Application of the Quilf Thermobarometer to the Peralkaline Trachytes and Pantellerites of the Eburru Volcanic Complex, East African Rift, Kenya

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10-2006 Application of the QUIlF thermobarometer to the peralkaline trachytes and pantellerites of the Eburru volcanic complex, East African Rift, Kenya. MInghua Ren University of Texas at El Paso

Peter Omenda Geothermal Development Corporation, Kenya

Elizabeth Y. Anthony University of Texas at El Paso

John C. White Eastern Kentucky University, [email protected]

Ray Macdonald Lancaster University

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Recommended Citation Ren, M., Omenda, P.A., Anthony, E.Y., White, J.C., Macdonald, R., and Bailey, D.K., 2006, Application of the QUIlF thermobarometer to the peralkaline trachytes and pantellerites of the Eburru volcanic complex, East African Rift, Kenya. In: Peralkaline Rocks: A Special Issue Dedicated to Henning Sørensen, PERALK2005 Workshop (G. Markl, Ed.) Lithos, v. 91, p. 109-124. (doi: 10.1016/ j.lithos.2006.03.011)

This Article is brought to you for free and open access by Encompass. It has been accepted for inclusion in EKU Faculty and Staff choS larship by an authorized administrator of Encompass. For more information, please contact [email protected]. Authors MInghua Ren, Peter Omenda, Elizabeth Y. Anthony, John C. White, Ray Macdonald, and D K. Bailey

This article is available at Encompass: http://encompass.eku.edu/fs_research/199 Lithos 91 (2006) 109–124 www.elsevier.com/locate/lithos

Application of the QUILF thermobarometer to the peralkaline trachytes and pantellerites of the Eburru volcanic complex, East African Rift, Kenya ⁎ Minghua Ren a, , Peter A. Omenda b, Elizabeth Y. Anthony a, John C. White c, Ray Macdonald d, D.K. Bailey e

a Department of Geological Sciences, University of Texas at El Paso, TX 79968, USA b Olkaria Geothermal Project, P.O. Box 785, KenGen, Moi South Lake Road, Naivasha 20117, Kenya c Department of Earth Sciences, Eastern Kentucky University, Richmond, KY 40475, USA d Environment Centre, Lancaster University, Lancaster LA1 4YQ, UK e Department of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK Received 11 July 2005; accepted 13 March 2006 Available online 10 July 2006

Abstract

The Quaternary Eburru volcanic complex in the south-central Kenya Rift consists of pantelleritic trachytes and pantellerites. The phenocryst assemblage in the trachytes is sanidine+fayalite+ferrohedenbergite+aenigmatite±quartz±ilmenite±magnetite ± pyrrhotite±pyrite. In the pantellerites, the assemblage is sanidine+quartz+ferrohedenbergite+fayalite+aenigmatite+ferrorichterite+pyrrhotite±apatite, although fayalite, ferrohedenbergite and ilmenite are absent from more evolved rocks (e.g. with SiO2 N71%). QUILF temperature calculations for the trachytes range from 709 to 793 °C and for the pantellerites 668–708 °C, the latter temperatures being among the lowest recorded for peralkaline silicic magmas. The QUILF thermobarometer demonstrates that the Eburru magmas crystallized at relatively low oxidation states (ΔFMQ +0.5 to −1.6) for both trachytes and pantellerites. The trachytes and pantellerites evolved along separate liquid lines of descent, the trachytes possibly deriving from a more mafic parent by fractional crystallization and the pantellerites from extreme fractionation of comenditic magmas. © 2006 Elsevier B.V. All rights reserved.

Keywords: Kenya; Eburru volcanic complex; Trachyte; Pantellerite; Peralkaline; QUILF

1. Introduction salic magmas, they are enriched in FeO*, Na2O, HFSE, REE and halogens, and are relatively low in Al2O3,CaO, Peralkaline magmas form mainly in extensional P2O5,SrandBa(Noble, 1968; Macdonald, 1974). Trace environments and hot spots. Compared to metaluminous element and isotopic characteristics of peralkaline silicic rocks are most commonly interpreted to show that the ⁎ Corresponding author. Tel.: +1 915 747 5843; fax: +1 915 747 magmas are ultimately mantle-derived, either by fraction- 5073. ation of basaltic magmas (Barberietal.,1975;Baconetal., E-mail addresses: [email protected] (M. Ren), 1981; Harris, 1983; Novak and Mahood, 1986; Bloomer et [email protected] (P.A. Omenda), [email protected] (E.Y. Anthony), [email protected] (J.C. White), al., 1989; Caroff et al., 1993; Civetta et al., 1998; Kar et al., [email protected] (R. Macdonald), 1998) or by remelting of underplated mafic rocks (Bailey [email protected] (D.K. Bailey). and Schairer, 1966; Mahood et al., 1990; Lowenstern and

