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Keith Bell - One of the best experts on this subject based on the ideXlab platform.
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nd pb and sr isotopic data from the mount elgon volcano eastern uganda western kenya implications for the origin and evolution of Nephelinite lavas
Lithos, 1995Co-Authors: Antonio Simonetti, Keith BellAbstract:Abstract Nd, Pb and Sr isotope ratios for Nephelinites from the Tertiary Mount Elgon alkaline volcanic centre, eastern Uganda-western Kenya, are highly variable and indicate open system behaviour. The variation in 143 Nd 144 Nd (0.51219-0.51286) and 87 Sr 86 Sr (0.70314-0.70604) ratios span almost the entire range documented for carbonatites from several East African alkaline complexes. The whole rock chemical data, mineralogy, composition of diopside phenocrysts, and variation in isotopic ratios from the Mount Elgon Nephelinites are similar to those from the Nephelinite lavas from the Tertiary Napak volcano, Uganda (Simonetti and Bell, 1994a). The diopside phenocrysts from Mount Elgon Nephelinite lavas reveal large core-to-rim compositional variations (which include normal, oscillatory and reverse zoning), and their Nd, Pb and Sr isotopic ratios are not in isotopic equilibrium with their host lavas. Microprobe data along with textural evidence from the Mount Elgon diopside phenocrysts support a model that involves crystallization in an open magma system that was undergoing continuous chemical and isotopic change. The large variation in Pb isotopic ratios (whole rocks- 206 Pb 204 Pb : 18.45-21.51; 207 Pb 204 Pb : 15.61-15.88; 208 Pb 204 Pb : 38.62-41.02), from the Mount Elgon lavas, best fit a model involving mixing between EM I and HIMU mantle components, and correlations in Pb-Sr and Pb-Nd isotopic plots partly support this interpretation. The isotopic data from Mount Elgon and Napak Nephelinites suggest complex evolutionary histories involving magma mixing, and support the presence of a heterogeneous sub-continental source beneath eastern Uganda, similar to that documented for various types of peralkaline Nephelinite lavas from the only active carbonatite-Nephelinite volcano, Oldoinyo Lengai, Tanzania (Bell and Dawson, 1995) and other East African volcanoes (e.g. Vollmer and Norry, 1983). The chemical data and large variation in isotopic ratios for the Mount Elgon Nephelinites suggests that each lava flow may represent a discrete partial melt derived from an isotopically inhomogeneous upper mantle. In the cases of the Mount Elgon and Napak lavas, magma mixing may have occurred between newly formed, ascending nephelinitic liquids and slightly older nephelinitic melts that ponded at shallower levels.
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nd pb and sr isotopic data from the napak carbonatite Nephelinite centre eastern uganda an example of open system crystal fractionation
Contributions to Mineralogy and Petrology, 1994Co-Authors: Antonio Simonetti, Keith BellAbstract:Nd, Pb and Sr isotopic data from Nephelinite lavas from the Tertiary Nephelinite-carbonatite complex of Napak, eastern Uganda, show large isotopic variations that can only be attributed to open-system behaviour. Possible explanations of the data include mixing between nephelinitic melts derived from an isotopically heterogeneous mantle, or interaction between a HIMU melt and mafic granulites. In both models crystal fractionation, involving olivine and clinopyroxene, played an important role. Major element chemistry, textural evidence and isotopic data from clinopyroxene phenocrysts from the olivine-bearing Nephelinites, suggest that the pyroxenes did not crystallize from their host liquids. The isotopic data from the clinopyroxene phenocrysts support an interpretation of crystal fractionation in an open magma system that was undergoing continuous isotopic change. This study emphasises the importance of using combined isotopic data from both whole rock and mineral phases to interpret the evolutionary history of a single eruptive centre.
