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  • 1
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    Constraints on the magmatic evolution of the oceanic crust from plagiogranite intrusions in the Oman ophiolite (2016)
    Springer
    In:  Contributions to Mineralogy and Petrology, 171 ( Art.No. 46).
    Details
    Publication Date: 2019-02-01
    Description: We present major and trace element as well as Sr, Nd, and Hf isotope data on a suite of 87 plutonic rock samples from 27 felsic crustal intrusions in seven blocks of the Oman ophiolite. The rock compositions of the sample suite including associated more mafic rocks range from 48 to 79 wt% SiO2, i.e. from gabbros to tonalites. The samples are grouped into a Ti-rich and relatively light rare earth element (LREE)-enriched P1 group [(Ce/Yb)N 〉 0.7] resembling the early V1 lavas, and a Ti-poor and LREE-depleted P2 group [(Ce/Yb)N 〈 0.7] resembling the late-stage V2 lavas. Based on the geochemical differences and in agreement with previous structural and petrographic models, we define phase 1 (P1) and phase 2 (P2) plutonic rocks. Felsic magmas in both groups formed by extensive fractional crystallization of olivine, clinopyroxene, plagioclase, apatite, and Ti-magnetite from mafic melts. The incompatible element compositions of P1 rocks overlap with those from mid-ocean ridges but have higher Ba/Nb and Th/Nb trending towards the P2 rock compositions and indicating an influence of a subducting slab. The P2 rocks formed from a more depleted mantle source but show a more pronounced slab signature. These rocks also occur in the southern blocks (with the exception of the Tayin block) of the Oman ophiolite implying that the entire ophiolite formed above a subducting slab. Initial Nd and Hf isotope compositions suggest an Indian-MORB-type mantle source for the Oman ophiolite magmas. Isotope compositions and high Th/Nb in some P2 rocks indicate mixing of a melt from subducted sediment into this mantle.
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  • 2
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    Chlorine-rich amphibole in deep layered gabbros as evidence for brine/rock interaction in the lower oceanic crust: A case study from the Wadi Wariyah, Samail Ophiolite, Sultanate of Oman (2018)
    Elsevier
    In:  Lithos, 323 . pp. 125-136.
    Details
    Publication Date: 2021-02-08
    Description: Highlights • Cl-rich amphibole (〈1.5 a.p.f.u. Cl) indicates equilibrium with Cl-rich fluid. • Interaction of amphibole with Cl-bearing fluid occurs in rock-dominated environment. • Fluid evolution and changing temperatures recorded in variations in amphibole composition. • Veins and dykelets in lower oceanic crust as signs of hydrothermal alteration. Abstract Hydrothermal veins and dykelets that cross-cut layered olivine gabbros deep in the plutonic section of the Samail Ophiolite, Sultanate of Oman, point towards the occurrence of hydrothermal circulation in the deep oceanic crust, and these features record interactions between rock and high temperature seawater-derived fluids or brines. Deep penetration of seawater-derived fluids down to 100 m above the Moho transition zone and the consequent interactions with the host rock lead to hydrothermal alteration from granulite facies grading down to greenschist facies conditions. Here we present a study of veins and dykelets formed by hydrothermal interaction cutting layered gabbro in the Wadi Wariyah, using petrographic, microanalytical, isotopic, and structural methods. We focus on amphiboles, which show a conspicuous compositional variation from high-Ti magnesiohastingsite and pargasite via magnesiohornblende and edenite, to Cl-rich ferropargasite and hastingsite (up to 1.5 a.p.f.u. Cl) and actinolite. These minerals record a wide range of formation conditions from magmatic to hydrothermal, and reveal a complex history of interactions between rock and hydrothermal fluid or brine in a lower oceanic crustal setting. Large variations in Cl content and cation configurations in amphibole suggest formation in equilibrium with fluids of different salinities at variable fluid/rock ratios. The presence of subsolidus amphibole extremely enriched in chlorine implies phase separation and brine/rock interactions. 87Sr/86Sr values of 0.7031 to 0.7039 and stable δ18O isotopic compositions of 4.1 to 5.6‰ of the different amphibole types suggest a rock-dominated environment, i.e. with low fluid/rock ratios. However, the slight departure from mean Oman isotope values may indicate there was some influence of seawater in the aforementioned fluid-rock interactions. Our study provides new petrological data for the subsolidus evolution of gabbro-hosted amphibole-rich veins in the presence of a seawater-derived fluid.
