Copper Systematics in Arc Magmas and Implications for Crust-Mantle Differentiation

Science ◽  
2012 ◽  
Vol 336 (6077) ◽  
pp. 64-68 ◽  
Author(s):  
Cin-Ty A. Lee ◽  
Peter Luffi ◽  
Emily J. Chin ◽  
Romain Bouchet ◽  
Rajdeep Dasgupta ◽  
...  
Keyword(s):  
Continental Crust ◽  
Redox State ◽  
Building Blocks ◽  
Magmatic Differentiation ◽  
Redox States ◽  
Cu Content ◽  
Arc Magmas ◽  
Reducing Conditions ◽  
Mid Ocean Ridge ◽  
Ocean Ridge

Arc magmas are important building blocks of the continental crust. Because many arc lavas are oxidized, continent formation is thought to be associated with oxidizing conditions. On the basis of copper’s (Cu’s) affinity for reduced sulfur phases, we tracked the redox state of arc magmas from mantle source to emplacement in the crust. Primary arc and mid-ocean ridge basalts have identical Cu contents, indicating that the redox states of primitive arc magmas are indistinguishable from that of mid-ocean ridge basalts. During magmatic differentiation, the Cu content of most arc magmas decreases markedly because of sulfide segregation. Because a similar depletion in Cu characterizes global continental crust, the formation of sulfide-bearing cumulates under reducing conditions may be a critical step in continent formation.

scholarly journals Subducting serpentinites release reduced, not oxidized, aqueous fluids

2019 ◽  
Vol 9 (1) ◽  
Author(s):  
F. Piccoli ◽  
J. Hermann ◽  
T. Pettke ◽  
J. A. D. Connolly ◽  
E. D. Kempf ◽  
...  
Keyword(s):  
Subduction Zone ◽  
Oxygen Fugacity ◽  
Mantle Wedge ◽  
Thermodynamic Modelling ◽  
Arc Magmatism ◽  
Redox Processes ◽  
Arc Magmas ◽  
Reducing Conditions ◽  
Mid Ocean Ridge ◽  
Ocean Ridge

AbstractThe observation that primitive arc magmas are more oxidized than mid-ocean-ridge basalts has led to the paradigm that slab-derived fluids carry SO2 and CO2 that metasomatize and oxidize the sub-arc mantle wedge. We combine petrography and thermodynamic modelling to quantify the oxygen fugacity (fO2) and speciation of the fluids generated by serpentinite dehydration during subduction. Silicate-magnetite assemblages maintain fO2 conditions similar to the quartz-fayalite-magnetite (QFM) buffer at fore-arc conditions. Sulphides are stable under such conditions and aqueous fluids contain minor S. At sub-arc depth, dehydration occurs under more reducing conditions producing aqueous fluids carrying H2S. This finding brings into question current models in which serpentinite-derived fluids are the cause of oxidized arc magmatism and has major implications for the global volatile cycle, as well as for redox processes controlling subduction zone geodynamics.

Element fluxes associated with subduction related magmatism

1991 ◽  
Vol 335 (1638) ◽  
pp. 393-405 ◽  
Keyword(s):  
Trace Element ◽  
Incompatible Element ◽  
Mantle Wedge ◽  
Isotope Ratios ◽  
Noticeable Effect ◽  
Arc Magmas ◽  
Mid Ocean Ridge ◽  
Ocean Ridge ◽  
Arc Magma ◽  
Subducted Sediment

