Composition of spherules and lower mantle minerals, isotopic and geochemical characteristics of zircon from volcaniclastic facies of the Mriya lamproite pipe

The article presents the results of studying the rocks of the pyroclastic facies of the Mriya lamproite pipe, located on the Priazovsky block of the Ukrainian shield. In them the rock's mineral composition includes a complex of exotic mineral particles formed under extreme reduction mantle conditions: silicate spherules, particles of native metals and intermetallic alloys, oxygen-free minerals such as diamond, qusongite (WC), and osbornite (TiN). The aim of the research is to establish the genesis of volcaniclastic rocks and to develop ideas of the highly deoxidized mantle mineral association (HRMMA), as well as to conduct an isotopic and geochemical study of zircon. As a result, groups of minerals from different sources are identified in the heavy fraction: HRMMA can be attributed to the juvenile magmatic component of volcaniclastic rocks; a group of minerals and xenoliths that can be interpreted as xenogenic random material associated with mantle nodules destruction (hornblendite, olivinite and dunite xenoliths), intrusive lamproites (tremolite-hornblende) and crystalline basement rocks (zircon, hornblende, epidote, and granitic xenoliths). The studied volcaniclastic rocks can be defined as intrusive pyroclastic facies (tuffisites) formed after the lamproites intrusion. Obviously, the HRMMA components formed under extreme reducing conditions at high temperatures, which are characteristic of the transition core-mantle zone. Thus, we believe that the formation of primary metal-silicate HRMMA melts is associated with the transition zone D".

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Woodcutters 55 Years Later: A New Look at an Old Discovery

SEG Discovery ◽

10.5382/segnews.2020-120.fea ◽

2020 ◽

pp. 15-21

Author(s):

Dave Shatwell

Keyword(s):

Chloride Content ◽

Mississippi Valley ◽

Deep Basin ◽

Widespread Occurrence ◽

Great Oxidation Event ◽

Basement Rocks ◽

Reducing Conditions ◽

Volcaniclastic Rocks ◽

Pine Creek ◽

Shallow Marine Sediments

Abstract The Woodcutters Zn-Pb-Ag deposit in the Rum Jungle district of the Pine Creek orogen in northern Australia was discovered in 1964 and produced 4.6 Mt of ore grading 12.3% Zn, 5.6% Pb, and 83 g/t Ag between 1985 and 1999. Woodcutters, together with several other polymetallic, uranium, and phosphate deposits, is within a Paleoproterozoic sequence of fluviatile and shallow marine sediments deposited in a deepening basin between ~2100 and 2025 Ma around the margins of an Archean granitic and gneissic dome. These sediments were overlain by turbidites and volcaniclastic rocks until the basin was inverted and the sediments and mineral deposits were deformed and metamorphosed at 1860 Ma. Whereas the polymetallic and uranium bodies at Rum Jungle are considered to be syngenetic or syndiagenetic, sulfides in the Woodcutters orebody replace dolomitic horizons in an otherwise carbonaceous unit. This suggests that Woodcutters is similar to Mississippi Valley-type mineralization and rules out affinities with younger sedimentary exhalative-style deposits elsewhere in the Pine Creek orogen. A model is proposed whereby metals were eroded from Archean basement rocks into Paleoproterozoic sandstone aquifers following the Great Oxidation Event, which also liberated sulfur by oxidation of pyrite. Evaporative conditions, as suggested by the widespread occurrence of dolomite and magnesite, may have increased the chloride content of seawater and enhanced its capacity to transport metals. Subsequently, deeply circulating seawater leached metals from the aquifers and ascended up a deep, basin-penetrating fault until it intersected carbonaceous sediments. In this environment, Zn and Pb sulfides were deposited under reducing conditions, while sulfur may have been provided by H2S from organic material. The Woodcutters and other deposits at Rum Jungle show how metals formerly locked up in Archean cratons were delivered by erosion under an oxygenated atmosphere to Paleoproterozoic shorelines, where they were further mobilized and concentrated by a variety of processes.

