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Lithium enrichment and isotopic variation in minerals from peridotite
xenoliths from northwestern Ethiopian plateau
Article  in  Journal of the Geological Society of India · January 2018
DOI: 10.1007/s12594-018-0825-x
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JOURNAL GEOLOGICAL SOCIETY OF INDIA
Vol.91, January 2018, pp.99-108
Lithium Enrichment and Isotopic Variation in Minerals from
Peridotite Xenoliths from Northwestern Ethiopian Plateau
Melesse Alemayehu1,2,*, Hong-Fu Zhang1,3, Patrick Asamoah Sakyi4 and Muhammed Haji5
1 State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box  9825,
Beijing 100029, China
2 School of Applied Sciences, Department of Applied Geology, Adama Science and Technology University, P.O. Box 1888, Adam,
Ethiopia
3 State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi’an 710069, China
4 Department of Earth Science, University of Ghana, P.O. Box LG 58, Legon Accra, Ghana
5 Division of Engineering Geology and Water Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences,
P.O. Box 9825, Beijing 100029, China
*E-mail: melesse555@yahoo.com
ABSTRACT indicate that the southern Ethiopian CLM formed between 0.9 to
We report Lithium (Li) concentrations and isotopic 2.8 Ga (Reisberg et al., 2004; Meshesha et al., 2011; Bianchini et al.,
compositions for co-existing olivine, orthopyroxene (opx), and 2014), providing strong indication that at least some parts of the
clinopyroxene (cpx) mineral separates from depleted and lithospheric mantle beneath the southern Ethiopian rift zone were older
metasomatised peridotite xenoliths hosted by basaltic lavas from than the plateau CLM. Many geophysical studies have showed low
northwestern Ethiopian plateau (Gundeweyn area). The peridotites velocity anomalies related to the presence of anomalously hot and
contain five lherzolites and one harzburgite and are variably buoyant mantle upwelling beneath various parts of the East African
depleted and enriched in LREE relative to HREE. In both depleted rift including the Ethiopian plateau and rift zone (e.g. Ebinger et al.,
and enriched lherzolites, Li is preferentially incorporated into 1989; Nyblade et al., 2000; Debayle et al., 2001; Benoit et al., 2006).
olivine (2.4-3.3 ppm) compared to opx (1.4-2.1 ppm) and cpx (1.4- The low velocity anomalies are related to one or two mantle plumes
2.0 ppm) whereas the Li contents of olivines (5.4 ppm) from an (e.g. Ebinger and Sleep, 1998; George et al., 1998; Rogers et al., 2000)
enriched harzburgiteare higher than those of lherzolites. Olivines or of mantle upwelling (African super plume, Janney et al. 2002;
from the samples show higher Li abundances than normal Furman et al. 2004, 2006b) that triggered lithospheric thinning beneath
mantle olivines (1.6-1.9 ppm) indicating the occurrence of Li East African rift (e.g. Ebinger and Casey 2001; Furman and Graham
enrichments through melt-preroditite interaction. The average 1999; Rooney 2010). The geophysical investigation integrated with
δ7 Li values range from +2.2 to +6.0‰ in olivine, from -0.1 to geochemical data of mantle xenoliths from Ethiopian Cenozoic basalts
+2.0‰ in opx and from -4.4 to -0.9‰ in cpx from the lherzolites. suggest that the present lithosphere is much hotter (60-150 mW/m2;
The Li isotopic composition (3.5‰) of olivines from harzburgite Meshesha et al., 2011; Alemayehu et al., 2016a) than the actual
fall within the range of olivine from lherzolites but the opxs show continental lithospheric mantle (40 mW/m2; Pollack and Chapman
low in δ7Li (-2.0‰). Overall Li isotopic compositions of olivines 1977). Moreover, the investigated physicochemical changes leads to
from the peridotites fall within the range of normal mantle olivine, the formation of the Ethiopian CLM during the Pan-African time (e.g.
