A novel mechanism of spheroidal weathering: a case study from the Monchepluton layered complex, Kola Peninsula, Russia

Arkov, Andrei Y. B; Aa, Nikiforov; Artin, Robert F. M

doi:10.17741/bgsf/87.2.003

Cited by 10 publications

(11 citation statements)

References 15 publications

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“…These spheroids observed in a dunitic rock of the Pados-Tundra complex (2.15 Ga) are similar to the patterns described recently, as a novel mechanism of spheroidal weathering, in a mineralized harzburgite of the Monchepluton layered complex (2.5 Ga), also from the Kola Peninsula, Russia (Barkov et al, 2015). Here, we compare textures, compositions and mineral associations which are characteristic of these two occurrences and discuss factors responsible for the origin of the spheroid type patterns of weathering in ultramafic rocks.…”

Section: Introductionsupporting

confidence: 86%

“…2a,b) are notably similar to those developed at Mount Kumuzh'ya in the Monchepluton complex (Figs. 2c, d;Barkov et al, 2015). At Pados-Tundra, the relief spheroids (1.5 to 4 cm across) are sparsely developed and hosted by an olivine-based matrix.…”

Section: Protruding Spheroidal Weathering At Pados-tundramentioning

confidence: 99%

“…The WDS point-analyses gave narrow compositional ranges documented for 30 grains of olivine and chromite each, and 10 grains of talc and tremolite each, analyzed in their variously textured forms present in the spheroidally weathered dunite at Pados-Tundra. Barkov et al, 2015) complexes. The observed spheroids (S), hosted by the olivine-rich matrix, are composed of orthopyroxene oikocrysts, which are entirely replaced by deuteric hydrous silicates (talc + tremolite) at Pados-Tundra.…”

Section: Protruding Spheroidal Weathering At Pados-tundramentioning

confidence: 99%

“…At Monchepluton, the spheroids (Figs. 2c, d) are 3-4 cm oikocrysts of a highly magnesian orthopyroxene: En 86 , which enclose aggregates of chromitemagnesiochromite, and host inclusions of high-Fo olivine and magnesian augite (Barkov et al, 2015).…”

Section: Comparison With Moncheplutonmentioning

confidence: 99%

“…(1) They both are associated with oikocrysts or spheroids of orthopyroxene (entirely replaced or well preserved) associated with olivine cumulates in the matrix; (2) Grains of chromian spinel in these rocks (dunite vs. harzburgite: Table 2) are aluminous magnesian chromite; their compositions are notably similar, with a somewhat higher content of Mg (magnesiochromite component) and Al, and correspondingly, lower Fe and Cr in the __________________________________________________________________________________________________________ Note: Spheroidal patterns of weathering from the Pados-Tundra complex (this study) are compared with the patterns described from Mount Kumuzh'ya in the Monchepluton complex (Barkov et al, 2015). mg# is 100Mg/(Mg+Fe 2+ ).…”

Section: Comparison With Moncheplutonmentioning

confidence: 99%

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The origin of spheroidal patterns of weathering in the Pados-Tundra mafic-ultramafic complex, Kola Peninsula, Russia

