Hydrous Fluids Research Articles

Late Neoarchean crustal growth under paired continental arc-back arc system in the North China Craton

The late Archean (~3.0–2.5 Ga) was a key period of continental growth globally, which is widely considered to reflect the onset of vigorous plate tectonic activity, although related continental growth modes remain contentious. Here we investigate a suite of late Neoarchean metavolcanic rocks from the southwest Qixia area of the Jiaobei terrane in the North China Craton. The rocks in this suite include amphibolites, clinopyroxene amphibolites, and hornblende plagioclase gneisses. We present zircon U-Pb isotopic data which indicate that the protoliths of these rocks formed during ~2549–2511 Ma.The (clinopyroxene) amphibolites correspond to meta-basaltic rocks, with some containing high modal content of titanite. These rocks show moderate to high FeOT (8.96–13.62 wt.%) and TiO2 (0.59–1.59 wt.%), flat to less fractionated REE patterns, and mildly negative Th, Nb, and Ta anomalies, resembling those of Fe-tholeiites. In addition, they display positive zircon ƐHf(t) values (+2.6 to +8.7), and are devoid of crustal contamination or fractional crystallization. Combined with the low Nb/Yb (mostly < 1.60) and (Hf/Sm)N (mostly < 0.95), low to moderate Th/Yb (0.08–0.54), and low V/Sc (5.53–9.19) ratios, these basaltic rocks are interpreted to have been derived from a relatively reduced and depleted mantle source that was mildly metasomatized by hydrous fluids. The hornblende plagioclase gneisses are meta-andesitic rocks, and occur interlayered with the basaltic rocks. They are transitional between tholeiitic and calc-alkaline rock series, and show fractionated REE patterns with evidently negative Th, Nb, and Ta anomalies. The depleted zircon ƐHf(t) values (+2.4 to +8.4) and quantitative chemical modeling suggest that the andesitic rocks were most likely generated by injection and mixing of juvenile felsic magmas with the tholeiitic basaltic magmas.In general, the chemical features and genesis of late Neoarchean meta-basaltic rocks in our study area resemble those of Mariana back-arc basin basalts. Combined with regional geological data, it is proposed that the Jiaobei terrane witnessed late Neoarchean crustal growth under a paired continental arc-back arc setting. On a regional context, we propose two distinct geodynamic mode of late Neoarchean continental growth across North China Craton (particularly the Eastern Block), i.e., (1) arc-continent accretion along northwestern part of the Eastern Block; and (2) paired continental arc-back arc system surrounding the ~3.8–2.7 Ga continental nuclei to the southeast.

Slab‐fluid metasomatism in the Early Paleozoic forearc mantle deduced from the Motai serpentinites, South Kitakami Belt, northeast Japan

AbstractThe present study examines the petrology and geochemistry of the Early Paleozoic Motai serpentinites, the South Kitakami Belt, northeast Japan, to reveal the subduction processes and tectonics in the convergent margin of the Early Paleozoic proto‐East Asian continent. Protoliths of the serpentinites are estimated to be harzburgite to dunite based on the observed amounts of bastite (orthopyroxene pseudomorph). Relic chromian spinel Cr# [=Cr/(Cr + Al)] increases with decreasing amount of bastite. The compositional range of chromian spinel is similar to that found in the Mariana forearc serpentinites. This fact suggests that the protoliths of the serpentinites are depleted mantle peridotites developed beneath the forearc regions of a subduction zone. The Motai serpentinites are divided into two types, namely, Types 1 and 2 serpentinites; the former are characterized by fine‐grained antigorite and lack of olivine, and the latter have coarse‐grained antigorite and inclusion‐rich olivine. Ca‐amphibole occurs as isolated crystals or vein‐like aggregates in the Type 1 serpentinites and as needle‐shaped minerals in the Type 2 serpentinites. Ca‐amphibole of the Type 1 serpentinites is more enriched in LILEs and LREEs, suggesting the influence of hydrous fluids derived from slabs. By contrast, the mineral assemblage, mineral chemistry, and field distribution of the Type 2 serpentinites reflect the thermal effect of contact metamorphism by Cretaceous granite. The Ca‐amphibole of the Type 1 serpentinites is different from that of the Hayachine–Miyamori Ophiolite in terms of origin; the latter was formed by the infiltration of melts produced in an Early Paleozoic arc–backarc system. Chemical characteristics of the Ca‐amphibole in the ultramafic rocks in the South Kitakami Belt reflect the tectonics of an Early Paleozoic mantle wedge, and the formation of the Motai metamorphic rocks in the forearc region of the Hayachine–Miyamori subduction zone system, which occurred at the Early Paleozoic proto‐East Asian continental margin.