δ7Li values of ~+4±2‰ within uncertainty, reflecting metasomatism Meshesha et al., 2011; Bianchini et al., 2014) followed by various
(enrichment) of the peridotites by isotopically heavy Li-rich types of metasomatic processes that have been related to eruption of
asthenospheric melt. Li isotope zonation is also observed in most plateau basaltic lavas connected to plume-related sub-alkaline
peridotite minerals. Majority of olivine grains display isotopically magmatism (Beccaluva et al. 2011). Thus, the Ethiopian xenoliths
heavy cores and light rims and the reverse case is observed for provide special opportunity to address mantle process and evolution
some olivine grains. Orthopyroxene and clinopyroxene grains show mechanism of modern lithospheric mantle, which is connected with
irregular distribution in δ7Li. These features of Li isotopic the rising of the Afar plume. It is usually agreed that the Pan-African
compositions within and between grains in the samples reflect the subduction and Cenozoic East African rifting are the two major
effect of diffusion-driven isotopic fractionation during melt- geological events that affect the CLM beneath Ethiopian plateau and
peridotite interaction and cooling processes. rift zone. The nature of the lithospheric mantle beneath Ethiopia also
modified from the typical refractory to fertile (Alemayehu et al., 2016a).
INTRODUCTION However, the processes related to peridotite-melt interaction are still
The continental lithospheric mantle (CLM) beneath Ethiopian not well constrained.
plateau and rift zone has experienced depletion and enrichment Many earlier worldwide studies have shown that most of the
processes at different cales and periods (Roger et al., 1999; Conticelli peridotites are affected by various degrees of peridotite-melt interaction
et al., 1999; Ayalew et al., 2003; Reisberg et al., 2004). This is shown (metasomatism), occurring at different lithospheric levels and causing
by the distinct compositions of mantle xenoliths found in the basaltic significant geochemical modifications of the lithospheric mantle (e.g.
rocks of plateau and Cenozoic rift (Ferrando et al. 2008; Frezzotti et Piccardo et al., 2007). Peridotite-melt interactions occurring in the
al. 2010; Meshesha et al. 2011; Beccaluva et al. 2011; Bianchini et al. Ethiopian CLM have been identified to be the result of multiple
2014). Peridotites from the Ethiopian plateau particularly from the metasomatic overprints, that were initially associated with the Pan-
Gundeweyn area (Fig. 1) have 0.5-0.9 Ga Lu–Hf depletion and African subduction and subsequently caused by small degree partial
0.4-0.6 Ga Sm-Nd enrichment ages (Alemayehu et al., 2016c). The melts from the asthenospheric mantle or from mantle plume sources
Sm–Nd, Lu–Hf and Re–Os isotope systematic of the peridotites during the development of the East African rift system (EARS)
0016-7622/2018-91-1-99/$ 1.00 © GEOL. SOC. INDIA    |    DOI: 10.1007/s12594-018-0825-x
plateau, on either side of the Main Ethiopian Rift (MER) and the Afar
depression (Berhe et al. 1987; Fig. 1a). The MER, which is bounded
by the Ethiopian plateau to the west and Somalian plateau to the east,
extends northeast to southwest and widen at the Afar depression (e.g.
Baker et al., 1972; Fig. 1). Volcanism of the Ethiopian plateau is mainly
confined to major deep-seated fractures, which were reactivated during
rifting of the EARS. Most eruptions were contemporaneous with the
collision of the Arabian and Eurasian plates (Hampton 1987), and
took place in northwest to southeast trend.
The thick sequence of late Eocene to early Miocene flood basalts
of Ethiopian plateau is overlain by less voluminous late Miocene to
Quaternary lavas erupted from several distinct shield volcanoes;
their ages vary from <29 to 3 Ma (Kieffer et al., 2004). The shield
volcanoes consist of alternating basaltic and rhyolitic lava flows,
tuffs and ignimbrites. Kieffer et al. (2004) found both tholeiitic and
alkaline volcanic rocks from shield volcanoes from northwestern
Ethiopian plateau with ages comparable with the flood basalts. They
described the large variation in the incompatible element contents of
the two volcanic series in terms of differences of source composition
and degree of partial melting, and concluded that the lavas from the
plateau were formed from a broad region of upwelling mantle, that
was thermally and compositionally heterogeneous. Conversely,
Tommasini et al. (2005) suggested that plateau mafic
lavas are derived from a fertile and vertically zoned enriched CLM
with relatively homogeneous lateral continuity at a pressure of
about 23.5 GPa. Finally, Furman et al. (2006) reported that the
geochemical difference between the Oligocene-Quaternary continental
flood basalts of the plateau and the Quaternary rift basalts are probably
Fig.1. (a) The distribution of Neoproterozoic basement rocksand not related to significant changes in lithospheric source composition
volcanic rocks in East Africa (Kampunzu and Mohr 1991) and the red but to depth and degree of melting as well as mixing processes
square denote the location of the study area, (b) Distribution of involving depleted, moderately enriched and metasomatised mantle
Cenozoic volcanic rocks in Ethiopian plateau and rift zone (Ferrando domains.