Mahood, 1991; Frost and Frost, 1997; Bohrson and Reid, rocks of the Proterozoic Gardar province (Markl et al., 1997, 1998; White et al., 2006-this volume). An exception 2001a,b; Marks and Markl, 2001; Marks et al., 2003). are the comendites of the Greater Olkaria Volcanic Complex (GOVC) in the south-central Kenya rift, where 2. Geological setting of the volcanoes of the Kenya Pb isotopes and U-series disequilibria and relatively high Dome LILE/HFSE ratios have been inferred to indicate an origin by crustal anatexis (Davies and Macdonald, 1987; The Kenya Rift is the segment of the East African Rift Macdonald et al., 1987; Black et al., 1997; Heumann System that extends from the Ethiopia–Kenya border into and Davies, 2002). However, the extreme depletion of Ba northern Tanzania. Geophysical studies have established and Sr in the Greater Olkaria Volcanic Complex (b5and that the depth to Moho varies from 20 km beneath b1 ppm, respectively) is not easily explained by anatexis Turkana in the north to 35 km beneath the Kenya Dome andseemstorequireanextendedperiodofcrystal (center) to 38 km in northern Tanzania (Keller et al., 1994; fractionation of the parental magmas, whatever their origin Mechie et al., 1997). The increased thickness of crust in (Davies and Macdonald, 1987; Heumann and Davies, the Kenya Dome reflects a 7 km thick lower crustal layer 2002). that has a P-wave velocity of 6.8 km/s. P-wave velocity in Petrogenetic information for individual systems can the uppermost mantle varies only slightly from 7.6 to be provided through the determination of magma 7.5 km/s in the Turkana to Kenya Dome segment. Seismic intensive parameters and mineral and whole-rock geo- velocity for the southernmost segment derives from an E– chemistry, which place constraints on parameters such W profile that passed through the Magadi area and thus is as pre-eruptive volatile concentrations of the magma less well constrained. A conservative estimate is 7.8 km/s ( f O2, f H2O) and magma temperature (Markl et al., (Birt et al., 1997). The existence of higher velocities south 2001a, 2003; Marks and Markl, 2001; Newman and of the Kenya Dome is consistent with gravity studies Lowenstern, 2002; White et al., 2005). Magma (Simiyu and Keller, 1997, 2001) that model a change in temperatures, for instance, provide a means to evaluate mantle densities from 3120 kg m−3 under the Kenya the competing hypotheses of fractional crystallization Dome to 3260 kg m− 3 in northern Tanzania. The from mantle-derived rocks vs. crustal anatexis. Since geophysical data thus imply that the Kenya Dome sits magmas as a rule do not achieve superheated conditions above a transition to thicker lithosphere as the Tanzanian (but rather generate more magma given more heat to the Craton is approached. East–west seismic profiles indicate system), temperatures substantially in excess of the a steep-sided velocity gradient between the low P-wave likely range of solidus temperatures for crustal rocks are velocities in the upper mantle of the axial region and more reasonably interpreted as resulting from crystalli- velocities of 8.1 km/s on the eastern flanks (Byrne et al., zation from mantle-derived magmas. On the other hand, 1997). Finally, the location of the Kenya Rift is temperatures at approximately the solidus for crustal structurally controlled; rift faults exploit weaknesses at compositions are permissive of crustal anatexis. Like- the contact between the Archean Tanzanian Craton to the wise, initial magma composition is often inherited or west and Proterozoic orogenic belts to the east (Smith and buffered by the oxidation state of the protolith, and thus Mosley, 1993; Smith, 1994; Stern, 1994). low oxidation states reflect direct derivation from A series of short wavelength gravity highs are mantle melting or anatexis of mantle-derived source superposed on the broad negative anomaly in the axial rocks (Frost and Frost, 1997; Anthony, 2005). region of the Kenya Rift. Swain (1992) modeled these Peralkaline rocks tend to lack mineral assemblages gravity highs as resulting from pervasive dike injection to a appropriate for determining intensive parameters; for depth of 22 km. The gravity interpretation of density bodies example, coexisting Fe–Ti oxides are rare in peralkaline is corroborated by seismic data from the KRISP experi- rhyolites. As a result, very few quantitative data for these ments (Simiyu and Keller, 1997, 2001). Simiyu and Keller parameters are available (Scaillet and Macdonald, 2001). modeled the high density bodies and their interpretation is However, the Eburru volcanic complex, which neigh- that dense mafic intrusions underlie the volcanic complexes bours the Greater Olkaria Volcanic Complex in the Kenya of Menengai, Eburru, Olkaria, and Suswa. The bodies rift, contains rocks with phenocryst assemblages contain- occur in the upper crust, perhaps as shallow as 7 to 12 km ing fayalite, clinopyroxene, ilmenite, and quartz, making and have densities of about 2900 kg/m3. This interpretation the use of the QUILF thermometer and oxygen barometer coincides with the interpretation by Swain (1992),who appropriate (Lindsley and Frost, 1992; Frost and Lindsley, envisaged each of the young (b2 Ma) complexes in the 1992). The application of QUILF is a similar approach to KenyaDomeasrepresentinganexus,wherearegionaldike that used recently in a series of papers on the peralkaline swarm has developed a shallow reservoir. M. Ren et al. / Lithos 91 (2006) 109–124 111