Hikurangi Plateau subduction a trigger for Vitiaz arc splitting and Havre Trough opening (southwestern Pacific)

AbstractSplitting of the Vitiaz arc formed the Tonga-Kermadec and Lau-Colville Ridges (southwestern Pacific Ocean), separated by the Lau Basin in the north and Havre Trough in the south. We present new trace element and Sr-Nd-Hf-Pb isotope geochemistry for the Kermadec and Colville Ridges extending ∼900 km north of New Zealand (36°S–28°S) in order to (1) compare the composition of the arc remnants with Quaternary Kermadec arc volcanism, (2) constrain spatial geochemical variations in the arc remnants, (3) evaluate the effect of Hikurangi igneous plateau subduction on the geochemistry of the older arc lavas, and (4) elucidate what may have caused arc splitting. Compared to the Kermadec Ridge, the Colville Ridge has higher more-incompatible to less-incompatible immobile element ratios and largely overlapping isotope ratios, consistent with an origin through lower degrees of melting of more enriched upper mantle in the Vitiaz rear arc. Between ca. 8 and 3 Ma, both halves of the arc (∼36°S–29°S) included a more enriched (EM1-type) composition (with lower 206Pb/204Pb and 207Pb/204Pb and higher Δ8/4 Pb [deviation of the measured 208Pb/204Pb ratio from a Northern Hemisphere basalt regression line] and 87Sr/86Sr) compared to older and younger arc lavas. High-Ti basalts from the Manihiki Plateau, once joined to the Hikurangi Plateau, could serve as the enriched Vitiaz arc end member. We propose that the enriched plateau signature, seen only in the isotope ratios of mobile elements, was transported by hydrous fluids from the western margin of the subducting Hikurangi Plateau or a Hikurangi Plateau fragment into the overlying mantle wedge. Our results are consistent with plateau subduction triggering arc splitting and backarc opening.

A new model for chromitite formation in ophiolites: Fluid immiscibility

Although the involvement of hydrous fluids has been widely invoked in formation of podiform chromitites in ophiolites, there is lack of natural evidence to signify the role and mechanism of fluids. In this study, a new model for the genesis of podiform chromitite is proposed on basis of revisits of comprehensive petrological, mineralogical and geochemical results of the well-preserved Kizildag ophiolite and the well-characterized Luobusa chromite deposit. In this model, ascending magmas intruding oceanic lithospheric mantle would presumably form a series of small magma chambers continuously connected by conduits. Tiny chromite nuclei would collect fluids dispersed in such magmas to form nascent droplets. They tend to float upward in the magma chamber and would be easily transported upward by flowing magmas. Chromite-rich droplets would be enlarged via coalescence of dispersed droplets during mingling and circulation in the magma chamber and/or transport in magma conduits. Crystallization of the chromite-rich liquid droplets would proceed from the margin of the droplet inward, leaving liquid entrapped within grains as precursor of mineral inclusions. With preferential upward transportation, immiscible chromite-rich liquids would coalesce to a large pool in a magma chamber. Large volumes of chromite would crystallize in situ , forming podiform chromitite and resulting in fluid enrichment in the chamber. The fluids would penetrate and compositionally modify ambient dunite and harzburgite, leading to significant fractionations of elemental and isotopic compositions between melts and fluids from which dunite and chromitite respectively formed. Therefore, fluid immiscibility during basaltic magma ascent plays a vital role in chromitite formation.

Tectonic Switching of the Trans‐North China Orogen in the Middle Paleoproterozoic: Insights From Mafic Magmatism in the Lüliang Complex

AbstractRecognition of tectonic switching is of great significance for understanding the formation and evolution of ancient accretionary orogens. In this study, whole‐rock geochemical and Sm‐Nd isotopic and zircon U‐Pb, Lu‐Hf isotopic, and trace element data are reported for two generations of metamorphosed mafic rocks from the Lüliang Complex, middle Trans‐North China Orogen. Although both episodes of mafic magmatism were derived from variable degrees of interaction between a subcontinental lithospheric mantle and slab‐derived hydrous fluids and melts, the younger ones (2.09–2.06 Ga) had higher whole‐rock εNd(t) and zircon εHf(t) values, higher (Ta/La)N ratios, and weaker Nb, Ta, and Ti anomalies in the primitive mantle‐normalized trace element diagram compared with the older ones (2.18–2.15 Ga), implying a less‐enriched mantle source and subdued subduction features. More importantly, a decrease of U/Yb and Ce/Ce* ratios of magmatic zircons and whole‐rocks V/Ti ratios from the older to younger ones implied less slab‐derived fluid addition, and a decrease of whole‐rock (Sm/Yb)N and Hf/Lu ratios suggests decreasing pressure of melt extraction and so lithosphere thickness. In consideration of other tectonothermal activity and paleogeography observed from the middle Trans‐North China Orogen, we propose a tectonic switching model to explain the middle to late Paleoproterozoic tectonic evolution caused by early subduction hinge retreat to later advance and ultimately continental collision.