et al. 2008). EVP and MER denote Ethiopian Volcanic Province and The Gundeweyn volcanic field is located in east Gojam,
Main Ethiopian Rift, respectively. The location of the study area is northwestern Ethiopia plateau and contains mantle xenoliths in the
shown by a cross (+). late Miocene alkali basalts dated at 23.1 Ma (Kieffer et al., 2004). The
volcanic field is mainly composed of lava flows, trachyte plugs and
(Rogers et al., 1999; Beccalva et al., 2011; Alemayehu et al., 2016b). scoria cones. The pyroclastic and volcanic rocks contain dominantly
Li is a mobile element that preferentially enters the fluid phase scoria and olivine-phyric basalts. The basaltic lavas and monogenetic
during partial melting and fluid/melt-rock reaction processes in mantle scoria cones enclose various types of fresh mantle xenoliths as
(Brenan et al., 1998). The large mass difference between the two Li described below.
(6Li ~7.59% and 7Li ~92.41%) isotopes (~16%) cause significant Li
isotopic fractionation during geological processes. The Li isotope Analysed Samples
system has been employed as a potential geochemical tool for tracing Six (five lherzolites and one harzburgite) peridotite xenoliths
various melt/fluid-related geological processes for example continental collected from the Gundeweyn volcanic field were studied in this
weathering (Rudnick et al. 2004; Teng et al. 2010), seafloor alteration work. The samples are very fresh and, show protogranular and
(Chan et al. 1992; Scholz et al. 2009), crust/mantle recycling (Tomascak porphyroclastic textures. The primary mineral assemblages are
et al. 2000; Brooker et al. 2004; Elliott et al. 2006) and peridotite- olivine, orthopyroxene, clinopyroxene and spinel with minor
melt/fluid interactions  (Rudnick and Ionov 2007; Zhang et al. 2010; amphiboles (Table 1). The minerals vary in size from 1 to 5 mm with
Tanget al. 2011; Su et al. 2012). subhedral to anhedral profile. A few orthopyroxene contains lamellae
In this work, new Li elemental and isotopic data on variably of clinopyroxene and vice versa. The olivines have Fo content, which
depleted and metasomatised mantle xenoliths entrained in alkaline is equivalent to Mg# (= 100 × Mg/(Mg + Fe), in the range from 89.3
basalt from northwestern Ethiopian plateau (Gundeweyn area) in order to 89.8 with higher values from the harzburgite (Table 1). Based on
to further explain the characteristics of early and late stage geological CI-normalized REE patterns, the clinopyroxene from lherzolites can
processes occurring beneath the area are presented. These new results be classified into two groups (Fig. 2): depleted (GT272, GT278 and
are integrated with the previous elemental and Sr isotopic studies GT2716) which are characterized by a distinct depletion in LREE and
(Alemayehu et al., 2016a) in order to further explore the compositional enriched (GT273 and GT275) which are characterized by enrichment
modification of CLM beneath Ethiopian plateau. in LREE relative to MREE and HREE. The clinopyroxene from
harzburgite (GT2713) is categorized into enriched group with lower
GEOLOGICAL SETTING AND ANALYSED SAMPLES MREE-HREE abundances than the lherzolites. Clinopyroxenes from
lherzolite have 87Sr/86Sr of 0.70227 to 0.70357, 143Nd/144Geological setting Nd of 0.51285to 0.51346, and 176Hf/177Hf of 0.28297 to 0.28360. These values range
The Ethiopian volcanic province is covered dominantly by Tertiary between depleted mantle and the HIMU mantle end-member. Overall,
and Quaternary basaltic rocks, ranging in thickness from 700 to the petrography combined with detailed major-trace element and Sr–
2000 m.  It covers an area of several hundred kilometers across on the Nd–Hf isotope characteristics of the Gundeweyn peridotites reflect
100 JOUR.GEOL.SOC.INDIA, VOL.91, JAN. 2018
Table 1. GPS location, modal abundances (vol.%), selected elemental composition and textures of Gundweyn (GT) mantle xenoliths
GPS location (degree) Modal mineralogy (Vol. %) Fo
