In contrast to the widely recognized aspects of serpentinization, initial stages of hydration and tectonic processes of unserpentinized peridotites are still unclear, but have important implications ...for understanding the lithospheric architecture of supra-subduction zones. This study provides petrological evidence from the Oeyama ophiolite, SW Japan, of the effects of high-temperature metasomatic hydration immediately before the cooling and ductile deformation of forearc peridotites. Key findings in this study are: 1) complex association of high-temperature metasomatic minerals: tremolitic amphibole, cummingtonite, phlogopite, chlorite, olivine and orthopyroxene in veins and in mylonites; 2) the systematic variation in Si and Na+K contents of the tremolitic amphibole, corresponding to its mode of occurrence and mineral association; and 3) the presence of thin (<0.7mm) veins of fine-grained olivine accompanied by a narrow diffusion zone of the host primary olivine. On the basis of petrography and mineral chemistry, the temporal sequence of hydration and deformation of the Oeyama ophiolite is considered as follows: 1) infiltration of slab-derived fluids, causing decomposition of primary pyroxene and chemical modification of primary olivine, 2) metasomatic formation of variable modal amounts of amphibole, phlogopite, chlorite, vein-forming olivine and secondary orthopyroxene at 650–750°C; 3) early-stage mylonitization of the hydrous peridotites in localized shear zones; and 4) syntectonic serpentinization at 400–600°C to form serpentinite mylonites. Paragenesis and amphibole compositions suggest comparable temperature conditions for metasomatism and early-stage mylonitization. Mylonitization occurred exclusively in hydrous peridotites, and the peridotite mylonites were preferentially overprinted by syntectonic serpentinization. Diffusion profiles of olivine cut by a vein suggest rapid cooling immediately after the metasomatic fluid infiltration. From these observations and calculations, it is concluded that the exhumation of the forearc peridotites was closely related to the infiltration of high-temperature metasomatic fluids and hydration occurred under a wide range of temperature conditions.
•Metasomatic amphibole and phlogopite widely occur in the Oeyama forearc ophiolite.•Temperature condition for the metasomatism was estimated to be 650–750°C.•Mylonitization of hydrous peridotites occurred at temperatures similar to those of the metasomatism.•Heterogeneous olivine compositions suggest rapid cooling after metasomatic veining.•All the data suggest exhumation immediately after the metasomatic fluid infiltration.
It is a general tendency that epidote, which is a typical greenschist facies mineral, is scarce in the lower oceanic crust, in spite of the widespread occurrence of the other minerals indicative of ...similar temperature conditions such as chlorite, actinolite, prehnite and serpentine. To find the cause of this, we carried out petrological analyses of lower crustal rocks of the Oman ophiolite sampled by the Oman Drilling Project of the International Continental Scientific Drilling Program (ICDP). Petrographic observations revealed the tendency, as expected, that the amount of epidote formed by static alteration of plagioclase decreases with depth. Because mineral assemblages indicative of a wide range of temperature conditions from amphibolite to subgreenschist facies occur throughout the cores without systematic variations of abundance, the decrease of epidote amount cannot be explained by the difference of temperature condition of alteration. Petrographic observations also revealed that epidote is absent or rare in rocks containing serpentinized olivine in contrast to prehnite showing a close association with serpentinization of olivine. In an exceptional sample containing both epidote and serpentinized olivine, epidote occurs with chlorite that cuts or replaces plagioclase, mantles adjacent olivine and is connected with chlorite + lizardite veins cutting mesh‐forming serpentine veins. The distribution and mode of occurrence of epidote suggest decoupling of its formation with the main stage of serpentinization. Serpentine veins cutting olivine to form mesh texture are typically lizardite with magnetite ribbons at vein centres and have compositions of lizardite–cronstedtite solid solution at vein margins or in magnetite‐free veins, suggesting a chemical condition with low silica and low oxygen potentials at an early stage of serpentinization. Thermodynamic modelling for olivine and plagioclase alteration at greenschist facies conditions indicates that silica potential for plagioclase alteration to form prehnite + chlorite and epidote + chlorite could be higher than for olivine serpentinization. On the other hand, oxygen potential for the prehnite + chlorite formation is lower than for the epidote + chlorite formation and is comparable with that for olivine serpentinization. From the observations and analyses, it is concluded that epidote formation is inhibited by olivine serpentinization, which maintains a reducing condition for alteration in the lower oceanic crust.
