Nitrogen recycling in subduction zones: A strong geothermal control
4
Citation
0
Reference
20
Related Paper
Citation Trend
Abstract:
Although several studies already dealt with N recycling in subduction zones, controversy still persists about the amount of N actually recycled to the deep mantle. From the study of fumaroles and hot springs in the Central America, Fischer et al. (2002) showed that subducted N can be efficiently transferred to the surface via arc volcanism and concluded that N is not recycled to the deep mantle. In contrast, occurrence of high amount of N in metamorphic microdiamonds from Kokchetav massif (Kazakhstan) indicates that it can be subducted to ultrahigh-pressures (Cartigny et al., 2001). The comparison between three sequences of subducted metasediments also demonstrates different behaviours of nitrogen during subduction. In the Catalina Schists (California) and the Erzgebirge Schists (Germany), N content decrease and δN increase with increasing metamorphic conditions indicate that N was strongly affected by devolatilization processes during subduction (Bebout and Fogel, 1992; Mingram and Brauer, 2001). On the contrary, a study of the Schistes Lustres metasediments (western Italian Alps) showed that N (together with other fluid-mobile elements, K, Rb, Cs, H) was entirely preserved during subduction down to 90 km depths (Busigny et al., 2003). All of these results can be reconciled if the thermal structure of the subduction zone is considered. While N is dramatically devolatilized in “warm” subduction zones, it can be deeply recycled in “cold” slab environment, which is the case of most current subduction zones. An important implication of this work concerns the evolution of N recycling through geological times. Because the thermal regime of the early Earth was hotter than today, the N recycling was certainly less efficient during this time. N was likely devolatilized and fractionated, producing an increase of the δN value of the remaining recycled nitrogen.Keywords:
Massif
Slab
Blueschist
Cite
Abstract Subduction zones are tectonic expressions of convergent plate margins, where crustal rocks descend into and interact with the overlying mantle wedge. They are the geodynamic system that produces mafic arc volcanics above oceanic subduction zones but high- to ultrahigh-pressure metamorphic rocks in continental subduction zones. While the metamorphic rocks provide petrological records of orogenic processes when descending crustal rocks undergo dehydration and anataxis at forearc to subarc depths beneath the mantle wedge, the arc volcanics provide geochemical records of the mass transfer from the subducting slab to the mantle wedge in this period though the mantle wedge becomes partially melted at a later time. Whereas the mantle wedge overlying the subducting oceanic slab is of asthenospheric origin, that overlying the descending continental slab is of lithospheric origin, being ancient beneath cratons but juvenile beneath marginal arcs. In either case, the mantle wedge base is cooled down during the slab–wedge coupled subduction. Metamorphic dehydration is prominent during subduction of crustal rocks, giving rise to aqueous solutions that are enriched in fluid-mobile incompatible elements. Once the subducting slab is decoupled from the mantle wedge, the slab–mantle interface is heated by lateral incursion of the asthenospheric mantle to allow dehydration melting of rocks in the descending slab surface and the metasomatized mantle wedge base, respectively. Therefore, the tectonic regime of subduction zones changes in both time and space with respect to their structures, inputs, processes and products. Ophiolites record the tectonic conversion from seafloor spreading to oceanic subduction beneath continental margin, whereas ultrahigh-temperature metamorphic events mark the tectonic conversion from compression to extension in orogens.
Forearc
Slab
Volcanic arc
Crustal recycling
Cite
Citations (230)
Understanding the exchange of volatiles between mantle, crust and atmosphere represents a central
issue of terrestrial geodynamics. Studies of uplifted tracts of subduction related metamorphic rocks
can provide detailed information regarding the extent of loss of volatile and fluid-mobile elements
during prograde metamorphism. Studies within SFB 574 have shown that in many cases fluid flow is
channelized rather than penetrative. The study of vein systems and metasomatic reaction zones is
therefore the key to understand fluid-related processes and mass transfer in subduction zones, as
they track pathways of migrating fluids and record fluid compositions. These processes were studied
in distinct parts of the subduction zones, representing (i) an accretionary wedge (e.g., Coastal
Cordillera Accretionary Complex, Chile), (ii) the subduction melange (e.g., Cyclades, Greece;
Tianshan, China) and (iii) the subducted oceanic slab with serpentinized lithospheric mantle, highpressure
metamorphic oceanic crust and sediments (e.g., Raspas Complex, Ecuador; Bantimala
Complex, Sulawesi; Zambesi Belt, Zambia). Case studies of all three mentioned parts of subduction
zones will be presented.
Furthermore, the role and evolution of fluids released during subduction were experimentally studied.
Instead of conducting single point analysis at a given P/T or single crystal analyses, to derive
partitioning data, we tracked a more dynamic approach and investigated the evolution, and thus the
change in the composition of basalts, gabbros and their respective fluids, when moving down along
the subducting slab.
