Petrology of Mafic Dykes from the Njimom Area (West-Cameroon): A Contribution to the Characterization of Late Paleozoic and Mesozoic Magmatism in the Southern Continental Part of the Cameroon Volcanic Line

Meteorites provide the only direct record of the chronology and nature of the processes that occurred in the early solar system. In this study, meteorites were examined in order to gain insight into the timing and nature of magmatism and silicate differentiation on asteroidal bodies in the first few million years of the solar system. These bodies are considered the precursors to terrestrial planets, and as such they provide information about conditions in the solar system at the time of planet formation. This study focuses on eucrites, which are basaltic meteorites that are believed to represent the crust of the Howardite-Eucrite-Diogenite (HED) parent body. The processes of silicate differentiation and the relationship between eucrites and the diogenitic mafic cumulate of the HED parent body are poorly understood. The major and trace element chemistry of the minerals in the eucrite suite was measured. There is little variability in mineral major element concentrations in eucrites, however considerable variability was observed in mineral trace element concentrations, particularly with respect to incompatible elements in the mineral phases. Magnesium was separated from digested eucrite samples, and the Mg isotope composition of the eucrites was measured to high precision in order to date the samples using the short-lived ²⁶Al–²⁶Mg chronometer and examine magmatic evolution on the HED parent body. Correlations between incompatible elements in pyroxene and ²⁶Mg anomalies, produced by the decay of ²⁶Al, indicate that the eucrite suite was formed from a single, evolving magma body. Large trace element and Mg isotopic differences between eucrites and diogenites indicate that the two meteorite groups did not, as previously suggested, originate from the same magma body. Instead they may have formed from two large magma bodies, which were spatially or temporally separated on the HED parent body. The application of the short-lived ²⁶Al–²⁶Mg chronometer to this suite of eucrites constrains the onset of eucrite formation to ~3 Myr after the formation of the solar system’s first solids, as a result of rapid accretion and melting of planetesimals due to heating from the decay of ²⁶Al.

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Timing and Mechanisms of Silicate Differentiation and Basalt Magma Generation on the Howardite-Eucrite-Diogenite Asteroid Parent Body

10.26686/wgtn.17142809.v1 ◽

2021 ◽

Author(s):

◽

Jessica Anne Dallas

Keyword(s):

Solar System ◽

Trace Element ◽

Isotope Composition ◽

Parent Body ◽

Basalt Magma ◽

Early Solar System ◽

Magma Body ◽

Element Concentrations ◽

Trace Element Chemistry ◽

Incompatible Elements

Meteorites provide the only direct record of the chronology and nature of the processes that occurred in the early solar system. In this study, meteorites were examined in order to gain insight into the timing and nature of magmatism and silicate differentiation on asteroidal bodies in the first few million years of the solar system. These bodies are considered the precursors to terrestrial planets, and as such they provide information about conditions in the solar system at the time of planet formation. This study focuses on eucrites, which are basaltic meteorites that are believed to represent the crust of the Howardite-Eucrite-Diogenite (HED) parent body. The processes of silicate differentiation and the relationship between eucrites and the diogenitic mafic cumulate of the HED parent body are poorly understood. The major and trace element chemistry of the minerals in the eucrite suite was measured. There is little variability in mineral major element concentrations in eucrites, however considerable variability was observed in mineral trace element concentrations, particularly with respect to incompatible elements in the mineral phases. Magnesium was separated from digested eucrite samples, and the Mg isotope composition of the eucrites was measured to high precision in order to date the samples using the short-lived ²⁶Al–²⁶Mg chronometer and examine magmatic evolution on the HED parent body. Correlations between incompatible elements in pyroxene and ²⁶Mg anomalies, produced by the decay of ²⁶Al, indicate that the eucrite suite was formed from a single, evolving magma body. Large trace element and Mg isotopic differences between eucrites and diogenites indicate that the two meteorite groups did not, as previously suggested, originate from the same magma body. Instead they may have formed from two large magma bodies, which were spatially or temporally separated on the HED parent body. The application of the short-lived ²⁶Al–²⁶Mg chronometer to this suite of eucrites constrains the onset of eucrite formation to ~3 Myr after the formation of the solar system’s first solids, as a result of rapid accretion and melting of planetesimals due to heating from the decay of ²⁶Al.

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Mantle xenoliths from Befang (Oku Massif) in the Cameroon Volcanic Line

10.5194/egusphere-egu2020-4235 ◽

2020 ◽

Author(s):

Sylvin S. T. Tedonkenfack ◽

Jacek Puziewicz ◽

Theodoros Ntaflos ◽

Sonja Aulbach ◽

Anna Kukula ◽

...

