particularly useful to infer phase equilibrium relations among minerals where experimental reactions are prohibitively slow at geologically reasonable temperatures or where fluid fluxing of mineral reactions is necessarily limited, as in investigations concerning the low-H2O-activity deep-crustal rocks, the granulites. Geo-experimental phase equilibrium diagrams, constructed largely from quantitative petrographic observations, have been successful in deducing polymorphism, order-disorder, and solid solution properties of such mineral systems as the aluminum silicate minerals, feldspars, epidote minerals, scapolites, and cordierite. The last minerals are particularly important in that they may retain a record of volatile components of an intergranular mineralizing fluid. Knowledge of the important effects of mixed volatile fluids has been inhibited by lack of experimental data on devolatilization, ion exchange, redox, sulfidation, and fluid-complexing reactions of the volatile-bearing minerals biotite, amphibole, apatite, scapolite, cordierite, carbonates, and sulfides with the major minerals of rocks of the deeper interior. A concerted approach among the experimental, thermodynamic, and petrographic disciplines is necessary to make progress on the still poorly understood role of volatiles in shaping the crust. The relatively young science of fluid inclusion analysis will have an increasingly important role in the interdisciplinary effort to interpret the evolution of the continental crust and its underpinnings.

The role of exhumation in metamorphic dehydration and fluid production

The generation of fluids as mountain belts form is thought to be related to the heating that accompanies rock burial. Mineral-equilibria modelling suggests that the exhumation of greywacke — a common rock type in mountain-forming regions — following heating also generates new fluid. When mountain belts form, crustal rocks undergo metamorphism, resulting in the breakdown of volatile-bearing minerals and the release of water-rich fluids. As these fluids move towards the Earth’s surface, they can cause generation of ore deposits, enhance deformation of the crust and change rock composition1. Generation of such fluids has long been considered to occur dominantly during heating associated with burial of rocks. In contrast, the exhumation of rocks that follows heating has not been expected to generate large amounts of fluid1,2,3. Here we use mineral-equilibria modelling to show that the erosion-induced exhumation of greywacke — a common rock type in mountain-forming regions — generates a continual supply of new fluid. Fluid formation is particularly pronounced at temperatures below about 500 ∘C. Such fluids can explain the pairing of seismic and electrical conductivity anomalies observed in the Southern Alps in New Zealand, as well as the formation of vein-infilled backshears there.

Enrichment of upper mantle peridotite: petrological, trace element and isotopic evidence in xenoliths from SE China

Trace element patterns of spinel- and garnet peridotite xenoliths from the Nushan and Mingxi basalts (SE China) reflect the interplay between partial melting and subsequent enrichment by metasomatism. Trace element patterns indicate the common addition of Th, Sr and LREE, but not of significant Ti and Zr, which suggests a metasomatising agent (or agents) with characteristics of H 2O-rich and CO 2-rich fluids. Laser Ablation Microprobe linked to an Inductively Coupled Plasma Mass Spectrometer (LAM-ICPMS) analyses of volatile-bearing minerals (including amphibole, phlogopite and apatite) and glasses show that they are important hosts for K, Rb, Sr, Ba, U and rare earth elements (REE) in the mantle, and play an important role in the fractionation of these elements in mantle processes. However, mass-balance calculations require that significant proportions of LREE, Th and Sr are situated in intergranular phases. 87Sr/ 86Sr ratios of whole-rock samples from each locality are higher, and 143Nd/ 144Nd lower, than those of the corresponding acid-leached clinopyroxenes (cpx), suggesting that highly radiogenic Sr and relatively unradiogenic Nd reside in these intergranular phases. The Sr/Nd isotopic compositions of clinopyroxenes separated from the peridotites are correlated with metasomatic style, as reflected in the trace element compositions of the clinopyroxenes. Clinopyroxenes in fertile peridotites from Nushan and Mingxi have high 143Nd/ 144Nd and low 87Sr/ 86Sr, reflecting long-term depletion of the mantle. Clinopyroxenes from metasomatised peridotites define trends toward lower 143Nd/ 144Nd and higher 87Sr/ 86Sr, consistent with a metasomatic fluid having a Sr/Nd isotopic composition near Bulk Earth, and 147Sm/ 144Nd≤0.09. Metasomatism by a similar type of fluid can explain the isotopic systematics of clinopyroxenes from most eastern China localities reported in the literature, but a few localities interpreted as on-craton would require fluids with still lower 143Nd/ 144Nd and higher 87Sr/ 86Sr. The minimum age of the major element depletion may be given by a Sm/Nd regression line of 733±340 Ma for cpx from unmetasomatised peridotites at Nushan. The age of the metasomatic event(s) cannot be constrained, but a few metasomatised samples with high 143Nd/ 144Nd indicate that a metasomatic event must have occurred shortly before eruption.

