The Viscosity of Carbonate‐Silicate Transitional Melts at Earth's Upper Mantle Pressures and Temperatures, Determined by the In Situ Falling‐Sphere Technique

ABSTRACT The “petrological Moho” recognized in the Jurassic Vourinos Ophiolite (northern Greece) was the first “crust-mantle” boundary described within a fossil oceanic lithosphere. Early observations suggested a Cenozoic brittle-field block rotation of the petrological Moho transition area resulting in an oblique clockwise rotation of ∼100°, but a brittle fault system responsible for the mechanism of this rotation was never located. A modern interpretation of research dating from the 1960s to the present documents the occurrence of a diverse set of ductile structures overprinting this primary intra-oceanic feature. The following observations from our original “Moho” studies in the Vourinos complex are still pertinent: the contact between the upper mantle units and the magmatic crustal sequence is in situ and intrusional in nature; high-temperature intragranular ductile deformation (mantle creep at temperatures from around 1200 °C down to ∼900 °C) fabrics terminate at the crust-mantle boundary; the overlying oceanic crustal rocks display geochemical fractionation patterns analogous to crustal rocks in the in situ oceanic lithosphere. Since these original studies, however, understanding the mechanisms of ductile deformation and ridge crest processes have advanced, and hence we can now interpret the older data and recent observations in a new paradigm of oceanic lithosphere formation. Our major interpretational breakthrough includes the following phenomena: lower temperature, intergranular deformation of ∼900 °C to 700 °C extends from the upper mantle tectonites up into the lower crustal cumulate section; the origin of mineral lineations within adcumulate crustal rocks as remnants of ductile deformation during early phases of magmatic crystallization; syn-magmatic folding and rotation of the cumulate section; the tectonic significance of flaser gabbro and late gabbroic intrusions in the crustal sequence; and the relevance and significance of a cumulate troctolite unit within the crustal sequence. These observations collectively point to an important process of a ductile-field, syn-magmatic rotation of the Moho transition area. The most plausible mechanism explaining such a rotation is proto-transform faulting deformation near the ridge crest. By recognizing and distinguishing structures that resulted from such initial rotational deformation in the upper mantle peridotites of ophiolites, future field-based structural, petrographic, and petrological studies can better document the mode of the initiation of oceanic transform faults.

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The Viscosity and Atomic Structure of Volatile-Bearing Melilititic Melts at High Pressure and Temperature and the Transport of Deep Carbon

Minerals ◽

10.3390/min10030267 ◽

2020 ◽

Vol 10 (3) ◽

pp. 267 ◽

Cited By ~ 2

Author(s):

Vincenzo Stagno ◽

Veronica Stopponi ◽

Yoshio Kono ◽

Annalisa D’Arco ◽

Stefano Lupi ◽

...

Keyword(s):

Experimental Data ◽

High Pressure ◽

Atomic Structure ◽

Ex Situ ◽

X Ray Diffraction ◽

High Pressure And Temperature ◽

X Ray ◽

Falling Sphere ◽

Basaltic Melts

Understanding the viscosity of mantle-derived magmas is needed to model their migration mechanisms and ascent rate from the source rock to the surface. High pressure–temperature experimental data are now available on the viscosity of synthetic melts, pure carbonatitic to carbonate–silicate compositions, anhydrous basalts, dacites and rhyolites. However, the viscosity of volatile-bearing melilititic melts, among the most plausible carriers of deep carbon, has not been investigated. In this study, we experimentally determined the viscosity of synthetic liquids with ~31 and ~39 wt% SiO2, 1.60 and 1.42 wt% CO2 and 5.7 and 1 wt% H2O, respectively, at pressures from 1 to 4.7 GPa and temperatures between 1265 and 1755 °C, using the falling-sphere technique combined with in situ X-ray radiography. Our results show viscosities between 0.1044 and 2.1221 Pa·s, with a clear dependence on temperature and SiO2 content. The atomic structure of both melt compositions was also determined at high pressure and temperature, using in situ multi-angle energy-dispersive X-ray diffraction supported by ex situ microFTIR and microRaman spectroscopic measurements. Our results yield evidence that the T–T and T–O (T = Si,Al) interatomic distances of ultrabasic melts are higher than those for basaltic melts known from similar recent studies. Based on our experimental data, melilititic melts are expected to migrate at a rate ~from 2 to 57 km·yr−1 in the present-day or the Archaean mantle, respectively.

