Crystallization In Magma Chambers Research Articles

Pre‐Eruptive Evolution and Timescales of Silicic Volcanism in the Tarim Large Igneous Province

AbstractThe widespread (∼48,000 km2) coexisting crystal‐rich monotonous intermediates (trachydacites) and crystal‐poor high‐silica rocks (rhyolites, ignimbrites) in the Tarim Large Igneous Province (T‐LIP) offer a rare opportunity to explore pre‐eruptive timescales and evolutionary patterns of different silicic magmas in LIPs. We combined information from petrography, geochemistry, crystal kinetics, and diffusion chronometry of felsic rocks from nine boreholes to investigate their pre‐eruptive history. The complex reverse Ti zoning patterns in quartz corresponding to temperature variations over 100°C, lognormal crystal size distributions, high‐Ti rims with faster growth rates, variable pressures, and Nd isotopic modeling, indicate silicic magmas in T‐LIP might have witnessed a universal evolutionary pattern of periodic recharge and rejuvenation, coarsening, and cooling and crystallization in magma chambers at different crustal levels, which we term here as RRCCC model. The uniform composition and crystallinity of trachydacites might result from latent heat buffering, and their eruption was triggered by melt‐induced fracturing. The chemical and thermal gradients preserved in rhyolites were generated by thermally variable magma inputs and extensive fractionation at different pressures. Co‐existence of these two types of rocks was controlled by interaction between mantle plume impingement and re‐melting of hydrous crust built up by early subduction. The best‐fitting Ti‐in‐quartz diffusion curves obtained by Neural Networks indicate that magma residence time lasted for ∼106 years, and eruption was triggered within ∼105 years after the last recharge event.

Linking surface deformation to thermal and mechanical magma chamber processes

Surface deformation at active volcanoes reflects a wide variety of magmatic and crustal processes, and the typical kinematic models employed to interpret these geodetic data are highly nonunique. This work presents a new model for surface deformation linked to magma chamber evolution in response to magma injection, eruptions, cooling and crystallization, volatile exsolution, and viscoelastic deformation of the crust. The model is applied to investigate surface displacements during 1) magma chamber crystallization and second boiling without magma injection; 2) a single episode of magma injection to the chamber; and 3) steady magma injection over multiple eruption cycles. Results indicate that the magnitude of surface velocities associated with crystallization and second boiling should not exceed ∼1 mm/yr unless the magma cooling rate is substantially elevated above normal conductive cooling rates. The surface response to an episode of magma injection depends on magma compressibility and how the timescale of injection compares to the timescale for viscous relaxation. When the injection time is short compared to the relaxation time, and when the magma is equally or less compressible compared to the wall rocks, post-injection deflation can occur without any volume contraction of the chamber. For more compressible magmas, or longer-duration injections, inflation continues over a protracted time following injection. Over multiple eruption cycles, uplift accumulates when more magma is added to the chamber than lost by eruptions; because both the eruption frequency and uplift rate are sensitive to the time-averaged magma supply and chamber volume, geologic data from the eruptive record can be integrated with geodetic data to place tighter constraints on the size and state of subvolcanic magma systems.

Chemical and boron isotopic compositions of tourmaline at the Baiyanghe Be[sbnd]U deposit, northwest China: Implications for Be[sbnd]U mineralization

