scholarly journals Quantitative chemical mapping of plagioclase as a tool for the interpretation of volcanic stratigraphy: an example from Saint Kitts, Lesser Antilles

2021 ◽  
Vol 83 (8) ◽  
Author(s):  
Oliver Higgins ◽  
Tom Sheldrake ◽  
Luca Caricchi
Keyword(s):  
Mineral Chemistry ◽  
Lesser Antilles ◽  
Chemical Mapping ◽  
Stratigraphic Sequence ◽  
Volcanic Stratigraphy ◽  
Chemical Zoning ◽  
Magmatic Processes ◽  
Quantitative Understanding ◽  
Reproducible Method ◽  
Textural Parameters

AbstractEstablishing a quantitative link between magmatic processes occurring at depth and volcanic eruption dynamics is essential to forecast the future behaviour of volcanoes, and to correctly interpret monitoring signals at active centres. Chemical zoning in minerals, which captures successive events or states within a magmatic system, can be exploited for such a purpose. However, to develop a quantitative understanding of magmatic systems requires an unbiased, reproducible method for characterising zoned crystals. We use image segmentation on thin section scale chemical maps to segment textural zones in plagioclase phenocrysts. These zones are then correlated throughout a stratigraphic sequence from Saint Kitts (Lesser Antilles), composed of a basal pyroclastic flow deposit and a series of fall deposits. Both segmented phenocrysts and unsegmented matrix plagioclase are chemically decoupled from whole rock geochemical trends, with the latter showing a systematic temporal progression towards less chemically evolved magma (more anorthitic plagioclase). By working on a stratigraphic sequence, it is possible to track the chemical and textural complexity of segmented plagioclase in time, in this case on the order of millennia. In doing so, we find a relationship between the number of crystal populations, deposit thickness and time. Thicker deposits contain a larger number of crystal populations, alongside an overall reduction in this number towards the top of the deposit. Our approach provides quantitative textural parameters for volcanic and plutonic rocks, including the ability to measure the amount of crystal fracturing. In combination with mineral chemistry, these parameters can strengthen the link between petrology and volcanology, paving the way towards a deeper understanding of the magmatic processes controlling eruptive dynamics.

scholarly journals Pervasive fluid-rock interaction in subducted oceanic crust revealed by oxygen isotope zoning in garnet

2021 ◽  
Vol 176 (7) ◽  
Author(s):  
Thomas Bovay ◽  
Daniela Rubatto ◽  
Pierre Lanari
Keyword(s):  
Oxygen Isotope ◽  
Western Alps ◽  
Large Contribution ◽  
Chemical Mapping ◽  
Rock Interaction ◽  
Rock Density ◽  
Chemical Zoning ◽  
Dehydration Reactions ◽  
Subducted Oceanic Crust ◽  
Eclogite Facies

AbstractDehydration reactions in the subducting slab liberate fluids causing major changes in rock density, volume and permeability. Although it is well known that the fluids can migrate and interact with the surrounding rocks, fluid pathways remain challenging to track and the consequences of fluid-rock interaction processes are often overlooked. In this study, we investigate pervasive fluid-rock interaction in a sequence of schists and mafic felses exposed in the Theodul Glacier Unit (TGU), Western Alps. This unit is embedded within metaophiolites of the Zermatt-Saas Zone and reached eclogite-facies conditions during Alpine convergence. Chemical mapping and in situ oxygen isotope analyses of garnet from the schists reveal a sharp chemical zoning between a xenomorphic core and a euhedral rim, associated to a drop of ~ 8‰ in δ18O. Thermodynamic and δ18O models show that the large amount of low δ18O H2O required to change the reactive bulk δ18O composition cannot be produced by dehydration of the mafic fels from the TGU only, and requires a large contribution of the surrounding serpentinites. The calculated time-integrated fluid flux across the TGU rocks is 1.1 × 105 cm3/cm2, which is above the open-system behaviour threshold and argues for pervasive fluid flow at kilometre-scale under high-pressure conditions. The transient rock volume variations caused by lawsonite breakdown is identified as a possible trigger for the pervasive fluid influx. The calculated schist permeability at eclogite-facies conditions (~ 2 × 10–20 m2) is comparable to the permeability determined experimentally for blueschist and serpentinites.

