Subduction initiation-induced rapid emplacement of garnet-bearing peridotites at a nascent forearc: Petrological and Os-Li isotopic evidence from the Purang ophiolite, Tibet

2021 ◽  
Author(s):  
Xiao-Han Gong ◽  
Ji-Feng Xu ◽  
Ren-Deng Shi ◽  
Ben-Xun Su ◽  
Qi-Shuai Huang ◽  
...  
Keyword(s):  
Oceanic Lithosphere ◽  
Garnet Peridotite ◽  
Chemical Compositions ◽  
Close Link ◽  
Geological Evidence ◽  
Subduction Initiation ◽  
Mid Ocean Ridge ◽  
Rapid Ascent ◽  
Melt Percolation ◽  
Ocean Ridge

Garnet-bearing peridotites commonly occur in the deeper parts of mature or thickened oceanic lithosphere, and are rarely exhumed and emplaced onto the seafloor. The Purang ophiolitic peridotites in south Tibet contain rare symplectite pseudomorphs after garnet, offering a unique window into the still poorly understood evolution of the deep oceanic lithosphere. Here, integrated petrologic and Os-Li isotopic data are used to constrain the evolution and dynamics of emplacement for these garnet peridotite protoliths. The Purang peridotites show wide variations of chemical compositions (spinel Cr#: 0.2−0.8) and Os model ages (up to 2.0 Ga), thus representing a piece of heterogeneous oceanic mantle lithosphere. Dunite channels show two distinctive groups of Cr# of spinels and Os-isotope compositions, with the low- to medium-Cr# (0.2−0.6) and high-Cr# (0.7−0.8) dunites reflecting the reaction of host lherzolites/harzburgites with percolating mid-ocean ridge basalt−like and boninitic melts, respectively. This confirms recent subduction initiation-related melt percolation in the Purang peridotites. Coexisting olivines and pyroxenes in the peridotites show systematic Li elemental and isotopic disequilibrium, suggesting fast cooling of the peridotites to Li closure temperature shortly after the melt percolations, likely during exhumation of the peridotites onto the seafloor. This supports a close link between subduction initiation and tectonic emplacement of the Purang peridotites. Combined with other geological evidence, we suggest the Purang peridotites may originate from the deep part of old, thick oceanic lithosphere of the Neo-Tethys. This thick oceanic lithosphere was progressively weakened and thinned likely during widespread plume-lithosphere interaction, triggering the transformation of garnet peridotite protoliths to spinel peridotites. Subsequently, initiation of a new subduction zone along the lithospheric weakness caused rapid ascent and emplacement of the Purang peridotites at a nascent forearc.

Fluids in Submarine Mid-Ocean Ridge Hydrothermal Settings

Elements ◽  
2020 ◽  
Vol 16 (6) ◽  
pp. 389-394
Author(s):  
Esther M. Schwarzenbach ◽  
Matthew Steele-MacInnis
Keyword(s):  
Heat Budget ◽  
Oceanic Lithosphere ◽  
Life Forms ◽  
Hydrothermal Systems ◽  
Hydrothermal Fluids ◽  
Early Earth ◽  
Geologic Time ◽  
Mid Ocean Ridge ◽  
Time Changes ◽  
Ocean Ridge

Seawater interaction with the oceanic lithosphere crucially impacts on global geochemical cycles, controls ocean chemistry over geologic time, changes the petrophysical properties of the oceanic lithosphere, and regulates the global heat budget. Extensive seawater circulation is expressed near oceanic ridges by the venting of hydrothermal fluids through chimney structures. These vent fluids vary greatly in chemistry, from the metal-rich, acidic fluids that emanate from “black smokers” at temperatures up to 400 °C to the metal-poor, highly alkaline and reducing fluids that issue from the carbonate–brucite chimneys of ultramafic-hosted systems at temperatures below 110 °C. Mid-ocean ridge hydrothermal systems not only generate signifi-cant metal resources but also host unique life forms that may be similar to those of early Earth.

