Gabbroic rocks recovered from deep holes drilled in the ocean floor provide us with valuable information about in-situ alteration processes of the lower oceanic crust. We found that the occurrence of ...biotite is widespread in gabbroic rocks recently drilled from IODP Hole U1473A at Atlantis Bank, near the Southwest Indian Ridge. Biotite is rare in oceanic gabbros, thus we analyzed textural and compositional details of biotite and associated minerals to better understand the conditions governing their formation.
In olivine gabbros from Hole U1473A and from nearby ODP Hole 735B, biotite occurs mainly in coronitic aggregates mantling olivine. It also forms monomineralic veins or occurs in biotite-chlorite-amphibole veins within plagioclase grains that contact the coronitic aggregates. The coronitic aggregates typically have an outer biotite-dominated zone and an inner zone mostly made up of Al-poor calcic amphibole. The biotite and the calcic amphibole zones frequently include Al-rich calcic amphibole (± cummingtonite) and talc, respectively. Plagioclase in direct contact with Al-rich calcic amphibole has 65–90 mol% anorthite. The coronitic aggregates also frequently have an outermost zone composed of submicron-scale biotite-chlorite mixtures, which show intermediate optical and chemical characteristics between biotite and chlorite, and a composite pattern of Raman-shift spectra. Most biotite-rich coronitic aggregates occur in proximity to felsic veins or to biotite or alkali feldspar microveins branched from felsic veins, whereas most biotite-chlorite coronas are connected to biotite-, chlorite- or amphibole-bearing microveins. Chlorite coronas around olivine, though rare in Atlantis Bank gabbros, occur in contact with chlorite-bearing microveins and show no relationship with felsic veins. Based on the plagioclase-amphibole equilibrium, we evaluated temperature of 750–850 °C for the formation of the biotite-rich coronitic aggregates.
From the modes of occurrence, compositions of minerals, and thermodynamic modeling, we conclude that the biotite coronas formed at higher temperatures and higher SiO2 and/or K+/H+ activities than chlorite coronas typically found in olivine gabbros from other mid-ocean ridge localities. The coronitic biotite-chlorite mixtures formed in response to lower SiO2 and/or K+/H+ activities, and possibly lower temperature, than the biotite coronas. Such a difference in physical and chemical conditions for corona formation probably reflects the distance from felsic vein/microvein or is related to the relative timing of reactions. The high-temperature metasomatic alteration of lower oceanic crustal gabbros shown in this study is most likely characteristic of oceanic core complexes from ultraslow-spreading ridges.
•Biotite is widely distributed in Atlantis Bank olivine gabbros.•The most common mode of biotite occurrence is corona mantling olivine.•Submicron-scale biotite-chlorite mixtures also occur in coronas.•The coronas are products of amphibolite-facies metasomatic alteration.•The biotite needs higher SiO2 and/or K+/H+ activities than biotite-chlorite mixtures.
Field observations and petrological and geochemical data are used to constrain a conceptual model for the formation of a gabbro‐peridotite section from Ligurian ophiolites (Italy). The studied ...section is attributed to an intraoceanic domain of the Jurassic Ligurian‐Piedmontese basin and is characterized by the lack of a basalt layer, similar to nonvolcanic segments from (ultra)slow spreading ridges. The proposed model shows a “hot” lithospheric evolution in which melt transport in the mantle under spinel to plagioclase facies conditions occurred mostly in the form of grain‐scale porous flow. We recognize a series of melt/peridotite interaction events, either diffuse or channeled, which modified the composition of the moderately depleted precursor mantle. In particular, localized infiltrations of MORB‐type melts gave rise to formation of spinel websterite layers close to the lithosphere‐asthenosphere boundary. The peridotite‐websterite association was involved in a spinel facies deformation attributed to emplacement of asthenospheric material at the base of the lithosphere. The “hot” lithospheric evolution is followed by an evolution characterized by melt transport through fractures, which started with crystallization of melt into troctolite to olivine gabbro dikes. Both mantle structures and gabbroic dikes are locally crosscut by gabbroic sills. As the mantle section cooled significantly, the dip of the melt migration structures evolved from subvertical to subhorizontal. The growth of a gabbroic pluton (up to ∼400 m thick) that is intruded into the mantle sequence is attributed to accretion of gabbroic sills. The tectonomagmatic history recorded by the gabbroic pluton after its solidification is characterized by ductile shearing developed from near‐solidus to amphibolite facies conditions.