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Nd and Sr isotope systematics of Shombole volcano, East Africa, and the links between Nephelinites, phonolites, and carbonatites
Geology, 1991Co-Authors: Keith Bell, T D PetersonAbstract:Nd and Sr isotope compositions of Nephelinites, carbonatites, and phonolites from Shombole, a Pliocene volcano in East Africa, show that the phonolites cannot be derived by simple fractional crystallization of Nephelinite magma. For a given initial {sup 87}Sr/{sup 86}Sr ratio, {sup 143}Nd/{sup 144}Nd is lower in most phonolites than in the Nephelinites and carbonatites. Interaction between nephelinitic magma and lower-crustal granulites can account for these differences. The similar ranges in isotopic composition of the carbonatites and Nephelinites are consistent with repeated melting events involving heterogeneous mantle. The carbonatites could have formed by immiscibility with Nephelinite magma or by direct partial melting of the same mantle source(s) as the Nephelinites.
Yuhui Ai - One of the best experts on this subject based on the ideXlab platform.
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the compressibility of caco3 li2co3 na2co3 k2co3 liquids application to natrocarbonatite and co2 bearing Nephelinite liquids from oldoinyo lengai
Contributions to Mineralogy and Petrology, 2015Co-Authors: Mary Catherine Oleary, Rebecca A Lange, Yuhui AiAbstract:To constrain the compressibility of natrocarbonate liquids, sound-speed measurements were made on 11 liquids in the CaCO3–Li2CO3–Na2CO3–K2CO3 quaternary system from 808 to 1323 K at 1 bar with a frequency-sweep acoustic interferometer. CaCO3 concentrations range from 15 to 50 mol% in four of the experimental liquids. The sound-speed data for all liquids were converted to isothermal compressibility (βT), which were fit to an ideal mixing model with respect to composition; the average residual is 1.2 %. Fitted values (±1σ) of the partial molar compressibility (10−2 GPa−1) at 1100 K were derived for CaCO3 (5.36 ± 0.13), Li2CO3 (8.09 ± 0.06), Na2CO3 (10.62 ± 0.07), and K2CO3 (14.09 ± 0.06); these values translate to bulk modulus values of 18.7, 12.4, 9.4, and 7.1 GPa, respectively, reflecting the relatively large compressibility of carbonate liquids. The data are additionally used to estimate the partial molar volume and compressibility of the CO2 component dissolved as carbonate in Nephelinite liquids; the density of this dissolved component at 1423 K and 1 GPa ranges from 1.62, 1.71 to 1.83 g/cm3 when it is complexed with K+, Na+, and Ca2+, respectively, and is estimated to be ~2.05 and 2.14 g/cm3 when complexed with Fe2+ and Mg2+, respectively. The results from this study can be applied to natrocarbonate liquids, such as those erupted from Oldoinyo Lengai volcano in Africa, and indicate a melt density of ~2.19 g/cm3 at 1150 °C and 1 GPa, which is ~18 % less dense than the average melt density (~2.67 g/cm3) calculated for associated Mg-poor Nephelinite liquids at the same conditions and volatile-free. However, the dissolution of 10.1 wt% H2O and 8.7 wt% CO2 in the average Nephelinite melt (based on volatile contents reported in the literature for these magmas) reduces its density to ~2.14 g/cm3 at 1150 °C and 1 GPa, eliminating the buoyancy contrast with the natrocarbonate melt. In turn, it is highly likely that the natrocarbonatite melts contained significant amounts of dissolved H2O and, as an example, the addition of 10 wt% H2O lowers the melt density by ~14 % to ~1.88 g/cm3, and therefore re-establishes a large density contrast with the volatile-rich Nephelinite melts.
