Experimental evidence for decompression melting of metasomatized mantle beneath Colima Graben, Mexico

Abstract Primitive subduction zone magmas provide information about the composition and thermal structure of the underlying mantle wedge. In the Colima Graben, Mexico, primitive lavas erupted from cinder cones range from high magnesium calc-alkaline basalts to high-K trachybasalts. This chemical diversity suggests that the sub-arc mantle wedge from which they derive is heterogeneous. To explore the conditions of magma generation in the wedge beneath Colima we used an inverse experimental approach to constrain multiple saturation points on the liquidus surface of a primitive high-K basanite (COM-1). Equilibrium piston-cylinder experiments were carried out between 1.0 and 2.4 GPa under hydrous (1.8–3.8 wt% H2O) and oxidizing (ƒO2 = − 0.5 to 4.3 log units relative to NNO) conditions. COM-1 + 3.8 wt% H2O is shown to be multiply-saturated with a phlogopite-bearing spinel pyroxenite assemblage (cpx + opx + phl + sp) close to its liquidus at 1.9–2.4 GPa and 1300 ºC. Experimental mapping of the liquidus surface reveals a multiple saturation point (MSP) where a lherzolitic phase assemblage of ol + cpx + opx + sp + phl coexist. The topology of the MSP indicates a peritectic of the form cpx + opx + phl + sp = liquid + ol. Four bracketing experiments define the MSP of COM-1 as 1300 ± 10 °C, 1.7 ± 0.1 GPa, ∆NNO = 3.4 ± 0.5 log units, for melt containing 3.6 ± 0.4 wt% H2O. The MSP olivine is too forsterite-rich (Fo92-94) to be in equilibrium with mantle lherzolite, but matches phenocryst core compositions in the natural basanite. Thus, experimental results indicate that COM-1 was produced by incongruent melting of an olivine-free, phlogopite-pyroxenite source that itself is the result of metasomatism of mantle wedge by slab-derived fluids. These conditions provide a valuable constraint on the thermal structure and chemical composition of the mantle wedge beneath Colima.

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The Oxidation State of Metasomatized Mantle Wedge: Insights from C-O-H-bearing Garnet Peridotite

Journal of Petrology ◽

10.1093/petrology/egp040 ◽

2009 ◽

Vol 50 (8) ◽

pp. 1533-1552 ◽

Cited By ~ 53

Author(s):

N. Malaspina ◽

S. Poli ◽

P. Fumagalli

Keyword(s):

Oxidation State ◽

Mantle Wedge ◽

Garnet Peridotite ◽

Metasomatized Mantle

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

10.5194/egusphere-egu2020-16224 ◽

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

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.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 &#176;C employing a piston cylinder apparatus. Single gold capsules were loaded with natural hematite, magnetite and synthetic haplogranite (Na0.56K0.38Al0.95Si5.19O12.2). Two sets of experiments were conducted: one with H2O-only fluids and the second one adding a 1.5 m H2O&#8211;NaCl solution. The capsule was kept frozen during welding to ensure no water loss.&#160;After quench, the presence of H2O in the quenched haplogranite glass was checked by Raman spectroscopy, while major elements were determined by microprobe analysis.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 FeOtot quenched in the haplogranite glass ranges from 0.60 wt% at 700 &#176;C, to 1.87 wt% at 900 &#176;C. In the presence of NaCl, we observed an increase in the amount of iron quenched in the glass (e.g., at 800 &#176;C from 1.04 wt% to 1.56 wt% of FeOtot).&#160;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.

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Porosity of metamorphic rocks and fluid migration within subduction interfaces

10.5194/egusphere-egu2020-6986 ◽

2020 ◽

Author(s):

Bruno Reynard ◽

Anne-Céline Ganzhorn ◽

Hélène Pilorgé

Keyword(s):

Experimental Approach ◽

Mantle Wedge ◽

High Pressures ◽

Planetary Science ◽

Seismogenic Zone ◽

Metamorphic Rocks ◽

Quantitative Evidence ◽

Seismic Rupture ◽

Large Earthquakes ◽

Forearc Mantle

&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160; Large earthquakes break the subduction interface to depths of 60 to 80 km. Current models hold that seismic rupture occurs when fluid overpressure builds in link with porosity cycles, an assumption still to be experimentally validated at high pressures. Porosities of subduction zone rocks are experimentally determined under pressures equivalent to depths of up to 90 km with a novel experimental approach that uses Raman deuterium-hydrogen mapping. Natural rocks (blueschists, antigorite serpentinites, and chlorite-schists) representing a typical cross-section of the subduction interface corresponding to the deep seismogenic zone are investigated. In serpentinite, and to a smaller extent blueschist, porosity increases with deformation, whereas chlorite-rich schists remain impermeable regardless of their deformation history[ 1]. Such a contrasting behavior explains the observation of over-pressurized oceanic crust and the limited hydration of the forearc mantle wedge. These results provide quantitative evidence that serpentinite, and likely blueschist, may undergo porosity cycles making possible the downdip propagation of large seismic rupture to great depths.&#160;[1] Ganzhorn, A.C., Pilorg&#233;, H., Reynard, B., 2019, Earth and Planetary Science Letters, 522: 107-117.