Sample Easting Northing olivine opx cpx spl amph olivine (La/Yb)N (Dy/Lu)N Texture Rock type
GT272 38.156 10.963 56 23 18 2 89.6 0.45 0.95 Protogranular Depleted lherzolite
GT273 38.158 10.964 51 28 19 2 trace 89.5 2.92 1.52 Protogranular Enriched lherzolite
GT275 38.159 10.964 60 24 15 1 trace 89.3 3.06 1.40 Protogranular Enriched lherzolite
GT278 38.161 10.965 51 28 19 2 89.6 0.24 1.06 Porphyroclastic Depleted lherzolite
GT2713 38.161 10.951 69 27 3 1 trace 89.8 34.5 0.86 Porphyroclastic Eriched harzburgite
GT2716 38.163 10.946 70 19 10 1 trace 89.6 0.25 1.18 Porphyroclastic Depleted lherzolite
Fo = forsterite content; Mg# = molar 100Mg/(Mg+Fe), nomenclature after Mercier and Nicolas (1975)
N = normalized to chondrite after Sun and McDonough (1989)
the occurrence of variable degree of partial melting and metasomatic fall in the range of previously published values within analytical
overprint at different times (Alemayehu et al. 2016a,c). uncertainty (δ7Li =4.51‰, -0.19‰ and -2.37‰, respectively; Su et
al., 2015).
ANALYTICAL  METHODS
Mineral separations and, Li elemental and isotope analysis were RESULTS
carried out at the State Key Laboratory of Lithospheric Evolution, Olivine, orthopyroxene and clinopyroxene Li abundances and
Institute of Geology and Geophysics, Chinese Academy of Sciences, isotope compositions of the peridotite are given in Table 2.
China. Olivine, orthopyroxene and clinopyroxene separates were Clinopyroxene from the lherzolites show negative Li anomaly relative
handpicked under binocular stereomicroscope. In situ Li concentration to the neighboring trace elements (Fig. 2). Because limited amount of
and isotope analyses of olivines and pyroxenes on gold coated grain clinopyroxene, Li content and isotopic compositions were not measured
mounts were performed using a Cameca IMS-1280 ion microprobe from harzburgites.A limited range in Li abundances and large variation
following the techniques of Zhang et al. (2010) and Su et al. (2012, in δ7Li values are observed either within or among mineral grains in a
2015). A 13 kV, 10-20 nA oxygen primary beam was focused on spot sample (Fig. 3a, b). LREE-depleted and enriched samples do not
size of 20 ìm in diameter. A 60 s pre-sputtering was applied without show clear systematic differences in both Li abundances and δ7Li
raster before analysis. A 10 kV was used to accelerate a positive (Fig. 3c, d).
secondary ion, which is measured at medium mass resolution In lherzolite, olivine exhibit variable Li abundances and δ7Li
(M/³%M~1100) with 125mm aperture without energy offset. The values, ranging from 2.4-3.5 ppm and 1.8 to 6.3‰ for depleted and,
primary beam position, entrance slits, contrast aperture, magnetic field 2.3-2.4 ppm and 2.4 to 4.2‰ for enriched samples (Fig. 4a). With the
and energy offset were automatically centered prior to each exception of some grains from GT275 and GT278, some olivines from
measurement. Secondary ions were counted on mono-collection pulse depleted and enriched samples have higher δ7Li in the cores than in
counting mode. 30 to 40 cycles were measured with counting time of the rims. Olivines from both depleted and enriched sample show higher
12 s for 6Li, 4 s for 7Li and 4 s for background at 6.5 mass. The Li contents relative to mantle olivines (1.6-1.9 ppm; Eggins et al. 1998;
counting rate on 7Li range from 30,000 to100,000 cps based on the Li Seitz and Woodland, 2000). Olivines reveal negative correlations
content of the sample and the primary beam intensity. Li isotopic ratios between Fo content and δ7Li (Fig. 4b). The range of Li contents and
are given in delta units using the δ7 Li notation (δ7Li = [(7Li/6Lisample)/ isotopic compositions in orthopyroxene of lherzolites range from 1.4-
(7Li/6LiLSVEC) - 1]*1,000 relative to the L-SVEC Li isotope standard, 2.2ppm and -1.1 to 1.3‰ for depleted with exception for one core
with 7Li/6LiLSVEC = 12.0192, Flesh et al., 1973)). 06JY31ol for olivine, grain of 6.8‰, and 1.3-2.1ppm and 0.7 to 1.3‰ for enriched samples
