The life cycle of the oceanic lithosphere commences in the spreading axis and ends in the subduction zone. To trace the cooling and evolutional history of the Earth, the change in thermochemical ...state during the life cycle of present‒day oceanic lithosphere is desired to be elucidated. In terms of the material science, spatial limitation of human‒accessible Earth interior is a bottleneck in reconstructing the thermochemical state of the oceanic lithosphere. Yet, by combining active sampling methods using ocean research vessels (ocean drilling, ocean bottom dredging, submersible survey, etc.) and passive sampling methods using Earth’s deep materials exposed to the surface owing to tectonic forces and volcanoes, we can collect samples that cover a considerable dimension. Here, I present efforts toward the elucidation of the thermochemical state of the oceanic lithosphere during its life cycle from the spreading axis to the subduction zone. The Oman ophiolite is presented as an analogue of oceanic lithosphere formed in the vicinity of a fast‒spreading axis, whereas the peridotite xenoliths from Tahiti Island are treated as an analogue of thermochemically disturbed oceanic lithosphere by a mantle plume, and those from petit‒spots are considered as an analogue less affected by thermochemical disturbance considering the lack of mantle plume beneath the petit‒spots. A heterogeneous thermal state corresponding to the segment structure is inferred in the fast‒spreading axis. The thermochemical state of the aging oceanic lithosphere is modified by mantle plume and petit‒spot magmatism, but pristine state can be reconstructed by using suitable peridotite xenoliths whose heating‒cooling and melting history is well characterized. The peridotite xenoliths from the petit‒spots can enhance a step toward reconstructing the thermochemical state of the deep oceanic lithosphere because deep‒rooted garnet‒stable peridotite xenoliths can be found.
The oceanic crust is in contact with sea water, and thus water is likely to penetrate downward into the solid earth. Meanwhile, the oceanic crust is a place where water is partitioned from the magma. ...Water that is incorporated into the oceanic crust as hydrous minerals is mainly 1. water in differentiated magma, 2. water released from subducted oceanic plate and supplied to magma via wedge mantle in the back-arc spreading system, and 3. sea water causing hydrothermal circulation. Here, I give an overview of behavior of the water partitioned from the magma into the lower oceanic crust, which is of particular difficulty to access, and the actual state of water is difficult to grasp. In addition, I present the deep-rooted hydrothermal circulation reaching to the lower oceanic crust.
A simple analytical procedure for determination of whole-rock major- and trace-element composition by micro-X-ray fluorescence (μXRF) spectrometer and inductively coupled plasma-mass spectrometer ...(ICP-MS) using fused-glass bead (sample + lithium tetraborate) is presented. In the case of peridotites, chromian spinel (Cr-spinel) is one of the minerals resistant to acids and interferes with accurate and reproducible determination of whole-rock composition. Such resistant minerals were not observed in the fused-glass beads prepared here, suggesting complete digestion of the samples including Cr-spinels. The μXRF spectrometer was employed to determine SiO2, MgO, and total Fe2O3 contents with the fused-glass beads. They were subsequently dissolved into a nitric acid solution to analyze thirty seven elements, Na, Al, P, K, Ca, Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Rb, Sr, Y, Zr, Nb, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, Pb, Th, and U with sector magnetic field ICP-MS (ICP-SFMS) and Q-pole mass filter ICP-MS (ICP-QMS). The analytical procedures were optimized and evaluated with geological reference materials of JP-1 and BIR-1a, and applied to various mantle xenoliths (lherzolite, harzburgite, dunite, wehrlite, olivine clinopyroxenite, and orthopyroxenite) from Tahiti and Moorea islands (Society Archipelago). Since only fused-glass beads are required as an analytical target to determine whole-rock major- and trace-element compositions, the sample amount consumed through the series of analytical procedure can be constrained to a minute amount (e.g., <0.4 g). The analytical procedures presented here are considered appropriate for tiny and/or precious samples, such as mantle xenoliths and rocks collected by dredging, diving, and drilling from the world’s ocean basins.