Destructive plate margin magmas may be subdivided into two groups on the basis of their rare earth element (REE) ratios. Most island arc suites have low Ce/Yb, and remarkably restricted isotope ratios of 87 Sr/ 86 Sr = 0.7033, 143 Nd/ 144 Nd = 0.51302, 206 Pb/ 204 Pb = 18.76 , 207 Pb/ 204 Pb = 15.57, and 208 Pb/ 204 Pb = 38.4. However, they also have Rb/Sr (0.03), Th/U (2.2) and Ce/Yb (8.5) ratios which are significantly less than accepted estimates for the bulk continental crust. The high Ce/Yb suites have higher incompatible element contents, more restricted heavy REE, and much more variable isotope ratios. Such rocks are found in the Aeolian Islands, Grenada, Indonesia and Philippines, and their isotope and trace element features have been attributed both to contributions from subducted sediment, and/or old trace element enriched material in the mantle wedge. It is argued that for isotope and trace element models the slab component can usefully be taken to consist of subducted sediment and altered mid-ocean ridge basalts, since these may contain ca. 80% of the water in the subducted slab, and the distinctive trace element features of arc magmas are generally attributed to the movement of material in hydrous fluids. The isotope data indicate that not more than 15% of the Sr and Th in an average arc magma were derived from subducted material, and that the rest were derived from the mantle wedge. The fluxes of elements which cannot be characterized isotopically are more difficult to constrain, but for most minor and trace elements the slab derived contribution in arc magmas is too small to have a noticeable effect on the residual slab.

scholarly journals Secular oxidation of Archean upper mantle caused by increasing crustal recycling

2021 ◽  
Author(s):  
Lei Gao ◽  
Shuwen Liu ◽  
Peter Cawood ◽  
Jintuan Wang ◽  
Guozheng Sun ◽  
...  
Keyword(s):  
Upper Mantle ◽  
Redox State ◽  
Incompatible Element ◽  
Volcanic Rocks ◽  
Crustal Recycling ◽  
Incompatible Elements ◽  
Mid Ocean Ridge ◽  
Redox Evolution ◽  
Ocean Ridge ◽  
Plate Subduction

Abstract The redox evolution of Archean mantle impacted Earth differentiation, mantle melting and the nature of chemical equilibrium between mantle, ocean and atmosphere of the early Earth. However, how and why it varies with time remain controversial. Archean mantle-derived volcanic rocks, especially basalts are ideal lithologies for reconstructing the mantle redox state. Here we show that the ~3.8-2.5 Ga basalts from fourteen cratons are subdivided geochemically into two groups, B-1, showing incompatible element depleted and modern mid-ocean ridge basalt-like features ((Nb/La)PM ≥ 0.75) and B-2 ((Nb/La)PM < 0.75), characterized by modern island arc basalt-like features. Our updated V-Ti redox proxy indicates the Archean upper mantle was more reducing than today, and that there was a significant redox heterogeneity between ambient and modified mantle presumably related to crustal recycling, perhaps via plate subduction, as shown by B-1 and B-2 magmas, respectively. The oxygen fugacity of modified mantle exhibits a ~1.5-2.0 log units increase over ~3.8-2.5 Ga, whereas the ambient mantle becomes more and more heterogeneous with respect to redox, apart from a significant increase at ~2.7 Ga. These findings are coincident with the increase in the proportions of crustal recycling-related lithologies with associated enrichment of associated incompatible elements (e.g., Th/Nb), indicating that increasing recycling played a crucial role on the secular oxidation of Archean upper mantle.

scholarly journals Lithium systematics in global arc magmas and the importance of crustal thickening for lithium enrichment

2020 ◽  
Vol 11 (1) ◽  
Author(s):  
Chen Chen ◽  
Cin-Ty A. Lee ◽  
Ming Tang ◽  
Kevin Biddle ◽  
Weidong Sun
Keyword(s):  
Source Rocks ◽  
Crustal Thickening ◽  
Volcanic Arcs ◽  
Continental Arcs ◽  
Arc Magmas ◽  
The Andes ◽  
Mid Ocean Ridge ◽  
Hydrologic System ◽  
Ocean Ridge ◽  
Lithium Enrichment

Abstract Much of the world’s Li deposits occurs as basinal brines in magmatic orogens, particularly in continental volcanic arcs. However, the exact origin of Li enrichment in arc magmatic systems is not clear. Here, we show that, globally, primitive arc magmas have Li contents and Li/Y ratios similar to mid-ocean ridge basalts, indicating that the subducting slab has limited contribution to Li enrichment in arc magmas. Instead, we find that Li enrichment is enhanced by lower degrees of sub-arc mantle melting and higher extents of intracrustal differentiation. These enrichment effects are favored in arcs with thick crust, which explains why magmatism and differentiation in continental arcs, like the Andes, reach greater Li contents than their island arc counterparts. Weathering of these enriched source rocks mobilizes and transports such Li into the hydrologic system, ultimately developing Li brines with the combination of arid climate and the presence of landlocked extensional basins in thickened orogenic settings.