Tourmaline records the physical and chemical conditions of the crystallization environment and preserves the information through younger magmatic-hydrothermal processes. Therefore, tourmaline is a favorable geochemical tracer and can be used to reveal processes that have contributed to the metallogenesis of hydrothermal deposits. The Baiyanghe deposit, located in the Xiemisitai Mountains, Northwest China, is the largest volcanogenic BeU deposit in Asia. The deposit has three types of ores: U type, Be type, and BeU type. These ores either occur separately or co-exist but have cross-cutting relationships. Major ore bodies occur as fracture fillings along contact zones between the Yangzhuang granite porphyry and the underlying Devonian volcanic rocks. Three types of tourmaline (e.g., hosted in the granite porphyry, rhyolitic crystal tuff, and basalt) were identified based on their occurrences, and seven sub-type tourmalines were clarified according to their textures and chemical compositions. They all belong to the alkali schorl group and rich in F. The low Al concentration (< 6 apfu) and Fe + Mg > 3 apfu tourmaline hosted in the rhyolitic crystal tuff suggest the substitution at Z site by Fe3+. The tourmalines hosted in the granite porphyry are rich in Li, Be, U, Ca, Ti, Cr, Y, Zr, Nb, Mo, Sn, Sb, Ta, and Th, whereas the tourmalines hosted in the rhyolitic crystal tuff and the basalt are rich in Mg, Sc, V, Co, Cu, Ni, Ga, and Sr, respectively. The δ11B value ranges from −5.94‰ to −4.48‰, from −8.01‰ to −6.93‰, and from −8.31‰ to −4.94% in the tourmalines hosted in the granite porphyry, the rhyolitic crystal tuff and the basalt, respectively. These data indicate that the chemical components that have contributed to the growth of tourmaline were derived from granitic magmas essentially. The oscillating zonation and the apparent variations of trace element compositions (e.g., Fe/(Fe + Mg) and Na/(Na + Ca) ratios) and REE patterns of the tourmalines indicate the involvement of periodic fluids and significant fluid-rock interaction, which are responsible for the precipitation of sub-type tourmalines. It is proposed that the tourmalines at the Baiyanghe deposit are the products of fluids exsolved from either a deep-rooted crystallizing magma chamber or one of the younger intrusions of the hosting composite granitic pluton in addition to the crystallizing the Yangzhuang granitic magma. The chemical and boron isotopic composition of tourmaline can provide a fingerprint to decipher the magmatic-hydrothermal process during the cyclic UBe mineralization.

New constraints on the open magma chamber processes in the formation of giant Hongge Fe-Ti-V oxide deposit

The Hongge layered intrusion in Emeishan Large Igneous Province (ELIP), SW China, contains the largest Fe-Ti-V oxide deposit in China (4500Mt ore reserve). The mechanism by which million tons of Fe-Ti-V oxide concentrated is debated. In this study, new petrography, mineral chemistry (~3600 microprobe analyses for olivine, clinopyroxene and plagioclase) and bulk rock major element chemistry (108 samples) are presented from samples collected over a 940 m drill core (ZK11003) in the southern part of the Hongge layered intrusion. In contrast to the Bushveld and Skaergaard layered intrusions, a broad normal fractionation trend upwards recorded by cumulus clinopyroxene and plagioclase is absent, whereas frequent reversals to more primitive compositions are observed in the Hongge intrusion. According to the mineral reversals, we divided the entire intrusion into eleven stratigraphic units. Notably, in the Middle Zone (MZ) and Lower Zone (LZ), the appearance of Fe-Ti-V oxide ore layers are closely related to these reversals. These distinct units are regarded as indicators for major replenishments of primitive high Ti ferrobasaltic/picritic magmas. In these samples, various clinopyroxene populations with both low and high Mg# are also observed and phases with high Mg# are interpreted to be inherited from the replenishing magmas.Previous studies have proposed that the replenishing magma might be anhydrous. However, the presence of a considerable amount of hydrous cumulus minerals including phlogopite and hornblende indicates that there is a high water content in melts at some stages of the formation of the Hongge layered intrusion, especially in the LZ and MZ. Contamination of crustal materials (e.g., introduction of aqueous fluids) or replenishing magmas with different water contents could lead to high water activities in the shallow crystallizing magma chamber. As a consequence, mixing of hydrous residual melts with replenishing magmas would trigger large scale precipitation of Fe-Ti-V oxide ores. Relatively high water activity, which could suppress the crystallization of cumulus plagioclase, would also explain the absence of this phase in the LZ and MZ. Thus, based on all these observations, we propose that the Hongge layered intrusion was formed by multiple emplacements of crystal-laden magmas that carried olivine and clinopyroxene already crystallized in deep-seated chambers.