The volcanic complexes associated with the Kenya drilling of six wells. The conclusion of the exploration Dome together form an important peralkaline province. campaign is that the Eburru volcanic complex has a The eruptive sequences of the province were evolved by geothermal reservoir that could support a 20 MW power complex combinations of fractional crystallization, station (Omenda and Karingithi, 1993). The area is not magma mixing, crustal remelting and possibly volatile developed at this time because the Greater Olkaria complexing. The magma reservoirs at each centre Volcanic Complex immediately to the south is more developed in different ways owing to variations in accessible and has higher steam production. The Eburru magma composition (including volatile content), cham- volcanic complex remains in consideration for future ber geometry and the effect of local tectonics. Com- geothermal energy development, making it important to positional zonation was repeatedly developed in the understand the evolution of the complex. magma chambers, at scales ranging from b1to30km3, A GIS/GPS/satellite and aerial photo study followed over time scales of 103–104 years. The greatest compo- the 1989 campaign and used the location of fumaroles sitional range most commonly was developed in the from that campaign. The study determined that 80% of magmas erupted as pyroclastics (Macdonald and the fumarole locations are associated with either the Scaillet, 2006-this volume). eastern ring structure or the north south flows that con- trol pantellerite eruption, corroborating stratigraphic 3. Volcanic history of the Eburru volcanic complex evidence that Et2 and Er2 are the youngest volcanic activity in the area. The Eburru volcanic complex is an east–west trending ridge with an area of 470 km2 and a maximum 4. Analytical methods elevation of 2850 m. Topographic highs in the west and the east are volcanic ring structures, with the western Whole-rock data are available on line; see Appen- pre-dating the eastern structure (Fig. 1). The western dix A. They are from sample collections at Lancaster ring structure, has an approximate diameter of two kilo- University (the KE samples) and the University of meters and is heavily mantled by pyroclastic deposits Texas at El Paso (UTEP) (the keb samples). All but appears to be composed dominantly of pantelleritic samples are flow rock except KE12, which is tuff. The lavas and welded tuffs (Clarke et al., 1990). West Hill, KE samples were analyzed for major elements at the one of two high peaks in the volcanic complex, is University of Reading by wet chemistry. Fluorine and located on the northwest margin of this ring structure. chlorine were determined colorimetrically and FeO Volcanic activity in the western part of the field was titrimetrically. Certain trace elements (Rb, Nb, Y, Zn followed in eastern Eburru by eruption of the Eburru and Zr) were determined by X-ray fluorescence (XRF) Trachyte Formation, which contains two divisions, the at Reading, and the remaining trace elements in the Older (Et1) and Younger members (Et2). The Older KE samples were determined by INAA at the United member is frequently offset by normal faults, whereas States Geological Survey in Reston, Virginia. Major the Younger member is unfaulted. Both members com- elements in the two BL samples were determined by prise trachyte lava flows, usually feldspar-phyric; cones wet chemistry at Lancaster University. Details of of pumiceous and scoriaceous blocks and welded pyro- analysis, including error estimates, are in Macdonald clastic rocks have also been recorded. The estimated age et al. (1987) and Bacon et al. (1981). One sample, for these eruptions is between 1.2 and 0.4 Ma. Activity KE17, is stratigraphically within the older member of was associated with the eastern ring structure, north– the Eburru Trachyte Formation (Et1), but is pantelleri- south trending normal faults and approximately fifty tic obsidian. We include it with the other Et samples in small craters (Clarke et al., 1990; Omenda, 1997; Vela- the data table, but it plots with pantellerites in Figs. 2 dor, 2003; Velador et al., 2003). and 3. The youngest volcanic activity in the area is the eastern For the keb samples from the UTEP sample collection, Eburru Pantellerite Formation (Er2). Lava flows, com- major elements and some trace elements were determined monly fresh, glassy obsidian, emanated along N–Sfaults by ICP-AES at Texas Tech University and the remaining that originate in the lowlands immediately north of the trace elements by INAA at the UTEP. Error estimates are Greater Olkaria Volcanic Complex and extend north to the given in Omenda (1997) and Barnes et al. (2002). mafic flows and cones of the Elmenteita area. Samples with mineral chemical data and QUILF In 1989 KenGen conducted an exploration program determinations are from this keb sample set. Mineral che- for geothermal resources, which included geologic map- mistry was obtained by electron probe microanalysis ping, chemical characterization of the fumaroles, and (EPMA) at UTEP on a Cameca SX50 instrument. 112 M. Ren et al. / Lithos 91 (2006) 109–124

Fig. 1. Geological map of Eburru shows contact relations between flow units and sample locations. Flow units based on Clarke et al. (1990).

Minerals were analyzed with a 15 keV accelerating and Na2O; and Ilmen Mountains Ilmenite (USNM 96189) voltage, 20 nA beam current, 5 μmbeamsize,and20s for TiO2, MnO, and FeO. For volatile components F, Cl, peak counting time. Calibration standards include Natural and S, Astimex Scientific Ltd. MINM25-53 standards Bridge Diopside (USNM 117733) for SiO2,MgO,and were used: fluorite for F, tugtupite for Cl, and marcasite CaO; Kakanui Anorthoclase (USNM 133868) for Al2O3 for S. Standards from both the Smithsonian and Astimex M. Ren et al. / Lithos 91 (2006) 109–124 113

Fig. 2. (a) TAS diagram shows the classification of the samples based on IUGS. The plot indicates the bimodal nature of the Eburru volcanic complex.

(b) Al2O3 vs. FeO* diagram from Macdonald (1974). collections were analyzed each day to monitor accuracy classification for granitic rocks of Frost et al. (2001). and precision of the analyses. Their characteristics – ferroan, alkalic, and peralkaline – place the Eburru volcanic complex with other extension- 5. Whole-rock chemistry related suites of A-type magma affinity. The reduced nature of the Eburru magmas, as deduced from mineral The samples included in this study are dominantly assemblages and compositions (see below), is consistent lava flows, with a few examples of pyroclastic units. with their very high Fe/Mg (Frost and Frost, 1997). 2 The lavas of the Younger Pantellerites (Er ) are glassy A combination of TAS and total FeO vs. Al2O3 obsidian flows. The trachytes (both Et1 and Et2) are (Macdonald, 1974) diagrams are used here to classify the usually devitrified. The minerals in these devitrified samples (Fig. 2a and b). The diagrams show that the flows are, however, unaltered. All the samples are pe- mapped units, which are stratigraphic, do contain flows of ralkaline (A.I. from 1.1 to 2.2) and strongly ferroan (Fe both trachyte and rhyolite. The samples of the Older number 0.97 to 0.98). The samples from the Eburru Trachyte unit (Et1) comprise one unevolved comenditic volcanic complex may be placed in a context with other trachyte (KE16g), pantelleritic trachyte, and two samples silica-rich rocks, both intrusive and extrusive, using the of pantelleritic rhyolite. Samples of the Younger Trachyte 114 M. Ren et al. / Lithos 91 (2006) 109–124

Fig. 3. FK/A (mol Fe+K/Al) vs. A.I. (mol Na+K/Al) diagram monitors the alkali variation based on criteria by White et al. (2003). (Solid line: A.I.= 0.6727(FK/A)+0.5954; dash line: 95% confidence interval).