Barkov¹,

Aa²,

Halkoaho³

et al. 2016

Bull Geol Soc Finland

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We document a new and unusual occurrence of patterns of protruding spheroidal weathering developed in a dunitic rock of the Pados-Tundra mafic-ultramafic complex of Early Proterozoic age, Kola Peninsula, Russia. It provides an example similar to that reported recently from a mineralized harzburgite in the Monchepluton layered complex in the same region. These patterns are genetically different from common results of "normal spheroidal weathering" sensu stricto. The spheroidally weathered dunite at Pados-Tundra consists of a high-Fo olivine, Ol (Fo 87.5 ), which is, in fact, not altered. Accessory grains of aluminous chromite are present. Relief spheroids (1.5 to 4 cm in diameter; up to ~5 vol. %) are distributed sparsely and heterogeneously. They are hosted by the olivine matrix and composed of talc, Tlc, and tremolite, Tr, (Mg# = 95-96) formed presumably at the expense of orthopyroxene, Opx, (i.e., pre-existing oikocrysts) during a deuteric (autometasomatic) alteration. In contrast, oikocrystic Opx (En 86.0 ) is quite fresh in related spheroids at Monchepluton, in which only minor deuteric alteration (Tlc + Tr) are observed. We infer that (1) the ball-shaped morphology of the weathered surface is a reflection of the presence of oikocrysts of Opx, which crystallized after Ol at the magmatic stage; they were entirely replaced by the deuterically induced Tlc + Tr at Pados-Tundra. (2) Differential rates of weathering are implied for rock-forming minerals in these ultramafic rocks, with a higher resistance of Opx vs. Fo-rich Ol, and Tlc + Tr vs. Fo-rich Ol. (3) The ball-like shape of the large spheroids, produced by magmatic processes, may likely represent an additional factor of their higher stability to weathering in the superficial environment. Similar patterns can be expected in other mafic-ultramafic complexes, especially in layered intrusions.

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Section: Introductionsupporting

confidence: 86%

Section: Protruding Spheroidal Weathering At Pados-tundramentioning

confidence: 99%

Section: Protruding Spheroidal Weathering At Pados-tundramentioning

confidence: 99%

Section: Comparison With Moncheplutonmentioning

confidence: 99%

Section: Comparison With Moncheplutonmentioning

confidence: 99%

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The origin of spheroidal patterns of weathering in the Pados-Tundra mafic-ultramafic complex, Kola Peninsula, Russia

Barkov¹,

Aa²,

Halkoaho³

et al. 2016

Bull Geol Soc Finland

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Compositional Variations of Apatite and REE-Bearing Minerals in Relation to Crystallization Trends in the Monchepluton Layered Complex (Kola Peninsula)

Barkov

Шарков

et al. 2021

Russian Geology and Geophysics

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––We have investigated the compositional variations of apatite (Ap) and rare-earth element (REE) minerals in the Monchepluton layered complex on the Kola Peninsula. On the basis of large sets of pertinent analytical data, we have estimated geochemical trends involving major, minor, and trace elements and studied their relation with the compositions of rock-forming silicate and oxide minerals. The variations observed in Ap differ considerably from trends reported for other layered intrusions. The composition fields of Ap are not consistent with the variations in the chemical composition of the bulk rocks and their constituent minerals, as determined along the representative cross sections of the entire complex. The compositional variations of Ap are fairly similar in all units of the complex. Chlorapatite (>6 wt.% Cl) is invariably abundant. There is no relationship between the Cl content of Ap and the degree of magnesium enrichment of the coexisting early magmatic silicates. In the F–Cl–OH diagram, broad fields of ternary solid solution are observed. There are no compositions along the Cl–F axis. The compositions of Ap are notably poor in Cl in the marginal series (the Nyud massif) and correspond to hydroxylapatite with a high content of fluorapatite component. Two composition fields of Ap are recognized in the Monchepluton complex: ≤3 wt.% and >6 wt.% Cl; there are, however, extensive overlaps. Two generations of apatite are thus implied. The first nucleated at the early stage of crystallization of H2O-bearing intercumulus melt as a result of substantial increase in the contents of P, F, Cl, and other incompatible components. The following stage of degassing of the crystallizing melt caused a decoupling of Cl and F. Fluorine remained mostly in the melt; in contrast, Cl was partitioned efficiently into an H2O-bearing fluid phase. At the early stage, the apatite incorporated combinations of hydroxylapatite and fluorapatite, with a low content of Cl. At the late stage, chlorapatite crystallized from a Cl-rich fluid, and ferrochloropargasite (4.1 wt.% Cl) formed in the Poaz massif as a result of autometasomatic alteration via reactions of this fluid with plagioclase and pyroxene. The apatite has high Sr contents (up to 4.1 wt.% SrO) in the highly magnesian cumulates of the Dunite block and the massifs of mounts Kumuzh’ya, Nittis, and Travyanaya. This enrichment illustrates the accumulation of Sr in the intercumulus melt, in which Ap was the only Sr-bearing phase in the absence or scarcity of intercumulus plagioclase. The REE contents also increased in the intercumulus melt and led to the formation of monazite-(Ce), REE-bearing Ap, and allanite-(Ce) in the remaining microvolumes of melt. Loveringite and Ap crystallized as coexisting phases in Mt. Sopcha. For the first time in a layered intrusion, an extensive range of compositions is documented in the Ce–La–Nd diagram for the REE-bearing phosphates (monazite and REE-rich apatite), which display a predominant La ↔ Nd substitution at the constant contents of Ce.