06JY31opx for orthopyroxene and 06JY31cpx for clinopyroxene (Su (Fig.4c). Some orthopyroxenes from depleted and enriched samples
et al. 2015) were used as standards and gave the average values of have lower δ7Li in the cores than in the rims and some of the grains
δ7Li=4.75±0.8‰, -0.24±0.9‰ and -2.47±1.0‰, respectively, which show irregular distribution in δ7Li, GT273 and GT278 samples. The
Li abundance and δ7Li obtained in clinopyroxene from depleted
lherzolite (1.4-2.0ppm; -5.6 to -0.5‰) is relatively wider compare to
enriched once (1.3-1.4ppm; 1.5 to -3.6‰) (Fig.4d). Similar to
orthopyroxenes, some clinopyroxenes from depleted and enriched
samples have lower δ7Li in the cores than in the rims and some
grains show the opposite trend (GT2716). In a harzburgite (GT2713),
olivines show homogeneous composition and higher Li abundances
(5.4-5.5 ppm) than olivines from lherzolite. The δ7Li values of olivines
vary from 3.2 to 3.8‰ and increase from the core to the rim (Fig. 3b).
Orthopyroxenes are also homogeneous in Li contents (1.3-1.4ppm)
and variable in δ7Li (-4.3 to -0.4‰) with irregular rim-core-rim
zonation.
In general, olivines Li concentration from each depleted and
enriched peridotites are usually homogeneous and higher than those
of coexisting pyroxenes (Fig. 3c). The clinopyroxenes show more or
less similar Li contents with coexisting orthopyroxenes (Fig. 3c).
Fig.2. Chondrite (CI) normalized REE and Li variation diagrams of Moreover, olivines δ7Li are mostly higher than those of coexisting
clinopyroxene from Gundeweyn mantle xenoliths. The data for REEs pyroxenes with clinopyroxene having lower δ7Li than coexisting
are from Alemayehu et al. (2016a) and the Li data is average analysis orthopyroxenes (Fig. 3d). With some exceptions, most olivines grains
from this study. Normalizing values are from Sun and McDonough show higher Li isotopic compositions in the core than in the rims and
(1989). the reverse case is observed for some olivine grains. Similarly, some
JOUR.GEOL.SOC.INDIA, VOL.91, JAN. 2018 101

abundances. Most of the peridotites show generally homogenous Li
abundances (Table 2) indicating the occurrence of Li enrichment for
longer time to homogenize the Li within the grains. The concentrations
of Li in olivines do not correlate well with the modal abundance of
olivine (Fig. 5a) and with the degree of LREE enrichment of the
peridotites, as measured by the chondrite-normalized (La/Yb)N ratio
of clinopyroxene (Fig. 5b). These features suggest that  Li addition
and LREE enrichment occur at different times within lithospheric
mantle beneath Gundeweyn. Moreover, the Li enrichments do not
appear to be correlated with the presence of metasomatic accessory
minerals for example amphibole (Table 1). Interestingly olivines from
the enriched harzburgites show Li enrichments (average 5.4 ppm) than
those from lherzolites. This characteristic could be related to
preferential enrichment of incompatible trace elements including Li
in olivine-rich matrices relative to pyroxene (Toramaru et al. 1986;
von Bargen et al. 1986; Van Orman et al. 2001; Rudnick et al. 2007).
Another interesting feature is that Li abundance in olivines from
harzburgite is higher than most olivines from worldwide peridotites
(Fig. 4a). These observations suggest that the lithospheric mantle
beneath the Gundeweyn region is influenced by large melt derived
from the asthenospheric mantle, probably connected with the rising
of the Afar plume which is near northeast of Gundeweyn. Like olivine,
(a) (b)
(c) (d)
(e) (f)
Fig.5. Lithium abundances (ppm) in olivine versus (a) modal olivine
(vol. %) and (b) (La/Yb)N in clinopyroxene. The gray field melt
depletion is from Eggins et al. (1998); Seitz and Woodland (2000)
and PM denotes primitive mantle from McDonough and Sun (1995).
Lithium isotopic compositions (‰) in olivine versus (c) modal olivine
(vol.%) and (d) (La/Yb)N in clinopyroxene. The vertical gray field
represents range of δ7Li for normal upper mantle (Tomascak 2004).