Early Paleozoic magmatic “flare-ups” in western Qinling orogeny, China: New insights into the convergence history of the North and South China Blocks at the northern margin of Gondwana

The geodynamic evolution of an accretionary orogeny commonly includes multiple magmatic episodes, which can be used to trace orogenic geodynamic processes using the geochronological and petrogenetic records preserved by igneous rocks. Paleomagnetic data indicate that the juxtaposed North China Craton (NCC) and South China Block (SCB) were located on the northern margin of the Gondwana supercontinent, but the geodynamic history of the two related blocks is still unclear. To address this issue, this paper integrates literature data with new geochronological, elemental, and isotopic data from early Paleozoic intermediate and silicic intrusions of the western North Qinling Terrane. We recognize three early Paleozoic flare-ups of intermediate–silicic intrusive magmatism in this area for the first time. Despite their similar lithology, the early Paleozoic igneous rocks formed during the three pulses have distinct geochemical compositions and petrogenesis, and record the evolution from subduction to continental collision. Geochronological data record: (1) An early Silurian (c. 441–434 Ma) pulse of diorite–quartz diorite–granite magmatism. The diorites and quartz diorites probably formed by fractionation of partial melts of mantle wedge modified by slab-derived hydrous fluids, while the granites formed by the partial melting of juvenile mafic lower crust. (2) A middle Silurian high-flux pulse of diorites, quartz diorites, and leucogranites (two-mica and muscovite granites) at c. 430–423 Ma. The diorites and quartz diorites were generated by crustal assimilation and fractional crystallization of basaltic partial melts of a pre-existing mantle source that had been altered by slab-derived hydrous fluids, and that was subsequently modified by slab-melts. The two-mica and muscovite granites formed by water-present melting of early–middle Silurian metamorphosed igneous rocks derived from enriched mantle and (meta-)graywackes, respectively. (3) A late Silurian (c. 415–409 Ma) magmatic pulse formed quartz diorites, A-type granites, and leucogranites (muscovite granites). The quartz diorites formed by differentiation of basaltic partial melts of enriched mantle, with a large contribution from depleted asthenospheric mantle. The A-type granites were generated by anatexis of juvenile mafic lower crust and subsequent fractional crystallization. The muscovite granites formed by biotite dehydration melting of (meta-)graywackes. The origins of these three early Paleozoic magmatic pulses were integrated with published regional paleomagnetic data and descriptions of sedimentary, metamorphic and igneous rocks, to provide a unique record of the geotectonic evolution of the Qinling Orogenic Belt that included: (1) slab rollback at c. 448–434 Ma; (2) initial continental collision at c. 433 Ma; (3) slab break-off at c. 430–423 Ma; (4) small-scale lithospheric foundering in a post-collisional setting at c. 415–409 Ma. Literature-derived geochronological records of collision-related magmatism and paleomagnetic data indicate that docking of the NCC and SCB with the northern Gondwana margin probably ended during the early Silurian (c. 433 Ma).

Warm and oxidizing slabs limit ingassing efficiency of nitrogen to the mantle

Nitrogen is a major and essential component of Earth's atmosphere, yet relative to other volatile elements, there are relatively few experimental constraints on the pathways by which nitrogen cycles between Earth's interior and exterior. We report mineral-melt and mineral-fluid partitioning experiments to constrain the behavior of nitrogen during slab dehydration and sediment melting processes. Experiments reacted rhyolitic melts with silicate and oxide minerals, in the presence of excess aqueous fluid, over temperatures between 725-925°C and pressures between 0.2 and 2.3 GPa. Oxygen fugacity ranged between iron metal saturation (∼NNO-5) to that in excess of primitive arc basalts (∼NNO+2). Our experiments demonstrate that hydrous fluid is the preferred phase for nitrogen over minerals (biotite, K-feldspar, and amphibole) and rhyolitic melts across all conditions explored. Relatively large effects of pressure (Δlog(Dmelt−fluidN)/Δ(GPa/K) = 761 ± 68 (1σ), Δlog(Dbiotite−fluidN)/Δ(GPa/K) = 462 ± 169) and moderate effects of oxygen fugacity (Δlog(Dmelt−fluidN)/ΔNNO = -0.20 ± 0.04, Δlog(Dbiotite−fluidN)/ΔNNO = -0.10 ± 0.04) modulate partitioning of nitrogen. We further document negligible partitioning effects related to mineral composition or Cl content of hydrous fluid. Of the minerals investigated, biotite has the largest affinity for N and should control the retention of N in slabs where present. Application of partitioning data to slab dehydration PT paths highlights the potential for highly incompatible behavior (Dbiotite−fluidN < 0.1) from the slab along warmer and oxidized (NNO+1) subduction geotherms, whereas dehydration along reduced and cooler geotherms will extract moderate amounts of nitrogen (Dbiotite−fluidN > 0.1). We find that slab melting is less effective at extracting N from slabs than fluid loss, at least under oxidized conditions (NNO+1). Ultimately, the conditions under which slabs lose fluid strongly affect the distribution of nitrogen between Earth's interior and exterior.

Volcanic spherules condensed from supercritical fluids in the Payenia volcanic province, Argentina

Spherules can be formed by high-temperature processes during volcanic eruptions, lightning strikes and meteorite impacts. Here we report four different types of spherules and spheroidal particles associated with tephra deposits from two separate volcanic fields in the southern Payenia province of Argentina. These silicate and carbonate spherules represent &lt;0.01% of the sampled material, with individual spherules &lt;200 µm in size. Thirty particles were imaged. Only the transparent spherules are smooth, perfect spheres; other morphologies include ellipsoids and aggregated dumbbells. The spheroids are either hollow or solid. Major element analyses show that the spherules and spheroids have silica-rich, Fe-rich, carbonate and basaltic compositions. Chemical analysis of the carbonate spheroids shows some variability in the trace element content between the cores and rims, suggesting element mobility and loss towards the margins. All the analysed carbonate spheroids have elevated Sr/Y, La/Y and La/Ce ratios outside the range for sedimentary carbonates. All four spherule types are considered to be volcanic in origin, with the excess CO 2 required for the formation of the carbonate spherules potentially sourced from the basement lithologies. Based on the major and trace element analyses, we conclude that the silica-rich and carbonate spherules formed by instantaneous condensation from supercritical CO 2 -rich hydrous fluids saturated with dissolved silicates. Supplementary material : Appendix A, containing the full analytical dataset collected by electron probe microanalysis and secondary ion mass spectrometry, is available at https://doi.org/10.6084/m9.figshare.c.5108689