Glass-bearing felsic nodules from the crystallizing sidewalls of the 1944 Vesuvius magma chamber

2000 ◽  
Vol 64 (3) ◽  
pp. 481-496 ◽  
Author(s):  
P. Fulignati ◽  
P. Marianelli ◽  
A. Sbrana
Keyword(s):  
Magma Chamber ◽  
Melt Inclusions ◽  
Mineral Chemistry ◽  
Glassy Matrix ◽  
Crystal Mush ◽  
Stratigraphic Sequence ◽  
Shallow Magma Chamber ◽  
Mineral Assemblages ◽  
Pressure Crystallization

AbstractIn the 1944 Vesuvius eruption, the shallow magma chamber was disrupted during the highly energetic explosive phases. Abundant cognate xenoliths such as subvolcanic fergusites and cumulates, hornfels, skarns and rare marbles occur in tephra deposits.Mineral chemistry, melt inclusions in minerals and glassy matrix compositions show that fergusites (highly crystalline rocks made of leucite, clinopyroxene, plagioclase, olivine, apatite, oxides and glass) do not correspond to melt compositions but result from combined sidewall accumulation of crystals, formed from K-tephriphonolitic magma resident in the chamber, and in situ crystallization of the intercumulus melt. Very low H2O contents in the intercumulus glass are revealed by FTIR and apatite composition. Whole rock compositions are essentially determined by the bulk mineral assemblages.Glass–bearing fergusites constitute the outer shell of the magma chamber consisting of a highly viscous crystal mush with a melt content in the range 20–50 wt.%. The leucite/(clinopyroxene+olivine) modal ratio, varies with the extraction order of magmas from the chamber, decreasing upwards in the stratigraphic sequence. This reflects a vertical mineralogical zonation of the crystal mush. These data contribute to the interpretation of the subvolcanic low–pressure crystallization processes at the magma chamber sidewalls affecting alkaline potassic magmas.

Alteration mineralogy and pathfinder element inventory in the footprint of the McArthur River unconformity-related uranium deposit, Canada

2021 ◽  
Vol 59 (5) ◽  
pp. 985-1019
Author(s):  
Nicholas Joyce ◽  
Daniel Layton-Matthews ◽  
Kurt Kyser ◽  
Matthew Leybourne ◽  
Kevin Ansdell ◽  
...  
Keyword(s):  
Rare Earth ◽  
Rare Earth Elements ◽  
Aluminum Phosphate ◽  
Mineral Chemistry ◽  
Chemical Mapping ◽  
Fe Oxide ◽  
Element Concentrations ◽  
Heavy Rare Earth Elements ◽  
Host Rocks ◽  
Pathfinder Elements