(e) 87Sr/86Sr against δ7Li (‰) in clinopyroxene (f) Oxygen isotope
compositions (‰) against δ7Li (‰) in olivine. Data for Sr and O
isotopic compositions are from Alemayehu et al. (2016a, d). Diamond
symbols represent data for Tanzanian peridotite (Aulbach et al. 2008;
Aulbach and Rudnick 2009) for comparison.
JOUR.GEOL.SOC.INDIA, VOL.91, JAN. 2018 103
Table 2: In-situ Li abundance and isotopic compositions of olivine, orthopyroxene and clinopyroxene from Gundeweyn (GT) mantle xenoliths
Li (ppm) 2σ δ7Li 2σ Li (ppm) 2σ δ7Li 2σ Li (ppm) 2σ δ7Li 2σ
Sample Position Olivine Sample Position Orthopyroxene Sample Position Clinopyroxene
GT272Ol@1 rim 2.55 0.05 5.51 0.85 GT272Opx@1 rim 2.21 0.04 -0.91 0.94 GT272Cpx@1 core 1.54 0.01 -2.60 1.46
GT272Ol@2 core 2.55 0.04 6.34 0.90 GT272Opx@2 core 1.72 0.02 6.78 1.07 GT272Cpx@2 rim 1.57 0.01 -0.88 1.55
GT272Ol@3 rim 2.52 0.04 6.09 0.86 GT272Opx@3 rim 2.19 0.04 0.14 0.94 GT273Cpx@1 core 1.44 0.01 -3.16 1.58
GT273Ol@1 rim 2.40 0.04 2.87 0.93 GT273Opx@1 rim 1.41 0.03 0.66 1.18 GT273Cpx@2 rim 1.38 0.01 1.46 1.56
GT273Ol@2 core 2.41 0.04 3.33 0.89 GT273Opx@2 core 1.32 0.03 0.83 1.21 GT275Cpx@1 core 1.38 0.01 -3.58 1.54
GT273Ol@3 rim 2.39 0.04 2.37 0.88 GT273Opx@3 rim 1.40 0.03 0.66 1.18 GT275Cpx@2 rim 1.33 0.01 -2.03 1.57
GT275Ol@1 rim 2.44 0.04 4.17 1.00 GT275Opx@1 rim 2.05 0.04 0.84 1.03 GT278Cpx@1 rim 1.96 0.01 -0.45 1.36
GT275Ol@2 core 2.49 0.04 3.82 0.89 GT275Opx@2 core 2.05 0.04 0.17 0.97 GT278Cpx@2 core 2.02 0.02 -2.13 1.28
GT275Ol@3 rim 2.52 0.04 3.64 1.05 GT275Opx@3 rim 2.07 0.04 1.30 0.97 GT278Cpx@3 rim 1.96 0.01 -1.95 1.28
GT278Ol1@1 rim 3.29 0.06 3.51 0.76 GT278Opx@1 rim 1.39 0.03 -0.08 1.18 GT2716Cpx@1 core 1.42 0.01 -4.36 1.52
GT278Ol1@2 core 3.21 0.06 4.60 0.86 GT278Opx@2 core 1.46 0.03 -0.68 1.15 GT2716Cpx@2 rim 1.44 0.01 -3.35 1.52
GT278Ol1@3 rim 3.19 0.05 4.42 0.77 GT278Opx@3 rim 1.40 0.03 -1.11 1.17 GT2716Cpx@3 rim 1.46 0.01 -5.60 1.50
GT278Ol2@1 rim 3.46 0.06 5.38 0.75 GT2713Opx@1 rim 1.30 0.02 -4.33 1.22
GT278Ol2@2 core 3.45 0.06 4.36 0.75 GT2713Opx@2 core 1.42 0.03 -0.95 1.17
GT278Ol2@3 rim 3.45 0.06 4.09 0.73 GT2713Opx@3 rim 1.40 0.03 -0.44 1.17
GT2713Ol@1 rim 5.36 0.09 3.83 0.72 GT2716Opx@1 rim 1.57 0.03 0.02 1.11
GT2713Ol@2 core 5.38 0.09 3.24 0.76 GT2716Opx@2 core 1.54 0.03 -0.40 1.11
GT2713Ol@3 rim 5.50 0.10 3.45 0.81 GT2716Opx@3 rim 1.53 0.03 0.16 1.12
GT2716Ol@1 rim 2.48 0.04 1.79 0.94
GT2716Ol@2 core 2.45 0.04 2.59 0.86
GT2716Ol@3 rim 2.43 0.04 2.07 1.04
orthopyroxene and clinopyroxene show limited range in Li contents processes is different from that of the metasomatic processes
(Fig. 4c, d; Table 2). It is understood that olivine crystallizes before responsible for LREE enrichment. Even though most of the earlier
pyroxenes at mantle depth and temperature. The higher Li abundance studies have underlined that Li isotopic disequilibria could not survive
in olivine than pyroxenes (Fig. 3c) indicates that Li is incorporated due to the very high diffusion rate of Li (e.g., Jeffcoate et al. 2007;
into mineral in the early stage of melt/magma fractionation at mantle Rudnick and Ionov 2007; Aulbach and Rudnick 2009; Halama et al.
