scholarly journals No evidence for carbon enrichment in the mantle source of carbonatites in eastern Africa

Geology ◽  
2020 ◽  
Vol 48 (10) ◽  
pp. 971-975
Author(s):  
Valentin Casola ◽  
Lydéric France ◽  
Albert Galy ◽  
Nordine Bouden ◽  
Johan Villeneuve
Keyword(s):  
Partial Melting ◽  
Mantle Source ◽  
Active Volcano ◽  
Mantle Xenoliths ◽  
Eastern Africa ◽  
East African ◽  
Δ13c Values ◽  
Low Degree ◽  
Isotopic Analyses ◽  
Rayleigh Distillation

Abstract Carbonatites are unusual, carbon-rich magmas thought to form either by the melting of a carbon-rich mantle source or by low-degree partial melting of a carbon-poor (<80 ppm C) mantle followed by protracted differentiation and/or immiscibility. Carbonate-bearing mantle xenoliths from Oldoinyo Lengai (East African Rift), the only active volcano erupting carbonatites, have provided key support for a C-rich mantle source. Here, we report unique microscale O and C isotopic analyses of those carbonates, which are present as interstitial grains in the silicate host lava, veins in the xenoliths, and pseudo-inclusions in olivine xenoliths. The δ18O values vary little, from 19‰ to 29‰, whereas δ13C values are more variable, ranging from –23‰ to +0.5‰. We show that such carbonate δ18O values result from the low-temperature precipitation of carbonate in equilibrium with meteoric water, rather than under mantle conditions. In this framework, the observed δ13C values can be reproduced by Rayleigh distillation driven by carbonate precipitation and associated degassing. Together with petrological evidence of a physical connection between the three types of carbonates, our isotopic data support the pedogenic formation of carbonates in the studied xenoliths by soil-water percolation and protracted crystallization along xenolith cracks. Our results refute a mechanism of C enrichment in the form of mantle carbonates in the mantle beneath the Natron Lake magmatic province and instead support carbonatite formation by low-degree partial melting of a C-poor mantle and subsequent protracted differentiation of alkaline magmas.

scholarly journals Supplemental Material: No evidence for carbon enrichment in the mantle source of carbonatites in eastern Africa

2020 ◽  
Author(s):  
Valentin Casola ◽  
et al.
Keyword(s):  
Mantle Source ◽  
Analytical Methods ◽  
Eastern Africa ◽  
Additional Information ◽  
Rayleigh Distillation

Additional information on sample location, sample description, carbon and oxygen analytical methods, and the Rayleigh distillation model.<br>

Geochronology, isotopic and Platinum-group elemental geochemistry of lavas and dykes from the western Guangxi in outer zone of Emeishan mantle plume, SW China

2021 ◽  
Author(s):  
Bing Zhao ◽  
Xijun Liu ◽  
Zhenglin Li ◽  
Wenmin Huang ◽  
Chuan Zhao
Keyword(s):  
Partial Melting ◽  
Mantle Source ◽  
Large Scale ◽  
Outer Zone ◽  
Large Igneous Province ◽  
Yangtze Block ◽  
Mafic Rocks ◽  
Emeishan Large Igneous Province ◽  
Low Degree ◽  
Igneous Province