Accretionary wedge
Forearc
Cite
Citations (0)
Forearc
Continental Margin
Cite
Citations (0)
Heat flow
Descent (aeronautics)
Cite
Citations (7)
Adakite
Eclogitization
Cite
Citations (154)
二个变形过程，即潜水艇先令脱水并且发生在 MORB，元沉积和潜水艇 ducted 的橄榄岩的部分融化海洋的岩石圈根据当模特儿的可得到的试验性的工作和阶段平衡被讨论。在很冷的 subduction 磅(压力温度)政体，在基本的层的水的大部分在蓝片岩外形的发作下面释放了的含水的 MORB 表演的相图(< 20 km )，并且在到通过 glaucophane 脱水的 lawsonite eclogite 外形的从 lawsonite 蓝色片岩的转变的深度( 60 70 km )；仅仅水的更小的部分将在对弧岩浆形成合适的深度范围通过 lawsonite 和硬绿泥石的脱水逃离平板；并且在 lawsonite 和 phengite 存储的水的很小的部分将褪色进更深的披风。为弧岩浆形成的闪石的角色仍然是可争辩的。在在在深度的 Al 富有的 metapelites 的冷 subduction 磅政体，在 Al 差的元沉积的绿泥石和滑石的脱水，和硬绿泥石和 carpholite，约 80 100 km 将做一些贡献到弧岩浆的形成。比较地，脱水在含水的橄榄岩蛇发生在 120 180 km 的深度，在弧 magmatism 起一个重要作用。沿着温暖的磅政体的海洋的外壳的 Subduction 将在 80 km 上在深度穿过稳固的 i，导致在在包含黑云母和 phengite 的元沉积，并且在包含绿帘石和闪石的碱性岩的浸透液体、液体不在的条件下面的部分融化。基本外壳的 melt 作文是在压力 < 的 adakitic 3.0 GPa，但是变得每铝土在更高的压力花岗石。
Lawsonite
Phengite
Glaucophane
Cite
Citations (0)
Wedge (geometry)
Hotspot (geology)
Cite
Citations (10)
Eclogitization
Adakite
Volcanic arc
Forearc
Lawsonite
Underplating
Cite
Citations (223)
The petrological research on the ultra high pressure metamorphism (UHP) of collisional orogen indicates that the upper crustal rocks is subducted to depths exceeding 100 km, and returned to the surface rapidly. In this study, we investigate the thermal structure of collisional orogen as a slab of continental lithosphere being subducted beneath an overriding wedge of continental lithosphere by the 2 D finite element method. The advection heat transfer due to the accretion of orogenic wedge is considered. The wedge is composed of the upper crust materials through the accretion from the down going plate to the upper plate. For identifying the significance of the geometric and/or kinetic factors on the thermal structure of continental subduction, the different combinations of parameters, including dip angle of subduction zone, accretion or erosion rates, and the convergence velocity etc., are used in modelling. The time span of continental subduction in our calculation is less than 30 Ma, according to the short duration of ultra deep subduction of continental slab suggested by the preservation of metastable pre peak low pressure mineralogy assemblage in the garnet of UHP rocks. Therefore, the steep dip angle of down going plate and/or low rate of accretion favour the ultra deep subduction of upper crust materials, especially for the slower down going slab. Meanwhile, taking the erosion rate as the level of exhumation rate of UHP rocks in some orogens (i.e., 1-2 km/Ma or more) does not result in the anatexis melting of crust of the overriding plate, due to the cooling effect of the rapid down going slab. However, the temperature structures of all models are generally cooler than those recovered by thermobarometric studies of the UHP rocks. This implies the significant increase of temperature after the rapid subduction of continental slab. Following the method of Davies and von Blackenburg (1998), we show that the slab breakoff can occur at the depth exceeding 100 km. Thermal modelling on the post subduction stage shows the heating related to the plate breakoff can cause the higher temperature recorded by the exhumed UHP rocks. The higher geotherm during post subduction stage leads to the weak strength of the orogenic wedge, and favours the faster upward movement of the UHP rock slices as ductile agents. The lower temperature gradient of the subduction slab predicted by modelling suggests the cold subducting slab could have transported significant fluids to mantle depth, not released during subduction. Accordingly, the absence of coeval calc alkalic magmatism in UHP orogens might resulted from the lower temperature as well as the fluid free circumstance, both are related to the rapid subduction of cold plate. Therefore, shear heating is not needed for explanation the thermal evolution of UHP orogen. On the other hand, the post collisional or late stage granitic plutonism is closely related to the deep seated heat producing materials of the accretion wedge.
Eclogitization
Orogeny
Slab
Convergent boundary
Anatexis
Cite
Citations (0)
Metasomatism
Forearc
Volcanic arc
Cite
Citations (56)