2 unit (Et ) are pantelleritic trachytes, and samples of the decreases, and K2O and FeO* remain about constant. 2 Younger Pantellerite unit (Er ) are pantelleritic rhyolites Na2O shows an overall increase; however, absolute va- with the exceptions of KE2 and KE4, which are lues are suspect given the Na loss on post-eruptive pantelleritic trachyte. It is clear from Fig. 2bthatthe crystallization discussed above. REE data for three Et1 1 2 dominantly trachytic formations Et and Et do not share rocks show modest LREE enrichment ([La/Yb]N =7.1, a common liquid line of descent with the pantelleritic 7.3) and distinct negative Eu anomalies (Eu/Eu*=0.37, rhyolites of Er2; instead, the Er2 pantelleritic rhyolites are 0.40). colinear with the comenditic rhyolites of the Greater In the Er2 pantelleritic rhyolites, increasing peralk- Olkaria Volcanic Complex. This observation raises the alinity is marked by increases in FeO* and SiO2 with possibility that the older units, which center on the ring decreasing Al2O3. These rocks have the trace element structures and the younger pantelleritic rhyolites, which characteristics typical of other pantelleritic rhyolites, erupted along the north–south fissure system, have a viz. very high HFSE abundances (e.g., 1494 to different petrogenesis. 2091 ppm Zr; 293 to 519 ppm Nb), low concentrations Before examining the major- and trace-element chem- of Ba (b10 ppm) and Sr (b2 ppm), and extreme de- istry in more detail, it is important to establish which pletion in transition elements (e.g., ≤0.5 ppm Co and samples may have experienced post-eruptive alkali loss, a b1 ppm Cr) (Macdonald and Bailey, 1973; Bailey and common phenomenon in peralkaline rocks. We use the Macdonald, 1975; Clarke et al., 1990; and this paper). criteria developed by White et al. (2003), which plots the Chondrite-normalised REE patterns show similar less mobile elements, Fe, K, and Al, against agpaitic index degrees of LREE enrichment to the pantelleritic trachy- 1 to evaluate the data. Fig. 3 shows that the Et data plot tes ([La/Yb]N =5.4–9.1), but the Eu anomalies are much below the correlation and thus have seen alkali loss. The Et2 deeper (Eu/Eu*=0.21±0.02). data plot at the lower limit of the 95% confidence interval With the exception of Zr, the Eburru pantelleritic and thus may have experienced some alkali loss, but not of rhyolites do not show as strong enrichment in HFSE and the magnitude of the Et1 samples. The only pantelleritic LILE as the less peralkaline Greater Olkaria Volcanic rhyolite that shows alkali loss is the Middle Flow (keb1). Complex comendites. They also have lower LILE/ Within the trachytes, Et1 rocks tend to be slightly less HFSE ratios than the Greater Olkaria Volcanic Complex evolved than Et2 rocks, in the sense of having higher rocks (e.g. Rb/Zr=0.13 vs. 0.30; Th/Ta=1.5 vs. 2.5). Al2O3, lower SiO2 contents, and lower peralkalinity. Bailey and Macdonald (1975) analyzed samples from the With increasing peralkalinity, SiO2 increases, Al2O3 Eburru volcanic complex and found that Cl correlates M. Ren et al. / Lithos 91 (2006) 109–124 115 strongly with Nb and Y, and F with Zr and Rb. They mineral compositions are uniform between grains, and emphasized the importance of fluid transport, in which grains are essentially unzoned. these cations form preferred complexes with either F or Cl. 6.1. Alkali feldspar 6. Phenocryst assemblages and mineral chemistry Subhedral to euhedral sanidine phenocrysts are com- The trachytic and rhyolitic samples are porphyritic, mon in all samples (Fig. 4b). Sanidine micro-crystals are with phenocryst contents ranging from 3 to 25 vol.% abundant as groundmass minerals in the trachytes. (Table 1). The principal textural difference between Phenocrysts are more albite-rich (Ab61–64) in the trachytes trachytes and pantellerites is that the matrix of than in the pantellerites (Ab48–61). All are Ca-poor trachytes is holo- or hypo-crystalline (with up to (Anb0.2%). The feldspars analyzed thus far in pantellerites 30% glass), while the pantellerites are predominantly exhibit discontinuous variation in Ab content, with one glassy. group at Ab48–50 and the other at Ab58–61.Themore The trachytes have sanidine+fayalite+ ferroheden- peralkaline rocks, e.g. the Middle Hill and Younger flows, bergite+aenigmatite±ilmenite±magnetite±pyrrhotite have low values. A composition gap exists between these ±pyrite. The less peralkaline pantellerites have sani- younger flows and the earlier, less peralkaline ones. Some dine+quartz+ferrohedenbergite+fayalite+ilmenite+ high-Or sanidines in the Middle flow have granophyric aenigmatite+ferro-richterite+pyrrhotite±apatite. Fer- textures (Fig. 4c), implying co-precipitation of quartz and rohedenbergite, fayalite and ilmenite are not present in sanidine. more peralkaline rocks, e.g., the Middle Hill (keb1) and Younger (keb15) flows. Quartz occurs as a 6.2. Clinopyroxene groundmass mineral in both trachytes and pantellerites and as phenocrysts in the pantellerite (Fig. 4a). Mineral Clinopyroxene occurs in all but the most chemically compositions were determined for eleven samples. evolved pantellerite flows. Phenocrysts are euhedral Representative analyses for clinopyroxene, fayalite, (Fig. 4d) to subhedral and ferrohedenbergitic in compo- and oxides are given in Tables 2 and 3.Datafor sition. Increases in the acmite component mirror the in- sanidine, aenigmatite, amphibole, and sulfide are creasing peralkalinity of the whole-rock composition: the available on line; see Appendix A. Within a sample, early trachytes (Et1) have the lowest acmite component