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The compressibility of CaCO3–Li2CO3–Na2CO3–K2CO3 liquids: application to natrocarbonatite and CO2-bearing Nephelinite liquids from Oldoinyo Lengai
Contributions to Mineralogy and Petrology, 2015Co-Authors: Mary Catherine O’leary, Rebecca A Lange, Yuhui AiAbstract:To constrain the compressibility of natrocarbonate liquids, sound-speed measurements were made on 11 liquids in the CaCO3–Li2CO3–Na2CO3–K2CO3 quaternary system from 808 to 1323 K at 1 bar with a frequency-sweep acoustic interferometer. CaCO3 concentrations range from 15 to 50 mol% in four of the experimental liquids. The sound-speed data for all liquids were converted to isothermal compressibility (βT), which were fit to an ideal mixing model with respect to composition; the average residual is 1.2 %. Fitted values (±1σ) of the partial molar compressibility (10−2 GPa−1) at 1100 K were derived for CaCO3 (5.36 ± 0.13), Li2CO3 (8.09 ± 0.06), Na2CO3 (10.62 ± 0.07), and K2CO3 (14.09 ± 0.06); these values translate to bulk modulus values of 18.7, 12.4, 9.4, and 7.1 GPa, respectively, reflecting the relatively large compressibility of carbonate liquids. The data are additionally used to estimate the partial molar volume and compressibility of the CO2 component dissolved as carbonate in Nephelinite liquids; the density of this dissolved component at 1423 K and 1 GPa ranges from 1.62, 1.71 to 1.83 g/cm3 when it is complexed with K+, Na+, and Ca2+, respectively, and is estimated to be ~2.05 and 2.14 g/cm3 when complexed with Fe2+ and Mg2+, respectively. The results from this study can be applied to natrocarbonate liquids, such as those erupted from Oldoinyo Lengai volcano in Africa, and indicate a melt density of ~2.19 g/cm3 at 1150 °C and 1 GPa, which is ~18 % less dense than the average melt density (~2.67 g/cm3) calculated for associated Mg-poor Nephelinite liquids at the same conditions and volatile-free. However, the dissolution of 10.1 wt% H2O and 8.7 wt% CO2 in the average Nephelinite melt (based on volatile contents reported in the literature for these magmas) reduces its density to ~2.14 g/cm3 at 1150 °C and 1 GPa, eliminating the buoyancy contrast with the natrocarbonate melt. In turn, it is highly likely that the natrocarbonatite melts contained significant amounts of dissolved H2O and, as an example, the addition of 10 wt% H2O lowers the melt density by ~14 % to ~1.88 g/cm3, and therefore re-establishes a large density contrast with the volatile-rich Nephelinite melts.
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The compressibility of CaCO_3–Li_2CO_3–Na_2CO_3–K_2CO_3 liquids: application to natrocarbonatite and CO_2-bearing Nephelinite liquids from Oldoinyo Lengai
Contributions to Mineralogy and Petrology, 2015Co-Authors: Mary Catherine O’leary, Rebecca A Lange, Yuhui AiAbstract:To constrain the compressibility of natrocarbonate liquids, sound-speed measurements were made on 11 liquids in the CaCO_3–Li_2CO_3–Na_2CO_3–K_2CO_3 quaternary system from 808 to 1323 K at 1 bar with a frequency-sweep acoustic interferometer. CaCO_3 concentrations range from 15 to 50 mol% in four of the experimental liquids. The sound-speed data for all liquids were converted to isothermal compressibility ( β _ T ), which were fit to an ideal mixing model with respect to composition; the average residual is 1.2 %. Fitted values (±1 σ ) of the partial molar compressibility (10^−2 GPa^−1) at 1100 K were derived for CaCO_3 (5.36 ± 0.13), Li_2CO_3 (8.09 ± 0.06), Na_2CO_3 (10.62 ± 0.07), and K_2CO_3 (14.09 ± 0.06); these values translate to bulk modulus values of 18.7, 12.4, 9.4, and 7.1 GPa, respectively, reflecting the relatively large compressibility of carbonate liquids. The data are additionally used to estimate the partial molar volume and compressibility of the CO_2 component dissolved as carbonate in Nephelinite liquids; the density of this dissolved component at 1423 K and 1 GPa ranges from 1.62, 1.71 to 1.83 g/cm^3 when it is complexed with K^+, Na^+, and Ca^2+, respectively, and is estimated to be ~2.05 and 2.14 g/cm^3 when complexed with Fe^2+ and Mg^2+, respectively. The results from this study can be applied to natrocarbonate liquids, such as those erupted from Oldoinyo Lengai volcano in Africa, and indicate a melt density of ~2.19 g/cm^3 at 1150 °C and 1 GPa, which is ~18 % less dense than the average melt density (~2.67 g/cm^3) calculated for associated Mg-poor Nephelinite liquids at the same conditions and volatile-free. However, the dissolution of 10.1 wt% H_2O and 8.7 wt% CO_2 in the average Nephelinite melt (based on volatile contents reported in the literature for these magmas) reduces its density to ~2.14 g/cm^3 at 1150 °C and 1 GPa, eliminating the buoyancy contrast with the natrocarbonate melt. In turn, it is highly likely that the natrocarbonatite melts contained significant amounts of dissolved H_2O and, as an example, the addition of 10 wt% H_2O lowers the melt density by ~14 % to ~1.88 g/cm^3, and therefore re-establishes a large density contrast with the volatile-rich Nephelinite melts.