Keyword(s):

Mantle Xenoliths ◽

Melt Crystallization ◽

Cameroon Volcanic Line ◽

Volcanic Chain ◽

Continental Part ◽

Continental Lithospheric Mantle ◽

Low Degrees ◽

Ree Patterns ◽

Ree Pattern

Cameroon Volcanic Line (CVL) is a ca. 1600 km long Cenozoic volcanic chain which&#160;crosses the boundary between ocean and continent in West Africa. Its origin, as well as &#160;the nature and age of the underlying continental lithospheric mantle (CLM), is still a matter of debate. Some of the CVL lavas contain peridotite xenoliths that can provide data elucidating the role of the CLM in the sustained magma generation along the line. In this abstract we describe xenolith suite from the Befang pyroclastic cone (< 1Ma) in the Oku Massif in the continental part of CVL, consisting of 14 spinel lherzolites, one spinel harzburgite and one websterite. The xenoliths are between 3 and 21 cm in diameter and have porphyroclastic to serial or equigranular texture, with porphyroclasts of olivine or orthopyroxene up to 9 mm in diameter. Some are weakly foliated. Olivine is Fo 88.6-90.4, contains 0.36 to 0.42 wt.% NiO and 180-750 ppm of Ca. Orthopyroxene (Mg# 0.89-0.91) contains 0.14 &#8211; 0.19 atoms of Al pfu, and clinopyroxene (Mg# 0.90-0.92) contains 0.24 &#8211; 0.31 atoms of Al pfu. The Cr# of lherzolite spinel is 0.09-0.15, in the harzburgitic one it is 0.18-0.19. Pyroxenes in all studied peridotites record a temperature range of 910 &#8211; 1010&#176;C (Brey and K&#246;hler 1990). Clinopyroxenes&#8217; REE patterns are flat at HREE-MREE and make a spectrum from slightly LREE-depleted to slightly LREE-enriched (LaN/LuN from 0.08 to 2.65). The trace-element patterns are flat except well-defined negative Nb-Ta and positive Th-U anomalies. Orthopyroxenes&#8217; REE patterns are variably depleted from HREE to LREE (LaN/LuN from 0.001 to 0.037). The REE pattern of clinopyroxene occurring in websterite exhibits enrichment from HREE towards LREE with hump in Sm/Nd, typical of silicate melt crystallization. The REE pattern of clinopyroxene The Befang lherzolites represent CLM metasomatised by melts produced by various, but generally low degrees of melting of DMM-like (Depleted MORB Mantle) source. Conversely, the harzburgite was formed by low degrees (few percent) of melting of DMM.Acknowledgements. The study was funded by Polish National Centre for Science project UMO-2017/27/B/ST10/00365 to JP. EPMA analyses were done thanks to the Polish-Austrian project WTZ PL 08/2018.References:Brey, G.P. & K&#246;hler, T. (1990). Geothermobarometry in four-phase lherzolites II. New&#160;thermobarometers and practical assessment of existing thermobarometers. Journal of Petrology 31, 1353-1378.

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Geochemical and isotopic compositions of Late Permian metavolcano–sedimentary sequence in the southern Songpan–Ganzi Orogen, eastern Tibetan Plateau

Geological Magazine ◽

10.1017/s0016756820000230 ◽

2020 ◽

Vol 157 (12) ◽

pp. 2004-2020

Author(s):

Yudi Zhu ◽

Yanpei Dai ◽

Lili Wang ◽

Di Xiu ◽

Chao Chen

Keyword(s):

Trace Element ◽

Tectonic Setting ◽

Crustal Contamination ◽

Detrital Zircons ◽

Large Igneous Province ◽

Stability Field ◽

Late Permian ◽

Sedimentary Sequence ◽

Low Degrees ◽

Trace Element Profiles

AbstractThe Permian metavolcano–sedimentary sequence of the Jianglang and Taka domes in the southern Songpan–Ganzi Orogen is composed of bedded marble and sandwiched metabasalt. This study presents geochemistry, C–O–Nd isotope systematics and zircon U–Pb geochronology data to explore the formation mechanism and tectonic setting of its protolith. The marble samples have high δ13CV-PDB (4.0 ‰ to −0.1 ‰) and δ18OV-SMOW (16.3 ‰ to 13.6 ‰) values, with similar εNd(t) values (−5.3 to −7.2) to Late Permian conodonts. They display seawater-like features, e.g. superchondritic Y/Ho ratios, negative Ce and positive La, Gd and Y anomalies, indicating a marine carbonate protolith in oxygen-rich palaeoseawater. Their pronounced positive Eu anomalies suggest a contribution (5 % to 1 %) of submarine high-T hydrothermal fluids. The metabasalt samples show low SiO2, high Fe2O3T and MgO contents. They have low La/Sm, (Th/Ta)PM and (La/Nb)PM ratios, without correlations between MgO and other major/trace elements. These signatures could be attributed to insignificant fractional crystallization and crustal contamination. Their rare earth elements (REE) patterns, trace element profiles and trace element ratios are comparable to those of mid-ocean ridge basalt (MORB), suggesting a depleted MORB-type mantle reservoir. Melt modelling illustrates low degrees (<10 %) of partial melting in the spinel stability field at a depth of 30–60 km. Moreover, the metabasalt contains 2453.7–258.9 Ma detrital zircons significantly related to the Palaeoproterozoic crystalline basement, Rodinia supercontinent, Caledonian events and c. 260 Ma Emeishan large igneous province. Combined with previous studies, we advocate that the investigated metavolcano–sedimentary suquence was most likely formed during the Late Permian in a mature back-arc basin of the Palaeo-Tethys ocean.

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