temperature. This is consistent with the earlier experimental and 2009), Vlaste´lic et al. (2009) argued that HIMU mantle has distinctly
empirical results that Li is moderately compatible element for olivine elevated δ7Li and that Li isotopic heterogeneities could survive
(Chan et al., 1992; Dohmen et al., 2010; Caciagli et al., 2011). diffusion over 1-2 billion years in the mantle. Therefore, it is considered
that the disequilibrium of Li isotopes in the mantle peridotites could
Variation of Li Isotope Compositions in Mantle Peridotite be preserved for a long period (Tang et al. 2009). The heavy δ7Li
Diffusion is an essential mechanism for Li isotopic variation in values (up to 6.2‰) in olivine from the HIMU lavas from the Cook-
mantle peridotites. The diffusion rate of 6Li about 2–3% faster than Austral volcanic chain reported by Chan et al. (2009). As a result, the
7Li (Richter et al. 2003; Coogan et al. 2005; Lundstrom et al. 2005) HIMU-like Li isotopic compositions in some olivine (up to 6.3‰)
regardless of other physical factors like temperature and pressure as grains indicate that Gundeweyn peridotites most probably experienced
well as melt composition. On the other hand, the Li diffusion rate is metasomatism by fluids/melts possibly derived from HIMU like
highly variable in different mantle minerals which might be linked to asthenospheric Afar plume (Beccaluva et al. 2011), which is very near
temperature (Wunder et al., 2006; Ionov and Seitz 2008; Dohmen to the study area (Fig. 1). Some ortopyroxene and clinopyroxene also
et al., 2010; Caciagli et al., 2011; Coogan, 2011; Yakob et al., 2012). reveal heavy rim and light core Li isotopic compositions and some
Accordingly, the isotopic fractionation of Li will be accompanied by other pyroxene grains show the reverse. Overall, the three minerals
the preferential incorporation of Li into minerals. Ionov and Seitz phase does not show systematic regular δ7Li zonation, indicating the
(2008) reported that the abnormal and disequilibrated isotopic presence of complex processes controlling the Li distribution in
compositions of Li in peridotite xenoliths might be linked to the re- samples. The irregular variation of δ7Li in core-rim of minerals
distribution of Li between minerals, but they attributed it to the post- grains from depleted and enriched Gundeweyn mantle xenoliths
eruptional cooling process. Minerals from our samples show systematic indicate recent diffusive fractionation of Li induced by percolation of
δ7Li variation (Fig. 3d; Table 2) with olivine (1.8 to 6.3‰) > metasomatic melt shortly before entrainment, transportation, eruption
othopyroxene (-1.1 to 1.3‰) > clinopyroxene (–5.6‰ to 1.5‰). The and cooling. This is consistent with observations reported by Xu et al.
overall decreasing δ7Li value from olivine to both orthopyroxene and (2013).