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Exhumation of subducted mafic rocks in a dynamically evolving thermal structure: constraints from phase equilibria modelling

10.5194/egusphere-egu21-13868 ◽

2021 ◽

Author(s):

Rilla C. McKeegan ◽

Victor E. Guevara ◽

Adam F. Holt ◽

Cailey B. Condit

Keyword(s):

Phase Equilibria ◽

Subduction Zone ◽

Subduction Zones ◽

Mantle Wedge ◽

Dynamic Models ◽

Thermal Structure ◽

Temporal Dependence ◽

Subduction Channel ◽

Mafic Rocks ◽

Point Of No Return

The dominant mechanisms that control the exhumation of subducted rocks and how these mechanisms evolve through time in a subduction zone remain unclear. Dynamic models of subduction zones suggest that their thermal structures evolve from subduction initiation to maturity. The series of metamorphic reactions that occur within the slab, resultant density, and buoyancy with respect to the mantle wedge will co-evolve with the thermal structure. We combine dynamic models of subduction zone thermal structure with phase equilibria modeling to place constraints on the dominant controls on the depth limits of exhumation. This is done across the temporal evolution of a subduction zone for various endmember lithologic associations observed in exhumed high-pressure terranes: sedimentary and serpentinite m&#233;langes, and oceanic tectonic slices.Initial modeling suggests that both serpentinite and sedimentary m&#233;langes remain positively buoyant with respect to the mantle wedge throughout all stages of subduction (up to 65 Myr), and for the spectrum of naturally constrained ratios of mafic blocks to serpentinite/sedimentary matrix. In these settings, exhumation depth limits and the &#8220;point of no return&#8221; (c. 2.3 GPa) are not directly limited by buoyancy, but potentially rheological changes in the slab at the blueschist-eclogite transition stemming from: the switch from amphibole-dominated to pyroxene-dominated rheology and/or dehydration embrittlement. These mechanisms may increase the possibility of brittle failure and hence promote detachment of the slab top into the subduction channel. For the range of temperatures recorded by exhumed serpentinite m&#233;langes, the locus of dehydration for altered MORB at the slab top coincides with the point of no return (2.3 GPa) between 35 and 40 Myr, suggesting a strong temporal dependence on deep exhumation in the subduction channel.&#160;Tectonic slices composed of 50% mafic rocks and 50% serpentinized slab mantle show a temporal dependence on the depth limits of positive buoyancy. For the range of temperatures recorded by exhumed tectonic slices, the upper pressure limit of positive buoyancy is ~2 GPa, and is only crossed between ~30 and 40 Myr after subduction initiation. Some exhumed tectonic slices record much higher pressures (2.5 GPa); thus, other mechanisms or lithologic combinations may also play a significant role in determining the exhumation limits of tectonic slices.&#160;Future work includes constraining how the loci of dehydration vary through time for different degrees of oceanic crust alteration, how exhumation limits and mechanisms may change with different subducting plate ages, and calculating how initial exhumation velocities may vary through time. Further comparison with the rock record will constrain the parameters that control the timing and limits of exhumation in subduction zones.

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Contemporaneous eruption of Nb-enriched basalts – K-adakites – Na-adakites from the 2.7 Ga Penakacherla terrane: implications for subduction zone processes and crustal growth in the eastern Dharwar craton, India

Canadian Journal of Earth Sciences ◽

10.1139/e2012-005 ◽

2012 ◽

Vol 49 (4) ◽

pp. 615-636 ◽

Cited By ~ 19

Author(s):

Robert Kerrich ◽

Chakravadhanula Manikyamba

Keyword(s):

Lower Crust ◽

Mantle Wedge ◽

Dharwar Craton ◽

Primitive Mantle ◽

Eastern Dharwar Craton ◽

High K ◽

Subduction Zone Processes ◽

Heavy Rare Earth Elements ◽

Ocean Ridge ◽

Lower Limits

An association of Nb-enriched basalts (NEB), high-MgO andesites (HMA), and flows with adakitic characteristics are interlayered with tholeiitic pillow basalts in the 2.7 Ga Penakacherla greenstone belt of eastern Dharwar craton. Two populations of basalt are present, a high-Mg# Ni (0.65–0.56, 106–52 ppm) and low-Mg# Ni (0.45–0.34, 32–13 ppm) counterpart; Nb spans 6.3–18 ppm relative to “normal” arc tholeiitic basalts, where Nb ∼3 ppm, and hence qualify as NEB. Basalts plot on the low-Ce/Yb trend of intraoceanic arcs, and have fractionated heavy rare-earth elements (HREE) indicative of melting with residual garnet at >90 km. Ratios of Nb/Ta (7.6 ± 0.7), Zr/Hf (44 ± 0.8), and Zr/Sm (27 ± 2.4) are systematically low, high, and similar to respective primitive mantle ratios of 17, 36, and 25, consistent with a mid-ocean ridge basalt-like mantle source in the sub-arc mantle wedge. Intermediate compositions are divided into high-K but low-Na (K2O 1.8–5.3; Na2O 0.5–2.1 wt.%) and low-K but high-Na (K2O 0.10–1.5; Na2O 4.1–5.6 wt.%) populations defining distinct magma series; accordingly, these are termed K-adakitic and Na-adakitic rocks, respectively. The Na-type has SiO2 ≥56 wt.%, MgO <3 wt.%, Mg# ∼0.5, Na2O ≥3.5 wt.%, K2O ≤3 wt.%, Yb ≤1.9 ppm, Cr ≥30 ppm, with slightly lower limits of Al2O3 ≥15 wt.% and La/Yb 7.5–8.2 versus ≥20, thus conforming to most criteria for Na-adakites. NEB are interpreted as melts of mantle wedge hybridized by adakitic melts having residual garnet; and Na-adakites are slab melts of low-Mg basalt in the garnet–amphibolite facies. K-adakitic flows are melts of mafic lower crust, or melts of lower crust delaminated into mantle wedge asthenosphere.