Dunite bands and dikes in ophiolitic mantle peridotites are interpreted as fossil melt channels within the suboceanic mantle. Concordant dunite bands (i.e. fossil melt channels transposed by outward ...transportation from the ridge axis via horizontal mantle flow) are particularly important as they possibly represent the melt channels through which the parental melts of mid-ocean ridge basalt (MORB) were transported to shallower depths beneath the paleo-ridge axis. We conducted field observations and sampling of concordant dunite bands (CDB) and their host harzburgite at selected outcrops covering a wide depth range in the mantle section along an inferred paleo-ridge segment in the northern to central part of the Oman ophiolite. The CDB increase in thickness and decrease in frequency upward. They are thicker and more frequent in the centre of the segment than near the segment ends when compared at the same stratigraphic level. The CDB consist mostly of olivine with minor spinel and very rare amounts of pyroxene. Clinopyroxene has a small grain size and an interstitial position relative to olivine. The constituent minerals in the CDB and their host harzburgite were analyzed by electron microprobe for major elements and by laser ablation inductively coupled plasma mass spectrometry for trace elements. Most of the CDB have refractory major element mineral compositions, such as high Fo 100Mg/(Mg+Fe) in olivine (>90.5), high Cr# Cr/(Cr+Al) in chromian spinel (>0.50), and low Al sub(2)O sub(3) (<3.5wt %) in clinopyroxene. Chondrite-normalized trace element patterns of clinopyroxene in the host harzburgites consistently show a gentle decrease from heavy REE (HREE) to middle REE (MREE) and a sharp decrease from MREE to light REE (LREE) (=highly depleted), but those in the CDB show weaker LREE depletion, which is more variable depending on the stratigraphic level and position along the paleo-ridge segment. In contrast, the HREE concentrations in clinopyroxene in the CDB are higher than or similar to those of the host harzburgites. Trace element compositions of clinopyroxene in the CDB and their host harzburgites are evaluated with a one-dimensional, steady-state, open-system decompressional melting-reaction model. The modeling results suggest that an LREE-enriched melt generated at high pressure was transported upwards through melt channels to the shallow mantle (up to the Moho transition zone), where it mingled with highly depleted melts accumulated from fractionally melted peridotites to generate normal (N)-MORB-like melts. The mantle started upwelling (=melting) in the garnet stability field in the segment centre, but either in the garnet or in the spinel stability field near the segment ends. This suggests a variation of geothermal gradient along the paleo-ridge segment: higher in the segment centre and lower near the segment ends. This inference is supported by the presence of thicker (up to 250cm) CDB as well as more frequent occurrence of CDB in the segment centre than near the segment end and by the geochemical evidence for chromatographic N-MORB-like melt percolation into the host peridotite only in the uppermost horizons near the segment ends.
Magnesium isotope compositions (δ26Mg) can provide valuable insights into Mg cycling through surface fluid-rock interactions. Isotopic variations of Mg within the oceanic lithosphere imply ...incorporation of isotopically different Mg, but the underlying mechanisms remain uncertain. In this study, we measure the Mg isotopic composition of the oceanic crust (n = 32) and mantle peridotite (n = 13) from the Oman ophiolite, which has experienced magmatism at the spreading center and subduction zone. The crustal section records δ26Mg values that range from −0.58 to −0.04 ‰. In the upper crustal section, higher Mg-bearing mineral abundance tends to have lighter δ26Mg values. However, the lower crustal section has no correlation with the δ26Mg values, indicating that the hydrothermal Mg sink formed by seawater was significant within the upper 2 km of the oceanic crust. In the mantle section, the δ26Mg values range from −0.39 ‰ to −0.07 ‰. Seven peridotite samples, with a loss of MgO by up to 5.3%, have slightly higher δ26Mg values than the global mantle, which may be attributed to low-temperature weathering. In contrast, three peridotite samples with higher melting degrees (spinel Cr# = Cr/(Cr + Al) atomic ratio) than the adjacent rocks are considered to have been affected by subduction-related interactions, forming lighter δ26Mg values. The isotopically light Mg source was subducted sediments, but its low concentration of Mg requires a water/rock ratio > > 1. Therefore, we conclude that the contribution of slab-derived Mg and modification of mantle δ26Mg values were localized.
Sagittal otoliths, typically composed of aragonite, are frequently laid down rather as vaterite during growth in hatchery-reared fish populations. Sagittal vateritization is believed to impair ...individual hearing/balancing abilities, but the causal mechanism remains unclear. Here we experimentally demonstrated that rearing in Sr-rich water induces sagittal vateritization in the HdrR-II1 inbred strain of the Japanese rice fish,
. Both sagittae were partly vateritized in 70% of individuals subjected to the Sr
treatment (
= 10), whereas fish reared in normal tap water showed no sagittal vateritization (
= 8). Our result is consistent with the theoretical prediction that vaterite becomes thermodynamically more stable than aragonite as the Sr
concentration in solution increases. A vateritic layer develops surrounding the original aragonitic sagitta in vateritized otoliths, some of which take on a comma-like shape. Electron probe microanalysis demonstrates that the vateritized phase is characterized by lower Sr
and higher Mg
concentrations than the aragonitic phase. It is unlikely that increased environmental Sr
is responsible for the sagittal vateritization in farmed fish. However, our findings likely help to establish an
assay using
to understand the physiological process underlying the sagittal vateritization in farmed fish.
