scholarly journals Abyssal Serpentinites: Transporting Halogens from Earth’s Surface to the Deep Mantle

Minerals ◽  
2019 ◽  
Vol 9 (1) ◽  
pp. 61 ◽  
Author(s):  
Lilianne Pagé ◽  
Keiko Hattori
Keyword(s):  
Mass Balance ◽  
Subduction Zones ◽  
Hydrothermal Fluids ◽  
Igneous Rocks ◽  
The Other ◽  
Ultrahigh Pressure ◽  
Conservative Estimate ◽  
Mantle Lithosphere ◽  
Deep Mantle ◽  
Subducting Slab

Serpentinized oceanic mantle lithosphere is considered an important carrier of water and fluid-mobile elements, including halogens, into subduction zones. Seafloor serpentinite compositions indicate Cl, Br and I are sourced from seawater and sedimentary pore fluids, while F may be derived from hydrothermal fluids. Overall, the heavy halogens are expelled from serpentinites during the lizardite–antigorite transition. Fluorine, on the other hand, appears to be retained or may be introduced from dehydrating sediments and/or igneous rocks during early subduction. Mass balance calculations indicate nearly all subducted F is kept in the subducting slab to ultrahigh-pressure conditions. Despite a loss of Cl, Br and I from serpentinites (and other lithologies) during early subduction, up to 15% of these elements are also retained in the deep slab. Based on a conservative estimate for serpentinite thickness of the metamorphosed slab (500 m), antigorite serpentinites comprise 37% of this residual Cl, 56% of Br and 50% of I, therefore making an important contribution to the transport of these elements to the deep mantle.

Experimental constraints on the partial melting of sediment-metasomatized lithospheric mantle in subduction zones

2020 ◽  
Vol 105 (8) ◽  
pp. 1191-1203
Author(s):  
Yanfei Zhang ◽  
Xuran Liang ◽  
Chao Wang ◽  
Zhenmin Jin ◽  
Lüyun Zhu ◽  
...  
Keyword(s):  
Upper Mantle ◽  
Lithospheric Mantle ◽  
Subduction Zones ◽  
Mantle Wedge ◽  
The Other ◽  
Central American ◽  
Bulk Sediment ◽  
Series Of Experiments ◽  
The Stability ◽  
Subducting Slab

Abstract Sedimentary diapirs can be relaminated to the base of the lithosphere during slab subduction, where they can interact with the ambient lithospheric mantle to form variably metasomatized zones. Here, high-pressure experiments in sediment-harzburgite systems were conducted at 1.5–2.5 GPa and 800–1300 °C to investigate the interaction between relaminated sediment diapirs and lithospheric mantle. Two end-member processes of mixed experiments and layered (reaction) experiments were explored. In the first end-member, sediment and harzburgite powders were mixed to a homogeneous proportion (1:3), whereas in the second, the two powders were juxtaposed as separate layers. In the first series of experiments, the run products were mainly composed of olivine + orthopyroxene + clinopyroxene + phlogopite in subsolidus experiments, while the phase assemblages were then replaced by olivine + orthopyroxene + melt (or trace phlogopite) in supersolidus experiments. Basaltic and foiditic melts were observed in all supersolidus mixed experiments (~44–52 wt% SiO2 at 1.5 GPa, ~35–43 wt% SiO2 at 2.5 GPa). In the phlogopite-rich experiment (PC431, 1.5 GPa and 1100 °C), the formed melts had low alkali contents (~<2 wt%) and K2O/Na2O ratios (~0.4–1.1). In contrast, the quenched melt in phlogopite-free/poor experiments showed relatively higher alkali contents (~4–8 wt%) and K2O/Na2O ratios (~2–5). Therefore, the stability of phlogopite could control the bulk K2O and K2O/Na2O ratios of magmas derived from the sediment-metasomatized lithospheric mantle. In layered experiments, a reaction zone dominated by clinopyroxene + amphibole (or orthopyroxene) was formed because of the reaction between harzburgite and bottom sediment-derived melts (~62.5–67 wt% SiO2). The total alkali contents and K2O/Na2O ratios of the formed melts were about 6–8 wt% and 1.5–3, respectively. Experimentally formed melts from both mixed and reaction experiments were rich in large ion lithosphile elements and displayed similar patterns with natural potassium-rich arc lavas from oceanic subduction zones (i.e., Mexican, Sunda, Central American, and Aleutian). The experimental results demonstrated that bulk sediment diapirs, in addition to sediment melt, may be another possible mechanism to transfer material from a subducting slab to an upper mantle wedge or lithospheric mantle. On the other hand, the breakdown of phlogopite may play an important role in the mantle source that produces potassium-rich arc lavas in subduction zones.

