Experimental investigation of the stability of clinopyroxene in mid-ocean ridge basalts: The role of Cr and Ca/Al

Lithos ◽  
2017 ◽  
Vol 274-275 ◽  
pp. 240-253 ◽  
Author(s):  
Martin Voigt ◽  
Laurence A. Coogan ◽  
Anette von der Handt
Keyword(s):  
Experimental Investigation ◽  
Mid Ocean Ridge ◽  
The Stability ◽  
Ocean Ridge

New  podiform chromitites Occurrence from the Masirah Ophiolite, Oman

2021 ◽  
Author(s):  
Sobhi Nasir
Keyword(s):  
Tectonic Setting ◽  
Shear Bands ◽  
Mineral Inclusions ◽  
Silicate Inclusions ◽  
Mid Ocean Ridge ◽  
Early Paleocene ◽  
Podiform Chromitites ◽  
Ocean Ridge ◽  
Mantle Section

<p>The Masirah ophiolite is one of the few true ocean ridge ophiolites that have been preserved (Rollinson, 2017) and lacks any indication that it formed in a subduction environment. The Masirah ophiolite in south-eastern Oman is a different and older ophiolite from the more famous northern Oman ophiolite. Chromite and copper ores comprise large deposits in the Samail ophiolite, northern Oman. In comparison, chromite and copper deposits have not been described in previous reports or previous exploration in Masirah ophiolite. Rollinson (2017) has proposed that the apparent absence of chromitites in the mantle section of Masirah ophiolite is an important discriminant between subduction related and ocean ridge ophiolites.  However, during recent studies on the Batain ophiolite mélange, and Masirah ophiolite, several chromitite pods have been discovered. The chromitites occur as separated small concordant, lenticular pods (3–10 m in thickness), which have been extensively altered and deformed, with the host pyroxenite serpentinites serpentinized harzburgites and dunites. The largest chromitite pods found within the pyroxenite and dunite of Masirah are up to 10 m across.  Unusual minerals and mineral inclusions (orthopyroxene, clinopyroxene, amphibole, phlogopite, serpentine, native Fe, FeO, alloy, sulfide, calcite, laurite, celestine and halite) within chromite have been observed in the chromitites from the  Masirah ophiolites.  The existence of hydrous silicate inclusions in the chromite calls for a role of hydration during chromite genesis. Both  phlogopite and hornblende were possibly formed from alkali-rich hydrous fluids/melts trapped within the chromite during the chromitite formation. High-T green hornblende and phlogopite included in the chromites is evidence of the introduction of water in the magma at the end of the chromite crystallization. Such paragenesis points to the presence of hydrous fluids during the activity of the shear bands. The chromitites parental magmas are rich in K, Na, LREE, B, Cs, Pb, Sr, Li, Rb and U relative to HREE, reflecting the alkalic fluids/melts that prevailed during the chromitites genesis.</p><p>The mineral inclusions  in association with host peridotites may have been brought by the uprising asthenosphere at mid-oceanic ridges due to the mantle convection. It appears that this chromite has been formed through reaction between amid-ocean-ridge basalt-melt with depleted harzburgite in the uppermost mantle.  The chromitite deposits have similar cr# (55-62% Al-chromitites), mg# Al2O3 and TiO2 contents to spinels found in MORB, and have been interpreted as having formed in amid-ocean ridge setting.  This suggests that this chromitites is residual from lower degree, partial melting of peridotite, which produced low-Cr# chromitites at the Moho transition zone, possibly in a mid-ocean-ridge setting. The chemistry of both mineral inclusions and chromite   suggests MORB-related tectonic setting for the chromitites that were crystallized at 1000 °C–1300 °C under pressures <3 GPa . The host peridotites were generated during the proto-Indian Ocean MORB extension and emplaced as a result of the obduction of the ophiolite over the Oman Continental margin during Late Cretaceous-Early Paleocene.</p><p>Rollinson, H., 2017. Geoscience Frontiers, 8: 1253–1262.</p>

scholarly journals Fractional crystallization causes the iron isotope contrast between mid-ocean ridge basalts and abyssal peridotites

