Capaccioni, . B.

Santo, Alba Patrizia

Tedesco D, . D.

Vaselli, Orlando

English

Santo, ALBA PATRIZIA

Florence Research

Petrographic and geochemical features of  the 2002 Nyiragongo lava flows63Acta Vulcanologica · Vol. 14 (1-2), · 15 (1-2), 2003: 63-66PETROGRAPHIC AND GEOCHEMICAL FEATURESOF THE 2002 NYIRAGONGO LAVA FLOWSAlba P. Santo1 · Bruno Capaccioni2 · Dario Tedesco3 · Orlando Vaselli11. Department of  Earth Sciences, Via G. La Pira, 4, I 50121 Firenze, Italy2. Institute of  Volcanology and Geochemistry, Loc. Crocicchia, I 61029 Urbino, Italy3. Department of  Environmental Sciences, Via Vivaldi 43, I 81100 Caserta, ItalyAbstractPreliminary data on the petrographic and geochemical characteristics of  some volcanic products of  the January 2002 Nyiragongo(Democratic Republic of  Congo) eruption are reported. Selected rocks were collected at three different sites: Shaheru fracture,Monigi vent and Virunga flow. They are represented by nephelinites, which closely resemble historical (1977 and 1982) productsof  the Nyiragongo volcano.Keywords: East African Rift, Nyiragongo volcano, 2002 eruption, nephelinites1. IntroductionThe Nyiragongo complex, belonging to the alkalineprovince of  the East African Rift (Western branch),consists of  three volcanoes (Baruta, Nyiragongo,Shaheru) aligned in a n-s direction and by numerousparasitic cones located on the volcano flanks alongradial fractures.Nyiragongo volcano is well known for its lava lakeintra-crateric activity (Tazieff 1949, 1977 and 1994). Inhistorical time the Nyiragongo volcano erupted in the1977 when rifting caused the opening of  several frac-tures in the upper part of  the volcano. On June 1982,volcanic activity resumed and magma erupted fromtwo vents located along two of  those fractures andforming a pseudo lava lake (Demant et alii 1984).During the last eruption of  the Nyiragongo Volcano( January 2002; see Komorowski et alii this volume, fordetails), the 1977 fracture systems were reactivated.The eruption started immediately north of  Shaherucrater; the fracture system rapidly propagated towardGoma, and partly covered the city. The eruption lastedabout 24 hours with a total volume of  25 × 106 m3 oflava ejected. The purpose of  this paper is to describethe petrographic and geochemical characteristics ofsome volcanic products from 2002 eruption and tocompare them with those collected from 1977 lava flowand 1982 lava lake activity.2. Classification and petrographyRock samples have been collected along the n-s fracturesystem connecting the Nyiragongo crater with Gomatown and precisely along the Shaheru fracture, close tothe Monigi vent and at Virunga – South of  Monigi (seeFigure 1 in Capaccioni et alii this volume). Exact samplelocations are reported in the Table 1. Rock samples arerepresented by strombolian scoriae and lava flows. Inthe chemical classification diagram of  Le Bas et alii(1986), these samples plot in the foidite field (Fig. 1). Allthe studied samples are hypocrystalline porphyritic,but differ in their texture and modal mineralogy. Rocksfrom Shaheru fracture display small amounts (< 10 vol.%) of  microlithes of  melilite and kalsilite, and minorclinopyroxene (Ti-augite) and rhoenite set in a glassygroundmass. Rocks from Monigi and Virunga exhibita phenocryst content ranging from 30 to 50 volume %.Phenocryst mineralogy consists of  nepheline, leuciteand lower amounts of  diopsidic clinopyroxene; olivine(Fo73-84) is present as microphenocryst; in addition toFig. 