Using zircon trace element composition to assess porphyry copper potential of the Guichon Creek batholith and Highland Valley Copper deposit, south-central British Columbia

The Late Triassic Guichon Creek batholith is a large (~ 1800 km2), composite, zoned batholith that hosts several large porphyry Cu-Mo deposits of the Highland Valley Copper district. The batholith consists of intrusive rocks that range in composition from gabbro to quartz monzonite. Adjacent to the mafic margin of the batholith is the Gump Lake granodiorite to quartz monzonite stock. A new U-Pb zircon age of 218 ± 0.18 for the Gump Lake stock indicates that magmatism in the region began at least seven million years prior to the emplacement of the main Guichon Creek batholith rocks at 211 Ma. Zircons from fifteen samples from the Guichon Creek batholith were analyzed by laser ablation ICP-MS to characterize the magmatic evolution and ore fertility of the batholith. The trace element composition of zircon record early, lower crustal, fractional crystallization followed by five pulses of magma recharge and mixing in an upper-crustal, oxidized, magma chamber as well as degassing of the magmatic-hydrothermal fluids that formed the porphyry copper deposits. Zircons from the early barren rocks have chondrite-normalized Eu/EuN* values of 0.19 to 0.56 and estimated temperatures of 850 to 750 °C. The middle to late intrusions that host porphyry copper mineralization have zircon Eu/EuN* values of 0.30 to 0.74 and slightly lower estimated temperatures of 800 to 600 °C. Late porphyritic stocks and dikes from the mineralized centers contain zircon crystals elevated in Y, Nb, Ta, and REE concentration relative to zircon from the earlier intrusions. This distinct change in zircon composition coincides with the copper mineralization, suggesting that zircon chemistry can be used as a tool to identify the genetic evolution of a crystallizing magma chamber and potential for mineralization.

Contemporaneous alkaline and calc-alkaline series in Central Anatolia (Turkey): Spatio-temporal evolution of a post-collisional Quaternary basaltic volcanism

This study focuses on spatio-temporal evolution of basaltic volcanism in the Central Anatolian post-collisional Quaternary magmatic province which developed along a NE-SW orientation in Turkey. This magmatic province consists of the stratovolcanoes Erciyes (ES) and Hasandag (HS), and the basaltic volcanic fields of Obruk-Zengen (OZ) and Karapınar (KA). The investigated samples range between basic to intermediate in composition (48–56wt% SiO2), and exhibit calc-alkaline affinity at ES whereas HS, OZ and KA are alkaline in composition. Based on new KAr ages and major element data, the oldest basaltic rock of ES is 1700±40ka old and exhibits alkaline character, whereas the youngest basaltic trachyandesite is 12±5ka old and calc-alkaline in composition. Most ES basaltic rocks are younger than 350ka. All samples dated from HS are alkaline basalts, ranging from 543±12ka to 2±7ka old. With the exception of one basalt, all HS basalts are ~100ka or younger in age. KAr ages range from 797±20ka to 66±7ka from OZ. All the basalt samples are alkaline in character and are older than the HS alkaline basalts, with the exception of the youngest samples. The oldest and youngest basaltic samples from KA are 280±7ka and 163±10ka, respectively, and are calc-alkaline in character. Based on thermobarometric estimates samples from OZ exhibit the highest cpx-liqidus temperature and pressure. For all centers the calculated crystallization depths are between 11 and 28km and increase from NE to SW. Multistage crystallization in magma chamber(s) located at different depths can explain this range in pressure. Harker variation diagrams coupled with least-squares mass balance calculations support fractional crystallization for ES and, to lesser extend for HS, OZ and KA. All basaltic volcanic rocks of this study are enriched in large-ion lithophile elements (LILE) and light rare earth elements (LREE). The lack of negative anomalies for high field strength elements (HFSE; Y, Yb) and the La/Nb >1 favor a shallow lithospheric source for ES, HS, OZ and KA basaltic volcanic rocks, whereas some samples bear the trace element signature of an asthenospheric mantle source. The lithospheric mantle beneath Central Anatolia may have not been affected from asthenospheric mantle directly. Negative Nb-Ta-Ti anomalies and a positive Pb spike of ES, HS, OZ and KA may be ascribed to crustal contamination or as the imprints of the previous subduction processes. According to this study, and previous studies, the effect of subduction and/or crustal contamination in Central Anatolia decreased from the Miocene to the Quaternary, and the origin of the Quaternary basaltic rocks mainly derived from subduction-related magmas enriched with sediment input rather than to slab-derived fluids. Our calculated eruption ages for the four basaltic complexes show that spatial differences predominate, whereas temporal trends are difficult to discern due to limited age resolution. According to the available geochronological, petrological and geochemical data, alkaline and calc-alkaline volcanism occurred simultaneously from distinct parental magmas.

