Several varieties of seafloor hydrothermal vents with widely varying fluid compositions and temperatures and vent communities occur in different tectonic settings. The discovery of the Lost City ...hydrothermal field in the Mid-Atlantic Ridge has stimulated interest in the role of serpentinization of peridotite in generating H2- and CH4-rich fluids and associated carbonate chimneys, as well as in the biological communities supported in highly reduced, alkaline environments. Abundant vesicomyid clam communities associated with a serpentinite-hosted hydrothermal vent system in the southern Mariana forearc were discovered during a DSV Shinkai 6500 dive in September 2010. We named this system the "Shinkai Seep Field (SSF)." The SSF appears to be a serpentinite-hosted ecosystem within a forearc (convergent margin) setting that is supported by fault-controlled fluid pathways connected to the decollement of the subducting slab. The discovery of the SSF supports the prediction that serpentinite-hosted vents may be widespread on the ocean floor. The discovery further indicates that these serpentinite-hosted low-temperature fluid vents can sustain high-biomass communities and has implications for the chemical budget of the oceans and the distribution of abyssal chemosynthetic life.
The interaction of hydrous fluids and melts with dry rocks of the lithospheric mantle inevitably modifies their viscoelastic and chemical properties due to the formation of compositionally distinct ...secondary phases. In addition, melt percolation and the associated metasomatic alteration of mantle rocks may also facilitate modification of the pre–existing rock texture and olivine crystallographic preferred orientation (CPO) and thus seismic properties. Here we explore the relationship between mantle metasomatism, deformation and seismic anisotropy using subduction–related mantle xenoliths from the Penghu Islands, western Taiwan. The investigated xenoliths have equilibrated at upper lithospheric mantle conditions (879 °C to 1127 °C) based on pyroxene geothermometry and show distinct variations in clinopyroxene chemical composition, texture and olivine CPO allowing for the classification of two distinct groups. Group 1 xenoliths contain rare earth element (REE) depleted clinopyroxene, show a porphyroclastic texture and olivine grains are mostly characterized by 100–axial pattern symmetries. In contrast, REE-enriched clinopyroxene from Group 2 xenoliths occur in a fine–grained equigranular texture and coexisting olivine frequently displays 010–axial pattern symmetries. The clinopyroxene compositions are indicative of cryptic and modal to stealth metasomatic alteration of Group 1 and Group 2 xenoliths, respectively. Furthermore, the observed olivine 100–axial pattern of Group 1 xenoliths reflects deformation by dislocation creep at high temperature, low pressure and dry conditions, whereas olivine 010–axial patterns of Group 2 xenoliths imply activation of olivine 001 glide planes along preferentially wet (010) grain boundaries. This correlation indicates that the variation in olivine CPO symmetry from 100– to 010–axial pattern in Penghu xenoliths results from deformation and intra-crystalline recovery by subgrain rotation during metasomatic alteration induced by melt percolation. The microstructural observations and olivine CPO combined with petrological and geochemical data suggest that Group 1 xenoliths preserve microstructural and chemical characteristics of an old, probably Proterozoic lithosphere, while Group 2 xenoliths record localized Miocene deformation associated with wall–rock heating and metasomatism related to melt circulation. Furthermore, the observed transition of olivine CPO from 100–axial pattern to 010–axial pattern by deformation in the presence of variable melt fractions and associated metasomatic alteration can be inferred to modify the physical properties of mantle rocks.
•Correlation of xenolith textures and metasomatism with olivine fabric symmetries•Olivine 100– to 010–axial symmetry transition during deformation in the presence of melt•Impact of mantle metasomatism on seismic properties
We tested plagioclase flow laws for rheological properties of the detachment shear zone developed in an oceanic core complex. Gabbroic mylonites occur extensively over the ∼125-km length of the ...Godzilla Megamullion, an enormous oceanic core complex situated in an extinct Philippine Sea back-arc basin. The mylonites were produced in the detachment shear zone at temperatures of 650–850 °C in the lower crust over a period of approximately 4 million years, corresponding to a slow spreading rate of 2.54±0.21 cm/yr (i.e. 8.05×10−10 m/s). Applying the rheological parameters for plagioclase flow laws, combined with the geochronological spreading rate, we calculated deformation mechanism maps of plagioclase as shear strain rates of 10−12 to 10−8 s−1, corresponding to shear zones in thickness of ∼0.1 to ∼1000 m. Our results show that, assuming a constant stress condition defined by a shear strain rate of 10−12 s−1 for grain size of 1000 μm in a temperature range between 650 and 850 °C, petrofabric parameters such as crystal-preferred orientations and dynamically recrystallized grain sizes along with estimated equilibrium temperatures can be possibly explained by the deformation mechanism maps of plagioclase for shear strain rates mostly of 10−11 to 10−9 s−1. It suggests that even if the entire thickness of the detachment shear zone may lie several hundred meters below the spreading center, the shear zone could be stratified, comprising many anastomosing narrow zones. It implies that, during the development of the detachment fault, strain localization would occur in the lower crust over a broad (∼1000 m) zone at high temperatures.