The planet beyond the plume hypothesis

Acceptance of the theory of plate tectonics was accompanied by the rise of the mantle plumerhotspot concept which has come to dominate geodynamics from its use both as an explanation for the origin of intraplate volcanism and as a reference frame for plate motions. However, even with a large degree of flexibility permitted in plume composition, temperature, size, and depth of origin, adoption of any limited number of hotspots means the plume model cannot account for all occurrences of the type of volcanism it was devised to explain. While scientific protocol would normally demand that an alternative explanation be sought, there have been few challenges to ''plume theory'' on account of a series of intricate controls set up by the plume model which makes plumes seem to be an essential feature of the Earth. The hotspot frame acts not only as a reference but also controls plate tectonics. Accommodating plumes relegates mantle convection to a weak, sluggish effect such that basal drag appears as a minor, resisting force, with plates having to move themselves by boundary forces and continents having to be rifted by plumes. Correspondingly, the geochemical evolution of the mantle is controlled by the requirement to isolate subducted crust into plume sources which limits potential buffers on the composition of the MORB-source to plume- or lower mantle material. Crustal growth and Precambrian tectonics are controlled by interpreta- tions of greenstone belts as oceanic plateaus generated by plumes. Challenges to any aspect of the plume model are thus liable to be dismissed unless a counter explanation is offered across the geodynamic spectrum influenced by ''plume theory''. Nonetheless, an alternative synthesis can be made based on longstanding petrological evidence for derivation of . intraplate volcanism from volatile-bearing sources wetspots in conjunction with concepts dismissed for being incompatible or superfluous to ''plume theory''. In the alternative Earth, the sources for intraplate volcanism evolve from the source residues of arc volcanism located along sutures in the continental mantle. Continental rifting and the lateral distribution of intraplate sources in the asthenosphere are controlled by Earth rotation. Shear induced on the base of the asthenosphere from the mesosphere as the Earth rotates is transmitted to the lithosphere as basal drag. Attenuation of the drag due to the low viscosity of the asthenosphere, in conjunction with plate motions from boundary forces, results in a rotation differential of up to 5c m yr y1 between the lithosphere and mesosphere manifest as westward plate lagreastward mantle flow. Continental rifting results from basal drag supplemented by local convection induced by lithospheric architecture. Large continental igneous provinces are generated by convective melting, with passive margin volcanic sequences following the axis of rifting and flood basalts overlying the intersection of sutures in the continental mantle. As rifting progresses, the convection cells expand, cycling continental mantle from sutures perpendicular to the rift axis to generate intraplate tracks in the ocean basin. Continental mantle not melted on rifting, or delaminated on continental collision, becomes displaced to the east of the continent by differential rotation, which also sets up a means for tapping the material to give fixed melting anomalies. When