Table 1 Modal abundances for Eburru mineral assemblages Unit Et1 Et2 Er2 Sample # keb8 keb13 keb5 keb12 keb6 keb16 keb2 keb17 keb10 keb1 keb15 Location West Horse Ol'Obonge Lion Cedar Eburru Southern Southern Northern Middle Younger Eburru Shoe Hill Hill Hill flow flow flow flow flow Phenocryst 20% 12% 6% 3% 25% 15% 20% 15% 3% 5% 20% sa: 17% sa: 11% sa: 5% sa: 3% sa: 20% sa: 12% sa: 15% sa: 10% sa: 2% sa: 2% sa: 15% fe-hd: fe-hd: fe-hd: aen: tr. aen: 3% fe-hd: fe-rich: fe-hd: 2% aen: b1% fe-rich: fe-rich: 2% b1% b1% 3% 2% 2% 2% fa: b1% aen: 1% aen: b1% ilm: tr. fe-rich: aen: 2% aen: 2% aen: 2% fe-rich: tr. qtz: 1% aen: 1% 1% ilm: b1% qtz: tr. qtz: tr. fe-hd: qtz: tr. fe-rich: qt: 1% fe-rich: qtz: tr. po: tr. qtz: 2% tr. tr. b1% aen: b1% fa: tr. ap: tr. fa: tr. fa: tr. qtz: tr. fe-hd: tr. qtz: b1% ap: tr. ap: tr. po: tr. ilm: tr. po: tr. mt: tr. qtz: tr. fa: tr. po: tr. ilm: tr. py: tr. mt: tr. afv: tr. qtz: tr. ap: tr. ilm: tr. ap: tr. po: tr. alnt: tr. ap: tr. mzt: tr. ilm: tr. po: tr. ap: tr. po: tr. po: tr. po: tr. mt: tr. Micro-crystal 70% 80% 90% 85% 15% 18% 10% 1% 15% 5% 3% in matrix Glass in matrix 30% 20% 10% 15% 85% 82% 90% 99% 85% 95% 97% Key: sa = sanidine; fe-hd = ferrohedenbergite; fa = fayalite; ilm = ilmenite; aen = aenigmatite; po = pyrrhotite; py = pyrite; alnt = allanite; ap = apatite; qtz = quartz; afv = arfvedsonite; fe-rich = ferrorichterite; mzt = monazite; tr. = trace amount. 116 M. Ren et al. / Lithos 91 (2006) 109–124 M. Ren et al. / Lithos 91 (2006) 109–124 117

Table 2 Eburru clinopyroxene compositions (representative analyses in weight percent oxides) Unit Et1 Et2 Er2 Sample # keb8 keb13 keb5 keb12 keb6 keb16 keb2 keb17 keb10

SiO2 47.88 47.90 48.81 48.26 48.10 47.79 48.81 47.85 48.99 TiO2 0.54 0.50 0.62 1.05 0.67 0.58 0.63 0.37 0.31 Al2O3 0.32 0.23 0.74 0.43 0.51 0.24 0.37 0.14 0.18 FeO 29.42 29.06 28.95 29.85 29.11 29.74 28.01 29.12 29.48 MnO 1.22 1.26 0.97 1.19 1.13 1.23 1.22 0.99 0.90 MgO 0.47 0.62 0.06 0.08 0.35 0.26 1.49 0.39 0.27 CaO 18.15 17.54 14.70 16.52 15.22 17.93 17.07 15.72 15.32

Na2O 1.48 1.86 3.81 2.33 2.70 1.67 2.24 3.43 3.91 K2O 0.05 0.01 0.12 0.10 0.13 0.01 0.02 0.01 0.01 F 0.01 0.02 0.02 0.03 0.01 0.01 0.01 0.01 0.01

SO2 0.03 0.03 0.04 0.03 0.02 0.03 0.02 0.02 Total 99.57 99.11 98.83 99.89 97.95 99.49 99.91 98.39 99.42

Wo mol% 40.20 38.86 33.02 36.90 34.70 39.67 37.36 34.63 32.20 En mol% 1.45 1.92 0.18 0.25 1.11 0.80 4.53 1.21 0.79 Fs mol% 52.42 51.75 51.33 53.42 53.05 52.84 49.23 50.48 52.08 Ac mol% 5.93 7.47 15.47 9.43 11.14 6.69 8.89 13.68 14.93

(5–7%), whereas the later trachytes and pantellerites have the pantellerites. One pantellerite, keb6, shows ilmenite up to 15%. inclusions in aenigmatite, which may support the stabiliza- tion of aenigmatite by the reaction of fayalite and ilmenite 6.3. Olivine with sodium-rich melt (Carmichael, 1962; White et al., 2005). Aenigmatite is also stabilized by low oxygen fuga- Olivine occurs as subhedral intergrowths with city in the melt (Ernst, 1962; Lindsley, 1971; Scaillet and ilmenite phenocrysts and as euhedral inclusions in sa- Macdonald, 2001). As discussed below, the units of the nidine (Fig. 4e, f). All the trachytes and the less peral- Eburru volcanic complex were erupted at very low f O2, kaline pantellerites are olivine-bearing, but the phase is consistentwiththeoccurrenceofaenigmatiteinallsamples. absent from the evolved pantellerites. The absence of olivine restricts the number of specimens to which the 6.6. Ilmenite QUILF thermobarometer can be applied. The composition of olivine averages Fa94Fo1Tp5. Ilmenite occurs as phenocrysts in the trachytes and less peralkaline pantellerites. It tends to be euhedral in the 6.4. Amphibole trachytes (Fig. 4g). In the pantellerites, as noted above, it occasionally occurs as inclusions in aenigmatite. Its Ferrorichterite occurs as phenocrysts in some panteller- composition is ∼0.92 mol% Ilm for the trachytes and ites. Amphibole has been found in only one of the trachytes, pantellerites, and ∼0.97 mol% Ilm for the more peralkaline the Et2 Ol'Obonge flow, in which it is a groundmass phase pantellerites. with an arfvedsonite–riebeckite composition. 6.7. Pyrrhotite 6.5. Aenigmatite Pyrrhotite forms euhedral grains within sanidine and Aenigmatite occurs as euhedral phenocrysts in trachytes corroded grains in pantellerite glass (Fig. 4h). Essentially and pantellerites. Compositions are relatively homoge- all samples contain pyrrhotite, with compositions that neous, with a slight shift towards higher Na2Ocontentsin range from 0.91 to 0.98 mol% FeS.