Compositional variation of olivine in serpentinized peridotites provides a significant constraint on modeling the redox conditions of serpentinization and the tectonothermal history of ophiolites. ...Here I report the variations of Fe, Mg, Mn, and Ni contents of olivine from the Oeyama ophiolite, SW Japan and show textural and chemical evidence for compositional modification of olivine related to high–temperature (T) serpentinization. The Fe–enrichment of olivine adjacent to antigorite without significant magnetite formation indicates a reducing condition for high–T serpentinization. Systematic variations of forsterite (Fo) component with distance from antigorite suggest Mg–Fe volume diffusion took place in olivine porphyroclasts under the conditions of high–T serpentinization. In addition, a similar diffusion pattern of Mn to Fe results in a retrograde trend in MnO–Fo diagram, which could be a useful indicator of high–T serpentinization. Retrograde antigorite is different from prograde antigorite in having a shape of elongated blade, lacking a significant amount of magnetite inclusion, and being more ferrous than lizardite. The existence of retrograde antigorite provides another piece of evidence for high–T serpentinization even if olivine has been decomposed by intense low–T serpentinization. Approximate estimation of time required for the observed Mg–Fe diffusion profiles of olivine porphyroclasts reveals that a cooling duration under the conditions of high–T serpentinization was much longer than that of amphibolite–facies metasomatism previously reported. This suggests a long residence time of the forearc peridotites within the serpentinized mantle wedge following rapid exhumation immediately after the amphibolite–facies metasomatism.
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.
To understand the similarity and diversity of serpentinization processes in different rock systems, gabbroic rocks recovered from IODP Site U1415 at the Hess Deep Rift were examined and compared with ...peridotites from the adjacent ODP Site 895. Textural observations, micro-Raman spectroscopic analyses and electron microprobe analyses indicated that most of the olivine-replacing serpentine in the gabbroic rocks lack the mixing with brucite, which is common in peridotites. At least three stages of serpentinization are observable in the gabbroic rocks; each generation is characterized by different submicroscopic mixtures or solid solutions of sheet silicates: 1) Mg-Fe2+ lizardite+ferri-lizardite+chlorite, 2) Mg-Fe2+ lizardite+ferri-lizardite, and 3) Mg-Fe2+ lizardite+ferri-lizardite+saponite. The first and third generations of serpentine and mixed minerals are relatively Fe-rich, whereas the second generation is Fe-poor and associated with abundant magnetite and pyrrhotite. The major difference between the alteration of gabbroic and peridotitic systems is probably best explained by the iron content and modal abundance of primary olivine and by rock-dominated fluid compositions with a high silica activity due to the alteration of plagioclase in gabbroic rocks. The mineralogical variations between the reported three generations of mixed sheet silicates in gabbroic rocks can be ascribed to variations of silica and/or oxygen activities in the associated fluids under decreasing temperature conditions. The abrupt increase of magnetite crystallization during serpentinization in gabbroic rocks could be caused by oxidation at a relatively high SiO2 activity without the olivine-serpentine-brucite buffering assemblage.
•Serpentinization is compared between gabbros and peridotites from the Hess Deep Rift.•Serpentine is associated with abundant magnetite and no brucite in the gabbros.•At least three stages of serpentinization are observable in the gabbroic rocks.•Lizardite formed with variable mixtures or solid solutions of other sheet silicates.•The observations can be explained by the variations of silica and oxygen activities.
Compositional variations of talc in peridotites and serpentinites could have significant implications for modeling of geochemical cycles involving the upper mantle but have been scarcely studied. We ...analyzed chemical compositions of prograde and retrograde talc and associated minerals in thermally metamorphosed serpentinites from Southwest Japan. The analyzed talc has variations of Si, Al, Mg, Fe, and Na contents. Most of the Si, Al, Mg, and Fe variations indicate mechanical mixing with serpentine and chlorite at a submicroscopic scale. Spatial distribution of talc–chlorite mixtures suggests their prograde metamorphic origin. Talc–serpentine mixtures could be formed by retrograde decomposition of talc–olivine assemblage and orthopyroxene at conditions of higher temperature and/or higher Si activity than serpentine–brucite mixtures, which are the typical products of serpentinization of olivine. Talc itself, regardless of prograde or retrograde origin, has compositional variations with Na enrichment as a likely result of solid solution or Na–mica mixing. The Na enrichment suggests that talc could be the most capable reservoir of Na in metamorphosed peridotites and serpentinites.