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  • 3
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    Depths of Partial Crystallization of H〈sub〉2〈/sub〉O-bearing MORB: Phase Equilibria simulations of Basalts at the MAR near Ascension Island (7-11°S) (2008)
    Oxford Univ. Press
    In:  Journal of Petrology, 49 (1). pp. 25-45.
    Details
    Publication Date: 2018-10-23
    Description: Phase equilibria simulations were performed on naturally quenched basaltic glasses to determine crystallization conditions prior to eruption of magmas at the Mid-Atlantic Ridge (MAR) east of Ascension Island (7–11°S). The results indicate that mid-ocean ridge basalt (MORB) magmas beneath different segments of the MAR have crystallized over a wide range of pressures (100–900 MPa). However, each segment seems to have a specific crystallization history. Nearly isobaric crystallization conditions (100–300 MPa) were obtained for the geochemically enriched MORB magmas of the central segments, whereas normal (N)-MORB magmas of the bounding segments are characterized by polybaric crystallization conditions (200–900 MPa). In addition, our results demonstrate close to anhydrous crystallization conditions of N-MORBs, whereas geochemically enriched MORBs were successfully modeled in the presence of 0·4–1 wt% H2O in the parental melts. These estimates are in agreement with direct (Fourier transform IR) measurements of H2O abundances in basaltic glasses and melt inclusions for selected samples. Water contents determined in the parental melts are in the range 0·04–0·09 and 0·30–0·55 wt% H2O for depleted and enriched MORBs, respectively. Our results are in general agreement (within ±200 MPa) with previous approaches used to evaluate pressure estimates in MORB. However, the determination of pre-eruptive conditions of MORBs, including temperature and water content in addition to pressure, requires the improvement of magma crystallization models to simulate liquid lines of descent in the presence of small amounts of water.
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  • 4
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    Crystallization and resorption in plutonic plagioclase: Implications on the evolution of granodiorite magma (Gęsiniec granodiorite, Strzelin Crystalline Massif, SW Poland) (2006)
    Elsevier
    Details
    Publication Date: 2023-01-19
    Description: Granodiorite from the Gęsiniec Intrusion, Strzelin Crystalline Massif, SW Poland contains complexly zoned plagioclases. Five chemically and structurally distinct zones can be correlated among crystals: ‘cores’ (25–35% An), inner mantles (∼40–45% An), outer mantles (40–25% An), resorption zones (35–50% An) and rims (35–30% An). Good structural and chemical (major and trace elements) correlation of zones between crystals indicates that zonation was produced by changes in conditions of crystallization on a magma chamber scale. Plagioclase, being the liquidus phase, records a time span from the beginning of crystallization to emplacement and rapid cooling of granodiorite as thin dykes. Crystallization began with the formation of inner mantles. The paucity and different sizes of inner mantles suggests slow crystallization in high temperature magma. Normally zoned inner mantles were formed under increasing undercooling. Compositional trends in mantles suggest closed system crystallization. The major resorption zones were caused by injection of less evolved magma as indicated by the strontium increase in plagioclase. The injection triggered a rapid rise of magma and plagioclase crystals facilitating mixing but also inducing fast, kinetically controlled growth of complex multiple, oscillatory zonation within resorption zones. The ascent of magma caused decompression melting of plagioclase and produced melt inclusions within inner mantles—the ‘cores’. The decompression range is estimated at a minimum of 2 kbar. Emplacement of granodiorite as thin dykes allow rapid cooling and preservation of magmatic zonation in plagioclases. Melt inclusions crystallized completely during post-magmatic cooling. The zonation styles of plutonic plagioclase differ markedly from volcanic ones suggesting different magma evolution. Zones in plutonic plagioclase are well correlated indicating crystallization in quiescent magma where crystals accumulation and compositional magma stratification may occur. Crystals probably did not travel between different regimes. Resorption occurred but as single albeit complex episodes. Good correlation of zones in plutonic plagioclases allows a distinction between the main processes controlling zonation and superimposed kinetic effects.