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Diamond formation in an electric field under deep Earth conditions

Science Advances ◽

10.1126/sciadv.abb4644 ◽

2021 ◽

Vol 7 (4) ◽

pp. eabb4644

Author(s):

Yuri N. Palyanov ◽

Yuri M. Borzdov ◽

Alexander G. Sokol ◽

Yuliya V. Bataleva ◽

Igor N. Kupriyanov ◽

...

Keyword(s):

Electric Fields ◽

Lithospheric Mantle ◽

Isotope Fractionation ◽

Diamond Formation ◽

Mantle Minerals ◽

Electrochemical Processes ◽

Mantle Mineral ◽

Deep Earth ◽

Diamond Crystallization ◽

Global Carbon

Most natural diamonds are formed in Earth’s lithospheric mantle; however, the exact mechanisms behind their genesis remain debated. Given the occurrence of electrochemical processes in Earth’s mantle and the high electrical conductivity of mantle melts and fluids, we have developed a model whereby localized electric fields play a central role in diamond formation. Here, we experimentally demonstrate a diamond crystallization mechanism that operates under lithospheric mantle pressure-temperature conditions (6.3 and 7.5 gigapascals; 1300° to 1600°C) through the action of an electric potential applied across carbonate or carbonate-silicate melts. In this process, the carbonate-rich melt acts as both the carbon source and the crystallization medium for diamond, which forms in assemblage with mantle minerals near the cathode. Our results clearly demonstrate that electric fields should be considered a key additional factor influencing diamond crystallization, mantle mineral–forming processes, carbon isotope fractionation, and the global carbon cycle.

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Jurassic–Cretaceous arc magmatism along the Shyok–Bangong Suture from NW Himalaya: Formation of the peri-Gondwana basement to the Ladakh Arc

Journal of the Geological Society ◽

10.1144/jgs2021-035 ◽

2021 ◽

pp. jgs2021-035

Author(s):

Wanchese M. Saktura ◽

Solomon Buckman ◽

Allen P. Nutman ◽

Renjie Zhou

Keyword(s):

Late Cretaceous ◽

Nw Himalaya ◽

Content Type ◽

Magmatic Arc ◽

Basement Rocks ◽

Ladakh Himalaya ◽

Accreted Terranes ◽

Volcaniclastic Rocks ◽

Supplementary Material ◽

Depositional Age

The Jurassic–Cretaceous Tsoltak Formation from the eastern borderlands of Ladakh Himalaya consists of conglomerates, sandstones and shales, and is intruded by norite sills. It is the oldest sequence of continent-derived sedimentary rocks within the Shyok Suture. It also represents a rare outcrop of the basement rocks to the voluminous Late Cretaceous–Eocene Ladakh Batholith. The Shyok Formation is a younger sequence of volcaniclastic rocks that overlie the Tsoltak Formation and record the Late Cretaceous closure of the Mesotethys Ocean. The petrogenesis of these formations, ophiolite-related harzburgites and norite sill is investigated through petrography, whole-rock geochemistry and U–Pb zircon geochronology. The youngest detrital zircon grains from the Tsoltak Formation indicate Early Cretaceous maximum depositional age and distinctly Gondwanan, Lhasa microcontinent-related provenance with no Eurasian input. The Shyok Formation has Late Cretaceous maximum depositional age and displays a distinct change in provenance to igneous detritus characteristic of the Jurassic–Cretaceous magmatic arc along the southern margin of Eurasia. This is interpreted as a sign of collision of the Lhasa microcontinent and the Shyok ophiolite with Eurasia along the once continuous Shyok–Bangong Suture. The accreted terranes became the new southernmost margin of Eurasia and the basement to the Trans-Himalayan Batholith associated with the India-Eurasia convergence.Supplementary material:https://doi.org/10.6084/m9.figshare.c.5633162

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Geochemical study on genesis of natural gases accumulated in deep volcaniclastic rocks.