scholarly journals Continental versus oceanic subduction zones

2016 ◽  
Vol 3 (4) ◽  
pp. 495-519 ◽  
Author(s):  
Yong-Fei Zheng ◽  
Yi-Xiang Chen
Keyword(s):  
Subduction Zones ◽  
Mantle Wedge ◽  
Ultrahigh Pressure ◽  
Metamorphic Rocks ◽  
Crustal Rocks ◽  
Incompatible Elements ◽  
Ultrahigh Temperature ◽  
Subducting Slab ◽  
Oceanic Slab ◽  
Metasomatized Mantle

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.

scholarly journals Extreme metamorphism and metamorphic facies series at convergent plate boundaries: Implications for supercontinent dynamics

Geosphere ◽  
2021 ◽  
Author(s):  
Yong-Fei Zheng ◽  
Ren-Xu Chen
Keyword(s):  
Subduction Zones ◽  
Plate Tectonics ◽  
The Other ◽  
Ultrahigh Pressure ◽  
Plate Boundaries ◽  
Lower Grade ◽  
Thermal Gradients ◽  
Mountain Building ◽  
Convergent Plate Boundaries ◽  
Metamorphic Facies

Crustal metamorphism under extreme pressure-temperature conditions produces characteristic ultrahigh-pressure (UHP) and ultrahigh-temperature (UHT) mineral assemblages at convergent plate boundaries. The formation and evolution of these assemblages have important implications, not only for the generation and differentiation of continental crust through the operation of plate tectonics, but also for mountain building along both converging and con- verged plate boundaries. In principle, extreme metamorphic products can be linked to their lower-grade counterparts in the same metamorphic facies series. They range from UHP through high-pressure (HP) eclogite facies to blueschist facies at low thermal gradients and from UHT through high-temperature (HT) granulite facies to amphibolite facies at high thermal gradients. The former is produced by low-temperature/pressure (T/P ) Alpine-type metamorphism during compressional heating in active subduction zones, whereas the latter is generated by high-T/P Buchan-type metamorphism during extensional heating in rifting zones. The thermal gradient of crustal metamorphism at convergent plate boundaries changes in both time and space, with low-T/P ratios in the compressional regime during subduction but high-T/P ratios in the extensional regime during rifting. In particular, bimodal metamorphism, one colder and the other hotter, would develop one after the other at convergent plate boundaries. The first is caused by lithospheric subduction at lower thermal gradients and thus proceeds in the compressional stage of convergent plate boundaries; the second is caused by lithospheric rifting at higher thermal gradients and thus proceeds in the extensional stage of convergent plate boundaries. In this regard, bimodal metamorphism is primarily dictated by changes in both the thermal state and the dynamic regime along plate boundaries. As a consequence, supercontinent assembly is associated with compressional metamorphism during continental collision, whereas supercontinent breakup is associated with extensional metamorphism during active rifting. Nevertheless, aborted rifts are common at convergent plate boundaries, indicating thinning of the previously thickened lithosphere during the attempted breakup of supercontinents in the history of Earth. Therefore, extreme metamorphism has great bearing not only on reworking of accretionary and collisional orogens for mountain building in continental interiors, but also on supercontinent dynamics in the Wilson cycle.

scholarly journals Melting of sediments in the deep mantle produces saline fluid inclusions in diamonds

2019 ◽  
Vol 5 (5) ◽  
pp. eaau2620 ◽  
Author(s):  
Michael W. Förster ◽  
Stephen F. Foley ◽  
Horst R. Marschall ◽  
Olivier Alard ◽  
Stephan Buhre
Keyword(s):  
Marine Sediments ◽  
Fluid Inclusions ◽  
Subduction Zones ◽  
Experimental Results ◽  
Mantle Lithosphere ◽  
Earth’S Mantle ◽  
Deep Mantle ◽  
Solid Phases ◽  
Alkali Chlorides ◽  
Low Pressures

Diamonds growing in the Earth’s mantle often trap inclusions of fluids that are highly saline in composition. These fluids are thought to emerge from deep in subduction zones and may also be involved in the generation of some of the kimberlite magmas. However, the source of these fluids and the mechanism of their transport into the mantle lithosphere are unresolved. Here, we present experimental results showing that alkali chlorides are stable solid phases in the mantle lithosphere below 110 km. These alkali chlorides are formed by the reaction of subducted marine sediments with peridotite and show identical K/Na ratios to fluid inclusions in diamond. At temperatures >1100°C and low pressures, the chlorides are unstable; here, potassium is accommodated in mica and melt. The reaction of subducted sediments with peridotite explains the occurrence of Mg carbonates and the highly saline fluids found in diamonds and in chlorine-enriched kimberlite magmas.