SSZ Semail Ophiolite vs MORB Masirah Ophiolite: A Perspective from Podiform Chromitites

AbstractOne of the major topics of debate in ophiolite geology is the original tectonic setting of ophiolites. New studies show that most ophiolites are formed more frequently in a supra‐subduction zone (SSZ) environment and that only a very small number of ophiolites have formed in an oceanic range (MOR). The Masirah ophiolite is one of the few oceanic ridge ophiolites that have been preserved, and the evidence that was formed in a subduction environment is missing (Moseley and Abbotts 1979, Dilek and Furnes, 2011; Rollinson, 2017). Masirah Island, the Batain and Ras Madrah areas of eastern Oman are almost entirely composed of a well‐developed ophiolite, known as the Masirah ophiolite (Fig. 1), which is, however, completely unrelated to the nearby Semail Ophiolite in the northern Oman Mountains (Fig. 2). The Masirah ophiolite is Jurassic in age and represents oceanic lithosphere derived from the Indian Ocean, but is about 15–20 Myr later than emplacement of mid‐Cretaceous Semail ophiolite in northern Oman. The presence of basaltic to rhyolitic lavas of calc‐alkaline affinity and boninites in the lava sequence of the Semail ophiolite led several researchers to propose a back‐arc basin model for this ophiolite (e.g. Tamura and Arai, 2006; Godard et al., 2008; Rollinson and Adetunji, 2015).The Masirah Ophiolite shows close affinities with MORB peridotites in general. Most of the olivine from the Masirah harzburgites show Fo contents that are similar to those of olivine from MORB. Both pyroxenes in these harzburgites have similar Mg# values, Al2O3 and Cr2O3 contents to those of pyroxenes from MORB peridotites. The observed primitive mantle‐normalized REE patterns showing enrichment in LREEs indicate that the Masirah peridotites have been modified by fluids or melts enriched in LREEs in a MORB environment.Podiform chromitites housed in ophiolites today interpreted as magmatic deposits formed during the reaction of molten rock in environments spike in the middle of the ocean (MOR) or suprasubduccion zone (SSZ) (Arai and Matsukage, 1998; Rollinson and Adetunji, 2015). The Masirah chromitites has a mineral chemistry similar to the mineral chemistry of chromite crystallized from MOR magmas. The Cr# values of chromite in the Masirah chromatite are similar to those of MOR peridotites. These findings suggest that the ultramafic and mafic cumulate rock assemblages overlying the upper mantle peridotites in the Masirah ophiolite represent the products of magma evolution in a MOR initiation stage within the proto Indian Ocean.Coexisting high‐ and low‐Cr# associations of chromitite and dunite have been found in the Semail ophiolite, which illustrates the common situation of ophiolites having both SSZ and MOR geochemical signatures. Cr# varies from 40–60 for shallow chromite bodies, and over the range 70–80 for the deep locations. This diversity of chromitite types suggests two stages of magmatic activity were responsible for the chromitite genesis, in response to a switch of tectonic setting. The first is residual from lower degree, partial melting of peridotite, which produced low‐Cr# chromitites at the Moho transition zone, possibly in a mid‐ocean‐ridge setting. The second chromitite‐forming event involves higher degree partial melting, which produced high‐Cr# discordant chromitite in the upper mantle, possibly in a supra‐subduction zone setting.Assemblages of mono‐ and poly‐phase silicate inclusions (including olivine, orthopyroxene, clinopyroxene, amphibole, phlogopite, serpentine, native Fe, FeO, alloy, sulfide, calcite, laurite, celestine and halite) within chromite have been observed in the low Cr# podiform chromitites from the Semail and Masirah ophiolites. The existence of hydrous silicate inclusions in the chromite calls for a role of hydration during chromite genesis. High‐T bright green hornblende–edenite included in the chromites is evidence of the introduction of water in the magma at the end of the chromite crystallization. Such paragenesis points to the presence of hydrous fluids during the activity of the shear bands.