2021 ◽  
Vol 2 (1) ◽  
Author(s):  
Yanhong Chen ◽  
Yaoling Niu ◽  
Meng Duan ◽  
Hongmei Gong ◽  
Pengyuan Guo
Keyword(s):  
Fractional Crystallization ◽  
Trace Element Geochemistry ◽  
Iron Isotope ◽  
Element Geochemistry ◽  
Mid Ocean Ridge ◽  
Atlantis Bank ◽  
The Indian Ocean ◽  
Ocean Ridge ◽  
Melt Evolution

AbstractThe iron isotope contrast between mid-ocean ridge basalts and abyssal peridotites is far greater than can be explained by mantle melting alone. Here we investigate a suite of mid-ocean ridge magma chamber rocks sampled by the Ocean Drilling Project Hole 735B in the Atlantis Bank of the Indian Ocean. We report major and trace element geochemistry from these rocks and measure their iron isotope compositions to investigate the potential role of fractional crystallization during melt evolution. We observe a large range of δ56Fe that defines a significant inverse curvilinear correlation with bulk rock MgO/FeOT. These data confirm that δ56Fe in the melt increases as fractional crystallization proceeds but, contrary to expectation, δ56Fe continues to increase even when oxides begin to crystallize. We conclude that iron isotope fractionation through fractional crystallization during the evolution of mid-ocean ridge basalts from abyssal peridotites reconciles the disparity in isotopic compositions between these two lithologies.

The hafnium paradox and the role of garnet in the source of mid-ocean-ridge basalts

Nature ◽  
1989 ◽  
Vol 342 (6248) ◽  
pp. 420-422 ◽  
Author(s):  
Vincent J. M. Salters ◽  
Stanley R. Hart
Keyword(s):  
Mid Ocean Ridge ◽  
Ocean Ridge

scholarly journals Experimental Study of the Behavior of Rectangular Excavations Supported by a Pipe Roof

2019 ◽  
Vol 9 (10) ◽  
pp. 2082 ◽  
Author(s):  
Xiongyao Xie ◽  
Mingrui Zhao ◽  
Isam Shahrour
Keyword(s):  
Experimental Study ◽  
Experimental Investigation ◽  
Experimental Tests ◽  
Soil Pressure ◽  
Sand Soil ◽  
Soil Settlement ◽  
Effective Role ◽  
The Stability ◽  
Pressure Analysis

This paper presents an experimental investigation of the role of pipe-roofs in the improvement of the stability of rectangular excavations constructed using pipe-roof technology. This technology is suitable for the construction of underground passages in crowded areas subjected to high requirements concerning soil settlement and stability during excavation construction. The design of a rectangular pipe-roof excavation required an understanding of the interaction between the soil, the pipe-roof and the excavation. This interaction is complex and plays an important role in the features of the pipe roof excavation. This paper presents a series of 1g physical experimental tests conducted in dry sand soil with an advanced monitoring system, which allows tracking of the soil settlement, the pipe deformation and the soil pressure. Analysis of these tests shows the effective role of the pipe-roof in reducing both the soil settlement and the soil pressure on the excavation. It also shows the influence of pipes on the deformation mechanism of the soil and its evolution from low deformation to the instability phase.

Petrological Implications of Seafloor Hydrothermal Alteration of Subducted Mid-Ocean Ridge Basalt

2020 ◽  
Author(s):  
David Hernández-Uribe ◽  
Richard M Palin ◽  
Kim A Cone ◽  
Wentao Cao
Keyword(s):  
Oceanic Crust ◽  
Hydrothermal Alteration ◽  
Phase Changes ◽  
Common Mechanism ◽  
Ridge Basalt ◽  
Mid Ocean Ridge ◽  
Subducted Oceanic Crust ◽  
Different Depths ◽  
The Stability ◽  
Ocean Ridge