2. First rocks on a dead Earth

2016 ◽  
pp. 13-28
Author(s):  
Jan Zalasiewicz
Keyword(s):  
Cooling Rate ◽  
Subduction Zones ◽  
Plate Tectonics ◽  
Igneous Rocks ◽  
Magma Ocean ◽  
Plate Collision ◽  
Ocean Ridges ◽  
Different Types ◽  
Fractional Melting

‘First rocks on a dead Earth’ describes the formation of the planet Earth from the collision of the precursor planets Tellus and Theia. The surface of the newly born Earth had a surface magma ocean. As this magma cooled, the first minerals formed. The earliest rocks on Earth date back to the Archaeon Eon. During that time, plate tectonics started up, which determined the nature of all subsequent rocks on Earth. The processes of fractional melting and impact of cooling rate on crystal sizes is explained along with the different types of igneous rocks—basalts, andesites, diorites, rhyolites, and granites—formed at mid-ocean ridges, subduction zones, and plate collision zones.

IV.—Allotropic Forms of Silica and their Significance as Constituents of Igneous Rocks

1906 ◽  
Vol 3 (3) ◽  
pp. 118-120
Author(s):  
Cosmo Johns
Keyword(s):  
Rock Mass ◽  
Igneous Rocks ◽  
The Other ◽  
Free Silica

That silica appears as a constituent mineral of igneous rocks in two distinct phases, viz. quartz and tridymite, has been known for some time. The writer is not aware that any explanation has been offered which would indicate the conditions determining the appearance of one or the other in a cooling fused rock-mass. He now proposes to describe certain experiments made with a view to explain why it is that though the free silica generally appears as quartz, yet occasionally, as in certain trachytes, it crystallizes out as tridymite.

Isotopic age determinations on clay minerals from lavas and tuffs of the Derbyshire orefield

1972 ◽  
Vol 109 (6) ◽  
pp. 501-512 ◽  
Author(s):  
P. R. Ineson ◽  
J. G. Mitchell
Keyword(s):  
Clay Minerals ◽  
Hydrothermal Alteration ◽  
Igneous Rocks ◽  
The Other ◽  
Mineral Deposits ◽  
Isotopic Age ◽  
Hydrothermal Mineralization ◽  
Pumice Tuff ◽  
Southern Section ◽  
Age Determinations

SummaryEpisodic hydrothermal mineralization has previously been recognized in the northern section of the Pennine orefield. Igneous rocks from the southern section (the Derbyshire orefield) have yielded isotopic ages, some of which are thought to represent ages of hydrothermal metasomatism (deuteric or subsequent). In order to ascertain whether epicyclic hydrothermal events gave rise to the Derbyshire mineral deposits, samples of highly altered doleritic lava and pumice tuff were collected adjacent to areas of mineralization. Clay-mineral concentrates from 34 samples were dated by the potassium–argon method. The conclusions drawn from these analyses support a hypothesis of repeated hydrothermal alteration of the clay minerals, reflecting at least two episodes of mineralization, one about 270 m.y., the other about 235 m.y. The geo-chronological significance of these and other results is considered.

Evolution of deep mantle sourced carbonated melt in the mantle lithosphere

2020 ◽  
Author(s):  
Guoliang Zhang
Keyword(s):  
Rare Earth ◽  
Field Strength ◽  
Rare Earth Elements ◽  
Lithospheric Mantle ◽  
High Field ◽  
Volcanic Rocks ◽  
Mantle Lithosphere ◽  
High Field Strength ◽  
Alkali Basalts ◽  
Deep Mantle