&lt;p&gt;The Emeishan flood basalts are part of an important large igneous province along the western margin of the Yangtze Block, Southwest China. The western Guangxi region in southwestern China is geologically a part of the Yangtze Block. Mafic rocks, comprising mainly lavas and dykes in western Guangxi belong to the outer part of the ~260 Ma Emeishan Large Igneous Province (ELIP). Here we present a systematic study of platinum-group elements (PGEs) combined with the LA-ICP-MS zircon U&amp;#8211;Pb age, whole-rock geochemical and isotopic data of the lavas and dykes in the Longlin area of outer zone of ELIP to constraints on their origin. On the basis of petrography and major elements characteristics, mafic lavas and dykes display an enrichment of LREE, LILE, HFSE, high (&lt;sup&gt;87&lt;/sup&gt;Sr/&lt;sup&gt;86&lt;/sup&gt;Sr)&lt;sub&gt;i&lt;/sub&gt; ratios (0.704227~0.705754), low &amp;#949;&lt;sub&gt;Nd&lt;/sub&gt;&lt;sub&gt;(t)&lt;/sub&gt; values(0.42~0.99), high &amp;#949;&lt;sub&gt;Hf&lt;/sub&gt;&lt;sub&gt;(t)&lt;/sub&gt; values(5.19~6.04), they are similar to those of Permian Emeishan high-Ti basalts and Ocean island basalts (OIB) features. The Longlin mafic rocks was formed in the Late Permian with the zircon U-Pb dated age of 256.3&amp;#177; 1.7 Ma. The age of the Longlin mafic rocks is close to the formation age of the ELIP large-scale magmatism, suggesting that these lavas and dykes probably belongs to part of the ELIP large-scale magmatism. The Longlin mafic rocks have low total PGE contents ranging from 1.56&amp;#215;10&lt;sup&gt;-9 &lt;/sup&gt;to 2.28&amp;#215;10&lt;sup&gt;-9&lt;/sup&gt;, with Os, Ir, Ru, Rh, Pt and Pd contents of 0.040~0.076, 0.046~0.076, 0.027~0.079, 0.037~0.056, 0.6374~1.053 and 0.715~1.021ppb, respectively. They show left-leaning primitive mantle-normalized PGE patterns with depletion in Iridium group(IPGE) and enrichment in Palladium group, which also have lower contents than mafic rocks from the inner zone of the ELIP, suggesting that a low degree of partial melting of the mantle source plays an important role. The Longlin mafic rocks exhibit a marked increase in Cu/Pd ratios (&gt;10&lt;sup&gt;5&lt;/sup&gt;,84655 to 174785) albeit with a narrow range of lower Pd/Ir ratios (&lt;50,13.4 to 18.7), different from the PGE-enriched basalts of the Siberian Traps, Emeishan Large Igneous Province (ELIP), East Greenland CFBs and Deccan Traps, indicating that their parent magmas was significantly depleted in chalcophile elements. Calculations based on the available trace element geochemistry reveal that the basalts were originated by low degree of partial melting(&lt;5%)&amp;#65292;with sulfides remain in the mantle during partial melting. Sulfide segregation could not happen during the evolution of the Longlin mafic rocks, due to the fact that neither significant fractional crystallization nor crustal contamination has been involved in their formation. Overall, mafic rocks from the outer zone of the ELIP show lower PGE contents than those in the inner zones, we find that the PGE contents in igneous rocks are related with the degrees of partial melting in the mantle source and the removal of sulfides before their emplacement.&lt;/p&gt;&lt;p&gt;This study was financially supported by the Guangxi Natural Science Foundation for Distinguished Young Scholars (2018GXNSFFA281009) and the Fifth Bagui Scholar Innovation Project of Guangxi Province (to XU Ji-feng).&lt;/p&gt;

scholarly journals Supplemental Material: No evidence for carbon enrichment in the mantle source of carbonatites in eastern Africa

2020 ◽  
Author(s):  
Valentin Casola ◽  
et al.
Keyword(s):  
Mantle Source ◽  
Analytical Methods ◽  
Eastern Africa ◽  
Additional Information ◽  
Rayleigh Distillation

Additional information on sample location, sample description, carbon and oxygen analytical methods, and the Rayleigh distillation model.<br>

Experimental study on a basanite from the McMurdo Volcanic Group, Antarctica: inference on its mantle source

2000 ◽  
Vol 12 (1) ◽  
pp. 105-116 ◽  
Author(s):  
Andrea Orlando ◽  
Sandro Conticelli ◽  
Pietro Armienti ◽  
Daniele Borrini
Keyword(s):  
Experimental Study ◽  
Partial Melting ◽  
Mantle Source ◽  
Volcanic Rocks ◽  
Liquidus Curve ◽  
Mantle Xenoliths ◽  
Primary Magma ◽  
Geochemical Evidence ◽  
Volcanic Group ◽  
Phase Relationships

Experiments to reconstruct the liquidus curve and establish the phase relationships of a basanite (Mg# = 72) from the McMurdo Volcanic Group, (thought to represent a nearly primary magma) used 1.0– 3.0 GPa and 1175–1550°C. The results suggest that this basanite could be generated by partial melting either of a spinel Iherzolite (at P = 1.5–2.0 GPa and T = 1390–1490°C) or of a garnet pyroxenite (at P > 3.0 GPa and T > 1550°C) source. Several lines of petrological and geochemical evidence support the latter hypothesis. Moreover, experimental results indicate the presence of mica in the source if it is assumed that the magma lost some water during its ascent to the surface. This is supported by the presence of mica and amphibole-bearing mantle xenoliths hosted in the most primitive volcanic rocks of the McMurdo Volcanic Group. These results and observations suggest that the source of magmas underwent metasomatism prior to partial melting.