Volatile-bearing minerals and lithophile trace elements in the upper mantle

Amphibole ( n = 11), mica ( n = 16), apatite ( n = 9) and clinopyroxene (n = 19) from mantle xenoliths in alkali basalts from Siberia, Mongolia and Spitsbergen were analysed for Rb, Sr, Ba, Y, Zr, Nb, Th, U, Pb, La, Ce by proton microprobe; some mineral separates were analysed by solution ICP-MS for a larger number of elements. These results are examined together with literature data to further constrain the variation ranges and elemental ratios for these elements in the volatile-bearing minerals and the role of the latter as hosts of lithophile trace elements in metasomatised upper mantle. Mica is the dominant host of Rb and Ba and has high but variable Rb/Sr (0.13—60) and Ba/Sr ratios. Amphibole and mica are major hosts for Nb in the peridotitec upper mantle and strongly affect its Zr/Nb ratio. Apatite is a major host for Th, U, Cl, Br, LREE and Sr, but some apatites are low in Th, U, Br. Clinopyroxene contains the bulk of Sr, Zr, Y, REE in fertile spinel lherzolites with disseminated amphibole and mica and can host significant enrichments of Th, U, Pb. There are significant differences in trace element composition between vein amphibole and mica and those disseminated in peridotites; in particular, the former commonly have higher Nb and Zr contents than the latter. Trace element partition coefficients between amphibole, mica, apatite and clinopyroxene in natural ultramafic rocks from this work generally agree with results of recent experimental studies and provide important constraints on the partition coefficients between these minerals and basaltic melts. The volatile-bearing minerals may play a major role in creating geochemical and radiogenic isotope characteristics of lithospheric mantle domains and source regions of mantle-derived melts. In particular, the long-term presence of mica is required to create lithospheric mantle reservoirs enriched in radiogenic Sr as the Rb/Sr ratio in amphibole, the only other common alkali-bearing mantle mineral, is usually below bulk earth value of 0.03. The abundance and melting relationships of amphibole, mica and apatite at low degrees of partial melting of metasomatised mantle may strongly affect the absolute and relative abundances of K, Rb, Ba, HFSE and other elements in the derivative melts.

Spinel peridotite xenoliths from the Atsagin-Dush volcano, Dariganga lava plateau, Mongolia: a record of partial melting and cryptic metasomatism in the upper mantle

Spinel peridotite xenoliths from the Atsagin-Dush volcanic centre, SE Mongolia range from fertile lherzolites to clinopyroxene(cpx)-bearing harzburgites. The cpx-poor peridotites typically contain interstitial fine-grained material and silicate glass and abundant fluid inclusions in minerals, some have large vesicular melt pockets that apparently formed after primary clinopyroxene and spinel. No volatile-bearing minerals (amphibole, phlogopite, apatite, carbonate) have been found in any of the xenoliths. Fifteen peridotite xenoliths have been analysed for major and trace elements; whole-rock Sr isotope compositions and O isotope composition of all minerals were determined for 13 xenoliths. Trace element composition and Sr-Nd isotope compositions were also determined in 11 clinopyroxene and melt pocket separates. Regular variations of major and moderately incompatible trace elements (e.g. heavy-rare-earth elements) in the peridotite series are consistent with its formation as a result of variable degrees of melt extraction from a fertile lherzolite protolith. The Nd isotope compositions of LREE (light-rare-earth elements)-depleted clinopyroxenes indicate an old (≥ 1 billion years) depletion event. Clinopyroxene-rich lherzolites are commonly depleted in LREE and other incompatible trace elements whereas cpx-poor peridotites show metasomatic enrichment that can be related to the abundance of fine-grained interstitial material, glass and fluid inclusions in minerals. The absence of hydrous minerals, ubiquitous CO2-rich microinclusions in the enriched samples and negative anomalies of Nb, Hf, Zr, and Ti in primitive mantle-normalized trace element patterns of whole rocks and clinopyroxenes indicate that carbonate melts may have been responsible for the metasomatic enrichment. Low Cu and S contents and high δ34S values in whole-rock peridotites could be explained by interaction with oxidized fluids that may have been derived from subducted oceanic crust. The Sr-Nd isotope compositions of LREE-depleted clinopyroxenes plot either in the MORB (mid-ocean-ridge basalt) field or to the right of the mantle array, the latter may be due to enrichment in radiogenic Sr. The LREE-enriched clinopyroxenes and melt pockets plot in the ocean island-basalt field and have Sr-Nd isotope signatures consistent with derivation from a mixture of the DMM (depleted MORB mantle) and EM (enriched mantle) II sources.