<p>Deep sourced magmas play a key role in distribution of carbon in the Earth’s system. Oceanic hotspots rooted in deep mantle usually produce CO<sub>2</sub>-rich magmas. However, the association of CO<sub>2</sub> with the origin of these magmas remains unclear. Here we report geochemical analyses of a suite of volcanic rocks from the Caroline Seamount Chain formed by the deep-rooted Caroline hotspot in the western Pacific. The most primitive magmas have depletion of SiO<sub>2</sub> and high field strength elements and enrichment of rare earth elements that are in concert with mantle-derived primary carbonated melts. The carbonated melts show compositional variations that indicate reactive evolution within the overlying mantle lithosphere and obtained depleted components from the lithospheric mantle. The carbonated melts were de-carbonated and modified to oceanic alkali basalts by precipitation of perovskite, apatite and ilmenite that significantly decreased the concentrations of rare earth elements and high field strength elements. These magmas experienced a stage of non-reactive fractional crystallization after the reactive evolution was completed. Thus, the carbonated melts would experience two stages, reactive and un-reactive, of evolution during their transport through in thick oceanic lithospheric mantle. We suggest that the mantle lithosphere plays a key role in de-carbonation and conversion of deep-sourced carbonated melts to alkali basalts. This work was financially supported by the National Natural Science Foundation of China (91858206, 41876040).</p>

Solubility of magnetite-hematite assemblages in slab-derived saline fluids

2020 ◽  
Author(s):  
Carla Tiraboschi ◽  
Carmen Sanchez-Valle
Keyword(s):  
Subduction Zones ◽  
Microprobe Analysis ◽  
Mantle Wedge ◽  
Sedimentary Cover ◽  
Low Pressure ◽  
Oceanic Lithosphere ◽  
Major Elements ◽  
Piston Cylinder ◽  
Water Saturated ◽  
Subducting Slab

<p>In subduction zones, aqueous fluids derived from devolatilization processes of the oceanic lithosphere and its sedimentary cover, are major vectors of mass transfer from the slab to the mantle wedge and contribute to the recycling of elements and to their geochemical cycles. In this setting, assessing the mobility of redox sensitive elements, such as iron, can provide useful insights on the oxygen fugacity conditions of slab-derived fluid. However, the amount of iron mobilized by deep aqueous fluids and melts, is still poorly constrained.</p><p>We experimentally investigate the solubility of magnetite-hematite assemblages in water-saturated haplogranitic liquids, which represent the felsic melt produced by subducted eclogites. Experiments were conducted at 1 GPa and temperature ranging from 700 to 900 °C employing a piston cylinder apparatus. Single gold capsules were loaded with natural hematite, magnetite and synthetic haplogranite (Na<sub>0.56</sub>K<sub>0.38</sub>Al<sub>0.95</sub>Si<sub>5.19</sub>O<sub>12.2</sub>). Two sets of experiments were conducted: one with H<sub>2</sub>O-only fluids and the second one adding a 1.5 m H<sub>2</sub>O–NaCl solution. The capsule was kept frozen during welding to ensure no water loss. After quench, the presence of H<sub>2</sub>O in the quenched haplogranite glass was checked by Raman spectroscopy, while major elements were determined by microprobe analysis.</p><p>Preliminary results indicate that a significant amount of Fe is released from magnetite and hematite in hydrous melts, even at relatively low-pressure conditions. At 1 GPa the FeO<sub>tot</sub> quenched in the haplogranite glass ranges from 0.60 wt% at 700 °C, to 1.87 wt% at 900 °C. In the presence of NaCl, we observed an increase in the amount of iron quenched in the glass (e.g., at 800 °C from 1.04 wt% to 1.56 wt% of FeO<sub>tot</sub>). Our results suggest that hydrous melts can effectively mobilize iron even at low-pressure conditions and represent a valid agent for the cycling of iron from the subducting slab to the mantle wedge.</p>

Ferrodiorite from the Isle of Skye

1962 ◽  
Vol 33 (256) ◽  
pp. 26-36 ◽  
Author(s):  
L. R. Wager ◽  
E. A. Vincent
Keyword(s):  
Igneous Rocks ◽  
The Other ◽  
Hybrid Origin ◽  
Skaergaard Intrusion ◽  
The One ◽  
Isle Of Skye

Summary As a preliminary to an account of the marscoite suite of the Western Redhills Tertiary igneous centre of Skye, a description is given here of one unit, the ferrodiorite. While accepting Harker's hypothesis of the hybrid origin of marscoite (1904, pp. 175–196) we consider that the ferrodiorite, with which marscoite on Marsco is associated, is not itself a hybrid, but one of the parents from which the hybrids were formed. Among Thulean igneous rocks the ferrodiorite has mineralogical and chemical affinities with Hebridean mugearites, on the one hand, and with the ferrogabbros of the Skaergaard intrusion on the other, but is probably significantly different from both.

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