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The distribution and abundance of halogens in eclogites: An in situ SIMS perspective of the Raspas Complex (Ecuador)

American Mineralogist ◽

10.2138/am-2020-6994 ◽

2020 ◽

Vol 105 (3) ◽

pp. 307-318 ◽

Cited By ~ 1

Author(s):

Benjamin M. Urann ◽

Véronique Le Roux ◽

Timm John ◽

Grace M. Beaudoin ◽

Jaime D. Barnes

Keyword(s):

Upper Mantle ◽

Subduction Zones ◽

Secondary Ion Mass Spectrometry ◽

Bulk Rock ◽

Ocean Island Basalts ◽

High Pressure Metamorphism ◽

Southern Ecuador ◽

And Behavior ◽

Secondary Ion

Abstract We present in situ secondary ion mass spectrometry (SIMS) and electron microprobe analyses of coexisting garnet, omphacite, phengite, amphibole, and apatite, combined with pyrohydrolysis bulk-rock analyses to constrain the distribution, abundance, and behavior of halogens (F and Cl) in six MORB-like eclogites from the Raspas Complex (Southern Ecuador). In all cases concerning lattice-hosted halogens, F compatibility decreases from apatite (1.47–3.25 wt%), to amphibole (563–4727 μg/g), phengite (610–1822 μg/g), omphacite (6.5–54.1 μg/g), and garnet (1.7–8.9 μg/g). The relative compatibility of Cl in the assemblage is greatest for apatite (192–515 μg/g), followed by amphibole (0.64–82.7 μg/g), phengite (1.2–2.1 μg/g), omphacite (<0.05–1.0 μg/g), and garnet (<0.05 μg/g). Congruence between SIMS-reconstructed F bulk abundances and yield-corrected bulk pyrohydrolysis analyses indicates that F is primarily hosted within the crystal lattice of eclogitic minerals. However, SIMS-reconstructed Cl abundances are a factor of five lower, on average, than pyrohydrolysis-derived bulk concentrations. This discrepancy results from the contribution of fluid inclusions, which may host at least 80% of the bulk rock Cl. The combination of SIMS and pyrohydrolysis is highly complementary. Whereas SIMS is well suited to determine bulk F abundances, pyrohydrolysis better quantifies bulk Cl concentrations, which include the contribution of fluid inclusion-hosted Cl. Raspas eclogites contain 145–258 μg/g F and at least 7–11 μg/g Cl. We estimate that ~95% of F is retained in the slab through eclogitization and returned to the upper mantle during subduction, whereas at least 95% of subducted Cl is removed from the rock by the time the slab equilibrates at eclogite facies conditions. Our calculations provide further evidence for the fractionation of F from Cl during high-pressure metamorphism in subduction zones. Although the HIMU (high U/Pb) mantle source (dehydrated oceanic crust) is often associated with enrichments in Cl/K and F/Nd, Raspas eclogites show relatively low halogen ratios identical within uncertainty to depleted MORB mantle (DMM). Thus, the observed halogen enrichments in HIMU ocean island basalts require either further fractionation during mantle processing or recycling of a halogen-enriched carrier lithology such as serpentinite into the mantle.

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Structure and equilibria among silicate species in aqueous fluids in the upper mantle: Experimental SiO2-H2O and MgO-SiO2-H2O data recorded in situ to 900°C and 5.4 GPa

Journal of Geophysical Research Solid Earth ◽

10.1002/2013jb010537 ◽

2013 ◽

Vol 118 (12) ◽

pp. 6076-6085 ◽

Cited By ~ 17

Author(s):

B. O. Mysen ◽

K. Mibe ◽

I.-M. Chou ◽

W. A. Bassett

Keyword(s):

Upper Mantle

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Age constraints for rare felsic mantle xenoliths from Elie Ness, Scottish Midland Valley

10.5194/egusphere-egu2020-20848 ◽

2020 ◽

Author(s):

Eszter Badenszki ◽

J. Stephen Daly ◽

Martin J. Whitehouse ◽

Brian G. J. Upton

Keyword(s):