Abstract Determining the mineralogical changes occurring in subducted oceanic crust is key to understanding short- and long-term geochemical cycles. Although numerous studies have explored the mineral assemblages that form in mid-ocean ridge basalt (MORB) at different depths below the Earth’s surface, it is widely recognized that seafloor hydrothermal alteration of the uppermost portion of the oceanic crust can change its composition between a ridge and a trench prior to subduction. In this study, we use petrological modelling to explore the effects of different types of pre-subduction hydrothermal alteration on the phase changes that occur during seafloor alteration of MORB-like compositions during subduction along an average Phanerozoic geotherm. We consider a representative composition of altered oceanic crust, as well as extreme end-member scenarios (pervasive spilitization, chloritization, and epidotization). Our models show that epidotization and chloritization of MORB strongly affects phase equilibria at different depths, whereas spilitization and an average style of alteration produce relatively fewer changes on the mineral assemblage to those expected in a pristine MORB. Devolatilization of MORB during subduction occurs mostly in the forearc region, although the type and extent of alteration strongly control the depth and magnitude of fluid released. Altered compositions carry significantly more H2O to sub- and postarc depths than unaltered compositions; the H2O carrying capacity of unaltered and altered compositions is further enhanced during subduction along colder geotherms. Extremely localized areas affected by epidotization can transport up to 22 times more H2O than unaltered MORB and up to two times more than average altered oceanic crust compositions to depths beyond the arc. Regardless of the extent and style of alteration, the stability of hydrous phases, such as epidote and phengite (important trace element carriers), is expanded to greater pressure and temperature conditions. Thus, hydrothermal alteration of the subducted oceanic slab-top represents a viable, and probably common, mechanism that enhances geochemical recycling between the Earth’s hydrosphere and shallow interior.

Mantle Plume – Spreading Ridge Interaction: A Comparative Study of the Red Sea and Reykjanes Ridges

2020 ◽  
Author(s):  
Zachary Molitor ◽  
Oliver Jagoutz ◽  
Leigh Royden ◽  
Stephanie Brown ◽  
Guido Port ◽  
...  
Keyword(s):  
Melting Temperature ◽  
Red Sea ◽  
Dynamic Pressure ◽  
Spreading Ridge ◽  
Mantle Plumes ◽  
Reykjanes Ridge ◽  
Mid Ocean Ridge ◽  
Mantle Temperature ◽  
Ocean Ridge

<p>As a young, mid ocean ridge, the Red Sea is a unique natural laboratory for studying the processes that drive continental rifting and breakup. The role of hot spots, frequently attributed to mantle plumes, in triggering or driving breakup and their impact on crustal structure and topography is not well understood. We have found that the Red Sea ridge bears a resemblance to the Reykjanes ridge in terms of bathymetry, morphology, geophysical properties, basalt chemistry, and modelled melting temperature and pressure of primary basalts. The results of modelling basalt melting temperature call into question the role of mantle temperature on generating excess melt beneath the Red Sea and Reykjanes ridges. Within 300 kilometers of a hotspot center, determined by seismic tomography, mantle excess temperatures are as high as 300 degrees relative to an ambient mantle temperature of about 1300 C. Outside of this radius excess temperatures are not significant (less than 50 C), and unlikely to cause significant melting anomalies. It is likely that the southern Red Sea and northern Reykjanes ridge are directly affected by hot, buoyant upwelling from the Afar and Iceland mantle plumes, and the central Red Sea and southern Reykjanes ridge may be affected by dynamic pressure related to actively upwelling mantle around the mantle plumes.</p>

Role of ancient, ultra-depleted mantle in Mid-Ocean-Ridge magmatism

2019 ◽  
Vol 511 ◽  
pp. 89-98 ◽  
Author(s):  
A. Sanfilippo ◽  
V. Salters ◽  
R. Tribuzio ◽  
A. Zanetti
Keyword(s):  
Depleted Mantle ◽  
Mid Ocean Ridge ◽  
Ocean Ridge
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