The intrinsic nature of antigorite breakdown at 3 GPa: Experimental constraints on redox conditions of serpentinite dehydration in subduction zones

Antigorite is considered as the most important source of water in subduction zones, playing a key role during arc magma genesis. Although, these magmas seem more oxidized than mid-oceanic ridge basalts (MORB), the possible inherent link between the oxidation state of arc magmas and serpentinite-derived hydrous fluids is still not well established. Here, we have performed dehydration experiments of natural antigorite serpentinite containing 5 weight percent (wt%) magnetite at 3 GPa and in a temperature range from 600 to 900 °C using a multi-anvil apparatus. These experiments aim to reproduce the different stages of H2O release, forming chlorite, olivine and orthopyroxene and water. Our experimental set up permits to preserve the intrinsic high oxygen fugacity (fO2) of serpentinite during dehydration. The new olivine and orthopyroxene which formed in equilibrium with antigorite, chlorite and magnetite have high XMg numbers setting up the oxygen fugacity to high values, between 3.1 and 4.1 log units above QFM (Quartz–Fayalite–Magnetite buffer). Hematite is observed concomitantly with high XMg in olivine, of 0.94–0.97, generally at low temperatures, below 800 °C, in coexistence with chlorite. Once the magnetite is destabilized, upon chlorite breakdown which occurs above 800 °C, the oxygen fugacity decreases to 3.7 due to the decrease of the XMg of silicates. This study demonstrates the highly oxidizing nature of the fluids released from antigorite dehydration. Thus, at high pressure and high temperature conditions, fO2-sensitive elements such as carbon and sulfur are expected to be mobilized under their oxidized form, providing an oxidizing context for arc magmas genesis and assuming that they are not completely reduced by their percolation through meta-gabbro, meta-basalts and meta-sediments.

The stability and composition of sulfate melts in arc magmas

The stability field and composition of immiscible sulfate melts in equilibrium with silicate magmas has been determined using experiments over a range of crustal pressures, allowing an assessment of their possible role in transporting sulfur into the sub-volcanic arc and porphyry copper deposit systems. Experimental starting materials were based on natural trachyandesite and trachydacite compositions, with 3.5–7 wt% H2O and 3.5–5.5 wt% sulfur added to produce large, analyzable amounts of sulfate phases. Conditions ranged over 800–1200 °C, 0.2-1GPa and ƒO2 > NNO + 2.5. Sulfate melts formed at temperatures above 1000 °C at 0.75 and 1 GPa and above 900 °C at 0.2 GPa, suggesting some pressure dependence on their stability. At temperatures below 1100 °C sulfate melts and anhydrite crystals commonly coexist. Sulfate melts quenched to an intergrowth that was difficult to prepare for analysis. However, the composition was approximated by EPMA and further constrained by mass balance calculations. Sulfate melts were dominated by CaO and SO3, but also contained, in order of decreasing abundance, Na2O, K2O, MgO, FeO, Cl and P2O5. Chlorine showed a particular preference for the sulfate melt relative to the coexisting silicate melt, and calculated partition coefficients for sulfate/silicate melts were 5–13 at 1200 °C, 0.75–1 GPa. Experimental data show that, in the absence of an exsolved, hydrous fluid phase, sulfate melts can form in natural arc magmas at near-liquidus temperatures ≥ 1000 °C, assuming that magmas are oxidized and contain sufficient sulfur (> 2000–3000 ppm S). These results suggest that sulfate melt could be an important component in transporting sulfur as well as chlorine to shallow levels in the crust for hydrous magmas under a specific range of conditions. Both the non-quenchable and water-soluble nature of sulfate melts (and anhydrite) make them difficult to identify, unless trapped as mineral inclusions similar to the “wormy anhydrite” trapped in high-temperature amphiboles from Yanacocha, Peru.

The distinctive peridotite of Taww, Northern flank of Jabal Nakhl, Oman

A small peridotite massif (~5 km x ~ 0.6 km) crops out on the Northern flank of the Jabal Nakhl close to the Taww oasis in the Northern Oman mountains. It rests directly on the autochthonous and is over-thrusted by the Hawasina nappes, which contrasts with the structural position of the Semail ophiolite. The lack of pervasive high-temperature mylonitization and the absence of a metamorphic sole at its base is a distinctive feature of the Taww massif which appears more structurally linked to the autochthonous than to the allochthonous units. Its exhumation was synchronous with the one of the autochthonous with no relative displacement during or after this event.The Taww peridotite is characterized by the abundance of clinopyroxene (Cpx); its modal composition is transitional between Cpx-harzburgite and lherzolite (> 5% Cpx). Cpx is associated in clusters with orthopyroxene porphyroclasts. This texture and mineral composition of the Taww peridotite are consistent with residues left after a moderate degree of partial melting of a fertile peridotite. The Taww peridotite has a mineralogical and chemical composition contrasting with that of the Semail peridotites made essentially of harzburgite generally devoid of Cpx and with trace element patterns pointing to melt-rock reaction. When present in the Semail harzburgite, Cpx is mostly interstitial and attributable to melt (+/− hydrous fluids) percolation and crystallization, an origin that can be excluded in the case of Taww Cpx.The Semail ophiolite evolved in a highly productive magmatic environment, likely a fast spreading centre. Conversely, the Taww peridotite is closer to peridotites from highly stretched continental margins or post-orogenic subcontinental mantle. The Taww massif clearly experienced a different emplacement, partial melting, melt migration and melt-rock interaction history than the Semail ophiolite. Whatever its origin, which remains puzzling, the Taww peridotite does not seem to belong to the same obducted units as the Semail and Hawasina nappes.