Journal of the Japanese Association for Petroleum Technology ◽

10.3720/japt.51.228 ◽

1986 ◽

Vol 51 (3) ◽

pp. 228-237 ◽

Cited By ~ 11

Author(s):

Susumu SAKATA ◽

Makoto TAKAHASHI ◽

Kazuo HOSHINO

Keyword(s):

Natural Gases ◽

Geochemical Study ◽

Volcaniclastic Rocks

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The sapphirine-bearing rocks in contact with the Lherz peridotite body: New mineralogical data, age and interpretation

BSGF - Earth Sciences Bulletin ◽

10.1051/bsgf/2019015 ◽

2020 ◽

Vol 191 ◽

pp. 5

Author(s):

Jessica Uzel ◽

Yves Lagabrielle ◽

Serge Fourcade ◽

Christian Chopin ◽

Pierre Monchoux ◽

...

Keyword(s):

High Temperature ◽

Contact Zone ◽

White Mica ◽

Lower Jurassic ◽

Upper Triassic ◽

Ultramafic Rocks ◽

Basement Rocks ◽

Petrographic Study ◽

Aluminous Spinel ◽

Different Sources

Sapphirine-bearing rocks are described in the Aulus Basin (Ariège, France) in a contact zone between the Lherz peridotitic body and Mesozoic metasediments which underwent the Pyrenean Cretaceous high-temperature, low-pressure metamorphic event (Monchoux, 1970, 1972a, 1972b). Sapphirine crystals occur in layered clastic deposits characterized by an uncommon suite of Al-Mg-rich minerals. A detailed petrographic study of sixteen samples representative of the diversity of the Lherz sapphirine-bearing rocks is presented. These rocks include breccias and microbreccias with various compositions. Some samples are composed of polymineralic clasts and isolated minerals that derive from regionally well-known protoliths such as ultramafic rocks, meta-ophites, “micaceous hornfels”, and very scarce Paleozoic basement rocks. Nevertheless, a large portion of the sapphirine-bearing clastic suite is composed of mono- and polymineralic debris that derive from unknown protolith(s). We define a "sapphirine-bearing mineral suite” (SBMS) composed of monomineralic debris including: sapphirine + enstatite + aluminous spinel + Mg-amphiboles + Ca-amphiboles + kornerupine + accessory minerals (apatite, diopside, rutile, serpentine, smectite, tourmaline, vermiculite and a white mica). We highlight the dominance of metamorphic Keuper clastic materials in the studied rocks and the presence of inclusions of anhydrite and F-, Cl-, Sr-rich apatite in minerals of the Al-Mg-rich suite. The brecciated texture and the presence of unequivocal sedimentary features suggest that the sapphirine-bearing rocks were mechanically disaggregated and then experienced winnowing in underwater conditions with poor mixing between the different sources. We measured U-Pb rutile age data in order to provide constraints on the age of (one of) the protolith(s) of those clastic deposits. The obtained age (98.6 + 1.2 Ma) is interpreted as the age of metamorphism of this protolith of the SBMS. Previous works interpreted the Lherz sapphirine-bearing rocks as crustal protoliths modified at depth along the contact with the ultramafic rocks of the Lherz body during their ascent towards shallower depths. These new data imply: (i) an Upper Triassic to Lower Jurassic origin for the main protolith of the sapphirine-bearing rocks; (ii) the metamorphism of this protolith along an active hot crust–mantle detachment during Cenomanian times with the involvement of metasomatic, brine-type fluids; and (iii) its brecciation during the exhumation of the material due to the evolution of the detachment, followed by subsequent sedimentary reworking of the metamorphic material.