The new discovery of Fe-monticellite in serpentinized peridotites of the Happo ultramafic complex, central Japan, shows it replacing olivine and coexisting with antigorite. The monticellite occurs in ...several forms: as discrete grains; partially disconnected rings or circular ribbons; fringing mantles or in veins cutting olivine; or as aggregates of small equant grains. It shows a textural contrast to the intensely sheared matrix of antigorite. The coexistence with antigorite and diopside, and the Fe-rich compositions of monticellite indicates that it formed at 300–350 °C. This extremely low-temperature for monticellite formation is consistent with the observed textures indicative of its static formation at a stage of serpentinization that followed mylonitization during high-temperature (400–600 °C) serpentinization in the Happo complex. The monticellite contains minute inclusions of awaruite, which indicates formation under reducing conditions, as is characteristic of serpentinization of olivine. The monticellite is richer in Fe and Mn than relict olivine, being commensurate with deficiencies of these elements in coexisting antigorite. There is no evidence of Ca addition from an external source. However, where tremolite is replaced by serpentine, diopside grows at the opposite side of relict olivine that is partly replaced by monticellite. Thus, tremolite is the likely source of Ca for monticellite formation. These textural relationships and the thermodynamic calculations for paragenesis indicate that monticellite formed by reactions between olivine and aqueous fluids, carrying Ca released from tremolite, that were driven by a local gradient of SiO2 chemical potential during serpentinization.
•Fe-monticellite occurs replacing olivine and coexisting with antigorite and diopside in serpentinized peridotites.•Textures indicate the static formation of monticellite following mylonitization of the serpentinite.•The paragenesis and Fe-rich compositions of monticellite indicate its formation at 300–350 °C.•The Fe-monticellite formed by reactions between olivine and aqueous fluids that carried Ca released from tremolite during serpentinization.
Aluminous spinel, corundum and diaspore are reported from intensely altered parts of primitive troctolites recovered from IODP Site U1415 at the Hess Deep Rift. The spinel is green-colored, has an ...irregular shape, has low Cr concentrations, and is so distinct from primary igneous chromite. Corundum and diaspore occur mainly at the rims of green spinel grains with a texture suggesting a sequential replacement of spinel by corundum, and then corundum by diaspore. The green spinel is associated with anorthite and pargasite, which is overgrown by tremolite that forms coronitic aggregates with chlorite around olivine. These petrographic observations are supported by pressure–temperature pseudosections, which predict spinel + pargasite stability field, and tremolite/hornblende + chlorite field at lower temperature conditions. From these pseudosections and simplified system phase diagrams, estimated formation temperature conditions calculated at 2 kbar are 650–750 °C for spinel + pargasite, 410–690 °C for tremolite/hornblende + chlorite, 400–710 °C for corundum, and <400 °C for diaspore. Because the aluminous spinel occurs in the domains that were previously occupied by magmatic plagioclase, and because spinel-bearing rocks characteristically have high Al
2
O
3
/CaO and Al
2
O
3
/SiO
2
ratios, it is likely that the stabilization of spinel was caused by the loss of Ca
2+
and SiO
2
(aq) in high-temperature hydrothermal fluids. The results of this study suggest that (1) the concentrations of aluminous phases in the lower oceanic crust are presently underestimated, and (2) chemical modification of the lower oceanic crust due to high-temperature hydrothermal metasomatic reactions could be common near spreading axes.
We report a new sampling strategy for collecting representative samples of drill core. By splitting the core with a diamond saw into working and archive halves, the saw cuttings constitute a ..."channel" sample, the best subsample from which to obtain an average mineralogical and geochemical composition of a core. We apply this procedure to sampling core of the lower oceanic crust in the Hess Deep obtained during Expedition 345 of the Integrated Ocean Drilling Program (now International Ocean Discovery Program).
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