ABSTRACT Pathfinder elements associated with the exploration footprint of the McArthur River unconformity-related U deposit include U, radiogenic Pb, V, Ni, Co, Cu, Mo, As, Zn, and rare earth elements. In this study, the mineralogical and paragenetic context for their occurrence was established by integrating in situ mineral chemistry and laser ablation mass spectrometry chemical mapping of interstitial assemblages, detrital grains, and cements with whole-rock analyses of drill core samples from the diagenetically altered background and the hydrothermally altered sandstone host rocks. Diagenetically altered background sandstones contain a matrix assemblage of illite and dickite, with trace to minor aluminum-phosphate-sulfate (APS) minerals, apatite, and Fe-Ti oxide minerals. Aluminum-phosphate-sulfate minerals account for the majority of the Sr and light rare earth element concentrations, whereas early diagenetic apatite, monazite, and apatite inclusions in detrital quartz and detrital zircon contribute significant U and heavy rare earth elements to samples analyzed with an aggressive leach (partial digestion) such as aqua regia. Hydrothermally altered sandstone host rocks also contain variable assemblages of Al-Mg chlorite (sudoite), alkali-deficient tourmaline, APS minerals, kaolinite, illite, Fe-oxide, and sulfide minerals. Late pre-mineralization chlorite accounts for a significant portion of the observed Ni concentrations, whereas Co, Cu, Mo, and Zn occur predominantly in cryptic sub-micron sulfide and sulfarsenide inclusions within clay mineral aggregates and in association with Fe-Ti oxides. Elevated concentrations of U were observed in cryptic micro-inclusions associated with sulfides in quartz overgrowths, with Fe-Ti oxide micro-inclusions in kaolinite, and in post-mineralization Fe-oxide veins. The distribution of pathfinder elements throughout the deposit footprint appears to be less related to the primary dispersion of alteration minerals from the hydrothermal system than to the secondary dispersion of elements post-mineralization. Their occurrence throughout pre-, syn-, and post-mineralization assemblages further demonstrates the limitations to defining geochemical footprints from pathfinder element concentrations expressed in lithogeochemical data sets without structural, lithological, and mineralogical context.

scholarly journals A Crustal Control on the Fe Isotope Systematics of Volcanic Arcs Revealed in Plutonic Xenoliths From the Lesser Antilles

2022 ◽  
Vol 9 ◽  
Author(s):  
G. F. Cooper ◽  
E. C. Inglis
Keyword(s):  
Trace Element ◽  
Mantle Source ◽  
Subduction Zones ◽  
Primary Objective ◽  
Chemical Diversity ◽  
Radiogenic Isotopes ◽  
Lesser Antilles ◽  
Volcanic Arc ◽  
Magmatic Processes ◽  
Isotope Systematics

Lavas produced at subduction zones represent the integration of both source heterogeneity and an array of crustal processes, such as: differentiation; mixing; homogenisation; assimilation. Therefore, unravelling the relative contribution of the sub-arc mantle source versus these crustal processes is difficult when using the amalgamated end products in isolation. In contrast, plutonic xenoliths provide a complementary record of the deeper roots of the magmatic plumbing system and provide a unique record of the true chemical diversity of arc crust. Here, we present the δ56Fe record from well characterised plutonic xenoliths from two distinct volcanic centres in the Lesser Antilles volcanic arc–the islands of Martinique and Statia. The primary objective of this study is to test if the Fe isotope systematics of arc lavas are controlled by sub-arc mantle inputs or during subsequent differentiation processes during a magma’s journey through volcanic arc crust. The Fe isotopic record, coupled to petrology, trace element chemistry and radiogenic isotopes of plutonic xenoliths from the two islands reveal a hidden crustal reservoir of heavy Fe that previously hasn’t been considered. Iron isotopes are decoupled from radiogenic isotopes, suggesting that crustal and/or sediment assimilation does not control the Fe systematics of arc magmas. In contrast to arc lavas, the cumulates from both islands record MORB-like δ56Fe values. In Statia, δ56Fe decreases with major and trace element indicators of differentiation (SiO2, Na2O + K2O, Eu/Eu*, Dy/Yb), consistent with fractionating mineral assemblages along a line of liquid descent. In Martinique, δ56Fe shows no clear relationship with most indicators of differentiation (apart from Dy/Yb), suggesting that the δ56Fe signature of the plutonic xenoliths has been overprinted by later stage processes, such as percolating reactive melts. Together, these data suggest that magmatic processes within the sub-arc crust overprint any source variation of the sub-arc mantle and that a light Fe source is not a requirement to produce the light Fe isotopic compositions recorded in volcanic arc lavas. Therefore, whenever possible, the complimentary plutonic record should be considered in isotopic studies to understand the relative control of the mantle source versus magmatic processes in the crust.

