scholarly journals A Role for Subducted Oceanic Crust in Generating the Depleted Mid‐Ocean Ridge Basalt Mantle

2020 ◽  
Vol 21 (8) ◽  
Author(s):  
Jonathan M. Tucker ◽  
Peter E. Keken ◽  
Rosemary E. Jones ◽  
Chris J. Ballentine
Keyword(s):  
Oceanic Crust ◽  
Ridge Basalt ◽  
Mid Ocean Ridge ◽  
Subducted Oceanic Crust ◽  
Ocean Ridge

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.

scholarly journals Reaction Between Mid‐Ocean Ridge Basalt and Lower Oceanic Crust: An Experimental Study

2019 ◽  
Vol 20 (9) ◽  
pp. 4390-4407 ◽  
Author(s):  
Alexandra Yang Yang ◽  
Chunguang Wang ◽  
Yan Liang ◽  
C. Johan Lissenberg
Keyword(s):  
Experimental Study ◽  
Oceanic Crust ◽  
Ridge Basalt ◽  
Mid Ocean Ridge ◽  
Ocean Ridge ◽  
Lower Oceanic Crust

scholarly journals Adakitic high-Al trondhjemites in the Proterozoic Østfold-Marstrand Belt, W Sweden

1999 ◽  
Vol 46 ◽  
pp. 165-179
Author(s):  
Bjørn Hageskov ◽  
Bente Mørch
Keyword(s):  
Oceanic Crust ◽  
Melt Extraction ◽  
Arc Magmas ◽  
Mid Ocean Ridge ◽  
Subducted Oceanic Crust ◽  
Deep Crustal Level ◽  
Partial Melts ◽  
Se Norway ◽  
Ocean Ridge ◽  
Orogenic Events

This paper investigates the first identified intrusives in SE Norway–W Sweden with the specific signature of adakitic arc magmas, which in recent settings are preferably explained as partial melts extracted from subducted oceanic crust. The studied adakitic high–Al trondhjemites occur as sheets in the Koster archipelago, W Sweden, where they form the oldest recognized granitoids in the metasupracrustals of the Stora Le–Marstrand formation. The trondhjemites were intruded during a short ca. 1.59–1.58 Ga interlude between the early and the main orogenic events of the Gothian orogeny (1.6–1.56 Ga, Åhäll et al. 1998). This interlude is otherwise characterized by ‘ordinary’ calcalkaline magmatism which on Koster is predated by the trondhjemites. The typical adakitic signature suggests that the trondhjemitic magma was extracted from a MORB (Mid Ocean Ridge Basalt) like source, and that a hornblende eclogite restite was left in the region of melting. The restite composition indicates melt extraction at PT conditions in the range of 18–25 kb/800°C to 13-15 kb/950–1050°C. These requirement can only be met by subduction of warm (young or shear heated) oceanic crust beneath a crust including early Gothian metamorphosed and deformed Stora Le–Marstrand formation or by melting of metabasaltic material at a deep crustal level. The latter is a less likely possibility and demands that the Stora Le–Marstrand formation at the time of melt extraction was part of a > 45 km thick crust.

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