J. B. Dawson - One of the best experts on this subject based on the ideXlab platform.
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Nephelinite–melilitite–carbonatite relationships: Evidence from Pleistocene–recent volcanism in northern Tanzania
Lithos, 2012Co-Authors: J. B. DawsonAbstract:Abstract The younger ( An integrated model involves carbonated olivine–Nephelinite magma generated in the asthenosphere by melting of carbonate-bearing peridotite. Small amounts of the melt migrate rapidly and are extruded as the olivine–Nephelinites and –melilitites. Larger amounts of magma pond in the lithosphere with which they react and, during variable periods of stagnation, varying degrees of olivine–pyroxene–spinel fractionation take place before upward migration of now-differing magma batches. Variable residence time within, and reaction with, variably enriched lithosphere accounts for the array between the HIMU and enriched mantle end-members in the isotope mixing model.
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Nephelinite melilitite carbonatite relationships evidence from pleistocene recent volcanism in northern tanzania
Lithos, 2012Co-Authors: J. B. DawsonAbstract:Abstract The younger ( An integrated model involves carbonated olivine–Nephelinite magma generated in the asthenosphere by melting of carbonate-bearing peridotite. Small amounts of the melt migrate rapidly and are extruded as the olivine–Nephelinites and –melilitites. Larger amounts of magma pond in the lithosphere with which they react and, during variable periods of stagnation, varying degrees of olivine–pyroxene–spinel fractionation take place before upward migration of now-differing magma batches. Variable residence time within, and reaction with, variably enriched lithosphere accounts for the array between the HIMU and enriched mantle end-members in the isotope mixing model.
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Mineral chemistry of a peralkaline combeite-lamprophyllite Nephelinite from Oldoinyo Lengai, Tanzania
Mineralogical Magazine, 1998Co-Authors: J. B. Dawson, P. G. HillAbstract:AbstractA peralkaline Nephelinite lava ([Na+K]/Al 2.15) from the active carbonatite volcano Oldoinyo Lengai, contains combeite, Ba lamprophyllite, a phase with affinities to delhayelite, CeSrNb perovskite, a CaNa phosphate high in Sr, Ba and K, and peralkaline glass; in addition to Fe-rich nepheline, aegirine-rich clinopyroxene and FeK-rich sodalite. The high alkali concentrations relative to alumina in the bulk rock could not have been achieved by fractionational crystallisation of the known Al-rich phenocryst phases (nepheline and sodalite) and some other process must be invoked.
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evolution of natrocarbonatite from a wollastonite Nephelinite parent evidence from the june 1993 eruption of oldoinyo lengai tanzania
The Journal of Geology, 1996Co-Authors: J. B. Dawson, David M. Pyle, Harry PinkertonAbstract:Lapilli and unusually thick, high viscosity lavas extruded from Oldoinyo Lengai in June, 1993 are composed of crystal-rich natrocarbonatite containing a small proportion of porphyritic silicate spheroids. The silicate spheroids are of wollastonite Nephelinite mineralogy, but they also contain natrocarbonatite mineral grains and aggregates (a) in glass inclusions within nepheline and pyroxene phenocrysts, and (b) within the fine-grained silicate groundmass of the spheroids. The chemical relationships of the natrocarbonatite component in the spheroids to the dominant silicate fraction are consistent with liquid immiscibility, as are the multiple stages of unmixing. These direct observations confirm the intimate relationship between natrocarbonatite and wollastonite Nephelinite inferred both from the field relationships at Oldoinyo Lengai, and from low-PT experimentation. The plutonic equivalent (wollastonite ijolite) of the magma type found to be parental to natrocarbonatite at Oldoinyo Lengai does occur in...