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The niobium and tantalum concentration in the mantle constrains the composition of Earth’s primordial magma ocean

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.2007982117 ◽

2020 ◽

Vol 117 (45) ◽

pp. 27893-27898

Author(s):

Dongyang Huang ◽

James Badro ◽

Julien Siebert

Keyword(s):

Diamond Anvil Cell ◽

Magma Ocean ◽

Core Formation ◽

The Core ◽

Bulk Silicate Earth ◽

Reducing Conditions ◽

Metal Silicate ◽

Diamond Anvil ◽

Laser Heated ◽

Tantalum Concentration

The bulk silicate Earth (BSE), and all its sampleable reservoirs, have a subchondritic niobium-to-tantalum ratio (Nb/Ta). Because both elements are refractory, and Nb/Ta is fairly constant across chondrite groups, this can only be explained by a preferential sequestration of Nb relative to Ta in a hidden (unsampled) reservoir. Experiments have shown that Nb becomes more siderophile than Ta under very reducing conditions, leading the way for the accepted hypothesis that Earth’s core could have stripped sufficient amounts of Nb during its formation to account for the subchondritic signature of the BSE. Consequently, this suggestion has been used as an argument that Earth accreted and differentiated, for most of its history, under very reducing conditions. Here, we present a series of metal–silicate partitioning experiments of Nb and Ta in a laser-heated diamond anvil cell, at pressure and temperature conditions directly comparable to those of core formation; we find that Nb is more siderophile than Ta under any conditions relevant to a deep magma ocean, confirming that BSE’s missing Nb is in the core. However, multistage core formation modeling only allows for moderately reducing or oxidizing accretionary conditions, ruling out the need for very reducing conditions, which lead to an overdepletion of Nb from the mantle (and a low Nb/Ta ratio) that is incompatible with geochemical observations. Earth’s primordial magma ocean cannot have contained less than 2% or more than 18% FeO since the onset of core formation.

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The redox state of the mantle during and just after core formation

Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences ◽

10.1098/rsta.2008.0147 ◽

2008 ◽

Vol 366 (1883) ◽

pp. 4315-4337 ◽

Cited By ~ 62

Author(s):

D.J Frost ◽

U Mann ◽

Y Asahara ◽

D.C Rubie

Keyword(s):

Oxygen Fugacity ◽

Oxidation State ◽

Upper Mantle ◽

Partition Coefficients ◽

Metal Loss ◽

Core Formation ◽

The Core ◽

Mantle Mineral ◽

Metal Silicate ◽

Siderophile Elements

Siderophile elements are depleted in the Earth's mantle, relative to chondritic meteorites, as a result of equilibration with core-forming Fe-rich metal. Measurements of metal–silicate partition coefficients show that mantle depletions of slightly siderophile elements (e.g. Cr, V) must have occurred at more reducing conditions than those inferred from the current mantle FeO content. This implies that the oxidation state (i.e. FeO content) of the mantle increased with time as accretion proceeded. The oxygen fugacity of the present-day upper mantle is several orders of magnitude higher than the level imposed by equilibrium with core-forming Fe metal. This results from an increase in the Fe 2 O 3 content of the mantle that probably occurred in the first 1 Ga of the Earth's history. Here we explore fractionation mechanisms that could have caused mantle FeO and Fe 2 O 3 contents to increase while the oxidation state of accreting material remained constant (homogeneous accretion). Using measured metal–silicate partition coefficients for O and Si, we have modelled core–mantle equilibration in a magma ocean that became progressively deeper as accretion proceeded. The model indicates that the mantle would have become gradually oxidized as a result of Si entering the core. However, the increase in mantle FeO content and oxygen fugacity is limited by the fact that O also partitions into the core at high temperatures, which lowers the FeO content of the mantle. (Mg,Fe)(Al,Si)O 3 perovskite, the dominant lower mantle mineral, has a strong affinity for Fe 2 O 3 even in the presence of metallic Fe. As the upper mantle would have been poor in Fe 2 O 3 during core formation, FeO would have disproportionated to produce Fe 2 O 3 (in perovskite) and Fe metal. Loss of some disproportionated Fe metal to the core would have enriched the remaining mantle in Fe 2 O 3 and, if the entire mantle was then homogenized, the oxygen fugacity of the upper mantle would have been raised to its present-day level.