Key Points
Compositional and structural modifications of the mantle section
Melt transport evolution in conjunction with exhumation
Similarities between studied ophiolite and modern (ultra)slow spreading ridges
The Jurassic Pineto ophiolite from Corsica exposes a ~1-km-thick troctolite–olivine-gabbro sequence, interpreted to represent a lowermost sector of the gabbroic oceanic crust from a (ultra-)slow ...spreading system. To constrain the petrogenesis of the olivine-gabbros, minor and trace element analyses of olivine (forsterite = 84–82 mol%) were carried out. Olivine from the olivine-gabbros is depleted in incompatible trace elements (Sc, V, Ti, Y, Zr and heavy rare earth elements) with respect to olivines from associated troctolites. Depleted incompatible element compositions are also shown by olivine (forsterite = 86 mol%) from a clinopyroxene-rich troctolite. The incompatible element compositions of olivine argue against a petrogenetic process entirely driven by fractional crystallization. We propose that melts migrating through an olivine–plagioclase crystal mush chemically evolved by reaction with the existing minerals, changing in composition as it flowed upward. The melt residual from these interactions led to partial dissolution of preexisting olivine and to crystallization of clinopyroxene, generating olivine-gabbro bodies within a troctolite matrix. Reactive flow was the major evolution process active in the ~1-km crustal transect exposed at the Pineto ophiolite, producing lithological variations classically attributed to fractional crystallization processes.
The gabbroic bodies from the Jurassic Ligurian ophiolites are structurally and compositionally similar to the gabbroic sequences from the oceanic core complexes of the Mid Atlantic Ridge. Initial ...cooling of the Ligurian gabbros is associated with local development of hornblende-bearing felsic dykes and hornblende vein networks. The hornblende veining is correlated with the widespread development of hornblende as coronas/pseudomorphs after the clinopyroxene in the host gabbros. In addition, the studied gabbroic body includes a mantle sliver locally containing hornblende gabbros and hornblendite veins. The hornblendes from the felsic dykes and the hornblende-rich rocks within the mantle sliver show a similar geochemical signature, characterized by low Mg#, CaO and Al2O3, negligible Cl, and high TiO2, K2O, REE, Y, Zr and Nb concentrations. The whole-rock Sm–Nd isotopic compositions of the felsic dykes and the hornblende-rich rocks define a Sm–Nd isochron corresponding to an age of 154±20Ma and an initial εNd of 9.2±0.5. The δ18O of the hornblendes and coexisting zircons from these rocks (about +4.5‰ and +5.8‰, respectively) do not indicate the presence of a seawater component in these melts. The formation of the felsic dykes and of the hornblende-rich rocks within the mantle sliver involved SiO2-rich silicate melts with negligible seawater component, which presumably were derived from high degree fractional crystallization of MOR-type basalts. The vein and the coronitic/pseudomorphic hornblendes show high Mg# and CaO, significant Cl (0.02–0.17wt%) and low TiO2 and K2O concentrations. The coronitic/pseudomorphic hornblendes have trace element compositions similar to those of the clinopyroxenes from the gabbros and δ18O values (+1.0‰ to 0.7‰) close to seawater, suggesting an origin by reaction between migrating seawater-derived fluids and the host gabbros. The vein hornblendes commonly show slight LREE enrichment, relatively high concentrations of Nb (up to 2.5ppm) and δ18O ranging from +3.7‰ to +0.8‰. The crystallization of these hornblendes most likely involved both seawater and magmatic components.
Microtextural and chemical evidence from gabbros indicates that melts may react with the crystal framework as they migrate through crystal mushes beneath mid-ocean ridges; however, the importance of ...this process for the compositional evolution of minerals and melts remains a matter of debate. Here we provide new insights into the extent by which melt-rock reaction process can occur in oceanic gabbros by conducting a detailed study of cryptic reactive melt migration as preserved in an apparently unremarkable, homogeneous olivine gabbro from deep within a section of the plutonic footwall of the Atlantis Bank core complex on the Southwest Indian Ridge (International ocean discovery program Hole U1473A). High-resolution chemical maps reveal that mineral zoning increases toward and becomes extreme within a cm-wide band that is characterized by elevated incompatible trace element concentrations and generates extreme more/less incompatible element ratios. We demonstrate that neither crystallization of trapped melt nor diffusion can account for these observations. Instead, taking the novel approach of correcting mineral-melt partition coefficients for both temperature and composition, we show that these chemical variations can be generated by intergranular reactive porous flow of a melt as it migrated through the mush framework, and whose composition evolved by melt-rock reaction as it progressively localized into a cm-scale reactive channel. We propose that the case reported here may represent, in microcosm, a preserved snapshot of a generic mechanism by which melt can percolate through primitive mafic (olivine gabbro) crystal mushes, and be modified toward more evolved compositions via near-pervasive reactive transport.