1. Total alkali-silica classification diagram (after Le Baset alii 1986). Fields: 1 = foidite; 2 = tephrite and basanite; 3= picro-basalt. 1977 lava flow and 1982 lava lake data arefrom Demant et alii 1994.Adress for correspondence: A. P. Santo: asanto@geo.unifi.itAlba P. Santo et alii64these minerals, melilite is present in the Virunga rocksample. The groundmass is glassy in the Monigi rocks;the Virunga groundmass contains the same phases asthe phenocrysts and microphenocrysts and smallamount of  glass. Accessory apatite and Fe-Ti oxides areubiquitous. The Shaheru and Monigi rocks can beclassified as nephelinites, whereas that from Virunga isa melilite-leucite bearing nephelinite (Le Maitre 2002).3. GeochemistryMajor and trace element compositions of  selected rocksamples are listed in Table 2.1 They are undersaturatedrocks (normative ne > 24 wt %; Table 2), and arecharacterised by K2O/Na2O ratio < 1. The low Mg#,Ni and Cr (Table 2) indicate that they represent differ-entiated magmas. On the alkali versus silica diagram(Fig. 1) the composition of  groundmass glasses, ana-lysed in three 2002 samples by electron microprobe, arealso reported (Table 3).2 In contrast with the ratherhomogenous composition of  whole rocks, the glassesdisplay variable composition (SiO2 = 36.7-41.5 %), al-though they still plot in the foidite field. Such a vari-ation may indicate mixing-mingling processes betweenFig. 2. Mantle normalised trace element patterns. GregoryRift (Kenia) data from Baker et alii (1977). Normalisationvalues from Sun and McDonough (1989).1. Major and trace element composition was obtained by X-Ray Fluo-rescence at the Dept. of  Geology, University of  Edinburgh, U.K., bymean of  a Philips PW 1400 XRF. Glass disks of  rock samples were used.2. Phase composition was determined by a JEOL JXA-8600 electronmicroprobe operating at 15 kV and 15 nA and equipped with a SeriesII Tracor Northern system. Correction for matrix effects were madeaccording to the method of  Bence and Albee (1968).Table 2. Major (%) and trace (ppm) element composition ofselected 2002 volcanics.Sample NY6 NY7 NY9 NY19 NY20SiO239.3 38.7 38.1 39.5 39.8TiO22.7 2.7 2.7 2.8 2.7Al2O3 14.6 14.4 14.2 14.6 14.6Fe2O313.2 13.2 13.1 13.3 13.2MgO 4.1 4.1 4.1 4.1 4.1CaO 12.2 12.2 12.2 12.3 12.1Na2O 5.6 5.8 5.8 5.8 5.7K2O 5.1 5.4 5.4 5.4 5.3MnO 0.29 0.29 0.29 0.29 0.29P2O51.5 1.5 1.4 1.5 1.5Mg# 0.38 0.38 0.38 0.38 0.38Sc 5.1 5.8 4.9 4.9 6.0V 275 279 252 272 272Cr 6.6 3.4 3.8 2.6 2.8Ni 34 36 30 29 31Rb 141 143 146 143 142Sr 2832 2875 2873 2871 2830Y 42 42 42 42 42Zr 319 319 315 319 315Nb 264 268 262 267 264Ba 2126 2127 2197 2126 2128La 191 192 184 185 185Ce 340 336 330 335 335Nd 122 122 117 121 120cipw norm calculation (wt %)Ne 25.9 24.5 24.2 24.8 25.1Le 24.2 25.7 26.0 25.4 25.0Di 16.3 4.3 0.6 8.4 13.3Ol 10.5 15.8 17.5 13.8 11.8La 10.0 14.6 16.3 13.0 10.9Ac 0.61 4.6 5.6 3.6 2.4Ilm 5.3 5.3 5.3 5.4 5.2Mt 3.6 1.6 1.1 2.1 2.7Ap 3.6 3.6 3.4 3.5 3.5Mg# = [mol Mg 2+/(Mg2+ +Fe2+)]liquids with different composition coexisting in thevolcanics system. Further studies will be directed atclarifying this point, which has strong petrological andvolcanological implications. All