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Nd and Sr Isotope Systematics of the Active Carbonatite Volcano, Oldoinyo Lengai
IAVCEI Proceedings in Volcanology, 1995Co-Authors: K. Bell, J. B. DawsonAbstract:The Nd and Sr isotopic compositions of natrocarbonatite, Nephelinite, phonolite and plutonic blocks from Oldoinyo Lengai form a distinct negative correlation, and closely follow the East African Carbonatite Line (EACL). The 143Nd/143Nd ratios of the lavas range from 0.51249 to 0.51269, those of plutonic blocks from 0.51174 to 0.51270. The 87Sr/86Sr of the lavas range from 0.70414 to 0.70512; plutonic blocks range from 0.70378 to 0.70861. The Nephelinites fall into two distinct groups; those with the more depleted isotopic signature include the combeitebearing Nephelinites. Published data from mantle xenoliths, both metasomatized and nonmetasomatized, fall along the same array and suggest the interaction of two components, one enriched and the other slightly depleted relative to bulk Earth and CHUR. Isotopic variations indicate that the rocks from Oldoinyo Lengai cannot be derived by magmatic differentiation alone; at least two spatially related components are involved in the origin of rocks from Oldoinyo Lengai.
Hannes B. Mattsson - One of the best experts on this subject based on the ideXlab platform.
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Fractional crystallization of Si-undersaturated alkaline magmas leading to unmixing of carbonatites on Brava Island (Cape Verde) and a general model of carbonatite genesis in alkaline magma suites
Contributions to Mineralogy and Petrology, 2016Co-Authors: Daniel Weidendorfer, Max W. Schmidt, Hannes B. MattssonAbstract:The carbonatites of Brava Island, Cape Verde hot spot, allow to investigate whether they represent small mantle melt fractions or form through extreme fractionation and/or liquid immiscibility from CO_2-bearing silicate magmas. The intrusive carbonatites on Brava Island are part of a strongly silica-undersaturated pyroxenite, ijolite, Nephelinite, nepheline syenite, combeite–foiditite, carbonatite series. The major and trace element composition of this suite is reproduced by a model fractionating olivine, clinopyroxene, perovskite, biotite, apatite, titanite, sodalite and FeTi oxides, all present as phenocrysts in the rocks corresponding to their fractionation interval. Fractionation of ~90 wt% crystals reproduces the observed geochemical trend from the least evolved ultramafic dikes (bulk X _Mg = 0.64) to syenitic compositions. The modelled fractional crystallization leads to alkali enrichment, driving the melt into the carbonatite–silicate miscibility gap. An initial CO_2 content of 4000 ppm is sufficient to saturate in CO_2 at the point where the rock record suggests continuing unmixing carbonatites from Nephelinites to nepheline syenites after 61 wt% fractionation. Such immiscibility is also manifested in carbonatite and silicate domains on a hand-specimen scale. Furthermore, almost identical primary clinopyroxene, biotite and carbonate compositions from carbonatites and Nephelinites to nepheline syenites substantiate their conjugate character and our unmixing model. The modelled carbonatite compositions correspond to the natural ones except for their much higher alkali contents. The alkali-poor character of the carbonatites on Brava and elsewhere is likely a consequence of the release of alkali-rich CO_2 + H_2O fluids during final crystallization, which cause fenitization in adjacent rocks. We propose a general model for carbonatite generation during alkaline magmatism, where the fractionation of heavily Si-undersaturated, alkaline parent melts results in alkali and CO_2 enrichment in the evolving melt, ultimately leading to immiscibility between carbonatites and evolved Si-undersaturated alkaline melts. Early saturation in feldspathoids or feldspars would limit alkali enrichment preventing the formation of carbonatites. The complete and continuous fractionation line from almost primitive melts to syenitic compositions on Brava underlines the possibly important role of intrusives for hot spot volcanism.