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Melting of lower continental crust in a young post-collision setting; a geochemical study of Plio-Quaternary acidic magmatism from central Sulawesi (Indonesia)

BSGF - Earth Sciences Bulletin ◽

10.2113/172.3.333 ◽

2001 ◽

Vol 172 (3) ◽

pp. 333-342 ◽

Cited By ~ 11

Author(s):

Mireille Polve ◽

Rene C. Maury ◽

Philippe Vidal ◽

Bambang Priadi ◽

Herve Bellon ◽

...

Keyword(s):

Middle Miocene ◽

Calc Alkaline ◽

Isotopic Study ◽

Crustal Rocks ◽

Basement Rocks ◽

Geochemical Study ◽

Metamorphic Basement ◽

Lower Crustal ◽

Central Sulawesi ◽

Rapid Uplift

Abstract Acidic potassic calc-alkaline (CAK) magmas have been emplaced in the central part of the western arm of Sulawesi from 6.5 to 0.6 Ma, mostly as peraluminous dacites, rhyolites and granites. They overlay or crosscut a high-grade metamorphic basement including lower crustal garnet peridotites and granulites, the latter showing evidences for incipient melting during rapid uplift. Major and trace element data coupled with a Sr, Nd and Pb isotopic study of the CAK magmas and their lower crustal basement rocks demonstrate that they share a number of common features, including radiogenic Sr and Pb and unradiogenic Nd signatures, consistent with those of Australian granulites and Indian Ocean sediments. We propose that the CAK magmas derived from the anatexis of lower crustal rocks of Australian origin (the Banggai-Sula microcontinent) during the phase of uplift which followed their collision with the Sundaland margin (the western arm of Sulawesi) during the Middle Miocene, and possibly the breakoff of the subducted Molucca Sea slab.

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Chapter 18: Geology of Round Mountain, Nevada: A Giant Low-Sulfidation Epithermal Gold Deposit

10.5382/sp.23.18 ◽

2020 ◽

pp. 375-397

Author(s):

David A. Rhys ◽

Nadia St. Jean ◽

Rodolfo Lagos ◽

David Emmons ◽

George A. Schroer ◽

...

Keyword(s):

Lacustrine Sediments ◽

Ore Formation ◽

Kinematic Indicators ◽

Welded Tuff ◽

Basement Rocks ◽

Epithermal Gold Deposit ◽

Volcaniclastic Rocks ◽

Volcanic Facies ◽

Low Sulfidation ◽

Late Stages

Abstract The Round Mountain low-sulfidation epithermal Au deposit occurs within the rhyolitic tuff of Round Mountain (26.86 Ma) on the northeast side of an elliptical volcanic center that has morphology and volcanic facies suggesting it originated as a caldera. The hosting tuff comprises three pyroclastic flow and fall deposits (units T1 to T3). These are overlain successively by lacustrine sediments and volcaniclastic rocks. which may contain paleowater table levels formed at the time of ore formation and a 26.4 Ma postmineralization tuff unit. A linear vertical drop in the basement contact coincides with thick tuff fill and megabreccia, which is interpreted to follow the position of a WNW-trending ring fissure or vent wall that may have focused the locations of subsequent hydrothermal upflow zones. Orebodies are developed in strata-bound zones that are most extensive in poorly welded tuff, focused below overlying impermeable welded tuff in a WNW-trending, gently NW-plunging corridor above and mantling the SW-dipping paleoslope of basement rocks. Ore comprises disseminated pervasive adularia-quartz-pyrite ± illite alteration with electrum. The disseminated mineralization surrounds, and is most intensely developed in association with, a low-displacement extensional fault-vein network composed of conjugate NE- and SW-dipping faults and steeply dipping extensional veins. Vein orientations and kinematic indicators suggest ore formation occurred during localized NE-SW-directed extension that may have been related to late stages of volcanic subsidence, potentially in association with deep resurgent magmatism into ring fissures approximately 0.5 m.y. after deposition of the host tuff sequence.

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