Fluorine in igneous rocks and minerals with emphasis on ultrapotassic mafic and ultramafic magmas and their mantle source regions

1996 ◽  
Vol 60 (399) ◽  
pp. 243-257 ◽  
Author(s):  
A. D. Edgar ◽  
L. A. Pizzolato ◽  
J. Sheen
Keyword(s):  
Partial Melting ◽  
Mantle Source ◽  
Alkali Basalt ◽  
Mantle Xenoliths ◽  
Igneous Rocks ◽  
Positive Correlation ◽  
Source Regions ◽  
Melting Experiments ◽  
Melt Phase

AbstractIn reviewing the distribution of fluorine in igneous rocks it is clear that F abundance is related to alkalinity and to some extent to volatile contents. Two important F-bearing series are recognized: (1) the alkali basalt—ultrapotassic rocks in which F increases with increasing K2O and decreasing SiO2 contents; and (2) the alkali basalt—phonolite—rhyolite series with F showing positive correlation with both total alkalis and SiO2. Detailed studies of series (1) show that F abundance in ultrapotassic magmas (lamproite, kamafugite, lamprophyre) occurs in descending order in the sequence phlogopite>apatite>amphibole>glass. Fluorine contents in the same minerals from fresh and altered mantle xenoliths may be several orders of magnitude less than those in the host kamafugite. For many lamproites, F contents correlate with higher mg# suggesting that F is highest in the more primitive magmas.Experiments at mantle conditions (20 kbar, 900–1400°C) on simplified F-bearing mineral systems containing phlogopite, apatite, K-richterite, and melt show that F is generally a compatible element. Additionally, low F abundance in minerals from mantle xenoliths suggests that F may not be available in mantle source regions and hence is unlikely to partition into the melt phase on partial melting. Melting experiments on the compositions of F-free and F-bearing model phlogopite harzburgite indicate that even small variations in F content produce melts similar in composition to those of lamproite.

scholarly journals Mitochondrial phylogeography of baboons (Papiospp.) – Indication for introgressive hybridization?

2009 ◽  
Vol 9 (1) ◽  
Author(s):  
Dietmar Zinner ◽  
Linn F Groeneveld ◽  
Christina Keller ◽  
Christian Roos
Keyword(s):  
Phylogenetic Relationships ◽  
Evolutionary History ◽  
Introgressive Hybridization ◽  
Distinct Species ◽  
Eastern Africa ◽  
East African ◽  
Hamadryas Baboons ◽  
Olive Baboons ◽  
Initial Separation ◽  
Molecular Studies

Abstract Background Baboons of the genus Papio are distributed over wide ranges of Africa and even colonized parts of the Arabian Peninsula. Traditionally, five phenotypically distinct species are recognized, but recent molecular studies were not able to resolve their phylogenetic relationships. Moreover, these studies revealed para- and polyphyletic (hereafter paraphyletic) mitochondrial clades for baboons from eastern Africa, and it was hypothesized that introgressive hybridization might have contributed substantially to their evolutionary history. To further elucidate the phylogenetic relationships among baboons, we extended earlier studies by analysing the complete mitochondrial cytochrome b gene and the 'Brown region' from 67 specimens collected at 53 sites, which represent all species and which cover most of the baboons' range. Results Based on phylogenetic tree reconstructions seven well supported major haplogroups were detected, which reflect geographic populations and discordance between mitochondrial phylogeny and baboon morphology. Our divergence age estimates indicate an initial separation into southern and northern baboon clades 2.09 (1.54–2.71) million years ago (mya). We found deep divergences between haplogroups within several species (~2 mya, northern and southern yellow baboons, western and eastern olive baboons and northern and southern chacma baboons), but also recent divergence ages among species (< 0.7 mya, yellow, olive and hamadryas baboons in eastern Africa). Conclusion Our study confirms earlier findings for eastern Africa, but shows that baboon species from other parts of the continent are also mitochondrially paraphyletic. The phylogenetic patterns suggest a complex evolutionary history with multiple phases of isolation and reconnection of populations. Most likely all these biogeographic events were triggered by multiple cycles of expansion and retreat of savannah biomes during Pleistocene glacial and inter-glacial periods. During contact phases of populations reticulate events (i.e. introgressive hybridization) were highly likely, similar to ongoing hybridization, which is observed between East African baboon populations. Defining the extent of the introgressive hybridization will require further molecular studies that incorporate additional sampling sites and nuclear loci.