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Fundamental changes in the activity of the natrocarbonatite volcano Oldoinyo Lengai, Tanzania
Bulletin of Volcanology, 2010Co-Authors: Jörg Keller, Jurgis Klaudius, Matthieu Kervyn, Gerald G. J. Ernst, Hannes B. MattssonAbstract:With a paroxysmal ash eruption on 4 September 2007 and the highly explosive activity continuing in 2008, Oldoinyo Lengai (OL) has dramatically changed its behavior, crater morphology, and magma composition after 25 years of quiet extrusion of fluid natrocarbonatite lava. This explosive activity resembles the explosive phases of 1917, 1940–1941, and 1966–1967, which were characterized by mixed ashes with dominantly nephelinitic and natrocarbonatitic components. Ash and lapilli from the 2007–2008 explosive phase were collected on the slopes of OL as well as on the active cinder cone, which now occupies the entire north crater having buried completely all earlier natrocarbonatite features. The lapilli and ash samples comprise nepheline, wollastonite, combeite, Na-åkermanite, Ti-andradite, resorbed pyroxene and Fe–Ti oxides, and a Na–Ca carbonate phase with high but varying phosphorus contents which is similar, but not identical, to the common gregoryite phenocrysts in natrocarbonatite. Lapilli from the active cone best characterize the erupted material as carbonated combeite–wollastonite–melilite Nephelinite. The juvenile components represent a fundamentally new magma composition for OL, containing 25–30 wt.% SiO_2, with 7–11 wt.% CO_2, high alkalies (Na_2O 15–19%, K_2O 4–5%), and trace-element signatures reminiscent of natrocarbonatite enrichments. These data define an intermediate composition between natrocarbonatite and Nephelinite, with about one third natrocarbonatite and two thirds Nephelinite component. The data are consistent with a model in which the carbonated silicate magma has evolved from the common combeite–wollastonite Nephelinite (CWN) of OL by enrichment of CO_2 and alkalies and is close to the liquid immiscible separation of natrocarbonatite from carbonated Nephelinite. Material ejected in April/May 2008 indicates reversion to a more common CWN composition.
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Fundamental changes in the activity of the natrocarbonatite volcano Oldoinyo Lengai, Tanzania. I. New magma composition during the 2007-2008 explosive eruptions
Bulletin of Volcanology, 2010Co-Authors: Jörg Keller, Jurgis Klaudius, Matthieu Kervyn, Gerald Ernst, Hannes B. MattssonAbstract:With a paroxysmal ash eruption on 4 September 2007 and the highly explosive activity continuing in 2008, Oldoinyo Lengai (OL) has dramatically changed its behavior, crater morphology, and magma composition after 25 years of quiet extrusion of fluid natrocarbonatite lava. This explosive activity resembles the explosive phases of 1917,1940-1941, and 1966-1967, which were characterized by mixed ashes with dominantly nephelinitic and natrocarbonatitic components. Ash and lapilli from the 2007-2008 explosive phase were collected on the slopes of OL as well as on the active cinder cone, which now occupies the entire north crater having buried completely all earlier natrocarbonatite features. The lapilli and ash samples comprise nepheline, wollastonite, combeite, Na-(a) over circle kermanite, Ti-andradite, resorbed pyroxene and Fe-Ti oxides, and a Na-Ca carbonate phase with high but varying phosphorus contents which is similar, but not identical, to the common gregoryite phenocrysts in natrocarbonatite. Lapilli from the active cone best characterize the erupted material as carbonated combeite- wollastonite-melilite Nephelinite. The juvenile components represent a fundamentally new magma composition for OL, containing 25-30 wt.% SiO2, with 7-11 wt.% CO2, high alkalies (Na2O 15-19%, K2O 4-5%), and trace-element signatures reminiscent of natrocarbonatite enrichments. These data define an intermediate composition between natrocarbonatite and Nephelinite, with about one third natrocarbonatite and two thirds Nephelinite component. The data are consistent with a model in which the carbonated silicate magma has evolved from the common combeite-wollastonite Nephelinite (CWN) of OL by enrichment of CO2 and alkalies and is close to the liquid immiscible separation of natrocarbonatite from carbonated Nephelinite. Material ejected in April/May 2008 indicates reversion to a more common CWN composition.