Melt inclusions in quartz phenocrysts: The acid rocks of the Bannaya–Karymshina area, Kamchatka

The Bannaya–Karymshina area is situated in southern Kamchatka west of the East Kamchatka Volcanic Belt in the backarc part of the Kuril–Kamchatka island arc. The area is unique in that it contains abundant ejecta of calc-alkaline, acid, mostly ignimbrite, volcanism for a period of 4 Ma. Three rock complexes can be identified with rhyolitic and rhyodacitic compositions: Middle Pliocene ignimbrites, crystalloclastic tuffs of Eopleistocene age that fill in the Karymshina caldera, and Early Pleistocene intrusions. All of these are composed of rocks with normal total alkalinity, while the concentration of potassium places them at the boundary between moderate and high-potassium rocks. We sought to determine the composition of primary acid melts by studying the composition of the silicate phase in homogeneous melt inclusions that were conserved in quartz phenocrysts hosted by volcanic rocks of varying ages. Practically all the melt inclusions we analyzed show increased total alkalies and are in the class of trachyrhyodacites and trachyrhyolites, with the varieties of the highest alkali content being alkaline rhyolites and comendites; the concentration of K2O classifies them as subalkaline rocks; one also notes the increased alumina of the acid melts. The compositions and spatial locations of the melt inclusions in quartz phenocrysts provide evidence of a three-phase crystallization in magma chambers at different depths. According to the experimental data, the quartz phenocrysts crystallized in a water-saturated melt at pressures of 0.1 to 3.5 kbars.

U-Pb geochronology and Sr-Nd isotopic systematics of minerals from the ultrabasic-alkaline massifs of the Kola province

This paper reports the results of U-Pb geochronological and Sr-Nd isotopic geochemical investigations (LA-ICP-MS) for perovskite, apatite, titanite, and calcite from the ultrabasic-alkaline rocks of the Paleozoic Kola alkaline province of the Fennoscandian Shield. Based on the obtained data, two main stages were distinguished in the history of Paleozoic intrusions in this province: (1) formation of ultrabasic-alkaline series of the Kovdor, Afrikanda, Turiy Mys, Ozernaya Varaka, Lesnaya Varaka, and other massifs, as well as the ultrabasic-alkaline series of the Khibiny and Lovozero massifs (385–375 Ma) and (2) formation of agpaitic syenites in the Khibiny and Lovozero calderas (375–360 Ma) and related apatite-nepheline deposits (370 Ma). The Sr-Nd isotopic geochemical investigations of perovskite, apatite, and titanite, which are the main hosts for the rare earth elements and Sr in ultrabasic-alkaline rocks, showed that variations in the Sr and Nd isotopic characteristics of these rocks are related to a large extent to crustal contamination during the ascent of their parental melts toward the surface and crystallization in magma chambers. As a result, the Sr and Nd isotopic characteristics of late minerals (apatite and titanite) do not reflect the initial Sr and Nd isotopic ratios of the primary magma. Initial ratios in the primary mantle melts are most closely approximated by the isotopic characteristics of phases crystallizing during early stages (e.g., perovskite).

Exploring fractionation models for Martian magmas

AbstractMartian primary compositions, i.e., magmas that did not experience fractionation and/or contamination after extraction from the mantle, occur as a subset of Martian meteorites and a few lavas analyzed on the planet's surface by rovers. Eruptions of primary magmas are rare on Earth and presumably on Mars. Previous studies of fractional crystallization of Martian primary magmas have been conducted under isobaric conditions, simulating idealized crystallization in magma chambers. Polybaric fractionation, which occurs during magma ascent, has not been investigated in detail for Martian magmas. Using the MELTS algorithm and the pMELTS revision, we present comprehensive isobaric and polybaric thermodynamic calculations of the fractional crystallization of four primary or parental Martian magmas (Humphrey, Fastball, Y‐980459 shergottite, and nakhlite parental melts) using various pressure‐temperature paths, oxygen fugacities, and water contents to constrain how these magmas might evolve. We then examine whether known Martian alkaline rock compositions could have formed through fractional crystallization of these magmas under the simulated conditions. We find that isobaric and polybaric crystallization paths produce similar residual melt compositions, but given sufficient details, we may be able to distinguish between them. We calculate that Backstay (Gusev Crater) likely formed by fractionation of a primary magma under polybaric conditions, while Jake_M (Gale Crater) may have formed through melting of a metasomatized mantle, crustal assimilation, or fractional crystallization of an unknown primary magma. The best fits for the Backstay composition indicate that consideration of polybaric crystallization paths can help improve the quality of fit when simulating liquid lines of descent.

