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RUSSIAN JOURNAL OF EARTH SCIENCES, VOL. 12, ES3004, doi:10.2205/2012ES000515, 2012
MORB-like mantle beneath Lanzerote Island, Canary Islands
A. F. Grachev1
Received 18 April 2012; accepted 20 April 2012; published 8 May 2012.
Our newly obtained data on the He-Ar and Sr-Nd isotopic systematics of mantle xenoliths and their host basalts suggest the absence of a mantle plume beneath Lanzerote Island. The R/Ra ratio of the xenoliths lies within the range typical of MORB. The He isotopic composition of basalts from Lanzerote Island provides evidence of the mixing of two sources: MORB and atmospheric. The He isotopic ratios of both the xenoliths and the basalts do not show any correlations with the Sr and Nd isotopic characteristics. KEYWORDS: Lanzerote Island; mantle xenoliths; basalts; He-Ar and Sr-Nd isotopes.
Citation: Grachev, A. F. (2012), MORB-like mantle beneath Lanzerote Island, Canary Islands, doi:10.2205/2012ES000515.
Russ. J. Earth. Sci., 12, ES3004,
Introduction
Lanzerote Island is one of the seven islands of the Canary Archipelago close to the African continent, and the geological evolution of this island was related to the opening of the Atlantic Ocean. The island sits within a zone of quiet mantle field of Jurassic age on transitional-type crust from continental to oceanic one [Arana and Ortiz, 1991].
The island is noted for active volcanism, which started earlier than 15 Ma, continues with just brief interludes until nowadays [Carracedo et al., 1998, 2002], and has formed more than 300 volcanoes. Modern volcanism is responsible for the 1730 fissure eruptions of basalts and has produced 30 cinder cones in the Timanafaya area and three volcanoes during the 1824 eruption [Arana and Ortiz, 1991]. Volcanoes on Lanzerote Island typically contain numerous ultramafic nodules in their lavas and cinders.
The composition of the ultramafic xenoliths, their mineralogy, and particularly, microtextures were examined in the dissertation by Sagredo [1969], who has established than the great majority of nodules in the lavas are harzburgites and dunites, whereas lherzolites, wehrlites, and pyroxenite are very rare. This conclusion was later confirmed in [Grachev et al., 1992, 1994; Neumann et al., 1995].
Studies of the He, Sr, Nd, and Pb isotopic composition of xenoliths in basalts from Lanzerote Island were launched under the international project “Teide Laboratory Volcano” in 1994 [Grachev et al., 1994; Ovchinnikova et al., 1995].
The very first data on the He isotopic composition of basalts and xenoliths from Lanzerote Island were obtained
1Schmidt Joint Institute of Physics of the Earth, Russian Academy of Sciences, Moscow, Russia
Copyright 2012 by the Geophysical Center RAS. http://elpub.wdcb.ru/journals/rjes/doi/2012ES000515.html
by Grachev et al. [1992] and Vance et al. [1992] and were later examined in basalts from other Canary Islands (La Gomera, Tenerife, El Hierro, and La Palma) [Day and Hilton, 2011; Grachev, 2001a, 2001b].
Our present research was focused on xenoliths and their host basalts from the Timanafaya volcanic field, from Tamia, Pico Partido, and Ermita de la Magdalena volcanoes, and from the area of the historical 1824 eruption (Figure 1). It is pertinent to mention that Vance et al. [1992] have also studied the He isotopic composition of a dunite xenolith from Pico Partido volcano.
Methods
Monomineralic separates were obtained from our samples of ultramafic xenoliths with the use of heavy liquids and the subsequent magnetic separation of minerals. If needed, the concentrates were then 95-99% purified by hand-picking.
Basalt samples were crushed to 3-5 cm, washed in cold 0.1 N HCl to get rid of surface contaminants, and then pulverized in an agate mortar to 200 mesh grain size.