Modelling garnet-fluid partitioning in H2O-bearing systems: a preliminary statistical attempt to extend the crystal lattice-strain theory to hydrous systems

Accurate geochemical models of magmatic processes require an understanding of crystal-melt partitioning of trace elements. Many major igneous processes at different tectonic environments in Earth occur in the presence of garnet as a residual phase. Since the pioneering crystal lattice-strain model, several attempts have been made to quantify garnet-melt partitioning coefficients over a wide range of conditions. However, high pressure high-temperature experimental data demonstrate distinct differences in partitioning determined at anhydrous conditions and partitioning determined in the presence of H2O. In this study, we present for the first time a constraint on the partitioning of REE, Y, and Sc between garnet and hydrous fluids as a function of the water content in the fluid phase. We analysed published hydrous experimental partitioning data using different statistical methods and modelled key parameters in the crystal lattice strain model (r0, D0, and E). We show a robust correlation between r0, temperature and garnet-fluid partitioning of Mg. We further show that D0 can be predicted using the garnet-fluid partitioning of Fe and that E can be predicted using various parameters describing the fluid phase. We validate and illustrate the ability to predict partitioning of REE in H2O-bearing systems using major element analysis of garnet and fluid only. Consequently, our statistical model paves the way to integrate thermodynamic models based on major element chemical equilibria with trace element studies on hydrous magmatic systems, allowing a description of the transfer of key trace elements in more realistic conditions.

Microstructures and Late-Stage Magmatic Processes in Layered Mafic Intrusions: Symplectites from the Sept Iles Intrusion, Quebec, Canada

AbstractReplacive symplectites (vermicular intergrowths of two or more minerals) are an important feature of layered igneous intrusions, recording evidence of late-stage reactions between interstitial liquid and crystals. They are common throughout the Layered Series of the 564 Ma Sept Iles layered intrusion in Quebec, Canada, and fall into three types: oxy-symplectites, ‘Type I’ symplectites, and ‘Type II’ symplectites. Oxy-symplectites are comprised of magnetite and orthopyroxene, nucleate on olivine primocrysts, and form via the reaction Olivine + O2 → Orthopyroxene + Magnetite; Type I symplectites (of which there are 3 distinct categories) are comprised of anorthitic plagioclase with pyroxene, amphibole, or olivine vermicules, grow from primocryst oxide grains, and replace primocryst plagioclase; and Type II symplectites (of which there are 2 distinct categories) are comprised of anorthitic plagioclase with orthopyroxene ± amphibole vermicules, grow from primocryst olivine grains, and replace primocryst plagioclase. Rare symplectites composed of biotite and plagioclase are also present. Symplectite growth occurred at 700–1030°C with pressure constraints of 1–2 kbar. We propose that Type I symplectites, and some Type II symplectites, formed from the interaction of primocrysts with residual Fe-rich liquid as a consequence of differential loss of an immiscible Si-rich liquid conjugate from the crystal mush. However, redistribution and concentration of hydrous fluids in incompletely solidified rock, or an increase in water activity of the interstitial melt, may be more plausible processes responsible for the formation of replacive symplectites comprising abundant hydrous mineral assemblages.

Petrogenesis of the Ultramafic Zone of the Stillwater Complex in North America: constraints from mineral chemistry and stable isotopes of Li and O

To investigate the petrogenesis of cyclic units in layered intrusions, we examined chromitite, dunite, poikilitic harzburgite and bronzitite from the Ultramafic Zone of the Stillwater Complex and measured stable isotopes of Li and O in their major minerals. The Li isotopes in olivine range from 4 to 26‰ in δ7Li with uniform Li contents of 1–3 ppm, whereas orthopyroxene and clinopyroxene have Li contents of 0.5–5 ppm and 4–8 ppm, and δ7Li ranges of −13 to 7‰ and −14 to −6‰, respectively. The δ18O values vary from 4.91 to 5.72‰ in olivine, from 5.11 to 5.87‰ in orthopyroxene, and from 4.64 to 5.86‰ in clinopyroxene. For a given sample, olivine displays more variable and higher δ7Li but lower δ18O values than orthopyroxene, indicating that olivine experienced more extensive compositional modification after crystallization relative to orthopyroxene. The general Li and O isotopic compositions are interpreted as the result of re-equilibration between interstitial liquids, from which pyroxenes crystallized, and cumulus minerals. The inter-mineral and inter-sample isotopic variations correlate with mineral assemblages, crystal sizes and major and trace element compositions, revealing that the interstitial liquids varied compositionally mainly due to mixing between fractionated magma and newly injected primitive magma. Abrupt mineralogical and geochemical changes from silicate rocks to chromitites imply that hydrous fluids, which collected on chromite surfaces and were later released from chromite seams, played an additional, critical medium of chemical exchange between minerals in the chromitites.