Calculation of Time-dependent Nucleation and Growth Rates from Quantitative Textural Data: Inversion of Crystal Size Distribution

Magmatictexturesprovideaninsufficientlyexploredtooltointerpretphysical processes and environmental variables that drive differenti-ation and crystallization in magma chambers. We derive a newmethod, which utilizes the crystal size distribution (CSD), to re-trieve the ratesof nucleation and growth from natural igneous rocks.However, a single CSD resultsfrom an arbitrary numberofcombin-ations of nucleation and growth rates; ifadditional parameters suchas crystallinity evolution are employed, the solution for rates becomesunique. Interpretation of representative log^linear CSD trendsshows that the nucleation rates are sensitive to even minor featuresof the CSD, whereas the growth rate functions are mainly related tothe crystallinity evolution. The reconstructed growth rates becomeminimal at intermediate crystallinities but diverge to very highvalues at the beginning and the end of crystallization.This generalresult is related to the small effective area of the solid^liquid inter-face close to the liquidus (few small grains) and near the solidus(largely solidified with diminishing melt pools). Assuming thatgrowth rate is related to environmental variables such as magmaundercooling, these results suggest that the solid fraction in manymagmas increases in a sigmoidal manner over their crystallizationinterval and indicate the tendency for largest undercooling at the be-ginning and at the end of crystallization. For crystallization timesconstrained by conductive cooling models, thegrowth ratescalculatedfor representative CSDs are of the order of10

Interaction between a mafic melt and host rocks during formation of the Kivakka layered intrusion, North Karelia

The study of interaction between mantle melts and crustal rocks is of great importance for deciphering the evolution of the Earth’s crust and for better understanding the composition of mantle sources, in particular, the degree of their compositional heterogeneity. This work presents the results of Rb-Sr and Sm-Nd isotopic studies of 37 samples taken from the Kivakka layered intrusion, host rocks, and rocks at the contact. The studies were aimed at verifying the hypothesis of possible crustal contamination of mafic melt during magma chamber crystallization. It was found that the section of the Kivakka layered massif is characterized by initial Sr and Nd isotopic heterogeneity, with negative correlation between initial Nd isotopic ratio and its content. The rocks of the massif have low ɛNd(T) values.

Time-dependent crystallization in magma chambers and lava lakes cooled from above: The role of convection and kinetics on nonlinear dynamics of binary systems

Motivated by geophysical applications we develop a number of mathematical models of the time-dependent crystallization in magma chambers, lava lakes and similar systems which involve nonlinear mechanisms of the melt convection and interfacial kinetics. The Stefan-type moving boundary models including important nonlinear physical dependences are solved analytically for different solidification stages. An influence of the convection intensity and undercooling degree on nonlinear solidification dynamics is discussed.

Compaction and Crystallization in Magma Chambers: Towards a Model of the Skaergaard Intrusion

The geochemical variations within the Skaergaard intrusion provide an opportunity to discover what features of the intrusion can be understood using the simplest form of the equations governing the two-phase flow of melt and matrix. The equations governing the conservation of mass, momentum and energy are first simplified by using the extended Boussinesq approximation, and then solved numerically to study the time-dependent behaviour of a compacting solidifying layer at the base of a magma chamber when variations in the horizontal plane can be neglected. The most important result is that the concept of a trapped liquid fraction, which has been widely used to model the bulk composition of layered intrusions, is a useful concept to describe the steady-state behaviour of compacting layers. This result is at first sight surprising, because there is relative movement between the melt and crystals during compaction, and the system is therefore open. The reason why it is useful is because both the melt and the crystals are moving downwards in a frame fixed to the upper surface of the compacting layer. Because the mass of all elements must be conserved, what goes into the top of the layer as melt and solid must come out of its bottom as a solid when the behaviour is not time dependent. However, when time-dependent behaviour occurs, the concept of a trapped liquid fraction ceases to be useful.The governing equations are used to model the concentration of phosphorus, uranium and rubidium in the lower part of the Skaergaard intrusion, where they behave incompatibly. The observed behaviour requires the viscosity of the solid part of the compacting layer to have a value of 10 Pa s.