He was extracted from rocks and minerals by the melting techniques [Kamensky et al., 1990] and by crushing the samples [Ikorsky and Kamensky, 1998] at the Laboratory of Isotopic Geochronology of the Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences. The crushing technique makes it possible to selectively extract gases from fluid inclusions and thus to minimize the effect of radiogenic gases accumulated in the crystal structure of minerals [Kaneoka et al., 1980]. To extract gases, 0.16-2.25 g of the material and steel rolling crushers were placed in a glass ampoule, which was then evacuated and welded. The material was crushed due to vibrations of the ampoule. The He isotopic composition and concentration were measured on a
mode, with the simultaneous recording of the ion currents of various isotopes of elements.
The chemical composition of the samples was analyzed by XRF with the application of an original analysis technique development at Sevzapgeologiya. The analytical XRF setup consisted of a 1000-channel pulse analyzer, spectro-metric amplifier, and a Si(Li) detector with 25 mm2 sensitive area and an energy resolution (5.9 keV) of 210 eV.
The material to be analyzed (20 g, 200 mesh) was placed into specialized trays. The characteristic radiation was excited by (i) an X-ray tube with an intermediate Ag target and (ii) the Am-241 radioactive isotope source. The XRF analyses were carried out with the VM, SGD-1A, SG-1A, ST-1A, SA-1, TV, and SGKHM-3 certified standards.
Samples
Figure 1. A Google-Earth map showing the location of the studied xenoliths samples. Insert: area of historical eruptions after [Romero et al., 1986].
MI-1201 no. 22-78 mass spectrometer with a He detection limit of 5 x 10-5 A/torr. The concentrations were calculated from the height of the peak accurate to ±5% (±1<r), and the errors of the measured isotopic ratios were ±20% at 3He/4He = n x 10-8 and ±2% at 3He/4He = n x 10-6. The blanks were conducted after reloading the cassette under the same conditions as the analyses of the samples.
Sm, Nd, Rb, and Sr were extracted for isotopic analysis at the same institute in compliance with the method described in [Richard et al., 1976]. The blanks were 0.003 ng for Rb, 0.2 ng for Sr, 0.03 ng for Sm, and 0.08 ng for Nd. The isotopic composition of these elements was determined on an Finnigan MAT-261 8-collector mass spectrometer in static
Our xenoliths can be classified into two groups according to their morphology and the age of their host basalts. The first group comprises xenoliths in basalts from Montana Tamia and Ermita de la Magdalena volcanoes belonging to suite 3, which was dated at the Early Quaternary [Sagredo,
1969]. Xenoliths of this group usually have geometric ellipsoidal morphologies and are up to 20 cm long (Figure 2a).
The second group consists of xenoliths from basalts and cinders at the Timanafaya volcanic field, which were produced by the 1730-1736 fissure eruptions. The xenoliths of this group are also large but are angular and covered with rinds of cinder or vitreous basaltic lava. It can be seen at contacts of the xenoliths with host rocks that the basalt melt penetrated into the xenoliths, although the contacts are usually sharp.
Another, although fairly rare, type of the xenoliths can be distinguished thanks to their reddish coloration (Figure 2b). Color changes are clearly pronounced along the boundaries of olivine grains and often affect much of some individual grains. The origin of the red suite is explained by man-
Figure 2. (a) - Typical rounded shape of mantle xenoliths (sample L-88-1), (b) - red-color type of xenolith (sample Tim-1).
Figure 3. Changes of trace elements abundances from centre to margin of olivine grain (sample L-88-1/7).
tle metasomatism and resultant changes mainly in the Fe, Mg, concentrations in the margins of olivine grains [Grachev, 2000]. These elements are concentrated within zones 50150 ^m thick (Figure 3). Zones of intense color changes are often related to partial melting. The occurrence of melt films along grain boundaries, where incompatible elements are concentrated, in mantle peridotites was first pointed out
in [Suzuki, 1987], and this effect is pronounced in the xeno-liths of the red suite in a change in their color.
The xenoliths have porphyroclastic textures with pronounced olivine porphyroblasts and neoblasts (Figure 4a).
Practically all olivine grains are deformed and contain kink bands (Figure 4b). The rocks also contain domains with traces of melting [Koreshkova, 1994].