•Plagioclase flow laws have been tested for the detachment shear zone of an OCC.•Shear strain rates of 10−11 to 10−9 s−1 can explain the rheology of plagioclase.•A broad shear zone may consist of many anastomosing narrow zones.•Strain localization would occur in the lower crust during asymmetric shearing.
B-type olivine fabrics are pervasive within highly depleted dunites of the small-sized Imono peridotite body located within the subduction-type Sanbagawa metamorphic belt of the southwest Japan arc. ...The dunites contain various microstructures, ranging from porphyroclastic to fine-grained intensely sheared textures. The Mg/(Mg
+
Fe) atomic ratios (Fo number) of olivine within these dunites are consistently around 0.9, as are the Cr/(Cr
+
Al) atomic ratios (Cr number) of chromian spinel, suggesting their evolution from a highly depleted magma (boninite). These data provide strong thermal constraints on the formation of the highly depleted dunites, as their formation requires hot, hydrous, shallow mantle (>
1250 °C at <
30 km depth) in the mantle wedge. Because the Sanbagawa metamorphic belt finally entrained these peridotites during progressive retrogression, B-type olivine fabrics probably developed in the fore-arc side of the subduction zone, above or along the subducting slab, possibly in association with dehydration fluids derived from the slab. The previously documented small magnitude of S-wave splitting can be explained by the seismic properties of B-type peridotites within an anisotropic layer of approximately several kilometers in thickness, oriented by flow parallel to the subducting slab, under maximum temperatures of 880–1030 °C depending on the flow stress. These findings indicate that such a B-type layer could constitute a dominant source of seismic S- and P-wave anisotropy in mantle wedge regions.
Measurements of crystallographic preferred orientations (CPO) and calculations of P- and S-wave velocities (Vp and Vs) and anisotropy were conducted on three quartz–mica schists and one felsic ...mylonite, which are representative of typical metamorphic rocks deformed in the middle crust beneath the southeastern Tibetan plateau. Results show that the schists have Vp anisotropy (AVp) ranging from 16.4% to 25.5% and maximum Vs anisotropy AVs(max) between 21.6% and 37.8%. The mylonite has lower AVp and AVs(max) but slightly higher foliation anisotropy, which are 13.2%, 18.5%, and 3.07%, respectively, due to the lower content and CPO strength of mica. With increasing mica content, the deformed rocks tend to form transverse isotropy (TI) with fast velocities in the foliation plane and slow velocities normal to the foliation. However, the presence of prismatic minerals (e.g., amphibole and sillimanite) forces the overall symmetry to deviate from TI. An increase in feldspar content reduces the bulk anisotropy caused by mica or quartz because the fast-axis of feldspar aligns parallel to the slow-axis of mica and/or quartz. The effect of quartz on seismic properties of mica-bearing rocks is complex, depending on its content and prevailing slip system. The greatest shear-wave splitting and fastest Vp both occur for propagation directions within the foliation plane, consistent with the fast Pms (S-wave converted from P-wave at the Moho) polarization directions in the west Yunnan where mica/amphibole-bearing rocks have developed pervasive subvertical foliation and subhorizontal lineation. The fast Pms directions are perpendicular to the approximately E-W orienting fast SKS (S-wave traversing the core as P-wave) directions, indicating a decoupling at the Moho interface between the crust and mantle beneath the region. The seismic data are inconsistent with the model of crustal channel flow as the latter should produce a subhorizontal foliation where vertically incident shear waves suffer little splitting.
•Departure of schists from transverse isotropy by quartz, amphibole and sillimanite•Feldspar generally reduces the bulk anisotropy caused by mica or quartz.•Decoupling between crust and upper mantle beneath west Yunnan (China)•Crustal channel flow cannot explain the decoupling at the Moho interface.