Fig. 4. Mineral images from trachyte and pantellerite flows, phenocrysts are in glassy groundmass in all. (a) Corroded quartz in Cedar Hill flow (keb6). (b) Euhedral sanidine and sanidine microcrystals in Lion Hill flow (keb12). (c) Quench crystallization of quartz and sanidine in Middle flow (keb1). (d) Euhedral ferrohedenbergite in Horseshoe flow (keb13). (e) Subhedral fayalite with ilmenite and clinopyroxene in Eburru Hill flow (keb16). (f) Euhedral fayalite included in sanidine in Lion Hill flow (keb12), note quench crystallization in skeletal fabric. (g) Euhedral ilmenite with ferrohedenbergite and sanidine in Eburru Hill flow (keb16). (h) Euhedral pyrrhotite and apatite in sanidine in Southern flow (keb17). 118 M. Ren et al. / Lithos 91 (2006) 109–124

2 7. Discussion and conclusions Er

7.1. Application of the QUILF thermobarometer 2

Et The QUILF thermobarometer (Frost et al., 1988; Lindsley and Frost, 1992; Frost and Lindsley, 1992) has been used in applications involving iron-rich, reduced

1 rocks (Marks and Markl, 2001; White et al., 2005). Use Et here is based on the program QUILF95 (Andersen et al., 1993). Its application for the Eburru samples is limited to olivine-phyric flows and thus excludes the most evolved pantellerites. With QUILF, temperature (T) is calculated based on Fe–Mg–Ca exchange between clinopyroxene and olivine from the following reactions:

Mg2SiO4 þ Fe2Si2O6 ¼ Fe2SiO4 ð1Þ þ Mg2Si2O6ðFeMgOlAugÞ; 2

Er Fe2SiO4 þ CaFeSi2O6 ¼ CaFeSiO4 ð2Þ þ Fe2Si2O6ðFeCaOlAugÞ; and 2

Et Mg2SiO4 þ CaMgSi2O6 ¼ CaMgSiO4 ð3Þ þ Mg2Si2O6ðMgCaOlAugÞ:

Temperature determined from clinopyroxene–olivine equilibria is strongly pressure (P)-dependent, as is silica

activity (aSiO2), which varies inversely with P.Thus,if 1 either aSiO2 or P is fixed, the other may be calculated if T is Et known with QUILF from the following reactions:

Mg2Si2O6 ¼ Mg2SiO4 þ SiO2ðMgOlQAugÞ; and ð4Þ

Fe2Si2O6 ¼ Fe2SiO4 þ SiO2ðFeOlQAugÞ: ð5Þ

Oxygen fugacity ( f O ) can be calculated from 2 2

Er equilibrium between fayalite, quartz, and the highly

dilute hematite component in ilmenite if T, P,andaSiO2 are known: 2 Et 2Fe2SiO4 þ O2 ¼ 2Fe2O3 þ 2SiO2ðFHQÞ: ð6Þ

We present QUILF results for T, P, f O2,andaSiO2 (relative to quartz saturation) for six samples: two thin- sections from the West Eburru Flow (keb8, Et1), Horseshoe Flow (keb13, Et1), Lion Hill Flow (keb12, 2 2

1 Et ), Eburru Hill Flow (keb16, Er ), and Cedar Hill 0.080.40 0.06 0.04 0.06 0.05 0.16 0.17 0.34 0.05 49.96 0.17 49.17 0.10 49.39 0.05 50.08 49.49 0.06 49.99 0.06 25.67 0.07 25.72 0.05 24.30 0.02 0.12 Fayalite Ilmenite Magnetite 28.79 29.40 29.26 29.23 29.04 0.03 0.17Flow 0.07 (keb6, 0.06 Er2). 0.07 Two calculations 0.04 0.27 were 0.16 performed 0.52 for each sample except keb6: one with P fixed at 1000 bar

with aSiO2 allowed to float (i.e., set as a trial value), and 3 2 2 O 0.11 0.02 0.07 0.12 0.02 0.02 0.03 0.02 0.01 0.02 0.02 0.02 0.02 0.07