Table 1. Chemistry (wt. %) of ultramafic xenoliths of the Lanzerote Island
Element L88-1 1 L88-2 2 L88-3 3 Tam-1 4 Tam-2 5 Tim-1 6 Tim-2 7 EN-4x 8 GC-1 9
SiO2 43.14 42.40 43.14 42.32 40.56 41.41 41.46 42.16 43.11
TiO2 0.16 0.04 0.28 0.09 0.05 0.04 0.04 0.05 0.19
AhO3 1.09 1.48 1.12 1.28 0.61 0.48 0.50 0.82 1.60
FeOt 7.79 7.55 7.86 8.84 9.63 9.12 9.03 8.84 8.48
MnO 0.25 0.25 0.24 0.11 0.13 0.12 0.13 0.12 0.13
MgO 46.09 45.12 45.29 46.62 47.70 47.39 48.45 46.10 44.56
CaO 0.60 1.45 0.63 0.58 0.52 0.49 0.43 0.89 1.20
Na2 O 0.20 0.20 0.24 0.36 0.31 0.29 0.46 0.29 0.58
K2O 0.09 0.05 0.09 0.13 0.06 0.04 0.38 0.02 0.10
P2O5 0.07 0.11 0.09 0.01 0.01 0.01 0.06 0.01 -
Cr2O3 0.27 0.40 0.32 - - - - - -
NiO 0.13 0.11 0.14 - - - - - -
Note: 1 — dunite, Ermita de la Magdalena, 2,3 — harzburgite, Pico Partida, 4,5 — harzburgite, Tamia, 6—8 — harzburgite, Timanafaya, 9 — harzburgite, caldera Vandama, Gran Canaria.
Figure 4. (a) - Olivine porphyblasts and neoblasts, (b) - kink bands (sample Tim1-1, thin section, cross-polarised light).
In terms of chemical composition (Table 1, Figure 5), all of the xenoliths affiliate with the strongly depleted mantle with high contents of MgO (45-48%) and low contents of CaO (0.4-1.4 wt%), and Al2O3 (0.5-1.5%).
He and Ar Isotopic Composition
Table 2 and Figure 6 and Figure 7 present our data on the He and Ar isotopic composition of the xenoliths, which were examined by the melting and crushing techniques. All of our xenolith samples, except for only two of them from Tamia
volcano (samples Tam-2 and Tam-3), which were hosted in Quaternary basalts, have 3He/4He ratios (R) much higher than the atmospheric ones (Ra = 1.39 x 10-6).
Regardless of the technique utilized to extract He, the xenoliths from lavas of the 1734 eruption have R/Ra ratios within the range typical of MORB (R/Ra =8 ± 1) [Kaneoka et al., 1980]. This means that the content of the cosmo-genic component in xenoliths from the young lavas is so low [Dunai and Wijbrans, 2000] that it does not any appreciably influence the results obtained by the melting and crushing techniques (Table 2).
10.00 ■
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Figure 5. MgO-Al2O3 diagram for the studied xenoliths.
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O
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O
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o
joo
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o
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O
□□
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O Xenoliths, melting 0 Xenoliths, crushing □ Basalts, melting
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0.20 040 0.60
4|He, 10"® cm^/g
0 80
Figure 6. 3He/4He (R/Ra) - 4He diagram for xenoliths
and basalts. Open circles and squares for gas released by melting procedure, close circles refer to crushing method data.