Mineral Chemistry and Petrography of Lava Successions From Kepsut-Dursunbey Volcanic Field, NW Turkey: Implications For Magmatic Processes and Crystallization Conditions

Kepsut-Dursunbey volcanic field (KDVF) is located in NW Turkey and contains various products of the post-collisional Neogene magmatic activity. Two distinct volcanic suites have been recognized; the Kepsut volcanic suite (KVS) and the Dursunbey volcanic suite (DVS). The KVS includes basaltic trachyandesite- basaltic andesite-andesite lavas and associated pyroclastic rocks. The DVS consists of dacite-rhyodacite lavas and extensive pumice-ash fall and flow deposits. Petrographical features (i.e. existence of xenocrysts, glomerocrysts, and mixing-compatible textures) and mineral chemistry of phenocryst assemblages of both suites provide evidence for magma mixing/AFC. Calculated crystallization pressures and temperatures give values of 5.7-7.0 kbar and 927-982 °C for the KVS and 3.7-5.3 kbar and 783-787°C for the DVS, indicating separate magma reservoirs and crystallization in magma chambers at deep and mid crustal levels, respectively. These observations support the establishment and evolution of KDVF magma system promoted by episodic basaltic inputs which may generate and mix with crustal melts.

Geochemistry of Telichian (Silurian) K-bentonites in Estonia and Latvia

In the Telychian section of Estonia and Latvia K-bentonites from 45 volcanic eruptions were discovered. The thickness of K-bentonite interbeds varies from a few millimetres to 20 cm. The sodium component concentration in sanidine phenocrysts measured by XRD ranges from 20 to 48 mol% and was used for establishing correlations. The Ti, Zr, Nb, Th and Sr concentrations and ratios show temporal trends indicating fractional crystallization in magma chambers. The analysis of biotite phenocrysts revealed magnesium and iron rich biotites in bentonites. Synthesis of these geochemical data enabled a classification of bentonites into seven geochemical types, which probably originate from seven different volcanic sources. Isopach schemes indicate ash transport from the west and north-west directions.

Persistent scatterer interferometric synthetic aperture radar for crustal deformation analysis, with application to Volcán Alcedo, Galápagos

While conventional interferometric synthetic aperture radar (InSAR) is a very effective technique for measuring crustal deformation, almost any interferogram includes large areas where the signals decorrelate and no measurement is possible. Persistent scatterer (PS) InSAR overcomes the decorrelation problem by identifying resolution elements whose echo is dominated by a single scatterer in a series of interferograms. Existing PS methods have been very successful in analysis of urban areas, where stable angular structures produce efficient reflectors that dominate background scattering. However, man‐made structures are absent from most of the Earth's surface. Furthermore, existing methods identify PS pixels based on the similarity of their phase history to an assumed model for how deformation varies with time, whereas characterizing the temporal pattern of deformation is commonly one of the aims of any deformation study. We describe here a method for PS analysis, StaMPS, that uses spatial correlation of interferogram phase to find pixels with low‐phase variance in all terrains, with or without buildings. Prior knowledge of temporal variations in the deformation rate is not required for their identification. We apply StaMPS to Volcán Alcedo, where conventional InSAR fails because of dense vegetation on the upper volcano flanks that causes most pixels to decorrelate with time. We detect two sources of deformation. The first we model as a contracting pipe‐like body, which we interpret to be a crystallizing magma chamber. The second is downward and lateral motion on the inner slopes of the caldera, which we interpret as landsliding.

