Nd and Sr isotopic compositions of East African carbonatites: Implications for mantle heterogeneity

Bell, Keith; Blenkinsop, John

doi:10.1130/0091-7613(1987)15<99:nasico>2.0.co;2

Cited by 150 publications

(59 citation statements)

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“…Panda Hill pyrochlore samples were from the Mbeya mountain range in southwest Tanzania. The corresponding carbonatite complex has been dated to an age of 116 ± 6 Ma [73]. The Panda Hill pyrochlores were brown in color with an idiomorphic octahedral form of 1-3 cm diameter.…”

Section: Sample Descriptionmentioning

confidence: 99%

Thermal annealing of natural, radiation-damaged pyrochlore

Zietlow

Beirau

Mihailova

et al. 2016

Zeitschrift Für Kristallographie - Crystalline Materials

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Radiation damage in minerals is caused by the α-decay of incorporated radionuclides, such as U and Th and their decay products. The effect of thermal annealing (400-1000 K) on radiation-damaged pyrochlores has been investigated by Raman scattering, X-ray powder diffraction (XRD), and combined differential scanning calorimetry/thermogravimetry (DSC/TG). The analysis of three natural radiation-damaged pyrochlore samples from Miass/Russia [6.4 wt% Th, 23.1 · 10 18 α-decay events per gram (dpg)], Panda Hill/Tanzania (1.6 wt% Th, 1.6 · 10 18 dpg), and Blue River/Canada (10.5 wt% U, 115.4 · 10 18 dpg), are compared with a crystalline reference pyrochlore from Schelingen (Germany). The type of structural recovery depends on the initial degree of radiation damage (Panda Hill 28 %, Blue River 85 % and Miass 100 % according to XRD), as the recrystallization temperature increases with increasing degree of amorphization. Raman spectra indicate reordering on the local scale during annealing-induced recrystallization. As Raman modes around 800 cm −1 are sensitive to radiation damage . The most radiation damaged pyrochlore (Miass) shows an abrupt recovery of both, its short-(Raman) and long-range order (X-ray) between 800 and 850 K, while the weakly damaged pyrochlore (Panda Hill) begins to recover at considerably lower temperatures (near 500 K), extending over a temperature range of ca. 300 K, up to 800 K (Raman). The pyrochlore from Blue River shows in its initial state an amorphous X-ray diffraction pattern superimposed by weak Bragg-maxima that indicates the existence of ordered regions in a damaged matrix. In contrast to the other studied pyrochlores, Raman spectra of the Blue River sample show the appearance of local modes above 560 K between 700 and 800 cm −1 resulting from its high content of U and Ta impurities. DSC measurements confirmed the observed structural recovery upon annealing. While the annealing-induced ordering of Panda Hill begins at a lower temperature (ca. 500 K) the recovery of the highly-damaged pyrochlore from Miass occurs at 800 K. The Blue-River pyrochlore shows a multi-step recovery which is similarly seen by XRD. Thermogravimetry showed a continuous mass loss on heating for all radiation-damaged pyrochlores (Panda Hill ca. 1 %, Blue River ca. 1.5 %, Miass ca. 2.9 %).

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Section: Sample Descriptionmentioning

confidence: 99%

Thermal annealing of natural, radiation-damaged pyrochlore

Zietlow

Beirau

Mihailova

et al. 2016

Zeitschrift Für Kristallographie - Crystalline Materials

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“…Previous theories of the genesis of carbonatites demonstrated that carbonatites are related to sedimentary limestones (Bell et al, 1982;Bell and Blenkinsop, 1987). The melting of this rock type is, in principle, possible, as shown by a number of experimental studies (Wyllie and Tuttle, 1960;Fanelli et al, 1986;Lentz, 1998;Lentz, 1999).…”

Section: Introductionmentioning

confidence: 96%

The case of the carbonatite-like dyke of the Madenska River complex at the Kriva Lakavica section in the Republic of Macedonia: oxygen and carbon isotopic constraints