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.

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.

Using Silica Activity to Model Redox-dependent Fluid Compositions in Serpentinites from 100 to 700 °C and from 1 to 20 kbar

2020 ◽  
Author(s):  
Codi Lazar
Keyword(s):  
Subduction Zones ◽  
Redox State ◽  
Deep Biosphere ◽  
Governing Parameter ◽  
Microbial Life ◽  
Elevated Pressures ◽  
Silica Activity ◽  
Mid Ocean Ridge ◽  
Ocean Ridge ◽  
Forearc Mantle

Abstract Serpentinization is a metamorphic process that can stabilize highly reduced hydrogen-rich fluids. Previous measurements of elevated CH4 and H2 concentrations in ultramafic-hosted submarine springs indicate that active serpentinization occurs along mid-ocean ridge systems at seafloor pressures (∼&lt;500 bar) and temperatures (∼&lt;350 °C). Serpentinites also exist at higher pressures in subduction zones; for example, during retrograde hydration of the forearc mantle wedge and during prograde deserpentinization within the subducted slab. However, many studies demonstrating the thermodynamic stability of reduced serpentinite fluids have been limited to terrestrial seafloor conditions. To investigate the redox state of serpentinite fluids at elevated pressures, phase equilibria and fluid compositions were computed for 100–700 °C and 1–20 kbar using aqueous silica activity (aSiO2(aq)) as a governing parameter. Silica-sensitive, redox-buffering assemblages were selected to be consistent with previously proposed reactions: SiO2(aq)–fayalite–magnetite (QFM), SiO2(aq)–Fe-brucite–cronstedtite, SiO2(aq)–Fe-brucite–Fe3+-serpentine, plus the silica-free buffer Fe-brucite–magnetite. Fluid species are limited to simple, zerovalent compounds. For silica-bearing redox reactions, aSiO2(aq) is buffered by coexisting ultramafic mineral assemblages in the system MgO–SiO2–H2O. Silica activity and fO2 are directly correlated, with the most reduced fluids stabilized by the least siliceous assemblages. Silica activity and fO2 increase with pressure, but are more strongly dependent on temperature, leading to greater silica enrichment and more oxidized conditions along shallow, warm subduction paths than along steeper, colder paths. Reduced fluids with mCH4/mCO2 &gt; 1 and fO2 below QFM are present only when serpentine is stable, and are favored along all subduction trajectories except shallow P–T paths at eclogite-grade. Values of mH2 and mCO/mCO2 depend strongly on P and T, but also on the choice of redox buffer, especially whether the Fe-serpentine component is cronstedtite or Fe3+-serpentine. Methane and H2S production are thermodynamically favored throughout the P–T range of the serpentinized forearc mantle and in other settings with similar conditions; for example, deep planetary seafloors. The model offers a generalized technique for estimating the redox state of a fluid-saturated serpentinite at elevated P and T, and yields results consistent with previous petrographic and thermodynamic analyses. High-pressure serpentinization may be an important source of reduced species that could influence prebiotic chemistry, support microbial life in the deep biosphere or in deep planetary oceans, or promote greenhouse warming on early Earth.