Four magmatic fabrics in the Tuolumne batholith, central Sierra Nevada, California (USA): Implications for interpreting fabric patterns in plutons and evolution of magma chambers in the upper crust

For the first time, four distinct magmatic fabrics are documented in a composite plutonic body, the Tuolumne batholith, central Sierra Nevada, California, USA. One type of fabric was formed by strain caused by highly localized magma flow (type 1), whereas the other three chamber-wide fabrics recorded strain increments during boundary processes along batholith margins (type 2) superimposed by increments of heterogeneous regional tectonic strain (types 3 and 4). Our present work indicates that, in contrast to studies that consider magmatic fabrics to be simple structures formed by a single process, magmatic lineations and foliations in plutons may reflect accumulated finite strain and form as composite structures recording multiple strain increments in relatively static, actively deforming and rheologically complex crystal mushes at lower melt percentages. We demonstrate that multiple magmatic fabrics in a single batholith may record remarkably different processes and thus preserve a temporal record of strain in the batholith from strain during internal chamber processes to postem-placement regional tectonic strain. However, it may be commonly very problematic to infer the exact nature of flow or even fabric-forming process from the preserved rock record. Our study also exemplifies how examination of magmatic fabric patterns in plutons, complemented with geochronology, may provide crucial constraints on the interplay among successive magma emplacement, fabric preservation, temporal evolution of strain fields in a crystallizing magma chamber, and the development of crystal-mush zones.

Accessory and rock forming minerals monitoring the evolution of zoned mafic–ultramafic complexes in the Central Ural Mountains

This study describes major and trace element compositions of accessory and rock forming minerals from three Uralian–Alaskan-type complexes in the Ural Mountains (Kytlym, Svetley Bor, Nizhnii Tagil) for the purpose of constraining the origin, evolution and composition of their parental melts. The mafic–ultramafic complexes in the Urals are aligned along a narrow, 900 km long belt. They consist of a central dunite body grading outward into clinopyroxenite and gabbro lithologies. Several of these dunite bodies have chromitites with platinum group element mineralization. High Fo contents in olivine (Fo 92–93) and high Cr/(Cr + Al) in spinel (0.67–0.84) suggest a MgO-rich (> 15 wt.%) and Al 2O 3-poor ultramafic parental magma. During its early stages the magma crystallized dominantly olivine, spinel and clinopyroxene forming cumulates of dunite, wehrlite and clinopyroxenite. This stage is monitored by a common decrease in the MgO content in olivine (Fo 93–86) and the Cr/(Cr + Al) value of coexisting accessory chromite (0.81–0.70). Subsequently, at subsolidus conditions, the chromite equilibrated with the surrounding silicates producing Fe-rich spinel while Al-rich spinel exsolved chromian picotite and chromian titanomagnetite. This generated the wide compositional ranges typical for spinel from Uralian–Alaskan-type complexes world wide. Laser ablation analyses (LA-ICPMS) reveal that clinopyroxene from dunites and clinopyroxenite from all three complexes have similar REE patterns with an enrichment of LREE (0.5–5.2 prim. mantle) and other highly incompatible elements (U, Th, Ba, Rb) relative to the HREE (0.25–2.0 prim. mantle). This large concentration range implies the extensive crystallization of olivine and clinopyroxene together with spinel from a continuously replenished, tapped and crystallizing magma chamber. Final crystallization of the melt in the pore spaces of the cooling cumulate pile explains the large variation in REE concentrations on the scale of a thin section, the REE-rich rims on zoned clinopyroxene phenocrysts (e.g. La Rim/La Core ∼ 2), and the formation of interstitial clinopyroxene with similar REE enrichment. Trace element patterns of the parental melt inferred from clinopyroxene analyses show negative anomalies for Ti, Zr, Hf, and a positive anomaly for Sr. These imply a subduction related geotectonic setting for the Uralian zoned mafic–ultramafic complexes. Ankaramites share many petrological and geochemical features with these complexes and could represent the parental melts of this class of mafic–ultramafic intrusions. Diopside from chromitites and cross cutting diopside veins in dunite has similar trace element patterns with LREE/HREE ratios (e.g. La/Lu = 5–60) much higher than those in diopside from all other lithologies. We suggest that the chromitites formed at high temperatures (800–900 °C) during the waning stages of solidification as a result of the interaction of an incompatible element-rich melt or fluid with the dunite cumulates.