Shock effects on hydrous minerals and implications for carbonaceous meteorites

New infrared absorption spectra, thermo-gravimetric analyses and optical-and scanning electron microscopy of shock-recovered specimens of antigorite serpentine (Mg3Si2O5(OH)4) from the pressure range between 25 to 59 GPa are reported. The infrared spectra show systematic changes in absorption peaks related to structural and molecular surface absorbed water. H2O absorption peaks increase at the expense of OH peaks with increasing shock pressure. Changes in SiO bond vibrational modes with increasing shock pressure parallel those seen for other, non-hydrous minerals. Thermogravimetric analyses of shock-recovered samples determine the amount of shock-induced water loss. For samples shocked in vented assemblies, the data define a relation between shock-induced water loss versus shock pressure. Results for samples shocked in sealed assemblies demonstrate a dependence of water loss on shock pressure and target confinement. For the vented assembly samples, a linear relation between shock pressure and both the length of dehydration interval and the effective activation energy for releasing post-shock structural water in antigorite is found. Optical and scanning electron miscroscopy of shocked antigorite reveal a number of textures thought to be unique to shock loading of volatile-bearing minerals. Gas bubbles, which probably are the result of shock-released H2O appear to be injected into zones of partial melting. This process may produce the vesicular dark veins which are distributed throughout heavily shocked samples. The present observations suggest several criteria which may constrain possible shock histories of the hydrous matrix phases of carbonaceous condrites. A model is proposed for explaining hydrous alteration processes occurring on carbonaceous chondrite parent bodies in the course of their accretion. We speculate that shock loading of hydrous minerals would release and redistribute free water in the regoliths of carbonaceous chondrite parent bodies giving rise to the observed hydrous alterations.

The evolution of an impact-generated atmosphere

Shock wave and thermodynamic data for rock-forming and volatile-bearing minerals are used to determine minimum impact velocities ( v cr) and minimum impact pressures ( p cr) required to form a primary H 2O atmosphere during planetary accretion from chondritelike planetesimals. The escape of initially released water from an accreting planet is controlled by the dehydration efficiency. Since different planetary surface porosities will result from formation of a regolith, v cr and p cr can vary from 1.5 to 5.8 km/sec and from 90 to 600 kbar, respectively, for target porosities between 0 and ∼45%. On the basis of experimental data, hydration rates for forsterite and enstatite are derived. For a global regolith layer on the Earth's surface, the maximum hydration rate equals 6 × 10 10 g H 2O sec −1 during accretion of the Earth. Attenuation of impact-induced shock pressure is modeled to the extent that the amount of released water as a function of projectile radius, impact velocity, weight fraction of water in the target, target porosity, and dehydration efficiency can be estimated. The two primary processes considered are the impact release of water bound in hydrous minerals (e.g., serpentine) and the subsequent reincorporation of free water by hydration of forsterite and enstatite. These processes are described in terms of model calculations for the accretion of the Earth. Parameters which lead to a primary atmosphere/hydrosphere are: an accretion time of ⪅ 1.6 × 10 8years, the use of an accretion model defined by Weidenschilling (1974, 1976), a mean planetesimal radius of 0.5 km, a hydration rate of 6 × 10 10 g H 2O sec −1 inferred from a mean porosity of ∼ 10% for the upper 1 km of the accreting Earth, and values for the dehydration efficiency, DE, of 0.55 and 0.07 for the maximum and minimum pressure decay model, respectively. Conditions which prohibit the formation of a primary atmosphere include an accretion time much longer than 1.6 × 10 8 years, a hydration rate for forsterite and enstatite well in excess of 6 × 10 10 g H 2O sec −1, and a dehydration efficiency DE < 0.07. We conclude that the concept of dehydration efficiency is of dominant importance in determining the degree to which an accreting planet acquires an atmosphere during its formation.