Table 2. He-Ar isotopes of ultramafic xenoliths and basalts of the Lanzerote island
Sample Rock, mineral Weight, g 4He (10-6 cm3 3He/4He /g) 40Ar (10-6 cm3/g) 40Ar/36Ar R/R0 Method
Xenoliths
L-88-1 Hrzb. 0.3731 0.20 8.1 0.89 1080 5.85 melting
L-88-1/2 Dunite 0.45 3.1 2.18 2500 2.23 melting
L-88-1/3 Hrzb. 0.25 10 6.6 1200 7.22 melting
L-88-1/3 Harz. Ol 0.02 7.0 1000 5.06 melting
L-88-1/4 Dunite 0.17 9.8 3.0 3800 7.08 melting
L-88-2/1 Dunite 0.30 10.1 0.55 1560 7.21 melting
L-88-2/4 Dunite 0.15 12 3.13 3680 8.67 melting
Tim-1 Ol 0.4296 0.04 12.6 9.0 melting
Tim-1 Opy 1.45 0.27 8.7 6.21 crushing
Tim-1 Ol 1.43 0.12 7.5 5.4 crushing
Tim-2 Ol 0.5126 0.05 7.02 5.07 melting
Tam-1 Ol 0.4745 0.04 8.74 6.31 melting
Tam-2 Ol 0.3062 0.10 3.84 2.77 melting
Tam-3 Ol 0.5250 0.30 3.98 2.88 melting
Lanz560 Cpy 1.05 0.25 9.70 6.93 crushing
Lanz560 Opy 0.40 0.40 8.60 6.14 crushing
Lanz560 Ol 2.00 0.17 9.70 6.93 crushing
Basalts
L-88-1 bas Ol 0.70 6.0 2.05 1070 4.28 melting
L-88-1/2 bas 0.05 6.1 1.21 325 4.35 melting
L-88-1 bas 0.24 2.8 1.03 420 2.0 melting
L-88-1/2 bas 0.21 2.8 1.4 390 2.0 melting
L-88-1/3 bas 0.05 0.6 0.6 352 0.43 melting
L-88-1/3 bas 0.21 0.6 1.68 409 0.43 melting
L-88-1/4 bas 0.25 0.1 1.13 491 0.07 melting
The host basalts are strongly degassed and have R/Ra ratios much lower than those of he xenoliths. It can be readily seen in the R/Ra —40 Ar/36 Ar diagram (Figure 7) that the 40Ar/36Ar ratio of the basalts is close to the atmospheric one, whereas this ratio of the xenoliths varies from 1000 to 4000.
0.10 —
0.01
_L
_i____________і________і______і і і і
О
О
Xenoliths
О Basalts
О
T
1000
40Ar/36Ar
40 л „ /36 ,
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10000
liths and basalts. A [Kaneoka, 1983].
The degassing of the basalts in the course of their melting accounts for their high CO2 concentrations (> 3 ncm3/g), whereas the analogous concentrations in the xenoliths never exceed 1 ncm3/g [Lokhov and Levskii, 1993].
As can be seen in the Sr-Nd diagram (Figure 8, Table 3), the xenoliths and basalts define a compact group of their
87 Cr/86,
Figure 7. 3He/4He (R/Ra) - Ar/ Ar diagram for xeno-
Sr/ SR
Atmosphere, M - MORB, P - Plume
Figure 8.
basalts.
Sr-Nd diagram for the studied xenoliths and
Table 3. Sm-Nd and Rb-Sr isotopic systematics of ultramafic xenoliths and basalts of the Lanzerote Island
N Sample Sm Nd Rb Sr 147Sm/144Nd 143Nd/144Nd ±2a 87Rb/86Sr 87Sr/86Sr ±2a
Xenoliths
1 Tam-1, Di 2.161 16.37 0.864 270.4 0.08007 0.512928±27 0.00924 0.703267 ± 14
2 Tam-2, Di 3.747 22.66 0.582 407.6 0.10028 0.512968 ± 16 0.00413 0.703147 ± 12
3 Tam-3, Di 0.814 5.662 0.668 192.7 0.08717 0.512897 ± 21 0.01002 0.703275 ± 21
4 Tim-1, Di 2.024 13.54 0.586 434.9 0.09065 0.512969 ± 12 0.0390 0.703175 ± 16
5 Tim-2, Di 8.688 36.13 2.648 285.3 0.14584 0.512770 ± 42 0.02684 0.703636 ± 283
6 Tim-3, Opy1 0.014 0.058 0.143672 0.512100 ± 35
7 Tim-3, Opy2 0.028 0.129 0.129453 0.512858 ± 29
8 Tim-3, Di 0.579 2.293 0.119727 0.512716 ± 17
9 L-88-1, wr 0.060 0.190 1.243 1.277 0.19022 0.512587 ± 16 2.81983 0.706367 ± 21
10 L-88-3, wr 0.720 1.491 4.754 5.964 0.29177 0.513095 ± 21 2.30788 0.715833 ± 22
11 GC-1, wr 0.008 0.054 0.034 1.381 0.09420 0.512820 ± 120 0.07070 0.704016 ± 40
Basalts
12 L-89-3 wr 6.84 27.18 14.73 397.9 0.15218 0.5129017 0.10298 0.703244±9
13 L-88-1/1 wr 6.50 25.70 14.26 391.8 0.15299 0.5129065 0.10256 0.703213 ± 19