Petrology of ultramafic xenoliths from Rayfield River, south-central British Columbia

1987 ◽  
Vol 24 (8) ◽  
pp. 1679-1687 ◽  
Author(s):  
Dante Canil ◽  
Mark Brearley ◽  
Christopher M. Scarfe
Keyword(s):  
British Columbia ◽  
Phase Equilibrium ◽  
Partial Melting ◽  
Upper Mantle ◽  
Mantle Xenoliths ◽  
Spinel Lherzolite ◽  
Equilibrium Constraints ◽  
Host Magma ◽  
South Central ◽  
Ultramafic Xenoliths

One hundred mantle xenoliths were collected from a hawaiite flow of Miocene–Pliocene age near Rayfield River, south-central British Columbia. The massive host hawaiite contains subrounded xenoliths that range in size from 1 to 10 cm and show protogranular textures. Both Cr-diopside-bearing and Al-augite-bearing xenoliths are represented. The Cr-diopside-bearing xenolith suite consists of spinel lherzolite (64%), dunite (12%), websterite (12%), harzburgite (9%), and olivine websterite (3%). Banding and veining on a centimetre scale are present in four xenoliths. Partial melting at the grain boundaries of clinopyroxene is common and may be due to natural partial melting in the upper mantle, heating by the host magma during transport, or decompression during ascent.Microprobe analyses of the constituent minerals show that most of the xenoliths are well equilibrated. Olivine is Fo89 to Fo92, orthopyroxene is En90, and Cr diopside is Wo47En48Fs5. More Fe-rich pyroxene compositions are present in some of the websterite xenoliths. The Mg/(Mg + Fe2+) and Cr/(Cr + Al + Fe3+) ratios in spinel are uniform in individual xenoliths, but they vary from xenolith to xenolith. Equilibration temperatures for the xenoliths are 860–980 °C using the Wells geothermometer. The depth of equilibration estimated for the xenoliths using geophysical and phase equilibrium constraints is 30–40 km.

Archaeology in Eastern Africa: Recent Developments and More Dates

1984 ◽  
Vol 25 (4) ◽  
pp. 369-393 ◽  
Author(s):  
Peter Robertshaw
Keyword(s):  
Iron Age ◽  
Poor Quality ◽  
Eastern Africa ◽  
East African ◽  
Long Distance ◽  
East African Coast ◽  
Northern Tanzania ◽  
Recent Developments ◽  
First Millennium ◽  
Historical Sequences

Obsidian hydration dating has been successfully applied to East African archaeological sites. Chemical sourcing of obsidian artefacts has documented long-distance movement of obsidian from the Central Rift valley. A date in the ninth or eighth century b.c. has been obtained for iron objects in the Er Renk District of the Southern Sudan. Tentative culture-historical sequences are available from excavations around the Sudd and in the Lake Besaka region of Ethiopia. Archaeological research has begun in the interior of Somalia. In northern Kenya, claims that Namoratunga II is an archaeo-astronomical site have been challenged. Excavations at Mumba-Höhle and Nasera have shed new light on the transition from the Middle to Later Stone Age in northern Tanzania perhaps 20,000 to 30,000 years ago. Knowledge of the Elmenteitan Tradition has been considerably advanced by excavations in south-western Kenya. Iron-smelting furnaces with finger-decorated bricks have been discovered in south-eastern Kenya, though not yet dated. New dates falling in the last few centuries have caused first millennium a.d. dates obtained previously for Engaruka to be rejected. Excavations at several sites on the East African coast indicate that the beginnings of coastal occupation from the Lamu archipelago to Mozambique fall in the ninth century a.d. In Malawi the Shire Highlands seem to have been settled around the tenth century a.d. Investigations of large smelting-furnaces in central Malawi indicate that they were used as concentrators of poor-quality iron ore. Excavations in rock-shelters on the southern edge of the Copperbelt have produced a culture-historical sequence spanning the last 18,000 years. The western stream of the Early Iron Age was established in the Upper Zambezi valley by about the mid fifth century a.d.