Vigorous Exsolution of Volatiles in the Magma Chamber beneath a Hydrothermal System on the Modern Sea Floor of the Eastern Manus Back-Arc Basin, Western Pacific: Evidence from Melt Inclusions

Image analyses and ion probe analyses were carried out on melt inclusions in olivine, pyroxene, and plagioclase phenocrysts of vesicular volcanic rocks that host the actively forming massive sulfides at the PACMANUS hydrothermal field in the eastern Manus back-arc basin, western Pacific. The melt inclusions, ranging in size from 150 μm, contain one or more bubbles. Most of them have bubble/melt inclusion volume ratios (Vb/Vm) of >5.5 vol percent, higher than those for the melt inclusions with their bubbles generated from shrinkage (Vb/Vm, 3.6 vol %) or postentrapment exsolution of volatiles (Vb/Vm, 3.7 vol %). This, together with the presence of vapor bubbles with little melt (Vb/Vm, 80 to >95 vol %) in pyroxene and olivine phenocrysts, suggests that a vapor phase was formed from vigorous exsolution of volatiles or the boiling of a magma in the crystallizing magma chamber. The Vb/Vm ratios show distinct distribution patterns in the phenocryst minerals, indicating that there was more than one vesiculation event in the magma chamber. Melt inclusions with no visible bubbles have the highest contents of H2O (2.1–2.5 wt %). The H2O contents of the glass in the melt inclusions are inversely related to the size of the coeval bubbles. The significant variation of H2O contents (1.0–2.5%) in the melt inclusions suggests that the H2O was degassed from the crystallizing magma. The positive correlations of S, Cl, and possibly F with H2O indicate that all these volatiles were exsolved from the melt into the vapor phase. The negative correlation of H2O with alkali contents in the glass of melt inclusions suggests that the magma was crystallizing in the presence of a fluid phase. It is estimated that the magma might have lost up to 65 percent of its volatiles while crystallizing. Significant amounts of magmatic fluid can be degassed from a magma during its crystallization in a shallow magma chamber. An individual vesiculation event may last at least 5 to 48 years, as estimated from the growth of the phenocrysts. This estimate is a minimum because not all the phenocrysts crystallize at the same time. The estimated life of a black smoker chimney is 2 to 10 years, which is within the duration of an individual vesiculation event. This implies that the magmatic fluid could be a major source of ore metals during the growth of individual sulfide chimneys. Through multiple vesiculation events in a crystallizing magma chamber, the magma could provide large quantities of ore metals and volatiles to a hydrothermal system over a long period of time to form a large massive sulfide deposit on the sea floor.

SEGMENTATION IN OMAN OPHIOLITE, IMPLICATIONS FOR FAST SPREADING RIDGES TECTONICS AND HIGH TEMPERATURE HYDROTHERMAL ACTIVITY

New, fine scale mapping in the large NW-SE ridge segment formerly identified in the Oman ophiolite (Nicolas and Boudier, 1995) has revealed that this structure is composed of smaller nestled segments, each being centered on the small mantle diapirs already mapped. The contacts with adjacent lithosphere and the tips of these segments have been mapped in detail. Their nature and structure depend on the difference in age between the two lithospheres. When the difference in age is in the range of 1 Myr, strike slip shear zones, 1-2 km wide, are developed in the mantle of the new segment. When this difference drops to ~0.5 Myr, the shear zones are small and diffuse but, in the mantle wedge at the tip of the segment which penetrates the older lithosphere, spectacular deformations are observed. The mantle and lower crust of the older lithosphere near the Moho are shovelled vertically and kilometer-sized folds develop in the gabbro unit. In contrast, the lid is not affected, suggesting that, at present day fast spreading ridges, similar major tectonic structures, as seen in Oman thanks to deep sections, may also be present. Contacts and tips of new segments are invaded by mafic dikes and sills issued from the segment magmatic activity and trapped against these colder boundaries. Melt reaction in the uppermost mantle at the tip of propagators generates large volumes of dunites and wehrlites. An important contribution to this magmatism results from massive seawater penetration down to the Moho, possibly favored by the segment tectonic activity. This water is heated at very high temperatures (VHT), 1000C° and possibly above. Inside crystallizing magma chamber, the hydrous reaction (Koepke et al.,2004) rejuvenates magmatic activity and generates orthopyroxene gabbros, injected and stretched parallel to the olivine gabbros layering. Outside the magma chamber, VHT seawater produces, by hydrous anatexis, copious melts which mix and react with the indigenous segment melts and crystallize as orthopyroxene- or pargasitebearing, clinopyroxene gabbros, diorites and trondjhemites. It is suggested that the geochemical signature of such hybrid melts should be looked for in present day ridges.