Seismic velocity structures of the oceanic lithosphere are variable due to the variation of alteration and heterogeneities in porosity. In order to interpret geophysically determined velocity ...structures, it is necessary to understand how the seismic velocity of the oceanic lithosphere is affected by alteration and porosity. We conducted petrological analysis and elastic wave velocity measurements at 200 MPa on mafic and ultramafic rocks drilled from the Samail ophiolite. The elastic wave velocity calculated from the mineral abundances of the ultramafic rocks was sensitive to serpentinization, whereas that of the mafic rocks was nearly independent of the alteration, suggesting that velocity changes with depth in the oceanic crust are mainly due to porosity reduction. Based on the effective medium theory and pressure dependence of the experimentally determined velocity, the change in the porosity was estimated to be 0.04%–0.42% at 200 MPa, indicating porosity reduction by the increase of lithostatic pressure in the oceanic crust in the fast‐spreading system such as the Pacific plate. We estimated the degree of hydration in the mantle to account for the geophysical observations based on the laboratory experiments. Both the crustal porosity and mantle serpentinization increase towards the trench due to fracture related plate bending, indicating extensive hydration of the oceanic lithosphere close to the trench. From these estimates, water contents in the oceanic lithosphere were calculated to be as high as 3 wt.% in the shallow crust and 6 wt.% in the upper most mantle, which is significant for water transport into Earth's interior.
Plain Language Summary
Geophysical observations have found evidence for hydration of the oceanic lithosphere, because the physical properties of rocks are controlled by mineralogy and pore fluids. To understand how the seismic velocity of the oceanic lithosphere is affected by mineral alteration and porosity, we conducted petrological analysis and elastic wave velocity measurements at high confining pressure on mafic and ultramafic rocks drilled from the Samail ophiolite, where the ancient fragments of oceanic lithosphere are emplaced on land. Our results indicate that the seismic velocity calculated from the mineral abundances of the ultramafic rocks was sensitive to serpentinization, whereas that of the mafic rocks was nearly independent of the mineral alteration, suggesting that the change of velocity in the oceanic crust are mainly due to porosity. Based on the laboratory experiments and the effective medium theory, we estimated the water contents in the Pacific plate from the geophysical observations. The water content increases toward trench due to fracture related plate bending, and the mantle may have more than twice the water content of crust, which significantly influences water cycling on Earth.
Key Points
The seismic velocity of ultramafic rocks is sensitive to serpentinization, but that of mafic rocks is nearly independent of alteration
Porosity variation in the oceanic crust is estimated to be 0.04%–0.42%, based on the pressure dependence of elastic wave velocities
Water contents in the oceanic lithosphere are calculated to be as high as 3 wt.% in the shallow crust and 6 wt.% in the upper‐most mantle
Peridotite formed by contact metamorphism and dehydration breakdown of an antigorite schist from the Happo area, central Japan shows a strong olivine crystallographic preferred orientation (Ol CPO). ...The lack of mesoscale deformation structures associated with the intrusion and the lack of microstructural evidence for plastic deformation of neoblastic grains suggest that olivine CPO in this area did not form as a result of solid-state deformation. Instead, the good correspondence between the original antigorite orientation and the orientation of the newly formed olivine implies the CPO formed by topotactic growth of the olivine after antigorite. Ol CPO is likely to develop by a similar process in subduction zone environments where foliated serpentinite is dragged down to depths where antigorite is no longer stable. The Happo Ol CPO has a strong a-axis concentration perpendicular to the lineation and within the foliation—commonly referred to as B-type Ol CPO. Seismic fast directions parallel to the ocean trench are observed in many convergent margins and are consistent with the presence of B-type Ol CPO in the mantle wedge of these regions. Experimental work has shown that B-type CPO can form by dislocation creep under hydrous conditions at relatively high stresses. There are, however, several discrepancies between the characteristics of natural and laboratory samples with B-type Ol CPO. (1) The formation conditions (stress and temperature) of some natural examples with B-type CPO fall outside those predicted by experiments. (2) In deformation experiments, slip in the crystallographic c-axis direction is important but has not been observed in natural examples of B-type CPO. (3) Experimental work suggests the presence of H2O and either high shear stress or relatively low temperatures are essential for the formation of B-type CPO. These conditions are most likely to be achieved close to subduction boundaries, but these regions are also associated with serpentinization, which prevents strong olivine CPO patterns from forming. We show B-type Ol CPO can form as a result of static topotactic growth of olivine after high-temperature breakdown of foliated serpentinite. These results resolve the discrepancies between experimental and natural examples of B-type CPO and show the need to rethink the formation process of olivine CPO in convergent margins. Topotactic growth of olivine after antigorite can account for the inferred distribution of B-type Ol CPO in the mantle wedge more successfully than dislocation creep.
•Olivine CPO can form by topotactic growth after aligned antigorite.•B-type olivine CPO is not necessarily the result of mantle flow.•B-type olivine CPO may be more widespread in the wedge mantle than previously thought.
