In situ oxygen-isotope, major-, and trace-element constraints on the metasomatic modification and crustal origin of a diamondiferous eclogite from Roberts Victor, Kaapvaal Craton

Abstract Aillikites are carbonate-rich ultramafic lamprophyres often associated with carbonatites. Despite their common field relationships, the petrogenetic links, if any, between aillikites and carbonatites remain controversial. To address this question, this study reports the results of a detailed geochemical and isotopic examination of the Permian Wajilitag aillikites in the northwestern Tarim large igneous province, including bulk-rock major-, trace-element and Sr-Nd isotope compositions, olivine major- and trace-element and (in-situ secondary ion mass spectrometry) oxygen isotope compositions, oxygen isotope data for clinopyroxene separates, and bulk-carbonate C-O isotopic analyses. Olivine in the aillikites occurs in two textural types: (i) microcrysts, 0.3-5 mm; and (ii) macrocrysts, 0.5-2.5 cm. The microcrysts exhibit well-defined linear correlations between Fo (79-89), minor and trace elements (e.g., Ni = 1304-3764 μg/g and Mn = 1363-3042 μg/g). In contrast, the olivine macrocrysts show low Fo79-81, Ni (5.3-442 μg/g) and Ca (477-1018 μg/g) and very high Mn (3418-5123 μg/g) contents, and are displaced from the compositional trend of the microcrysts. The microcrysts are phenocrysts crystallized from the host aillikite magmas. Conversely, the lack of mantle-derived xenoliths in these aillikites suggests that the macrocrysts probably represent cognate crystals (i.e., antecrysts) that formed from earlier, evolved aillikite melts. Olivine phenocrysts in the more primitive aillikite dykes (Dyke 1) have relatively higher Fo82-89 and mantle-like oxygen isotope values, whereas those in the more evolved dykes (Dyke 2 and 3) exhibit lower Fo79-86 and oxygen isotope values that trend toward lower than mantle δ18O values. The decreasing δ13C values of carbonate from Dyke 1 through to Dyke 2 and 3, coupled with the indistinguishable Sr-Nd isotopes of these dykes, suggest that the low δ18O values of olivine phenocrysts in Dyke 2 and 3 resulted from carbonate melt/fluid exsolution from a common progenitor melt. These lines of evidence combined with the overlapping emplacement ages and Sr-Nd isotope compositions of the aillikites and carbonatites in this area suggest that these exsolved carbonate melts probably contributed to the formation of the Tarim carbonatites thus supporting a close petrogenetic relationship between aillikites and carbonatites.

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Recent fluid processes in the Kaapvaal Craton, South Africa: coupled oxygen isotope and trace element disequilibrium in polymict peridotites

Earth and Planetary Science Letters ◽

10.1016/s0012-821x(99)00311-8 ◽

2000 ◽

Vol 176 (1) ◽

pp. 57-72 ◽

Cited By ~ 44

Author(s):

H.-F Zhang ◽

D.P Mattey ◽

N Grassineau ◽

D Lowry ◽

M Brownless ◽

...

Keyword(s):

South Africa ◽

Trace Element ◽

Oxygen Isotope ◽

Kaapvaal Craton

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Enigmatic Alluvial Sapphires from the Orosmayo Region, Jujuy Province, Northwest Argentina: Insights into Their Origin from in situ Oxygen Isotopes

Minerals ◽

10.3390/min9070390 ◽

2019 ◽

Vol 9 (7) ◽

pp. 390 ◽

Cited By ~ 2

Author(s):

Ian T. Graham ◽

Stephen J. Harris ◽

Laure Martin ◽

Angela Lay ◽

Eduardo Zappettini

Keyword(s):

Trace Element ◽

Oxygen Isotope ◽

Oxygen Isotopes ◽

Significant Variation ◽

Trace Element Geochemistry ◽

Trace Element Chemistry ◽

Element Chemistry ◽

Study Results ◽

Individual Grains

This study sought to investigate in situ oxygen isotopes (δ18O) within alluvial colorless-white to blue sapphires from the Orosmayo region, Jujuy Province, NW Argentina, in order to provide additional constraints on their origin and most likely primary geological environment. Analyses were conducted using the in situ SIMS oxygen isotope technique on the same grains that were analyzed for their mineral inclusions and major and trace element geochemistry using EMPA and LA–ICP–MS methods in our previous study. Results show a significant range in δ18O across the suite, from +4.1‰ to +11.2‰. Additionally, akin to their trace element chemistry, there is significant variation in δ18O within individual grains, reaching a maximum of 1.6‰. Both the previous analyses and δ18O results from this study suggest that these sapphires crystallized within the lower crust regime, involving a complex interplay of mantle-derived lamprophyres and carbonatites with crustal felsic rocks and both mantle- and crustal-derived metasomatic fluids. This study reinforces the importance of the in situ analysis of gem corundums, due to potential significant variation in major and trace element chemistry and ratios and even oxygen isotope ratios within discrete zones in individual grains.

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Trace-element content and partitioning in calcite, dolomite and apatite in carbonatite, Phalaborwa, South Africa

Mineralogical Magazine ◽

10.1180/0026461036750151 ◽

2003 ◽

Vol 67 (5) ◽

pp. 921-930 ◽

Cited By ~ 50

Author(s):

J. B. Dawson ◽

R. W. Hinton

Keyword(s):

South Africa ◽

Trace Element ◽

Oxygen Isotope ◽

Trace Element Content ◽

The Other ◽

Element Content ◽

Isotope Data ◽

Oxygen Isotope Data ◽

Carbon And Oxygen Isotope

AbstractA carbonatite sample from Phalaborwa, South Africa, consists of apatite, magnetite and a calcitedolomite ‘perthite’ which is interpreted as being due to exsolution of dolomite from a high-Mg calcite precursor. Carbon and oxygen isotope data indicate that the carbonates are equilibrated. In situ ionmicroprobe analyses for Fe, Mn, Na, Si, Y, the REEs, Pb, Th and U give the following average concentrations (in ppm) in the sequence apatite, calcite, dolomite: Fe 98, 1680, 8190; Mn 61, 510, 615; Na 1171, 627, 125; Si 368; 1.6, 0.2; Sr 4447, 5418, 2393; Ba 37, 2189, 75; La 1245, 300, 67; Y 121, 50, 5.8; Pb 16, 5.4, 1.4; Th 20, 0.02, 0; U 2.4, 0, 0.01. The concentrations are reasonably uniform in both apatite and dolomite, but in calcite are more variable. Na, Si, Y, the REEs, Pb, Th and U partition into apatite relative to both carbonates (and, hence, the precursor carbonate); KD ap/cc for REE decreases from ∽4 for La to ∽2 for Tm. There is almost equal partitioning of Sr between apatite and calcite. During separation of dolomite from calcite, Sr and Ba partition strongly into calcite and all the other analysed elements, except Fe and Mn, also preferentially enter calcite. The REEs prefer calcite relative to dolomite, and the KD dol/cc is reasonably constant, only varying from 0.23 to 0.17. Sr, Ba and Pb in the carbonates, and their partitioning between the calcite and dolomite, differ from other carbonatite carbonates reported in the literature.

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