scholarly journals Ridge Jumps and Mantle Exhumation in Back-Arc Basins

Geosciences ◽  
2021 ◽  
Vol 11 (11) ◽  
pp. 475
Author(s):  
Valentina Magni ◽  
John Naliboff ◽  
Manel Prada ◽  
Carmen Gaina
Keyword(s):  
Continental Crust ◽  
Oceanic Crust ◽  
Numerical Models ◽  
Spreading Ridge ◽  
Transient Nature ◽  
Mid Ocean Ridge ◽  
Thinned Continental Crust ◽  
Mantle Exhumation ◽  
Back Arc ◽  
Ocean Ridge

Back-arc basins in continental settings can develop into oceanic basins, when extension lasts long enough to break up the continental lithosphere and allow mantle melting that generates new oceanic crust. Often, the basement of these basins is not only composed of oceanic crust, but also of exhumed mantle, fragments of continental crust, intrusive magmatic bodies, and a complex mid-ocean ridge system characterised by distinct relocations of the spreading centre. To better understand the dynamics that lead to these characteristic structures in back-arc basins, we performed 2D numerical models of continental extension with asymmetric and time-dependent boundary conditions that simulate episodic trench retreat. We find that, in all models, episodic extension leads to rift and/or ridge jumps. In our parameter space, the length of the jump ranges between 1 and 65 km and the timing necessary to produce a new spreading ridge varies between 0.4 and 7 Myr. With the shortest duration of the first extensional phase, we observe a strong asymmetry in the margins of the basin, with the margin further from trench being characterised by outcropping lithospheric mantle and a long section of thinned continental crust. In other cases, ridge jump creates two consecutive oceanic basins, leaving a continental fragment and exhumed mantle in between the two basins. Finally, when the first extensional phase is long enough to form a well-developed oceanic basin (>35 km long), we observe a very short intra-oceanic ridge jump. Our models are able to reproduce many of the structures observed in back-arc basins today, showing that the transient nature of trench retreat that leads to episodes of fast and slow extension is the cause of ridge jumps, mantle exhumation, and continental fragments formation.

Geophysical studies of the continental margin northeast of Newfoundland

1968 ◽  
Vol 5 (3) ◽  
pp. 483-500 ◽  
Author(s):  
D. K. B. Fenwick ◽  
M. J. Keen ◽  
Charlotte Keen ◽  
A. Lambert
Keyword(s):  
Continental Crust ◽  
Continental Margin ◽  
Magnetic Anomalies ◽  
Ocean Basin ◽  
Labrador Sea ◽  
Mobile Belt ◽  
Magnetic Survey ◽  
Continental Plate ◽  
Mid Ocean Ridge ◽  
Ocean Ridge

A seismic and magnetic survey has been made of an area that straddles the continental margin northeast of Newfoundland from the edge of the shelf to the ocean basin of the Labrador Sea. The results bear on the question of the extension of the mobile belt of the Appalachian System in Newfoundland to the margin. Our seismic studies show that the crust is approximately 30 km thick beneath the edge of the shelf northeast of Newfoundland, and thins to approximately 12 km at the foot of the slope. Seismic studies by Lamont Geological Observatory suggest that the mobile belt continues to the upper part of the slope; our studies support this. Large magnetic anomalies run nearly parallel to the edge of the shelf over the slope and rise, at right angles to the strike of the Appalachian System. Their amplitude is 1400 gammas and the belt of anomalies is 180 km wide. It runs continuously for at least 220 km. One interpretation is that the anomalies owe their origin to the juxtaposition of a magnetic continental plate 30 km thick against a thinner magnetic oceanic plate 10 to 12 km thick. We suggest that the continental crust is particularly magnetic, and the 'shelf-edge' anomalies particularly large, because it represents the basic mobile belt of the Appalachian System terminating beneath the lower part of the continental slope. The contrast in magnetization between non-magnetic oceanic mantle and magnetic continental crust is due to this, and perhaps also to higher temperatures in the oceanic mantle. They could be particularly high if the Labrador Sea is the site of a mid-ocean ridge. Some evidence from the magnetic survey suggests that a fault with dextral slip runs on the continent side of the margin in a northeasterly direction. It does not continue into the ocean basin.

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