The occurrence, origin, and fate of water in chromitites in ophiolites

Abstract We present petrological investigations and mineral chemistry of several Tethyan ophiolites to reveal the occurrence, origin, and fate of water in podiform chromitites. The results show that clinopyroxene and olivine in chromitites have H2O contents of 801–366 and 53–17 ppm, respectively. The highest water contents of olivine occur in massive chromitite and the lowest always in the clinopyroxene-bearing ores because much of the available hydrous fluids was taken up by the clinopyroxene during crystallization. The major and trace elemental and Li isotopic compositions of clinopyroxene associated with chromite and olivine in podiform chromitites indicate formation from a mixture of surface hydrous fluids on chromite grains and evolved melts from which olivine crystallized. The hydrous fluids initially originated from dehydration of a subducting slab as revealed by Li isotopic compositions of clinopyroxene and olivine in the chromitites. High fluid/rock ratios facilitated concentration of chromite to form chromitite, suppressing crystallization of olivine. The hydrous fluids that were collected on the chromite grain surface during crystallization allowed chromite grains to rise via decreasing density in the form of bubbles, thus promoting their gathering and concentration. The fate of these hydrous fluids depends on ambient physical and chemical conditions. Mostly they hydrate adjacent olivine grains in the chromitite or penetrate the surrounding dunite envelope. In some cases, the fluids dissolve into silicate melts to produce water-bearing clinopyroxene and/or hydrous minerals, such as amphibole, or infiltrate silicate and chromite grains to form inclusions, which may exsolve later in the form of mineral lamellae. Our investigations provide direct natural evidence for the presence and importance of water in the formation and evolution of chromite deposits, as inferred by earlier experimental studies.

A new concept for the genesis of felsic magma: the separation of slab-derived supercritical liquid

Felsic magmas produced at subduction zones have played an important role in the generation and evolution of the continental crust. For the origin of felsic magmas, processes such as fractional crystallization of mafic magmas, partial melting of crustal materials, partial melting of subducting slabs, and partial melting of pyroxenitic mantle wedge components have been proposed. Recent experimental studies have predicted that felsic melt can also be produced in the mantle wedge by the separation of slab-derived supercritical liquid beyond depths corresponding to the critical point. To date, however, the presence of felsic magma of this origin has not yet been reported. In this study, we investigated dacitic lavas and preceding calc-alkaline andesite lavas from the Rishiri Volcano, located at the rear of the Kuril arc. We show that hydrous felsic melt and aqueous fluid were separated from slab-derived supercritical liquid in the mantle wedge. The former erupted as dacitic magma whilst the aqueous fluid induced the generation of primary basaltic magma involved in creating calc-alkaline andesite magma. We infer that slab-derived supercritical liquid is an efficient transport medium for moving silicate-rich components from subducting slabs to the Earth’s surface, and that this process may have contributed to the growth of the continental crust.

Geochronological and geochemical features of the Xiaowulangou complex plutons, Xilinhot, Inner Mongolia, and their geological significance

The southern margin of the Central Asian Orogenic Belt (CAOB) is pivotal to understanding the ultimate closure process of the Palaeo‐Asian Ocean; however, controversial issues regarding the petrogenesis and tectonic setting of its Late Palaeozoic igneous rocks have caused uncertainty regarding the Late Palaeozoic oceanic and continental evolution of the area. Herein, the Xiaowulangou complex plutons near Xilinhot, Inner Mongolia, were considered as a case study for the southern margin of the CAOB, and they were analysed via multiple geological methods, including field geology, petrology, zircon U–Pb isotopic geochronology, and geochemistry to achieve greater clarity on these issues. Lithologically, the plutons consist of diorite, quartz diorite, granodiorite, and monzonitic granite. Zircon U–Pb dating gives weighted mean ages of the diorite, quartz diorite, and granodiorite plutons as 329 ± 1 (mean squared weighted deviation [MSWD] = 0.22) and 306 ± 1 (MSWD = 0.83), 310 ± 1 (MSWD = 0.23), and 328 ± 1 (MSWD = 0.40) and 300 ± 1 Ma (MSWD = 1.4), respectively, corresponding to the late Early Carboniferous to the Late Carboniferous. The diorites and the quartz diorites constitute a suite of low‐K tholeiitic calc‐alkaline I‐type granitoids. They have low rare earth element (REE) content with a flat distribution pattern lacking pronounced negative Eu anomalies (δEu = 0.74–1.06). Geochemically, they are more enriched in large‐ion lithophile elements (LILEs) such as Rb, Th, K, and Sr than in high‐field‐strength elements (HFSEs) such as Nb, Ta, Ti, and P, indicating that these rocks were formed in a subduction setting as a result of partial melting of a mantle wedge that had been metasomatized by subduction fluids. The granodiorites and monzonitic granites belong to the peraluminous and calc‐alkaline to high‐K calc‐alkaline series and feature high silicon and alkali content; low P2O5 and CaO content; enrichment in Rb, Th, K, Zr, and Hf; depletion in Nb, Ta, Sr, P, and Ti; and significant negative Eu anomalies (δEu = 0.66–0.95 and 0.06–0.46, respectively). The granodiorites and monzonitic granites show the characteristics of I‐type granite that were mainly derived from partial melting of the crust. The petrological and geochemical characteristics of the rocks indicate that the plutonic complex is a result of superposed injection of a mixture of mantle‐ and crust‐derived magmas with different degrees of evolution. This was probably triggered by underplating of mantle magmas that originated from the partial melting of a wedge mantle metasomatized by hydrous fluids from the subducted slab. The diorites and quartz diorites were produced by fractional crystallization of mantle‐derived magma. The granodiorites and monzonitic granites crystallized from magma formed by mixing of the intermediate to basic magma and crust‐derived acidic magma. Considering their petrological and geological features, the Xiaowulangou complex plutons were formed in an active continental margin setting related to the northward subduction of the Palaeo‐Asian Ocean from the Early Carboniferous to the Late Carboniferous. Furthermore, consequently, a tectonic evolution model was established from the Late Palaeozoic to the Early Mesozoic in the Xilinhot–Xi Ujimqin Banner regions based on these new data and the regional geology.

