Bushveld symplectic and sieve-textured chromite is a result of coupled dissolution-reprecipitation: a comparison with xenocrystic chromite reactions in arc basalt

Yudovskaya, Marina A.; Costin, Gelu; Shilovskikh, Vladimir V.; Chaplygin, Ilya; McCreesh, Matthew; Kinnaird, Judith A.

doi:10.1007/s00410-019-1613-3

Cited by 21 publications

(12 citation statements)

References 73 publications

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“…We restrict ourselves to suggesting the unusual, amoeboidal textures of some of the Cr-spinel grains are indicative of several stages of growth and re-equilibration, i.e., consistent with multiple replenishment events. Yudovskaya et al (2019) came to a similar conclusion from a more detailed study of similar Cr-spinels from the Turfspruit locality.…”

Section: Discussionsupporting

confidence: 63%

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Non-Sequential Injection of PGE-rich Ultramafic Sills in the Platreef Unit at Akanani, Northern Limb of the Bushveld Complex: Evidence from Sr and Nd Isotopic Systematics

Scoon

Costin

Mitchell³

et al. 2020

Journal of Petrology

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The Platreef Unit is a deceptively complex sequence of layered cumulates located in the northern limb of the 2.055 Ga-old Bushveld Complex. The unit includes the Platreef, a thick, richly mineralized stratabound PGE orebody which differs markedly from the comparatively thin, predominantly stratiform Merensky Reef found in the Upper Critical Zone of the eastern and western limbs. The Platreef Unit is, however, interpreted as a localized facies of the Upper Critical Zone, despite layering being neither as systematic nor as clearly defined as in the equivalent stratigraphy found in the other limbs. The Platreef Unit in the Akanani project area includes well-defined layers of feldspathic harzburgite and norite, in addition to the ubiquitous feldspathic orthopyroxenite-melanorite that characterizes other sections. The paucity of floor-rock xenoliths is an additional feature. The relatively well-developed nature of the layering and paucity of xenoliths in the Platreef Unit at Akanani is explained by separation of the unit from the floor of the intrusion by a thick succession of ultramafics assigned to the Lower Critical Zone. We identify three lithological subgroups in the Platreef Unit at Akanani. They do not define an upward-younging stratigraphy. The primary stratigraphy, or PU1 subunit, is dominated by multiple layers of feldspathic orthopyroxenite, melanorite, and norite. This subunit built up from incremental addition of relatively small magma pulses. Repeated magma replenishment induced concomitant partial melting of earlier-formed layers. The PU1 subunit includes thin chromite stringers that contain Cr-spinels with unusual, amoeboidal textures consistent with several stages of growth and re-equilibration. The feldspathic harzburgite of the younger PU2 subunit was emplaced non-sequentially into the already complexly-layered PU1 subunit as a series of sinuous lenses or syn-intrusive sills. One of the PU2 sills contains the richest and most consistent of the mineralized sections at Akanani, i.e., the Main Mineralized Reef (MMR). The irregularly-developed pegmatoidal lithologies of the PU3 subunit are ascribed to recrystallization of earlier-formed cumulates (PU1 and PU2). Whole rock isotopic data for a section of the Platreef Unit, together with the overlying Lower Main Zone and underlying Lower Critical Zone, mostly from drill-hole ZF-1, demonstrate a complex pattern in both Sr87/Sr86 initial ratios and ԐNd values. These patterns are consistent with multiple lineages of parental magmas. The Lower Main Zone and the majority of the Platreef Unit are characterized by anomalously high Sr initial ratios (with a large degree of scatter) and low ԐNd values (relatively tightly constrained). Harzburgite layers from the Lower Critical Zone have a low Sr initial ratio and a relatively high ԐNd value. The new isotopic data suggest these sequences crystallized from multiple magma batches, broadly constrained within the U-type (ultramafic) and A-type (tholeiitic) lineages, derived from mantle sources and/or staging chambers which experienced varying degrees of crustal contamination. The MMR crystallized from a specific pulse of the U-type magma lineage characterized by a high Sr87/Sr86 initial ratio (average of 0.71113) and a markedly low ԐNd value (average of -11.35). The olivine-saturated magmas associated with the MMR were derived from a localized mantle source and yet underwent an unusually high degree of crustal contamination. Some of layered PGE orebodies in the Bushveld Complex, including the Platreef and Merensky Reef, were emplaced as syn-magmatic sills which crystallized from anomalously PGE-rich parental magmas with an unique isotopic fingerprint.

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Section: Discussionsupporting

confidence: 63%

“…Some of the amoeboidal grains contain prominent silicate cores. Similar features have been described byYudovskaya et al (2019) from the Turfspruit locality. Euhedral grains of Cr-spinel may contain silicate inclusions(Figs.…”

supporting

confidence: 80%

Non-Sequential Injection of PGE-rich Ultramafic Sills in the Platreef Unit at Akanani, Northern Limb of the Bushveld Complex: Evidence from Sr and Nd Isotopic Systematics

Scoon

Costin

Mitchell³

et al. 2020

Journal of Petrology

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“…However, spinel exsolution, which is characteristic of high-T magnetite 36 , makes it unlikely that fusible alkali-rich carbonates and salts were entrapped as solids also. Rather, hybrid entrapment of alkali-bearing carbonatite melt and crystal phases took place, as has been suggested for a number of different intrusive rocks 43 – 45 (Fig. 5 b).…”