the studied rocks showsimilar mantle-normalised patterns of  incompatibleelements (Fig. 2). Overall, the Nyiragongo rocks dis-play a strong element fractionation, with enrichmentsin Ba and Nb, and a negative spike of  K. The compari-son with rocks from the eastern branch of  the EastAfrican Rift (Gregory Rift, Kenia; Fig. 2) shows similarpatterns, except for Ti, but with a stronger enrichmentof  all the elements in the Nyiragongo rocks.As observable in Figure 1 and Table 3, lava compositionTable 1. Sample location and description.Sample NY6 NY7 NY9 NY19 NY20Local name Shaheru Shaheru Virunga Monigi MonigiCoordinates 01°32’56.4" S 01°32’56.4" S 01°39’36.5" S 01°38’34.9"S 01°38’34.9" S29°15’02.0" E 29°15’02.0" E 29°13’59.5"E 29°14’45.1" E 29°14’45.1" EDescription juvenile scoria juvenile scoria lava lava juvenile scoriaPetrography melilite, kalsilite, melilite, kalsilite+ melilite, nepheline, nepheline, leucite+ nepheline, leucite+Ti-augite+apatite apatite and Fe-Ti leucite + diopside, diopside, olivine, diopside, olivine,and Fe-Ti oxides oxides olivine, apatite and apatite and Fe-Ti apatite and Fe-TiFe-Ti oxides oxides oxidesPetrographic and geochemical features of  the 2002 Nyiragongo lava flows65Table 3. Major element (%) composition of  1977, 1982 lavas and2002 glasses used for comparison with 2002 rock samples.* *Sample 1977 1982 NY20 NY7 NY6mean lavaflow lava lake gdm glass gdm glass gdm glassSiO2 39.9 40.0 41.3 39.5 36.7Al2O315.0 14.5 15.0 15.0 14.3Fe2O313.8 13.2 12.0 12.6 17.5MgO 4.2 4.3 3.8 4.2 6.3CaO 12.2 12.5 10.6 11.8 8.2Na2O 5.3 5.9 6.2 5.9 5.1K2O 5.3 5.3 5.8 5.4 5.1TiO22.7 3.0 2.8 2.6 4.3MnO 0.28 0.26 0.31 0.31 0.27P2O5 1.4 1.3 1.2 1.5 n.a.Mg# 0.38 0.39 0.38 0.40 0.42* = data from  Demant et al., 1994; gdm = groundmass; n.a.= not nalysed;Mg# = [mol Mg 2+/(Mg2+ +Fe2+)]from 2002 eruption closely resembles those of  1977 and1982 eruptions at least in major element abundances.The studied 2002 Nyiragongo volcanics display veryhomogeneous geochemical composition but show differ-ent textures and modal mineralogy. This may indicatemagma ponding and crystallisation at different depths,a hypothesis to be tested by accurate geothermo-barometric studies on rocks from different vents.AcknowledgementsThe authors would like to thank A. Peccerillo, Univer-sity of  Perugia, for critical reading and improvementof  the manuscript.ReferencesBaker B. H., Goles G. G., Leeman W. P., Lindstrom M. M. (1977).Geochemistry and petrogenesis of  a basalt-benmoreite-trachyte suitefrom the southern part of  the Gregory Rift, Kenya. «Contrib.Mineral. Petrol.», 64, 303-332.Bence A. E., Albee A. L. (1968). Empirical correction factors for theelectron microanalyses of  silicates and oxides. «J. Geol.», 76, 382-483.Capaccioni B., Vaselli O., Santo A. P. and Yalire M. (this volume).Monogenic and polygenic volcanoes in the area between theNyiragongo summit crater and the Lake Kivu shoreline «ActaVulcanol.» 129-136.Demant A., Lestrade P., Lubala T. R., Kampunzu A. B. and DurieuxJ. 1994. 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Petrographic and geochemical features of the 2002 Nyiragongo lava flows.

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Petrographic and geochemical features of the 2002 Nyiragongo lava flows.

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