Abstract
Exposures of the Earth’s crust-mantle transition are scarce, thus, limiting our knowledge about the formation of subcontinental underplate cumulates, and their significance for metal storage ...and migration. Here, we investigated chalcophile metals to track sulfide crystallization within the Contact Series, an <150-m-thick pyroxenite-gabbronorite sequence, formed by mantle-derived melts, highlighting the boundary between the Balmuccia mantle peridotite and gabbronoritic Mafic Complex of the Ivrea-Verbano Zone. Within the Contact Series, numerous sulfides crystallized in response to the differentiation of mantle-derived underplated melts. Such sulfide-controlled metal differentiation resulted in anomalous Cu contents (up to ~380 ppm), compared to reference mantle (~19 ppm) and crustal samples (~1 ppm). We propose that the assimilation of continental crust material is a critical mechanism driving sulfide segregation and sulfide-controlled metal storage. Our results evidence that sulfides are trapped in the underplated mafic-ultramafic cumulates and that their enrichment in Cu may provide essential implications for crustal metallogeny.
This work presents new field and petrological data on a poorly known lower crustal section from the Alpine Jurassic ophiolites, the Pineto gabbroic sequence from Corsica (France). The Pineto gabbroic ...sequence is estimated to be ~1.5 km thick and mainly consists of clinopyroxene-rich gabbros to gabbronorites near its stratigraphic top and of troctolites and minor olivine gabbros in its deeper sector. The sequence also encloses olivine-rich troctolite and mantle peridotite bodies at different stratigraphic heights. The composition and the lithological variability of the Pineto gabbroic sequence recall those of the lower crustal sections at slow- and ultra-slow-spreading ridges. The gabbroic sequence considered in this study is distinct in the high proportion of troctolites and olivine gabbros, which approximately constitute 2/3 of the section. In particular, the lower sector of the Pineto gabbroic sequence shows the existence of large-scale fragments of the deepest oceanic crust displaying a highly primitive bulk composition. The mineral chemical variations document that the origin and the evolution of the Pineto gabbroic rocks were mostly constrained by a process of fractional crystallisation. The clinopyroxenes from the olivine gabbros and the olivine-rich troctolites also record the infiltration of olivine-dissolving, Cr
2
O
3
-rich melts that presumably formed within the mantle, into replacive dunite bodies. Cooling rates of the troctolites and the olivine gabbros were evaluated using the Ca in olivine geospeedometer. We obtained high and nearly constant values of −2.2 to −1.7 °C/year log units, which were correlated with the building of the Pineto gabbroic sequence through multiple gabbroic intrusions intruded into a cold lithospheric mantle.
The present study focuses on the petrographic and petrological characteristics of mantle bodies included in Upper Cretaceous sedimentary melanges of the External Ligurian units (Northern Appennine), ...within the Monte Gavi and Monte Sant'Agostino areas. Two distinct pyroxenite-bearing mantle sections were recognized, mostly based on their plagioclase-facies evolution. The Monte Gavi mantle section is nearly undeformed and records a process of melt infiltration and reaction under plagioclase-facies conditions. The melt-rock interaction event involved both peridotites (mostly harzburgites) and enclosed spinel pyroxenite layers, and is estimated to have occurred at 0.7–0.8 GPa. In the Monte Gavi peridotites and pyroxenites, the spinel-facies clinopyroxene was partially replaced by plagioclase and new orthopyroxene (± secondary clinopyroxene). The reactive melt migration led to relatively high TiO2 contents in relict clinopyroxene and spinel (up to 2.3 wt% and 1.0 wt%, respectively, in the pyroxenites), with the latter also having high Cr# (up to 35 in the peridotites). The Monte Sant'Agostino mantle section displays a widespread ductile shearing and no evidence for melt-rock interaction under plagioclase-facies conditions. The main deformation phase recorded by the Monte Sant'Agostino peridotites (mostly lherzolites) is estimated to have occurred at 750–780 °C and 0.3–0.6 GPa, and gave rise to protomylonitic to ultramylonitic textures characterized by 10–50 μm neoblasts. The enclosed pyroxenite layers yielded relatively high temperature and pressure estimates (870–930 °C and 0.8–0.9 GPa). Presumably, in the Monte Sant'Agostino mantle section, plagioclase crystallization occurred earlier in the pyroxenites than in enclosing lherzolites, thereby enhancing strain localization and formation of mylonite shear zones in the entire mantle section. We propose that subcontinental mantle section from the External Ligurian units consists of three distinct mantle domains, developed in response to the rifting evolution that ultimately formed a Middle Jurassic ocean-continent transition: (1) a spinel tectonite domain that underwent no significant deformation and melt-rock reaction under plagioclase-facies conditions, characterized by static plagioclase development, (2) a plagioclase mylonite domain experiencing melt-absent deformation, and (3) a nearly undeformed domain that underwent melt infiltration and reaction under plagioclase-facies conditions. We relate mantle domains (1,2) to a rifting-driven uplift in the late Triassic accommodated by large-scale shear zones consisting of plagioclase mylonites.
•Rifting evolution of two distinct peridotite-pyroxenite mantle sequences•Thermobarometry of plagioclase-facies mantle rocks•Reactive melt migration under plagioclase-facies conditions•Melt-absent ductile shearing and mylonite-ultramylonite formation•New constraints on the mantle evolution in the Western Tethys OCT
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