Section: Discussionmentioning

confidence: 54%

Origin of alkali-rich volcanic and alkali-poor intrusive carbonatites from a common parental magma

Chayka

Kamenetsky

Vladykin

et al. 2021

Sci Rep

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The discrepancy between Na-rich compositions of modern carbonatitic lavas (Oldoinyo Lengai volcano) and alkali-poor ancient carbonatites remains a topical problem in petrology. Although both are supposedly considered to originate via fractional crystallization of a “common parent” alkali-bearing Ca-carbonatitic magma, there is a significant compositional gap between the Oldoinyo Lengai carbonatites and all other natural compositions reported (including melt inclusions in carbonatitic minerals). In an attempt to resolve this, we investigate the petrogenesis of Ca-carbonatites from two occurrences (Guli, Northern Siberia and Tagna, Southern Siberia), focusing on mineral textures and alkali-rich multiphase primary inclusions hosted within apatite and magnetite. Apatite-hosted inclusions are interpreted as trapped melts at an early magmatic stage, whereas inclusions in magnetite represent proxies for the intercumulus environment. Melts obtained by heating and quenching the inclusions, show a progressive increase in alkali concentrations transitioning from moderately alkaline Ca-carbonatites through to the “calcite CaCO3 + melt = nyerereite (Na,K)2Ca2(CO3)3” peritectic, and finally towards Oldoinyo Lengai lava compositions. These results give novel empirical evidence supporting the view that Na-carbonatitic melts, similar to those of the Oldoinyo Lengai, may form via fractionation of a moderately alkaline Ca-carbonatitic melt, and therefore provide the “missing piece” in the puzzle of the Na-carbonatite’s origin. In addition, we conclude that the compositions of the Guli and Tagna carbonatites had alkali-rich primary magmatic compositions, but were subsequently altered by replacement of alkaline assemblages by calcite and dolomite.

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“…Альтернативная точка зрения на происхождение кристаллов хромшпинелида подобной морфологии недавно была высказана в работе (Yudovskaya et al, 2019) на примере расслоенных ультрамафитов Бушвельда и мантийных ксенолитов в базальтах Курильских островов. Авторы интерпретируют образование ветвящихся (fish hook) кристаллов хромшпинелидов как результат «растворения-кристаллизации» при взаимодействии кристаллов с просачивающимися расплавами.…”

Section: Discussionunclassified

“…Он ксеноморфен по отношению к силикатным минералам, имеет футляровидный габитус, и, вместе с тем, нельзя не отметить стремление его внешних контуров к принятию кристаллографических очертаний, характерных для минералов кубической сингонии. Описанный хромшпинелид сходен с кристаллами типа фишхук (fish hook), но этот термин чаще используется при описании ксеноморфных хромшпинелидов из расслоенных интрузий (Yudovskaya et al, 2019).…”

Section: фактический материалunclassified

Morphological and Compositional Features of Chromian Spinel from Mantle Ultramafic Rocks of The Nurali Massif (South Urals)

Saveliev

2019

Минералогия (Mineralogy)

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Accessory chromian spinels of lherzolites and dunites from a mantle section of the Nurali ophiolite massif are described in the paper. Lherzolites typically host anhedral chromian spinel grains associated with olivine, pyroxenes and plagioclase. The compositions of silicates and chromian spinels are typical of those from ophiolite mantle sections. Olivine and orthopyroxene are characterized by high Mg content (forsterite and enstatite); clinopyroxene is diopside. The compositions of chromian spinel on the Al–Cr–Fe+3 plot occur close to the Al–Cr side. The #Cr and #Mg values of chromian spinels increase from lherzolites to dunites. Both vermicular spinels trapping olivine and orthopyroxene fragments (type I) and symplectite-like intergrowths of chromian spinel and plagioclase (type II) are most genetically interesting. Type I formed during synkinematic growth in deformed silicate matrix. Type II possibly formed as a result of decompression breakdown of a high-P mineral phase enriched in Cr, Al and Ca (a knorringite-type garnet?). In dunites, numerous tiny chromian spinel rods (type IV) in plastic deformed olivine are observed along with typical euhedral chromian spinel (type III) with inclusions of olivine and pargasite. Latter ones locally occur closely to fne pargasite grains. The formation of chromian spinel rods is explained as a result deformation-induced segregation of trace elements on the structural defects of the olivine lattice. Figures 7. Tables 4. References 48.

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Bushveld symplectic and sieve-textured chromite is a result of coupled dissolution-reprecipitation: a comparison with xenocrystic chromite reactions in arc basalt

Cited by 21 publications

References 73 publications

Non-Sequential Injection of PGE-rich Ultramafic Sills in the Platreef Unit at Akanani, Northern Limb of the Bushveld Complex: Evidence from Sr and Nd Isotopic Systematics

Non-Sequential Injection of PGE-rich Ultramafic Sills in the Platreef Unit at Akanani, Northern Limb of the Bushveld Complex: Evidence from Sr and Nd Isotopic Systematics

Origin of alkali-rich volcanic and alkali-poor intrusive carbonatites from a common parental magma

Morphological and Compositional Features of Chromian Spinel from Mantle Ultramafic Rocks of The Nurali Massif (South Urals)

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