A Distal, High-grade Irish-type Orebody: Petrographic, Sulfur Isotope, and Sulfide Chemistry of the Island Pod Zn-Pb Orebody, Lisheen, Ireland

2021 ◽  
Author(s):  
Aileen L. Doran ◽  
Steven P. Hollis ◽  
Julian F. Menuge ◽  
Stephen J. Piercey ◽  
Adrian J. Boyce ◽  
...  
Keyword(s):  
Sulfur Isotope ◽  
Isotope Composition ◽  
Spatial Scales ◽  
Isotope Analysis ◽  
Mineral Chemistry ◽  
Hydrothermal Activity ◽  
Hydrothermal Fluids ◽  
High Grade ◽  
Complex Deformation ◽  
Chemical Zoning

Abstract Irish-type Zn-Pb deposits are important global sources of zinc, but despite a fundamental understanding of ore genesis within the Irish orefield, a detailed understanding of fluid migration and chemical evolution pathways related to sulfide and carbonate precipitation is lacking. We present the first petrographic, paragenetically constrained sulfur isotope and mineral chemistry study of mineralization at the Island Pod orebody, Lisheen deposit. The Island Pod orebody comprises high-grade mineralization that is less deformed than elsewhere in the Irish orefield. Consequently, studies of the Island Pod orebody and its mineralization provide information on the evolving nature of hydrothermal fluids involved in ore deposition. The Island Pod orebody consists almost exclusively of pyrite, sphalerite, and galena, with several stages of calcite and dolomite precipitation. Pre-ore, diagenetic pyrite is commonly overgrown by early main ore-stage pyrite, with both phases frequently replaced by main ore-stage sphalerite. In many cases, early main ore-stage pyrite is texturally zoned and exhibits chemical zoning patterns, reflecting that episodic influxes of hydrothermal fluids contained variable concentrations of As, Co, Ni, and Tl. The main ore stage was dominated by the formation of sphalerite and galena from mineralizing fluids that were depleted in these trace elements (e.g., As, Co, Tl) compared to the early main ore stage. Sulfur isotope analysis reveals four distinctive but slightly overlapping isotopic groupings, corresponding to different mineral and paragenetic stages: (1) δ34S values range from –47.7 to –30.7‰, associated with diagenetic pyrite; (2) δ34S values range from –34.3 to –14.7‰, related to early main ore-stage pyrite; (3) δ34S values range from –15.5 to + 1.7‰, corresponding to main ore-stage sphalerite; and (4) δ34S values range from –11.1 to + 17.4‰, associated with galena. Large variations in S isotope composition are common at intragrain and at other small spatial scales. The textures, paragenetic sequence, and ranges in δ34S values are consistent with hydrothermal sulfide deposition where the fluids containing bacteriogenic sulfide mixed with metal-bearing fluids. Replacement and remobilization from other Lisheen orebodies may have contributed to some of the higher sulfur isotope ratios observed in the Island Pod orebody. The excellent preservation of sulfide textures in the Island Pod orebody observed during this study demonstrates that it is an ideal location to study hydrothermal fluid evolution, including episodic fluid flow, mixing, precipitation, and compositional variations during the early main ore stage. In other Irish Zn-Pb orebodies, these early-ore textures are often obscured due to more complex dissolution and replacement processes, making interpretation of the early hydrothermal activity challenging. Consequently, the petrographic, mineral chemistry, and sulfur isotope studies of the Island Pod orebody presented here contribute to an enhanced understanding of ore-forming processes in similar deposits, where mineralization is often associated with more complex deformation or repeated pulses of hydrothermal activity.