Everton Marques Bongiolo - One of the best experts on this subject based on the ideXlab platform.
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The nephelinitic–phonolitic volcanism of the Trindade Island (South Atlantic Ocean): Review of the stratigraphy, and inferences on the volcanic styles and sources of Nephelinites
Journal of South American Earth Sciences, 2016Co-Authors: Gustavo Luiz Campos Pires, Everton Marques BongioloAbstract:Abstract Trindade Island is located in the South Atlantic Ocean, 1170 km from the Brazilian coast, and represents the eastern end of the E–W Vitoria–Trindade Chain. It shows the youngest plume-induced (ca. 3.7 to 2.4 Ma) and the Desejado (DF, ∼2.4 to 1.5 Ma), Morro Vermelho (MV, The nephelinitic volcanism may represent Strombolian and Hawaiian–type activity of low viscosity and volatile–rich lavas interlayered with pyroclastic successions (fall–out deposits). Phonolitic deposits record explosive Vulcanian–style episodes of volatile–rich and higher–viscosity lavas interlayered with pyroclastic deposits (mostly pyroclastic flows). Geochemical data allowed the individualization of Nephelinites as follows: (1) MV olivine–rich Nephelinites and all olivine–free varieties are low K 2 O/Na 2 O, K 2 O/TiO 2 and intermediate CaO/Al 2 O 3 that may be derived from N–MORB and HIMU mantle components; (2) the VF olivine–rich Nephelinites have high K 2 O/Na 2 O, K 2 O/TiO 2 and CaO/Al 2 O 3 that indicates both EM and HIMU mantle sources and; (3) the PF olivine–rich Nephelinites show high K 2 O/TiO 2 similar to those from VF, and intermediate CaO/Al 2 O 3 as Nephelinites from MV rocks, suggesting a mixed source with EM + HIMU > N–MORB components. We suggest that the HIMU and EM mantle types resulted from metasomatic episode(s) in the peridotitic mantle beneath the Trindade Island during the Brasiliano Orogeny and later, as previously pointed out by Marques et al. (1999). Thus, the major HIMU component would relate to recycled oceanic crust or lithospheric mantle (mostly CO 2 –eclogites) whereas the less important EM component to recycled marine or continental sediments.
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geochemical modeling and nd sr data links Nephelinite phonolite successions and xenoliths of trindade island south atlantic ocean brazil
Journal of Volcanology and Geothermal Research, 2015Co-Authors: Everton Marques Bongiolo, Gustavo Luiz Campos Pires, Mauro Cesar Geraldes, Anderson Costa Dos Santos, Reiner NeumannAbstract:Abstract The Trindade Island corresponds to the eastern end of the submarine E–W Vitoria–Trindade Chain as part of the Trindade plume track on the South American plate. It is a suitable site for petrogenetic investigations, since a series of unusual rock compositions crops out within Nephelinite–phonolite successions and scarce xenoliths. Software-based geochemical modeling and Nd–Sr analyses, coupled with field work, petrography and literature data were used to evaluate and model the petrogenetic processes that led to the formation of variable rock compositions. Results show the formation of: (1) Nephelinites at 1490 °C and 3 GPa, from 0.1 to 7% of partial melting of an enriched garnet–lherzolitic source or from 1 to 5% partial melting of TiO2-rich garnet–phogopite lherzolite (up to 2.5 wt.