The conversion tectonics from spreading to subduction: Paleostress analysis of dike swarms during the subduction initiation in the Oman Ophiolite

2019 ◽  
Vol 132 (5-6) ◽  
pp. 1333-1343 ◽  
Author(s):  
Susumu Umino ◽  
Yuki Kusano ◽  
Atsushi Yamaji ◽  
Takahiro Fudai ◽  
Akihiro Tamura ◽  
...  
Keyword(s):  
Stress Field ◽  
Clockwise Rotation ◽  
Oman Ophiolite ◽  
Subduction Initiation ◽  
Mid Ocean Ridge ◽  
Abrupt Changes ◽  
Rift Zones ◽  
Dike Swarms ◽  
Ocean Ridge ◽  
Paleostress Analysis

Abstract We present paleostress analyses of dike swarms intruded during the subduction initiation in the northern Oman Ophiolite to understand the tectonomagmatic environment. Five swarms of subparallel dikes extending WNW-ESE are 1–5 km in width and are spaced every 5 km N-S. Each swarm has a core of 100% sheeted dikes 1–2 km in width, which emanated from the dunite-wherlite-clinopyroxenite-gabbronorite-diorite-tonalite complexes below and intruded through V1 and into V2 extrusive rocks. Individual dike strikes are varied but generally subparallel to the overall trend of the swarm. Paleostress analyses indicate subvertical σ1, ∼σ2, and subhorizontal σ3 with high magma pressures, resulted in the mutually intrusive, extensional shear dikes and abrupt changes in dike strike at high angles. These occurrences suggest intrusions under a more compressive environment compared to the extensional stress field that formed the N-S–striking sheeted dikes of V1 spreading stage. Most E-W–striking dikes possess both boninitic and tholeiitic geochemistry. The latter resemble the V1 flows and dikes with affinities of mid-ocean ridge basalt. Some tholeiitic dikes strike N-S, which are mutually intrusive to E-W–striking dikes. Tholeiitic dikes are more intensely altered than boninite, suggesting their older ages. Conversion of the stress field from a N-S–running spreading axis to inextensional E-W–running rift zones associated with the change in magma geochemistry agree with the relatively compressive V2 arc above a forced subduction zone, which originated from intraoceanic thrusting caused by the clockwise rotation of a microplate including the future northern ophiolite.

scholarly journals Abiotic hydrogen (H2) sources and sinks near the Mid-Ocean Ridge (MOR) with implications for the subseafloor biosphere

2020 ◽  
Vol 117 (24) ◽  
pp. 13283-13293 ◽  
Author(s):  
Stacey L. Worman ◽  
Lincoln F. Pratson ◽  
Jeffrey A. Karson ◽  
William H. Schlesinger
Keyword(s):  
Control Volume ◽  
Preliminary Investigation ◽  
Oceanic Lithosphere ◽  
Analytical Framework ◽  
Ocean Crust ◽  
Global Estimate ◽  
Mid Ocean Ridge ◽  
Free Hydrogen ◽  
Microbial Consumption ◽  
Ocean Ridge

Free hydrogen (H2) is a basal energy source underlying chemosynthetic activity within igneous ocean crust. In an attempt to systematically account for all H2within young oceanic lithosphere (<10 Ma) near the Mid-Ocean Ridge (MOR), we construct a box model of this environment. Within this control volume, we assess abiotic H2sources (∼6 × 1012mol H2/y) and sinks (∼4 × 1012mol H2/y) and then attribute the net difference (∼2 × 1012mol H2/y) to microbial consumption in order to balance the H2budget. Despite poorly constrained details and large uncertainties, our analytical framework allows us to synthesize a vast body of pertinent but currently disparate information in order to propose an initial global estimate for microbial H2consumption within young ocean crust that is tractable and can be iteratively improved upon as new data and studies become available. Our preliminary investigation suggests that microbes beneath the MOR may be consuming a sizeable portion (at least ∼30%) of all produced H2, supporting the widely held notion that subseafloor microbes voraciously consume H2and play a fundamental role in the geochemistry of Earth’s ocean–atmosphere system.