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Ultramagnesian Olivine in the Monchepluton (Fo96) and Pados-Tundra (Fo93) Layered Intrusions (Kola Peninsula)

Barkov

Martin

Изох

et al. 2021

Russian Geology and Geophysics

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—The paper focuses on compositional variations of olivine and chromian spinel in the Monchepluton and Pados-Tundra layered intrusions, which host significant chromitite mineralization. Ore-bearing dunite (with up to 25–30 vol.% Mcr) in the Sopcheozerskoe chromite deposit from the Monchepluton complex, Kola Peninsula, Russia, bears an assemblage of phases with exceptionally high magnesium contents: Fo96 + augite (Mg# = 94) + magnesiochromite, Mcr (Mg# ≈ 65); Mg# = 100·Mg/(Mg + Fe2+ + Mn). However, olivine in the host dunite has normal maximum values of Mg# comparable to those in cumulus olivine from layered intrusions worldwide (Fo≤91–92). The Fo96 phase in the Sopcheozerskoe deposit shows the most primitive composition ever reported from any layered intrusion. Magnesiochromite occurs as unzoned homogeneous euhedral crystals unaffected by subsolidus exchange or metasomatic effects. Olivine in ore-bearing dunite (20–25 vol.% magnesian chromite) from the Pados-Tundra complex attains Fo93, with the Mg# value notably higher than the range (Fo85.5–90.6) in olivine from orthopyroxenite, harzburgite, and dunite within the intrusion. Olivine and chromian spinel in the two complexes behave coherently, with covarying patterns of Mg# and Ni contents in olivine at R = 0.75 (n = 160) and positive correlation between Mg# in coexisting chromian spinel and olivine grains at R = 0.8 (n = 150). This behavior indicates that the two phases attained equilibrium during crystallization. It appears unlikely that the extremely high Mg enrichment in olivine (Fo96), as well as in all associated phases of the Monchepluton complex, would result from a subsolidus reaction between olivine and chromian spinel or low-temperature alteration of olivine. We suggest a more realistic explanation that the olivine (+ high-Mg augite)–chromian spinel assemblage crystallized from komatiitic magma under the conditions of progressively increasing oxygen fugacity (fO2). The high Mg# in the Mcr-chromite-enriched system, above the maximum values common in cumulus olivine from layered intrusions (up to Fo96 against Fo≤91–92), may be caused by shortage of ferrous iron.

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A novel mechanism of spheroidal weathering: a case study from the Monchepluton layered complex, Kola Peninsula, Russia

Cited by 10 publications

References 15 publications

The origin of spheroidal patterns of weathering in the Pados-Tundra mafic-ultramafic complex, Kola Peninsula, Russia

The origin of spheroidal patterns of weathering in the Pados-Tundra mafic-ultramafic complex, Kola Peninsula, Russia

Compositional Variations of Apatite and REE-Bearing Minerals in Relation to Crystallization Trends in the Monchepluton Layered Complex (Kola Peninsula)

Ultramagnesian Olivine in the Monchepluton (Fo96) and Pados-Tundra (Fo93) Layered Intrusions (Kola Peninsula)

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