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Archean granitoids of the Aravalli Craton, northwest India

Geological Society London Special Publications ◽

10.1144/sp489-2018-195 ◽

2020 ◽

Vol 489 (1) ◽

pp. 215-234 ◽

Cited By ~ 1

Author(s):

Iftikhar Ahmad ◽

M. E. A. Mondal ◽

Md Sayad Rahaman ◽

Rajneesh Bhutani ◽

M. Satyanarayanan

Keyword(s):

Partial Melting ◽

Continental Crust ◽

Mantle Wedge ◽

Nd Isotope ◽

Oceanic Plateau ◽

Slab Breakoff ◽

Northwest India ◽

Aravalli Craton ◽

Metasomatized Mantle ◽

Asthenospheric Upwelling

AbstractThe Archean granitoids of the Aravalli Craton (NW India) are represented by orthogneisses (3.3–2.6 Ga) and undeformed granitoids (c. 2.5 Ga). Here we present whole-rock geochemical (elemental and Nd-isotope) data of the granitoids from the Aravalli Craton with an aim of understanding the evolution of the continental crust during the Archean. These Archean granitoids have been classified into three compositional groups: (1) TTG – tonalite–trondhjemite–granodiorite; (2) t-TTG – transitional TTG; and (3) sanukitoids. Based on the geochemical characteristics, it is proposed that the TTGs have formed from the partial melting of subducting oceanic plateau. The t-TTG formed owing to reworking of an older continental crust (approximately heterogeneous) in response to tectonothermal events in the craton. For the formation of the sanukitoids, a two-stage petrogenetic model is invoked which involves metasomatization of the mantle wedge, followed by slab breakoff and asthenospheric upwelling, which leads to the melting of asthenosphere and the metasomatized mantle wedge. It is also proposed that subducted sediments contributed to the genesis of sanukitoid magma.

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Two-Stage Origin of K-Enrichment in Ultrapotassic Magmatism Simulated by Melting of Experimentally Metasomatized Mantle

Minerals ◽

10.3390/min10010041 ◽

2019 ◽

Vol 10 (1) ◽

pp. 41 ◽

Cited By ~ 3

Author(s):

Michael W. Förster ◽

Stephan Buhre ◽

Bo Xu ◽

Dejan Prelević ◽

Regina Mertz-Kraus ◽

...

Keyword(s):

Trace Element ◽

Sedimentary Rock ◽

Experimental Approach ◽

Sedimentary Rocks ◽

Igneous Rocks ◽

Two Stage ◽

Second Stage ◽

Stage Process ◽

First Time ◽

Metasomatized Mantle

The generation of strongly potassic melts in the mantle requires the presence of phlogopite in the melting assemblage, while isotopic and trace element analyses of ultrapotassic rocks frequently indicate the involvement of subducted crustal lithologies in the source. However, phlogopite-free experiments that focus on melting of sedimentary rocks and subsequent hybridization with mantle rocks at pressures of 1–3 GPa have not successfully produced melts with K2O >5 wt%–6 wt%, while ultrapotassic igneous rocks reach up to 12 wt% K2O. Accordingly, a two-stage process that enriches K2O and increases K/Na in intermediary assemblages in the source prior to ultrapotassic magmatism seems likely. Here, we simulate this two-stage formation of ultrapotassic magmas using an experimental approach that involves re-melting of parts of an experimental product in a second experiment. In the first stage, reaction experiments containing layered sediment and dunite produced a modally metasomatized reaction zone at the border of a depleted peridotite. For the second-stage experiment, the metasomatized dunite was separated from the residue of the sedimentary rock and transferred to a smaller capsule, and melts were produced with 8 wt%–8.5 wt% K2O and K/Na of 6–7. This is the first time that extremely K-enriched ultrapotassic melts have been generated experimentally from sediments at low pressure applicable to a post-collisional setting.

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Continental versus oceanic subduction zones

National Science Review ◽

10.1093/nsr/nww049 ◽

2016 ◽

Vol 3 (4) ◽

pp. 495-519 ◽

Cited By ~ 88

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.

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