4. Subduction of oceanic lithosphere

2015 ◽  
pp. 53-76
Author(s):  
Peter Molnar
Keyword(s):  
Oceanic Lithosphere ◽  
Great Depth ◽  
Continental Margins ◽  
Island Arcs ◽  
The Andes ◽  
Mid Ocean Ridge ◽  
Subducting Plate ◽  
Ocean Ridge ◽  
Large Earthquakes ◽  
Adjacent Rock

‘Subduction of oceanic lithosphere’ begins with the notion that for the Earth not to expand, the sum total of new lithosphere made at a spreading centre (or mid-ocean ridge) must be matched by the removal, by subduction, of an equal amount of lithosphere elsewhere. The subduction process is asymmetric: one plate will slide beneath the other at island arcs and continental margins like the Andes of South America. Before it plunges beneath the island arc, the subducting plate of lithosphere bends down gently to cause a deep-sea trench. The subducting plate slides beneath the region between the trench and volcanoes, commonly in large earthquakes, and plunges to great depth, pulled down by gravity acting on the dense slab of subducted lithosphere. Water carried to depth by the subducting plate lowers the melting temperature of the adjacent rock and enables volcanoes to form.

The Othris Ophiolite, Greece: A snapshot of subduction initiation at a mid-ocean ridge

Lithos ◽  
2008 ◽  
Vol 100 (1-4) ◽  
pp. 234-254 ◽  
Author(s):  
Matthias G. Barth ◽  
Paul R.D. Mason ◽  
Gareth R. Davies ◽  
Martyn R. Drury
Keyword(s):  
Subduction Initiation ◽  
Mid Ocean Ridge ◽  
Ocean Ridge

scholarly journals Variations of Chemical Compositions of Mid-ocean Ridge Basalts (MORB) and their Origin

2008 ◽  
Vol 117 (1) ◽  
pp. 124-145 ◽  
Author(s):  
Hiroshi SATO ◽  
Hidenori KUMAGAI ◽  
Natsuki NEO ◽  
Kentaro NAKAMURA
Keyword(s):  
Chemical Compositions ◽  
Mid Ocean Ridge ◽  
Ocean Ridge

scholarly journals Global variation of seismic energy release with oceanic lithosphere age

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Nicolás Pinzón ◽  
Carlos A. Vargas
Keyword(s):  
Cross Sections ◽  
Thermal Evolution ◽  
Seismic Energy ◽  
Oceanic Lithosphere ◽  
Physical Factors ◽  
Additional Tool ◽  
Mid Ocean Ridge ◽  
Mid Atlantic Ridge ◽  
Spreading Rates ◽  
Ocean Ridge

AbstractVariations in Mid Ocean Ridge seismicity with age provide a new tool to understand the thermal evolution of the oceanic lithosphere. The sum of seismic energy released by earthquakes during a time, and for an area, is proportional to its lithospheric age. Asthenospheric temperatures emerge on ridge centers with new crust resulting in high seismic activity; thus, the energy released sum is highest on the young lithosphere and decreases with age. We propose a general model that relates the systematic variation of seismic energy released with the lithospheric age. Our analysis evaluates the main physical factors involved in the changes of energy released sum with the oceanic lithosphere age in MOR systems of different spreading rates. These observations are substantiated based on three cross-sections of the East Pacific Rise, six sections in the Mid Atlantic Ridge, and three profiles in the Central Indian Ridge. Our global model provides an additional tool for understanding tectonic processes, including the effects of seismicity and mid-plate volcanism, and a better understanding of the thermal evolution for the young oceanic lithosphere.

Geochemistry and tectonic significance of the Mesoproterozoic Kgwebe metavolcanic rocks in northwest Botswana: implications for the evolution of the Kibaran Namaqua-Natal Belt

1998 ◽  
Vol 135 (5) ◽  
pp. 669-683 ◽  
Author(s):  
A. B. KAMPUNZU ◽  
P. AKANYANG ◽  
R. B. M. MAPEO ◽  
B. N. MODIE ◽  
M. WENDORFF
Keyword(s):  
Chemical Composition ◽  
Lower Crust ◽  
Chemical Compositions ◽  
Mafic Rocks ◽  
Metavolcanic Rocks ◽  
High K ◽  
Mid Ocean Ridge ◽  
Tectonic Significance ◽  
Ocean Ridge ◽  
Subducting Slab