Recently, small volcanic knolls known as petit-spot are highlighted from seismic viewpoint for a functional necessity to suppress the coseismic displacement during the 2011 Tohoku-Oki earthquake. To ...better understand physical mechanism of shallow oceanic plate disturbance by the petit-spot magma intrusion, we aim at elucidating the interaction of petit-spot magma and wet sediment in subseafloor volcanic environment. We used petit-spot peperites that are mixtures generated by mingling of magma and wet sediment. The peperites were collected at a petit-spot knoll located east of the outer trench swell in the northwestern Pacific (offshore of northern Japan). The sedimentary portions in the peperites are hardly to poorly consolidated due to heat supply from the magma body, and in places include radiolarian microfossils and zircon grains. Mineral phases of the sedimentary portions were identified to illite, quartz and hydrated sodium aluminum silicate. The radiolarian species and zircon grains were dated at ca. 10.6 to 15.6 Ma and ca. 1 to 108 Ma, respectively. The wide age range can be interpreted that whole sediments on the Early Cretaceous basaltic basement were entirely disturbed and transported to the surface via recent (younger than 1 million years) violate petit-spot magmatism. The subduction of petit-spot with annealed sediment “armor” functioned to prevent coseismic tsunamigenic slip propagation during the 2011 Tohoku-Oki earthquake as the seismic data predicted.
•Sediment layer is entirely disturbed via violate petit-spot magmatism.•Sediments were dehydrated by heat supply from petit-spot magma body.•Sediment-armored petit-spots can function to prevent coseismic tsunamigenic slip.
Diopsidite, mainly composed of diopsidic clinopyroxene (diopside for simplicity), was precipitated from a high‐temperature hydrothermal solution, rich in silicate components mainly in the mantle ...peridotite. We present here a new type of diopsidite in the uppermost mantle section, which contains Cr‐rich minerals such as chromite, Cr‐rich diopside (<2.5 wt.% Cr2O3) and Cr‐rich grossular (<7 wt.% Cr2O3). The chromites are varied in shape, subhedral to anhedral, and sometimes contain globular inclusions of grossular and chlorite. Film‐like chromite branches off from coarse partially dissolved chromite grains, filling diopside grain boundaries. The diopside is Cr‐rich solely around the chromite seam or chromite concentrate, but is almost free from Cr elsewhere. The diopsidite is surrounded by a whitish rock, which is composed of tremolite with a trace amount of chromite, diopside and grossular. The coarse chromite grain (<1 mm) in the whitish rock is highly vermicular in shape. The petrological and chemical characteristics of the mantle diopsidite suggest that a hydrothermal solution had collected Cr by partial to complete dissolution of chromite when replacing the mantle peridotite, and possibly transported Cr upward to the crust. Abundances of trace elements (rare earth elements(REE), Sr, Zr, Ti and Y) in the mantle diopsidite indicate their mobility in a hydrothermal solution together with Cr.