% of CO2). Nephelinites represent low viscosity (18 Pa s), high-temperature (1170 °C) and high-density (2.79 g/cm3) lavas; (2) pyroxenite, jacupirangite and melteigite cumulates at 900 °C and 0.5 GPa, after 46%, 49% and 56% fractional crystallization of Nephelinites, and leaving phonotephrites as residual liquids; (3) monchiquites, from fractional melting of enriched, CO2-bearing garnet–amphibole–phlogopite peridotites or from hydrated Nephelinite magmas. They may evolved to sannaite via fractional crystallization or may generate phonolites through extraction of pyroxene and amphibole cumulates at 1080 to 970 °C and 1.2 to 0.9 GPa; (4) sannaites may represent ‘hydrous’ phonotephrites, and can evolve to phonolites and phonolitic foidites via fractional crystallization; and (5) bebedourite cumulates at 850 °C and 0.38 to 0.4 GPa, from phonotephritic or sannaitic magmas after 20% and 26% of fractional crystallization, leaving phonolites and phonolite–foidites as residual liquids. Phonolites and phonolitic foidites, the most evolved rocks of the Nephelinite–phonolite association, represent higher-viscosity (1.1 × 106 Pa s), lower-temperature (950 °C), and lower-density (2.48 g/cm3) lavas. Both the geochemical modeling and the Nd–Sr isotopic data show that the studied rocks are genetically associated. They evolved via fractional crystallization after partial melting of a very homogeneous reservoir with inherited asthenospheric heterogeneities. Our work refines and validates some of the assumptions of previous contributions, and it can be used as a basis for further petrogenetic investigations on oceanic islands.
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Geochemical modeling and Nd–Sr data links Nephelinite–phonolite successions and xenoliths of Trindade Island (South Atlantic Ocean, Brazil)
Journal of Volcanology and Geothermal Research, 2015Co-Authors: Everton Marques Bongiolo, Gustavo Luiz Campos Pires, Mauro Cesar Geraldes, Anderson Costa Dos Santos, Reiner NeumannAbstract:Abstract The Trindade Island corresponds to the eastern end of the submarine E–W Vitoria–Trindade Chain as part of the Trindade plume track on the South American plate. It is a suitable site for petrogenetic investigations, since a series of unusual rock compositions crops out within Nephelinite–phonolite successions and scarce xenoliths. Software-based geochemical modeling and Nd–Sr analyses, coupled with field work, petrography and literature data were used to evaluate and model the petrogenetic processes that led to the formation of variable rock compositions. Results show the formation of: (1) Nephelinites at 1490 °C and 3 GPa, from 0.1 to 7% of partial melting of an enriched garnet–lherzolitic source or from 1 to 5% partial melting of TiO2-rich garnet–phogopite lherzolite (up to 2.5 wt.% of CO2). Nephelinites represent low viscosity (18 Pa s), high-temperature (1170 °C) and high-density (2.79 g/cm3) lavas; (2) pyroxenite, jacupirangite and melteigite cumulates at 900 °C and 0.5 GPa, after 46%, 49% and 56% fractional crystallization of Nephelinites, and leaving phonotephrites as residual liquids; (3) monchiquites, from fractional melting of enriched, CO2-bearing garnet–amphibole–phlogopite peridotites or from hydrated Nephelinite magmas. They may evolved to sannaite via fractional crystallization or may generate phonolites through extraction of pyroxene and amphibole cumulates at 1080 to 970 °C and 1.2 to 0.9 GPa; (4) sannaites may represent ‘hydrous’ phonotephrites, and can evolve to phonolites and phonolitic foidites via fractional crystallization; and (5) bebedourite cumulates at 850 °C and 0.38 to 0.4 GPa, from phonotephritic or sannaitic magmas after 20% and 26% of fractional crystallization, leaving phonolites and phonolite–foidites as residual liquids. Phonolites and phonolitic foidites, the most evolved rocks of the Nephelinite–phonolite association, represent higher-viscosity (1.1 × 106 Pa s), lower-temperature (950 °C), and lower-density (2.48 g/cm3) lavas. Both the geochemical modeling and the Nd–Sr isotopic data show that the studied rocks are genetically associated. They evolved via fractional crystallization after partial melting of a very homogeneous reservoir with inherited asthenospheric heterogeneities. Our work refines and validates some of the assumptions of previous contributions, and it can be used as a basis for further petrogenetic investigations on oceanic islands.