The c. 1.1 Ga Kgwebe metavolcanic rocks exposed in the northwest of Botswana are late Kibaran rocks. They represent a bimodal suite of Within-Plate low titanium-phosphorus (LTP) continental tholeiites and post-orogenic Within-Plate high-K rhyolites. The chemical compositions of the Kgwebe mafic rocks are characterized by low values of Ce/Pb (<10) and high La/Nb ratios (average c. 2, maximum 4). Mid-ocean ridge basalts (MORB)-normalized spidergrams show marked enrichment in mobile elements (Sr, K, Rb, Ba) and negative anomalies in Nb. These features suggest they may have originated in a mantle, enriched during a previous subduction event. The Kgwebe metarhyolites are marked by Y>60 ppm, Sr/Y<1, Rb/Th>20 and high K-contents. They cannot therefore be the product of melting of sediments or a subducting slab. It is inferred that they represent felsic magmas resulting from melting of Mesoproterozoic (Kibaran) calcalkaline rocks underplated in the middle and/or lower crust. The Kgwebe bimodal metavolcanic rocks pre-date the Neoproterozoic Ghanzi Group rocks which are correlated with the lower part of the Damara sequence. The chemical composition and field relations suggest that these metavolcanic rocks were emplaced during a late orogenic collision-associated extensional collapse. This collapse affected a crust thickened during the Kibaran orogeny in the Namaqua-Natal Belt of southwest Africa.

scholarly journals Recycling of Marine Carbonate Induced Calcium Isotope Heterogeneity of Arc Magmas in Subduction Zones

2021 ◽  
Author(s):  
Xue-Gang Chen ◽  
Tao Wu ◽  
Qin Gao ◽  
Yu-Ming Lai
Keyword(s):  
Subduction Zones ◽  
Essential Element ◽  
Chemical Compositions ◽  
Carbon Cycles ◽  
Arc Magmas ◽  
Marine Carbonate ◽  
Depleted Mantle ◽  
Mid Ocean Ridge ◽  
Sedimentary Carbonate ◽  
Ocean Ridge

Calcium (Ca) is an essential element constituting sedimentary carbonate in subducting sediments. Ca isotopic characteristics of subduction-related rocks could provide insight into the behavior and budget of carbonate and carbon cycles in subduction zones, due to the distinctive &delta;44/40Ca ranges of sedimentary carbonate with respect to the mantle. Here, we studied the Ca isotopic compositions of arc magmas from the Northern Luzon arc (NLA), which are evolved from a depleted mantle metasomatized by slab-derived fluids and sediment melts. The &delta;44/40Ca values range from 0.76 &plusmn; 0.04&permil; to 1.01 &plusmn; 0.03&permil; and cover the typical ranges for bulk silica earth (BSE, ~ 0.94&permil;) and fresh mid-ocean ridge basalt (MORB, ~ 0.83&permil;). The Ca isotopes of NLA volcanics are not dominantly determined by the effects of mantle partial melting or fractional crystallization, nor significantly modified by secondary alteration. Instead, the &delta;44/40Ca values of NLA volcanics are controlled by the subduction-related metasomatism. The metasomatism by slab-derived fluids (mainly expelled from altered oceanic crust, AOC) dramatically elevated the contents of fluid-mobile elements (e.g., Ba and Pb) with respect to fluid-immobile elements (e.g., Ce). This process, however, rarely modified the Ca isotopes, possibly ascribed to the &delta;44/40Ca similarity between AOC and the depleted mantle. The &delta;44/40Ca values significantly correlated with subduction indicators (e.g., Sr-Nd isotopes, Ba/Nb, Ce/Pb, and Nb/La), demonstrating the Ca isotopes of NLA volcanics are mainly controlled by the metasomatism of sediment melts subducting from the South China Sea (SCS). Based on the thermal structures and chemical compositions of sediments subducting into global trenches, we propose that carbonate Ca isotopic signals can only be observed in the arcs with high sedimentary Ca fluxes and temperature-pressure conditions well beyond the solidus of H2O-saturated sediment melting, e.g., NLA, Nicaragua, Guatemala, Colombia, Peru, South Chile, North Vanuatu, New Zealand, and Kermadec. The absence of such signals in other arcs suggests either limited sedimentary fluxes or much of the subducting sedimentary carbonate has been survived during plate subduction to enter the deep mantle.

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