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  • 5
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    A Reference Section Through Fast‐Spread Lower Oceanic Crust, Wadi Gideah, Samail Ophiolite (Sultanate of Oman): Trace Element Systematics and REE Crystallization Temperatures – Implications for Hybrid Crustal Accretion (2022)
    AGU (American Geophysical Union) | Wiley
    Details
    Publication Date: 2024-02-07
    Description: Oceanic gabbros are the most abundant rocks close to Earth’s surface. Here we present new data from a consistent profile through the paleocrust of the Samail ophiolite (Oman), which is thought to provide the best analogue for modern fast-spreading oceanic crust. Incompatible trace elements of co-existing plagioclase and clinopyroxene fractionate from the mineral core to rim and up section from layered to foliated to varitextured gabbros. Layered gabbro parental melts correspond to mid-ocean-ridge basalts (MORB), and plagioclase Ca# shows a pronounced inverse zonation. Likely, they crystallized in situ from hydrous melts, compositionally buffered by replenishment at equilibrium to MORB and near steady-state boundary conditions. Further upsection, the compositional variability increases. Foliated gabbro rim and core compositions indicate increased fractionation and disequilibrium to MORB, triggered by open-system fractional crystallization within a heterogeneous magma plumbing structure, characterized by magma mixing, varying ambient water activities, and boundary conditions. Varitextured gabbros are chemically diverse with parental melts partially more primitive than MORB, suggesting that primitive melts directly reach the axial melt lens (AML). REE-in-plagioclase-clinopyroxene thermometry compared to and supported by anorthite-in-plagioclase thermometry reveals a relationship of urn:x-wiley:21699313:media:jgrb55525:jgrb55525-math-0001 [°C]=6.1±0.2*An+706±19. Crystallization temperatures of the layered gabbros cover a narrow range of 1216±14°C. Considerable temperature variability of 1077-1231°C is observed further upsection, featuring a thermal minimum within the foliated gabbros. This minimum is assumed to represent a zone where the fractionated descending crystal mushes originating from the AML meet with evolved liquids expelled from deeper crustal levels. Our findings suggest hybrid accretion of fast-spread crust.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
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  • 6
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    Shallow recycling of lower continental crust: The Mahoney Seamount at the Southwest Indian Ridge (2023)
    Elsevier
    Details
    Publication Date: 2024-02-07
    Description: The Mahoney Seamount is a recently discovered volcanic edifice located 4 km north of the ultra-slow spreading Southwest Indian Ridge (SWIR). The SWIR is one of the slowest spreading ridges worldwide with a full spreading rate of ∼14 mm/year and low magmatic productivity. We report that highly vesicular basalts from the Mahoney Seamount have unradiogenic Nd-Hf together with radiogenic Sr isotopic compositions. Their distinct low 206Pb/204Pb isotope signature combined with high 207Pb/204Pb and 208Pb/204Pb is best explained by melting of a mantle that has been strongly influenced by stranded lower continental crust. The geographic distribution of the isotopic variability favors the idea of shallow recycling of lower continental crust isolated for a longer period contributing to melts forming Mahoney Seamount through off-axis fault systems. The isotopic composition of Mahoney Seamount lavas shares many characteristics with EM-1 sources and the DUPAL signature. Previous isotopic studies of the SWIR basalts proposed recycling of ancient subcontinental lithospheric mantle (SCLM) or pelagic sediments with oceanic crust to be responsible for this enriched isotopic signature. Lu/Hf and Sm/Nd ratios of pelagic sediments would result in decoupled 143Nd/144Nd and 176Hf/177Hf ratios. This decoupling is also observed in Ejeda-Bekily dikes from Madagascar, but those are believed to sample the SCLM dispersed in the Indian Ocean. However, Mahoney Seamount shows no decoupling in those isotopic systems and the restricted occurrence of the extreme lower continental crustal signature at Mahoney Seamount implies that the enriched isotopic signature has a different origin.
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