Relationships of lithologies in uppermost mantle section of Oman ophiolite are highly complicated, harzburgites especially being closely associated with dunites, wehrlites, and gabbros. The petrology ...and geochemistry of the uppermost mantle section provide constrains on how MORB (mid-ocean ridge basalt) magmas migrate from the mantle to crust. We conducted detailed sampling at the uppermost mantle section of the northern Oman ophiolite (along Wadi Fizh), and it provides us with centimeter-scale lithological and mineral chemical heterogeneity. In particular, we found peculiar plagioclase-free harzburgites that have not been recorded from the current ocean floor, which contain high-Mg# Mg/(Mg + Fe
2+
) atomic ratio clinopyroxenes that are almost in equilibrium (saturated) with MORB in terms of REE concentrations. They are from the uppermost mantle section underlying the wehrlite-dunite layer (=Moho transition zone; MTZ) just beneath the layered gabbro. MORBs cannot be in equilibrium with harzburgites; however, we call the peculiar harzburgites as “MORB-saturated harzburgite” for simplicity in this paper. The MORB-saturated harzburgites exhibit slightly enriched mineralogy (e.g., spinels with higher Ti and ferric iron, and clinopyroxenes with higher Ti and Na) and contain slightly but clearly more abundant modal clinopyroxene (up to 3.5 vol.%) than ordinary Oman depleted harzburgites (less than 1 vol.% clinopyroxene), which are similar to abyssal harzburgites. Gabbro-clinopyroxenite bands, which were melt lenses beneath the ridge axis, are dominant around the MTZ. Detailed sampling around the gabbro-clinopyroxenite bands revealed that the MORB-saturated harzburgites appear around the bands. The interaction between a melt that was MORB-like and an ordinary harzburgite induced incongruent melting of orthopyroxenes in harzburgites, and the melt chromatographically intruded into the wall harzburgite and was modified to coexist with olivine and two pyroxenes at low melt/harzburgite ratios. The modified melt left clinopyroxene (not clinopyroxene + plagioclase as in plagioclase-impregnated abyssal harzburgite) to form the MORB-saturated harzburgites in the vicinity (harzburgite) of the fracture, which are left as gabbro-clinopyroxenite bands. This local modification mimics the whole lithological and chemical variation of the MTZ and makes chemical variation of MORB suite at fast-spreading ridge.
IODP Expedition 357 used two seabed drills to core 17 shallow holes at 9 sites across Atlantis Massif ocean core complex (Mid-Atlantic Ridge 30°N). The goals of this expedition were to investigate ...serpentinization processes and microbial activity in the shallow subsurface of highly altered ultramafic and mafic sequences that have been uplifted to the seafloor along a major detachment fault zone. More than 57 m of core were recovered, with borehole penetration ranging from 1.3 to 16.4 meters below seafloor, and core recovery as high as 75% of total penetration in one borehole. The cores show highly heterogeneous rock types and alteration associated with changes in bulk rock chemistry that reflect multiple phases of magmatism, fluid-rock interaction and mass transfer within the detachment fault zone. Recovered ultramafic rocks are dominated by pervasively serpentinized harzburgite with intervals of serpentinized dunite and minor pyroxenite veins; gabbroic rocks occur as melt impregnations and veins. Dolerite intrusions and basaltic rocks represent the latest magmatic activity. The proportion of mafic rocks is volumetrically less than the amount of mafic rocks recovered previously by drilling the central dome of Atlantis Massif at IODP Site U1309. This suggests a different mode of melt accumulation in the mantle peridotites at the ridge-transform intersection and/or a tectonic transposition of rock types within a complex detachment fault zone. The cores revealed a high degree of serpentinization and metasomatic alteration dominated by talc-amphibole-chlorite overprinting. Metasomatism is most prevalent at contacts between ultramafic and mafic domains (gabbroic and/or doleritic intrusions) and points to channeled fluid flow and silica mobility during exhumation along the detachment fault. The presence of the mafic lenses within the serpentinites and their alteration to mechanically weak talc, serpentine and chlorite may also be critical in the development of the detachment fault zone and may aid in continued unroofing of the upper mantle peridotite/gabbro sequences.
New technologies were also developed for the seabed drills to enable biogeochemical and microbiological characterization of the environment. An in situ sensor package and water sampling system recorded real-time variations in dissolved methane, oxygen, pH, oxidation reduction potential (Eh), and temperature and during drilling and sampled bottom water after drilling. Systematic excursions in these parameters together with elevated hydrogen and methane concentrations in post-drilling fluids provide evidence for active serpentinization at all sites. In addition, chemical tracers were delivered into the drilling fluids for contamination testing, and a borehole plug system was successfully deployed at some sites for future fluid sampling. A major achievement of IODP Expedition 357 was to obtain microbiological samples along a west–east profile, which will provide a better understanding of how microbial communities evolve as ultramafic and mafic rocks are altered and emplaced on the seafloor. Strict sampling handling protocols allowed for very low limits of microbial cell detection, and our results show that the Atlantis Massif subsurface contains a relatively low density of microbial life.
•Seabed rock drills and real-time fluid monitoring for first time in ocean drilling•First time recovery of continuous sequences along oceanic detachment fault zone•Highly heterogeneous rock type and alteration in shallow detachment fault zone•High methane and hydrogen concentrations in Atlantis Massif shallow basement•Oceanic serpentinites potentially provide important niches for microbial life
PDF