Microstructural and petrologic analyses of 7 gabbroic rocks sampled from the medial area of the Godzilla Megamullion (site KH07-02-D18), located along the Parece Vela Basin spreading ridge (Parece ...Vela Rift), Philippine Sea, reveal the development of a high-temperature ductile shear zone associated with hydrothermal metamorphism in the lower crust. The deformed gabbroic rocks are petrographically classified into mylonites and an ultramylonite, and are characterized by porphyroclastic textures consisting mainly of coarse plagioclase and clinopyroxene/amphibole porphyroclasts in a fine-grained matrix. Plagioclase crystallographic-preferred orientations vary from (010)100 and (001)100 patterns in the mylonites to a weak (001)100 pattern in the some mylonites and ultramylonite, suggesting a change in the deformation mechanism from dislocation creep to grain-size-sensitive creep with increasing intensity of deformation. The chemical composition of matrix plagioclase is generally more sodic than that of porphyroclasts. Secondary amphibole is ubiquitous, consisting mainly of pargasite and magnesiohornblende (brown hornblende) and actinolite (green hornblende). The mineral assemblage is consistent with the hydrothermal metamorphic reaction: clinopyroxene
+
calcic plagioclase
+
fluid
→
amphibole
+
sodic plagioclase. Compared with deformed gabbroic rocks from the breakaway and termination areas of the Godzilla Megamullion, the samples record ductile shearing under high temperature conditions, possibly related to the development of the Godzilla Megamullion, although hydrothermal activity in the medial area appears to have been less intense than in both the breakaway and termination areas.
► Fe-Ti oxide gabbros at the medial area of the Godzilla Megamullion deformed under high temperature with hydrous fluid. ► Hydrothermal alteration resulted in retrograde metamorphism associated with deformation of the Fe-Ti oxide gabbros. ► The deformed gabbros in this study possibly relate to the development of the Godzilla Megamullion. ► Hydrothermal activity associated with the deformation of gabbroic rocks seems to be distinct from breakaway, medial to termination areas at the Godzilla Megamullion.
Sampling the upper mantle via scientific ocean drilling remains elusive. Although the technologies required for drilling to the Moho still don’t exist, we have made significant progress over the last ...five decades in piecing together the complex geology of the oceanic crust. Here, we highlight key findings that reveal the architecture of oceanic crust and the thermal, physical, and chemical processes that are responsible for the growth and structure of the oceanic lithosphere. These advances result from enduring efforts to drill and collect downhole geophysical logs of oceanic crust near both slow and fast spreading ridges.
We calibrated the magnitude and symmetry of seismic anisotropy for 132 mica‐ or amphibole‐bearing metamorphic rocks to constrain their departures from transverse isotropy (TI) which is usually ...assumed in the interpretation of seismic data. The average bulk Vp anisotropy at 600 MPa for the chlorite schists, mica schists, phyllites, sillimanite‐mica schists, and amphibole schists examined is 12.0%, 12.8%, 12.8%, 17.0%, and 12.9%, respectively. Most of the schists show Vp anisotropy in the foliation plane which averages 2.4% for phyllites, 3.3% for mica schists, 4.1% for chlorite schists, 6.8% for sillimanite‐mica schists, and 5.2% for amphibole schists. This departure from TI is due to the presence of amphibole, sillimanite, and quartz. Amphibole and sillimanite develop strong crystallographic preferred orientations with the fast c axes parallel to the lineation, forming orthorhombic anisotropy with Vp(X) > Vp(Y) > Vp(Z). Effects of quartz are complicated, depending on its volume fraction and prevailing slip system. Most of the mica‐ or amphibole‐bearing schists and mylonites are approximately transversely isotropic in terms of S wave velocities and splitting although their P wave properties may display orthorhombic symmetry. The results provide insight for the interpretation of seismic data from the southeast Tibetan Plateau. The N‐S to NW‐SE polarized crustal anisotropy in the Sibumasu and Indochina blocks is caused by subvertically foliated mica‐ and amphibole‐bearing rocks deformed by predominantly compressional folding and subordinate strike‐slip shear. These blocks have been rotated clockwise 70–90° around the east Himalayan Syntaxis, without finite eastward or southeastward extrusion, in responding to progressive indentation of India into Asia.
Key Points
Departure of schists from transverse isotropy by amphibole, sillimanite, and quartz
Decoupling between crust and upper mantle beneath the southeast Tibetan Plateau
No southeastward extrusion for the Sibumasu and Indochina blocks since 45 Ma
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