clinopyroxene implies that fractional crystallization can result in Li Many recent studies have suggested that Li diffusion in
isotopic fractionation of the melts/magmas followed by melt additions clinopyroxene is faster than in olivine (Jeffcoate et al. 2007; Rudnick
into the peridotites. The Li rich and high δ7Li character of olivines in and Ionov 2007; Parkinson et al. 2007). If so, the low δ7Li
Fe-rich peridotites and the deep-seated garnet-bearing peridotites from clinopyroxene relative to δ7Li olivine in the Gundeweyn peridotites
Tanzanian are described to be related to a prolonged melt-peridotite (Fig. 3d), coupled with the lack of correlations between ä7Li
interaction event linking to plume related silicate melts connected with clinopyroxene and other parameters (5e), may reflect recent diffusion
the East Africa rift (Aulbach et al. 2008). This is consistent with present of Li into clinopyroxene and associated kinetic isotope fractionation,
observations. It is expected that Gundeweyn peridotites, including possibly during transport in the basaltic host rock. Furthermore,
olivine, have experienced metasomatic overprinting during earlier diffusion-driven Li isotopic fractionation will produce lower δ7Li in
phases of East Africa rift magmatism, which may be related to Pan- clinopyroxene than in coexisting olivines (Aulbach and Rudnick 2009;
African subduction (Alemayehu et al. 2016a). The Li isotopic Ionov and Seitz 2008; Rudnick and Ionov 2007; Tang et al. 2007)
compositions of olivines from Gundeweyn peridotites fall within the because the diffusivity of Li is much higher in pyroxene (6Li diffuses
range of normal mantle olivine, δ7Li values of ~+4±2‰ within faster than 7Li) than in olivine (Dohmen et al. 2010; Parkinson et al.
uncertainty (Fig. 5c), reflecting metasomatism (enrichment) of the 2007). There is no systematic correlation between δ7Li olivine and
peridotites by isotopically heavy Li-rich asthenospheric melt. LREE enrichment and Sr isotope in clinopyroxene (Fig. 5d,e), and
Moreover, the rough negative correlations between δ7Li and forsterite oxygen isotope composition in olivine (Fig. 5f). This interpretation is
content in olivine (Fig. 4b) suggest that the peridotites are the results also consistent with the lack of correlation between Li in olivine and
of interaction between lithospheric mantle and isotopically heavy Li (La/Yb)N in clinopyroxene and modal abundances of the minerals
rich melt. Comparable interaction between δ7Li depleted lithospheric indicating that diffusion-driven Li isotopic fractionation related to melt-
mantle with a Li rich and isotopically heavy melt has been suggested peridotite interaction and incompatible enrichment events occur at
for south Africa cratonic peridotites (Bell et al., 2005), but it is unclear different times. The clinopyroxene from Gundeweyn peridotites are
whether the relatively light Li in ancient refractory mantle is due to fall within the range of the Tanzanian clinopyroxene periodotite in
earlier melt addition or whether it reflects a secular evolution toward δ7Li. This could be related diffusion fractionation δ7Li that can occur
more isotopically heavy mantle due to addition of high δ7Li crustal during peridotite-melt/fluid interaction before or coincide with the
components (Bell et al. 2005). However, Aulbach et al. (2008) infer entrainment into host magmas and the transport of the mantle xenolith
that the Tanzania peridotites are the results of interaction between to the surface. Similar interpretations are also made from worldwide
isotopically light ancient lithospheric mantle and isotopically heavy peridotite (e.g. Aulbach and Rudnick, 2009; Aulbach et al., 2008;
Li rich melt. Similar interpretation can be applied for Gundeweyn Gallagher and Elliott, 2009; Halama et al., 2009; Ionov and Seitz,
peridotites that the lithospheric mantle beneath northwestern Ethiopia 2008; Rudnick and Ionov, 2007).
experienced isotopically heavy Li rich melt-peridotite reactions.
Majority of olivine grains from Gundeweyn peridotite display SUMMARY AND CONCLUSION
isotopically heavy cores and light rims and few of the olivine grains The development of Li elemental and isotope analysis has been
show the reverse relation. These indicate that initially the peridotite is used to trace several geological processes related to melt/fluid-rock
metasomtized with isotopically heavy Li-rich melt and later upon reaction or metasomatism (Rudnick and Ionov, 2007; Ackerman et
cooling the rim becomes low in δ7Li due to diffusion-driven Li isotopic al., 2013; Gu et al., 2016), cooling processes upon eruption (Ionov
fractionation (e.g. Xiao et al. 2015). It is thus proposed that for the and Seitz, 2008), and the interaction of the peridotite with the host
Gundeweyn samples, olivine best records the δ7Li of the metasomatic magma during entrainment and ascent (Aulbach and Rudnick, 2009).
104 JOUR.GEOL.SOC.INDIA, VOL.91, JAN. 2018
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108 JOUR.GEOL.SOC.INDIA, VOL.91, JAN. 2018
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