Chasing the Magma Chamber: MT meets Geodynamics and Seismology – A numerical case study of magmatic plumbing at Oldoinyo Lengai Volcano

2021 ◽  
Author(s):  
César Daniel Castro ◽  
Miriam Christina Reiss ◽  
Arne Spang ◽  
Philip Hering ◽  
Luca de Siena ◽  
...  
Keyword(s):  
Magma Chamber ◽  
Transfer Functions ◽  
Geophysical Methods ◽  
Gravity Anomalies ◽  
Active Volcano ◽  
Rift System ◽  
East African Rift System ◽  
Plumbing System ◽  
East African ◽  
Oldoinyo Lengai

&lt;p&gt;How well can geophysical methods image magmatic systems? Geophysical methods are commonly used to image magmatic systems; however, synthetic studies which give insights into the resolution of such methods and their interpretational scope are rare. Gravity anomalies, magnetotelluric, seismological and geodynamical modelling all have a different sensitivity to the rock parameters and are thus likely complementary methods. Our study aims to better understand their interplay by performing joint modelling of a synthetic magmatic system. &amp;#160;Our model setup of a magma chamber is inspired by seismological observations at the Natron plumbing system including active volcano Oldoinyo Lengai within the East African Rift system. The geodynamic modelling is guided by shear-wave velocity anomalies and it is constrained by a large Bouguer gravity anomaly which is modelled by a voxel-based gravity code. It yields the 3D distribution of several geological parameters (pressure, temperature, stress, density, rock type). The parameters are converted into a 3D resistivity distribution. By 3D forward modelling including the topography, synthetic MT transfer functions (phase tensor, induction vectors) are calculated for a rectangular grid of 441 sites covering the area. The variation of geodynamic parameters and/or petrological relations alters the related resistivity distribution and thus yields the sensitivity of MT responses to geodynamic parameters. In turn, MT observations may constrain geodynamic modelling by inverting MT transfer functions. The inversion is performed allowing for the recent seismicity distribution beneath the Natron plumbing system, assuming that active seismic areas are related to enhanced resistivity. The inversion is performed for a realistic distribution (in view of logistic accessibility) of about 40 MT sites.&lt;/p&gt;&lt;p&gt;By combining multiple forward models, this study yields insights into the sensitivity of different observables and thus provides a valuable base on how MT, gravity and seismological observations can help imaging a complex geological setting.&lt;/p&gt;

scholarly journals Alkali basalt from the Seifu Seamount in the Sea of Japan: post-spreading magmatism in a back-arc setting

Solid Earth ◽  
2020 ◽  
Vol 11 (1) ◽  
pp. 23-36
Author(s):  
Tomoaki Morishita ◽  
Naoto Hirano ◽  
Hirochika Sumino ◽  
Hiroshi Sato ◽  
Tomoyuki Shibata ◽  
...  
Keyword(s):  
Partial Melting ◽  
Sea Of Japan ◽  
Alkali Basalt ◽  
Spinel Peridotite ◽  
The Sea Of Japan ◽  
Low Degree ◽  
Tsushima Basin ◽  
Arc Setting ◽  
Element Enrichment ◽  
Back Arc

Abstract. We present geochemical and 40Ar∕39Ar age data for a peridotite xenolith-bearing basalt dredged from the Seifu Seamount (SSM basalt) in the northeast Tsushima Basin, southwest Sea of Japan. An 40Ar∕39Ar plateau age of 8.33±0.15 Ma (2σ) was obtained for the SSM basalt, indicating that it erupted shortly after the termination of back-arc spreading in the Sea of Japan. The SSM basalt is a high-K to shoshonitic alkali basalt that is characterized by light rare earth element enrichment. The trace element features of the basalt are similar to those of ocean island basalt, although the Yb content is much higher, indicating formation by the low-degree partial melting of spinel peridotite. The Nd, Sr, and Pb isotopic compositions of the SSM basalt differ from those of back-arc basin basalts in the Sea of Japan. The Sr–Nd isotopic composition of the SSM basalt suggests its source was depleted mid-ocean ridge mantle containing an enriched mantle (EM1) component. The SSM basalt was formed in a post-back-arc extension setting by the low-degree partial melting of an upwelling asthenosphere that had previously been associated with the main phase of back-arc magmatism.

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