Multiple hydrothermal and metamorphic events in the Kidd Creek volcanogenic massive sulphide deposit, Timmins, Ontario: evidence from tourmalines and chlorites

1989 ◽  
Vol 26 (4) ◽  
pp. 694-715 ◽  
Author(s):  
John F. Slack ◽  
Paul R. Coad
Keyword(s):  
Hydrothermal System ◽  
Mineral Chemistry ◽  
Massive Sulphide ◽  
Hydrothermal Fluids ◽  
Massive Sulphide Deposit ◽  
Sulphide Deposit ◽  
Small Areas ◽  
Chemical Zoning ◽  
The North ◽  
Growth Zones

Tourmaline and chlorite are the principal ferromagnesian silicate minerals in the Kidd Creek massive sulphide deposit. Tourmaline is most common in sphalerite-rich peripheral margins of the chalcopyrite stringer zone. Within the north orebody, samples typically contain <1% tourmaline, but small areas (hand-specimen scale) may have 10–20%. Chlorite is more widely distributed and in places constitutes 30–50% of rock volumes. Associated assemblages may include quartz, sulphides (principally chalcopyrite, sphalerite, and (or) pyrite), carbonate, albite, sericite, and rare fluorite, allanite, or zoisite(?).The tourmalines and chlorites record a series of multiple hydrothermal and metamorphic events. Paragenetic studies suggest that tourmaline was deposited during several discrete stages of mineralization, as evidenced by brecciation and cross-cutting relationships. Most of the tourmalines have two concentric growth zones defined by different colours (green, brown, blue, yellow). Some tourmalines also display pale discordant rims that cross-cut and embay the inner growth zones and polycrystalline, multiple-extinction domains. Late sulphide veinlets (chalcopyrite, pyrrhotite) transect the inner growth zones and pale discordant rims of many crystals. The concentric growth zones are interpreted as primary features developed by the main ore-forming hydrothermal system, whereas the discordant rims, polycrystalline domains, and cross-cutting sulphide veinlets reflect post-ore metamorphic processes.Detailed electron microprobe analyses of tourmalines show a wide compositional range, from Fe-rich dravite nearly to end-member schorl, with Fe/(Fe + Mg) ratios varying from 0.33 to 0.92; only minor amounts of Ca are present, yielding uniformly high Na/(Na + Ca) ratios of 0.84–0.99. Two sets of chemical zoning trends are identified in the tourmalines, involving systematic changes in Fe/(Fe + Mg), Na/(Na + Ca), Al, and Ti that are believed to reflect internal coupled substitutions (e.g.,  + Ti = Na + Al) and local mineral equilibria (e.g., tourmaline–chlorite). Analyses of the pale discordant reaction rims show consistent depletion of Fe, Ca, and Ti, presumably by fluid–solid reactions during post-ore metamorphism.Chlorites also show an extensive range in composition, from ripidolite nearly to end-member daphnite, with Fe/(Fe + Mg) ratios of 0.43–0.98 and Si cation values of 5.00–5.39. Chlorites from the fringes of the footwall stringer zone have narrow compositional ranges, whereas chlorites near footwall rhyolite sills in the core of the stringer zone display major variations in Fe/(Fe + Mg) ratios, including one sample with a range of 0.68–0.95. The former group of chlorites has Fe/(Fe + Mg) ratios that correlate well with those of coexisting tourmalines (exclusive of late reaction rims). Data for the latter group, in contrast, fall off equilibrium KD curves, indicating that the tourmalines and chlorites within these samples are not in chemical equilibrium. The chlorites are believed to have been altered (overprinted) by Fe-rich hydrothermal fluids apparently generated during intrusion of the rhyolite sills. The tourmalines, however, are unaffected and retain primary chemical signatures.Variations in mineral proportions and mineral chemistry within the deposit mainly depend on fluctuations in temperature, pH, water/rock ratios, and amounts of entrained seawater. The major proposed control is mixing between high-temperature, Fe-rich end-member hydrothermal fluids and cold, Mg-rich entrained seawater. Fe/(Fe + Mg) variations in footwall tourmalines (and equilibrium chlorites) are believed to largely reflect the progressive infiltration of Mg-rich seawater into the margins and top of the hydrothermal system. The more Fe-rich compositions of Kidd Creek tourmalines relative to those from sediment-hosted massive sulphide deposits (e.g., Sullivan, British Columbia) may be related to the preferential generation of end-member hydrothermal fluids in proximal volcanic environments like that at Kidd Creek.

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