O one with a fixed at 1.0 (quartz-saturated) with P

2 SiO O 0.06 0.01 0.05 0.12 0.01 0.02 0.05 0.03 0.01 0.02 0.01 0.05 0.03 0.06

2 2 2 TotalXfo mol%Xla mol% 98.50 0.008Xfa mol% 0.007Xtp mol% 0.011 0.938 100.30 0.007 0.047 0.003 0.929 99.28 0.011 0.052 0.007 0.936 0.006 0.049 0.004 99.26 0.944 0.007 0.043 Xil mol% 0.940 100.07 Xhem mol% 0.049 Xgk mol% 0.927 0.033 Xpy 98.19 mol% 0.911 0.001 0.038 97.46 0.039 0.920 0.002 0.042 0.049 0.0003 97.89 0.916 0.041 0.037 0.001 0.918 0.043 99.55 0.043 0.0005 0.974 0.025 0.039 98.40 0.001 Nti Nmg Nmn 0.042 97.34 0.001 0.749 0.053 0.0001 0.742 95.53 0.045 0.0017 0.712 0.043 97.03 95.76 K Table 3 Eburru fayalite and oxides compositions (representative analyses in weight percent oxides) Sample #SiO keb8 keb13 keb12 keb6 keb16 keb8 keb13 keb12 keb6 keb16 keb17 keb13 keb12 keb6 Unit Et Al FeOMnOMgOCaO 65.09Na 3.24 0.30 66.23 0.38 3.69 65.61 0.46 0.39 3.41 65.75 0.14 0.60 2.98 66.62 0.28 0.31 46.18 3.44 0.16 45.39 0.38 1.78 0.03 46.62 0.01 2.20 0.06 46.85 0.10 1.69 46.64 0.01 0.01 1.98 45.23 0.01 0.01 1.77 67.66 0.01 0.03 68.93 1.91 0.03 0.01 68.70 1.63 0.01 0.04 1.39 0.00 0.29 1.30 0.02 0.20 TiO allowed to float. Since all of the rocks and minerals M. Ren et al. / Lithos 91 (2006) 109–124 119 investigated in this study are extremely Mg-poor and 2003). In fact, ∼710 °C best fits the solidus curves thus subject to greater analytical error, the measured for low water content, whereas, as discussed below, the Mg-rich end member components of clinopyroxene pantelleritic magmas may have been water-rich, allow- (En) and ilmenite (Gk) were also allowed to float in ing solidus temperatures as low as about 650 °C for calculations. (Calculations that allowed the Fo compo- these samples. nent in olivine to float instead of En provided nearly The pressure results are consistent with an experi- identical results.). mental study on anhydrous pantellerites from Eburru Calculated values are very close to trial values for which showed that at Pb1 kbar, alkali feldspar is the these components in each determination, which sole phase on the liquidus; at P≈1 kbar, alkali feldspar suggest that these minerals are in equilibrium with is joined by quartz on the liquidus; and at PN1 kbar, each other. We emphasize that detailed petrography, quartz replaces alkali feldspar as the sole liquidus phase including the backscatter images presented above, are (Bailey et al., 1974). Eburru trachytes have phyric alkali crucial to establishing equilibrium textures. QUILF feldspar, but only groundmass quartz, i.e., silica acti- results are summarized in Table 4 and Fig. 5. vities just below unity, and equilibrated below 1 kbar; Mineral input data are average values from multiple Eburru pantellerite has phyric alkali feldspar and quartz spots on a number of mineral grains per thin-section. (and unit silica activities) and equilibrated at about The worksheet of mineral compositions and standard 1 kbar. deviations is available on line; see Appendix A. The minerals are unzoned and homogeneous in composi- 7.2. Comparison of QUILF determinations and experi- tion within a thin-section, resulting in small standard mental data, and petrogenesis of the Eburru volcanic deviations. complex QUILF results suggest equilibration temperatures between 707–715 °C for the older trachyte and app- The correlations among field relations, whole-rock roximately 790 °C for the younger trachyte, at oxygen compositions, mineral assemblages, and QUILF thermo- fugacities approximately one log unit below the FMQ barometry constrain models for the petrogenesis of the buffer, pressures less than 1000 bar, and silica activities Eburru volcanic complex. Important results are the low f O2 near unity. Results from the pantellerites suggest and temperatures of equilibration of the Eburru samples. maximum equilibration temperatures of 708 °C at oxy- Achievement of the low temperatures has undoubtedly gen fugacities approximately 0.6 to 0.7 log units below been in part due to the high halogen contents of the melts the FMQ buffer, pressures near 1000 bar, and unit silica (up to 1 wt.% F+Cl). However, we note that the comen- activities. The temperatures represent some of the dites of the Greater Olkaria Volcanic Complex were lowest temperatures recorded for peralkaline rocks (Fig. water-rich; glass (melt) inclusion and experimental stu- 5). This conclusion is one of the principal findings of dies have shown that the magmas contained up to 6 wt.% the present study. We believe these temperatures to H2O(Wilding et al., 1993; Scaillet and Macdonald, 2001, reflect the magmatic conditions at time of eruption for 2003). Indirect evidence for high water content comes two reasons. First, the petrography of these samples, from the large proportion of pyroclastic rocks in the with their unzoned phenocrysts and fresh glass, implies complex, which have so far received little study. that we are working with quench samples, which have The low temperatures are permissive of an origin either not seen subsolidus re-equilibration. This is contrary by crustal anatexis or extreme fractionation of mafic the substantial subsolidus history documented for the magmas. Problems with an anatectic origin include the Gardar rocks by Markl et al. (2001a,b), for example. difficulty of generating peralkaline melts from metalumi- The chemical homogeneity of the minerals, reflected in nous or peraluminous crustal lithologies, which are their small standard deviations, results also in small presumed to dominant the crustal volume. So far as we errors on the least squares fit of the QUILF calcula- know, no experimental study of crustal anatexis for these tions. We have experimented with QUILF calculations compositions has produced peralkaline melts. Volatile- at the high and low end of the compositional ranges and induced melting, involving especially halogens, has been find that the temperatures vary by less than 30 °C invoked as a way of adding alkalis to the crustal protoliths (available on line). Second, these calculated low (Macdonald et al., 1970; Bailey and Macdonald, 1975, temperatures agree with low solidus temperatures at 1987; Macdonald et al., 1987), but there is as yet no low oxygen fugacity established by experimental work experimental verification of this process. An alternative on natural samples from Eburru and the Greater Olkaria crustal protolith would be alkali gabbro, mafic granulite, or Volcanic Complex (Scaillet and Macdonald, 2001, syenite, as has been suggested for peralkaline rocks from 120

Table 4 Results of QUILF95 calculations for Eburru samples Unit Et1 Et2 Er2 Sample keb8 2keb8 keb13 keb12 keb16 keb6

aSiO2 P aSiO2 P aSiO2 P aSiO2 P Input Calc Input Calc Input Calc Input Calc Input Calc Input Calc Input Calc Input Calc Input Calc Input Calc