Upper Mantle ◽

Solid Phase ◽

Mantle Xenoliths ◽

Solid Phase Reaction ◽

Phase Reaction ◽

Magma Intrusion ◽

Accessory Zircon ◽

Complex Source ◽

Age Constraints

EN-101, a rare albitite [Pl +Fe-Ti oxide +Ap +Zrn] xenolith from Elie Ness, Scottish Midland Valley, is hosted by a c. 290 Ma old alkali basaltic diatreme [1, 2].&#160; EN-101 is considered to belong to the Scottish &#8220;anorthoclasite suite&#8221; comprising xenoliths and megacrysts of various compositions which are interpreted as samples from the upper mantle &#8211; lower crust where they form (syenitic) vein or dyke-like bodies e.g., [3, 4, 5]. The &#8220;anorthoclasite suite&#8221; has been found in all Scottish terranes suggesting that the presumed dyke system must be extensive.Xenoliths of the &#8220;anorthoclasite suite&#8221; primarily consist of Na-rich and Ca-poor feldspar megacrysts, with generally high Na/K ratios [3] that are typically accompanied by accessory zircon, apatite, biotite, magnetite and Fe-rich pyroxene whereas garnet and corundum with Nb-rich oxides are only occasionally present [3, 4, 5]. Upton et al. [4, 5] argued that the parental melt of the &#8220;anorthoclasite suite&#8221; formed though small&#8211;fraction melting of metasomatized mantle and subsequent melt&#8211;solid phase reaction was also involved.&#160; Upton et al. [5] proposed that crystallization of the anorthoclasite suite samples occurred shortly prior to- or contemporaneously with their entrainment. However so far no in-situ dating has been carried out on these samples.Early attempts to date the anorthoclasite suite using zircon and feldspar megacrysts from Elie Ness suggested at least a two-stage formation mechanism, where zircon megacrysts yielded a U-Pb age of c. 318&#160;Ma, while euhedral feldspar xenocrysts are significantly younger and roughly coeval with the host volcanism yielding a K-Ar whole-rock age of c. 294 Ma [6].&#160; In this study we present the first in situ U-Pb dating of zircon, which yielded a concordia age of 328 &#177; 2 Ma (MSWD=0.19; n=12) for EN-101. Zircons &#949;Hf328 values range from +5.2 to +7.5 consistent with a mildly depleted source refreshed by metasomatism. These results may indicate that the proposed extensive syenitic veining within the Scottish upper mantle not only has a complex source [5], but is possibly the result of repeated episodes of magma intrusion.References:<ol><li>Gernon, T.M. et al. 2013 Bulletin of Volcanology. 75:1-20.</li> <li>Gernon, T.M. et al. 2016 Lithos. 264:70-85.</li> <li>Aspen, P. et al. 1990 European Journal of Mineralogy 2:503-17.</li> <li>Upton, B.G.J. et al. 1990 Journal of Petrology.40:935-56.</li> <li>Upton, B.G.J. et al. 2009 Mineral Mag. 73:943-56.</li> <li>Macintyre, R.M. et al. 1981 Transactions of the Royal Society of Edinburgh: Earth Sciences. 72:1-7.</li> </ol>

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Amphibole in a spinel lherzolite xenolith: evidence for volatiles and partial melting in the upper mantle beneath southern British Columbia

Canadian Journal of Earth Sciences ◽

10.1139/e84-111 ◽

1984 ◽

Vol 21 (9) ◽

pp. 1067-1072 ◽

Cited By ~ 7

Author(s):

Mark Brearley ◽

Christopher M. Scarfe

Keyword(s):

British Columbia ◽

Partial Melting ◽

Upper Mantle ◽

Stability Field ◽

Spinel Lherzolite ◽

Ultramafic Xenolith ◽

Metasomatic Fluid ◽

Small Grains ◽

First Time

Pargasitic amphibole has been observed for the first time in an ultramafic xenolith from British Columbia. The xenolith is a chrome diopside-bearing spinel lherzolite trapped within an alkali basaltic lava flow at Lightning Peak, near Vernon, British Columbia. Amphibole (<5%) occurs within the xenolith as small grains, interstitial between other xenolith mineral phases, and always shows evidence of melting. Microprobe analyses of the amphibole reveal that it is a pargasite rich in MgO (MgO = 17.1–17.7 wt.%; Mg/(Mg + Fe2+) = 0.89) and CaO (10.4–10.7 wt.%). Textural and chemical evidence suggests that the pargasite is in equilibrium with the other phases in spinel lherzolite. The pargasite probably crystallized within the spinel stability field of the upper mantle from a volatile-rich metasomatic fluid that was produced by dehydration of subducted material. Melting in the amphibole may have been caused by one of three processes: superheating by the host alkali basalt, decompression as the magma ascended, or by in situ partial melting within the upper mantle. The partial melting of amphibole-bearing spinel lherzolite provides a possible mechanism for the generation of late Cenozoic alkalic magmas of the Intermontane Belt of British Columbia.

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