Continuously Changing Quartz‐Albite Saturated Melt Compositions to 330 °C With Application to Heat Flow and Geochemistry of the Ocean Crust

AbstractMelt compositions in equilibrium with quartz and albite at 0.1 GPa and temperatures from 819 to 330 °C are reported for cold seal experiments in the Na2O‐Al2O3‐SiO2‐H2O system. At 60–70 °C intervals, melt compositions continuously change from rhyolite‐like melts with &lt;10 wt.% H2O at the quartz‐albite eutectic to melts of nearly endmember Na2Si2O5 having &gt;30 wt.% H2O at 330 °C (vapor undersaturated). Melt Na2O increases while SiO2 and Al2O3 decrease down temperature. Experiments performed at T &gt; 600 °C are vapor saturated with &lt;9.9–21 wt.% H2O while those T &lt; 600 °C are vapor undersaturated with 18–37 wt.% H2O. Because of the continuous change in the liquid's composition down temperature, all compositions can be called melts; in reality, their high water contents may also lead to being termed hydrous fluids. These results indicate that igneous processes continue to temperatures &gt;300 °C below the haplogranite solidus. The water‐rich melts at low temperature have low viscosities and densities suggesting that they buoyantly ascend by reactive flow at low melt porosities (&lt;5%). Formation of low‐temperature melt will occur along the sides of gabbro bodies of mid‐ocean ridges, resulting in large‐scale convection that removes heat from the lower crust. This circulation may move heat off axis influencing observed heat flow. Melt reaction processes result in silicic differentiates (plagiogranites) and formation of greenschist metabasalts. This frees calcium from silicate bonding, providing an important sink for CO2 in Earth's carbon cycle. Sr isotopes of the ocean crust change with ocean crust production rates indicating mid‐ocean ridge systems are a major control on Earth's climate over time.

Petrogenesis of the ~740 Korab Kansi mafic-ultramafic intrusion, South Eastern Desert of Egypt: Evidence of Ti-rich ferropicritic magmatism

The Neoproterozoic Korab Kansi mafic-ultramafic intrusion is one of the largest (100 km2) intrusions in the Southern Eastern Desert of Egypt. The intrusion consists of Fe-Ti-bearing dunite layers, amphibole peridotites, pyroxenites, troctolites, olivine gabbros, gabbronorites, pyroxene gabbros and pyroxene-hornblende gabbros, and also hosts significant Fe-Ti deposits, mainly as titanomagnetite-ilmenite. These lithologies show rhythmic layers and intrusive contacts against the surrounding granites and ophiolitic-island arc assemblages. The wide ranges of olivine forsterite contents (Fo67.9-85.7), clinopyroxene Mg# (0.57–0.95), amphibole Mg# (0.47–0.88), and plagioclase compositions (An85.8-40.9) indicate the role of fractional crystallization in the evolution from ultramafic to mafic rock types. Clinopyroxene (Cpx) has high REE contents (2–30 times chondrite) with depleted LREE relative to HREE, like those crystallized from ferropicritic melts generated in an island-arc setting. Melts in equilibrium with Cpx also resemble ferropicrites crystallized from olivine-rich mantle melts. Cpx chemistry and its host rock compositions have affinities to tholeiitic and calc-alkaline magma types. Compositions of mafic-ultramafic rocks are depleted in HFSE (e.g. Nb, Ta, Zr, Th and U) relative to LILE (e.g. Li, Rb, Ba, Pb and Sr) due to the addition of subduction-related hydrous fluids (rich in LILE) to the mantle source, suggesting an island-arc setting. Fine-grained olivine gabbros may represent quenched melts approximating the primary magma compositions because they are typically similar in assemblage and chemistry as well as in whole-rock chemistry to ferropicrites. We suggest that the Korab Kansi intrusion crystallized at temperatures ranging from ~700 to 1100 °C from ferropicritic magma derived from melting of metasomatized mantle at <5 Kbar. These hydrous ferropicritic melts were generated in the deep mantle and evolved by fractional crystallization under high ƒO2 at relatively shallow depth. Fractionation formed calc-alkaline magmas during the maturation of an island arc system, reflecting the role of subduction-related fluids. The interaction of metasomatized lithosphere with upwelling asthenospheric melts produced the Fe and Ti-rich ferropicritic parental melts that are responsible for precipitating large quantities of Fe-Ti oxide layers in the Korab Kansi mafic-ultramafic intrusion. The other factors controlling these economic Fe-Ti deposits beside parental melts are high oxygen fugacity, water content and increasing degrees of mantle partial melting. The generation of Ti-rich melts and formation of Fe-Ti deposits in few layered intrusions in Egypt possibly reflect the Neoproterozoic mantle heterogeneity in the Nubian Shield. We suggest that Cryogenian-Tonian mafic intrusions in SE Egypt can be subdivided into Alaskan-type intrusions that are enriched in PGEs whereas Korab Kansi-type layered intrusions are enriched in Fe-Ti-V deposits.