Ilmenite 109 (2006) 91 Lithos / al. et Ren M. Hem 0.033 0.033 0.026 0.038 0.038 0.042 0.042 0.043 0.043 0.041 Gk 0.001 0.001 0.001 0.001 0.001 0.001 0.002 0.002 0.002 0.002 0.0003 0.001 0.0003 0.001 0.0005 0.001 0.0005 0.001 0.0005 0.001 Py 0.039 0.039 0.039 0.049 0.049 0.037 0.037 0.039 0.039 0.043

Olivine Fo 0.008 0.008 0.008 0.012 0.012 0.004 0.004 0.004 0.004 0.0075 La 0.007 0.007 0.007 0.007 0.007 0.011 0.011 0.007 0.007 0.006

Augite En 0.018 0.018 0.018 0.018 0.022 0.024 0.026 0.024 0.026 0.003 0.008 0.003 0.008 0.010 0.009 0.010 0.009 0.014 0.014 Wo 0.451 0.451 0.434 0.452 0.452 0.439 0.439 0.451 0.451 0.42 – 124 Activities SiO2 (Qtz) 1.000 0.983 1.000 1.000 1.000 0.948 1.000 1.000 0.972 1.000 1.000 0.995 1.000 1.000 P (bar) 1000 1000 711 1000 1000 1000 104 1000 1000 472 1000 1000 922 1000 T (°C) 709 707 713 715 709 793 788 708 707 668 log f O2 −17.7 −17.8 −18.0 −17.4 −17.6 −15.8 −15.8 −17.4 −17.4 −18.3 ΔFMQ −1.1 −1.0 −1.4 −0.9 −0.8 −1.1 −1.0 −0.7 −0.6 −0.5

Column aSiO2 is the application of QUILF by setting pressure as fixed value and SiO2 activity as trial value. Column P is the application of QUILF by setting SiO2 activity as fixed value and pressure as trial value. Italics signify values in the “Input” column that were set as trial values, the values calculated by QUILF95 (Andersen et al., 1993) are in normal font in the adjacent “Calc” column. ΔFMQ is log f O2 relative to FMQ buffer at P, T. ΔFMQ=log f O2 −FMQ(P,T ). M. Ren et al. / Lithos 91 (2006) 109–124 121

Fig. 5. Temperature and oxygen fugacity data from this study and the literature with the aenigmatite–ilmenite stability curve (White et al., 2005) plotted relative to the FMQ buffer (ΔFMQ=log f O2 −FMQ(T); Frost et al., 1988). The aen–ilm stability curve for Eburru is based on aNds =1.0 at quartz saturation, with ilmenite and hematite activities calculated following Andersen and Lindsley (1988) for sample keb8 (Et1). Eburru samples are close to ilm-out line and correspond to the petrographic observation that some ilmenite occur as inclusions inside aenigmatite. The Eburru samples are located at low temperature and oxidation state relative to most samples from other peralkaline systems. (Key to the legend: C67/91: Carmichael, 1967, 1991; B80: Bizouard et al., 1980; M81: Mahood, 1981; WW81: Wolff and Wright, 1981; C84: Conrad, 1984; NM86: Novak and Mahood, 1986; V89: Vogel et al., 1989; and W05: White et al., 2005). the Trans-Pecos Magmatic Province (White et al., 2006- a mechanism for generating strongly peralkaline liquids. As this volume) and for other ferroan associations world-wide long as calcic pyroxene is stable, the peralkalinity of (Lowenstern and Mahood, 1991; Frost and Frost, 1997; residual liquids increase only slightly. Low f O2, however, Bohrson and Reid, 1997, 1998). Syenite does exist in the causes clinopyroxene to break down, resulting in derivative subsurface in the Kenya Rift: it has been encountered in liquids that are more peralkaline than coexisting feldspar. drill core in the Eburru volcanic complex (Omenda, 1997) Continued feldspar fractionation then leads to strongly and as large xenoliths in other Kenya rift volcanoes peralkaline residual liquids (Markletal.,2001a;Macdo- (McCall, 1970; McCall and Hornung, 1972). Omenda nald and Scaillet, 2006-this volume). (1997) and Scaillet and Macdonald (2001) proposed partial We argued above that the pantelleritic trachytes melting of syenite as a possible mechanism for generation formed a different liquid line of descent to the pantel- of the Greater Olkaria Volcanic Complex comendites. lerites, although the trachytic lineage also produced Whatever the appropriate protolith, a potentially important pantellerites. petrogenetic mechanism for the origin of the Eburru We cannot preclude the possibility that the lineages pantelleritic rhyolites may be extreme fractionation of were generated by fractionation of parental magmas comenditic magmas. Scaillet and Macdonald (2003) expe- with different Fe/Al characteristics, or by remelting of rimentally demonstrated that crystallization of Greater different (syenitic?) protoliths. We are currently testing Olkaria Volcanic Complex comendites with A.I. ∼1.35 at the options for all these petrogenetic models through low f O2 (bFMQ) and Tb800 °C resulted in melts with isotopic analysis. Of particular interest, however, would major element compositions indistinguishable from the be an experimental study of the crystallization path of Eburru pantellerites (Fig. 2 — from Scaillet and Macdo- the trachytes. Would they trend towards the same low- nald, 2003). However, the composition of Eburru samples, temperature composition as the pantellerites on the total which are more peralkaline and have lower incompatible FeO–Al2O3 plot, a composition which represents the trace elements (except Zr) and F concentrations, precludes natural low-variancy point for peralkaline magmas the Greater Olkaria Volcanic Complex comendites as direct (Bailey and Schairer, 1966)? If experimental data were parental melts. to support this liquid line of descent, these data would Theimportanceoflowf O2 for the Kenyan lavas was remove the apparent obstacle to comagmatic derivation also established by these experiments, which demonstrated of Eburru pantellerite from Eburru trachyte. 122 M. Ren et al. / Lithos 91 (2006) 109–124

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