14 L88-1/2 wr 6.77 26.88 13.98 399.3 0.15235 0.5129156 0.10127 0.703277 ± 14
15 ND-6 wr 7.99 39.31 nd nd 0.12283 0.51300612 nd 0.703212 ± 13
16 ND-7 wr 9.03 45.65 33.52 730.2 0.11950 0.5129448 0.13273 0.703212 ± 13
Note: Sm, Nd, Rb and Sr concentrations in ppm (isotope dilution, precision is about 0.5%), error estimations on 147Sm/144Nd and 87Rbr/86Sr are ±0.3% and 0.5% respectively. During the period of analytical work the weighted mean of 10 La Jolla Nd standart runs yielded 0.511852 ± 4 (25), using 0.241579 for 143Nd/144Nd to normalize; and NBS-987 standard yeilded 0.710255 ± 15 (25), using 8.375210 for 87Sr/86Sr to normalize. Total procedure blanks for Nd and Sm are 0.08 and 0.03 ng respectively, and for Sr and Rb are 0.3 and 0.4 ng respectively. All isotopic analyses were carried out on the Finningan MAT-261 solid source machine under multicollector static mode. Chemical preparation of rock samples and elements separation were done using standart procedure similar to [Richards et al., 1976] in the Precambrian Geolody and Geochemistry Institute (St. Petersburg) and in the Geological Institute of the Kola Scientific Centre of the Russian Academy of Sciences.
data points with typical MORB 87Sr/86Sr and 143Nd/144Nd ratios, but these ratios are not correlated with the he isotopic composition.
Our results are in good agreement with data on the He isotopic composition of olivine phenocrysts in basalts from the islands of La Palma [Grachev, 2001a, 2001b; Hilton et al., 2000] and Tenerife.
Discussion
Our data on the He, Sr, and Nd concentrations and isotopic composition in xenoliths from basalts from Lanzerote Island testify to a strongly depleted type of the mantle, and these mantle xenoliths should be regarded as such of refractory residual material after the derivation of basalts.
The alkali basalts of Lanzerote Island have a He isotopic composition principally different from that of the typical plume basalts of Hawaii and Reunion Island, whose R/Ra ratios are greater than 20 [Kaneoka et al., 1980]; although xenoliths (predominantly dunites) in tholeiite basalts at Oahu Island are also of residual nature [Jackson and Wright,
1970], they have R/Ra ratios typical of MORB [Kaneoka et al., 1980]. Recent studies at Oahu Island resulted in the discovery of nodules with garnet, and moreover, the xenoliths were determined to contain nanodiamonds, which suggests
that the xenoliths were formed under pressures higher than 50-60 kbar [Keshav et al., 2007].
The He-Ar isotopic composition of xenoliths from Lanze-rote Island is closely similar to that of xenoliths from seamounts in the northwestern Pacific Ocean with typically MORB signatures [Yamamoto et al., 2009].
It follows that the isotopic parameters of both the xeno-liths and the basalts from Lanzerote Island do not show any indications of a mantle plume. With regard for data on other islands of the Canary Archipelago [Day and Hilton, 2011; Grachev, 2001a, 2001b; Grachev et al., 1992; Vance et al., 1992], this led us to conclude that no mantle plume occurs beneath all of the Canary Islands.
Acknowledgments. The author thanks V. Arana for help with the fieldwork.
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A. F. Grachev, Schmidt Joint Institute of Physics of the Earth, Russian Academy of Sciences. (afgrachev@gmail.com)