Dolenec¹,

Serafimovski²,

Daneu³

et al. 2015

Turkish J Earth Sci

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The oxygen and carbon isotope compositions of carbonate samples from the carbonatite-like dyke of the Madenska River complex at the Kriva Lakavica section, samples of calcite skarns from the Damjan Fe-ore deposit and the Sasa Pb-Zn ore deposit, and samples of marbles from the Pohorje Mountains were analyzed to provide critical evidence in favor of the magmatic or sedimentary origin of this carbonatite-like dyke. We suggest that the carbonatite-like dyke is not a "normal" carbonatite, but instead represents melted carbonates, which can be associated with an unexposed, deep-seated, causative magmatic body. This dyke has a fluidal texture and carries xenoliths of ultramafic rocks that can be up to 35 cm in size. Its isotopic composition plots between primary unaltered carbonatites and marine carbonates, and ranges between 13.79‰ and 18.89‰ for δ 18 O VSMOW and between -1.22‰ and 1.31‰ for δ 13 C VPDB . These values are significantly lower than those observed in carbonatites analyzed during this study and range between 6.53‰ and 8.10‰ for δ 18 O VSMOW and between -5.82‰ and -4.32‰ for δ 13 C VPDB , which is the primary isotope signature of most magmatic carbonatites. Similarly high δ 18 O and δ 13 C values were found in skarns of the Damjan Fe deposit close to the Madanska River complex and in the Sasa Pb-Zn deposit, as well as in highgrade regional metamorphic calcitic marbles of the Pohorje massive. The emplacement levels of the carbonatite-like dyke, due to several tectonic processes, are uncertain. The type of country rocks (sedimentary carbonates, ultramafic, mafic, and granitic rocks), hydrothermal alternation, and metasomatic and regional metamorphic processes seem to be the most important parameters that affect the O and C isotopic patterns found in the Kriva Lakavica carbonatitelike dyke and in the investigated samples.

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“…Evidence for "open-system" behavior at Oka was already evident from the TIMS generated whole rock data as these define a range of Sr and Nd isotope values that are well outside the typical in-run analytical precision ( Figure 12). In Figure 12B, the Nd and Sr isotope compositions for apatite and perovskite from Oka are compared to those for well-established mantle components (i.e., HIMU, EMI, EMII (enriched mantle 2), and DMM (depleted mid-ocean ridge basaltic mantle)) [76] and East African carbonatite complexes [58]. The Nd and Sr isotope data from Oka plot proximal to the field for the HIMU mantle component and most lie along a HIMU-EMI mixing array.…”

Section: Relationship Between Oka Monteregian Igneous Province (Mip)mentioning

confidence: 99%

“…The in-situ Sr and Nd isotope data for both perovskite and apatite overlap the entire range defined by previously reported whole rock data for carbonatite from Oka [12], but definitely indicate a larger variation ( 87 Sr/ 86 Sr: 0.70312-0.70367; 143 Nd/ 144 Nd: 0.51270-0.51286), and is not consistent with closed-system behavior ( Figure 12A). Of interest, the Nd and Sr isotope data from Oka overlap the upper end of the East African Carbonatite Line (EACL; Figure 12B) [58]. …”

Section: Radiogenic Isotope Datamentioning

confidence: 99%

Evidence for the Multi-Stage Petrogenetic History of the Oka Carbonatite Complex (Québec, Canada) as Recorded by Perovskite and Apatite

Chen¹,

Simonetti²

2014

Minerals

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Abstract:The Oka complex is amongst the youngest carbonatite occurrences in North America and is associated with the Monteregian Igneous Province (MIP; Québec, Canada). The complex consists of both carbonatite and undersaturated silicate rocks (e.g., ijolite, alnöite), and their relative emplacement history is uncertain. The aim of this study is to decipher the petrogenetic history of Oka via the compositional, isotopic and geochronological investigation of accessory minerals, perovskite and apatite, using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The new compositional data for individual perovskite and apatite grains from both carbonatite and associated alkaline silicate rocks are highly variable and indicative of open system behavior. In situ Sr and Nd isotopic compositions for these two minerals are also variable and support the involvement of several mantle sources. U-Pb ages for both perovskite and apatite define a bimodal distribution, and range between 113 and 135 Ma, which overlaps the range of ages reported previously for Oka and the entire MIP. The overall distribution of ages indicates that alnöite was intruded first, followed by okaite and carbonatite, whereas ijolite defines a bimodal emplacement history. The combined chemical, isotopic, and geochronological data is best explained by invoking the periodic generation of small volume, partial melts generated from heterogeneous mantle.

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Nd and Sr isotopic compositions of East African carbonatites: Implications for mantle heterogeneity

Cited by 150 publications

References 0 publications

Thermal annealing of natural, radiation-damaged pyrochlore

Thermal annealing of natural, radiation-damaged pyrochlore

The case of the carbonatite-like dyke of the Madenska River complex at the Kriva Lakavica section in the Republic of Macedonia: oxygen and carbon isotopic constraints

Evidence for the Multi-Stage Petrogenetic History of the Oka Carbonatite Complex (Québec, Canada) as Recorded by Perovskite and Apatite

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