Petrological features of picrobasaltic melts on Lanzarote, Canary Islands

: The paper discusses issues related to the petrology of picrobasaltic melts forming lava flows on Lanzarote, Canary Islands. During ascientific and educational expedition on the aforementioned island, lava flows of picro basalt of the third phase of the 1731-1732 eruption were studied and tested. Preference was given to the well-preserved streams in the west and south of the island thatare accessible for direct study. Volcanogenic-sedimentary associations were identified in the studied outcropsand described, the structural and textural features of the flows were characterized, and samples with mantle rock xenoliths were taken for petrological reconstructions. New geochemical data onrocks and minerals are presented. It was established that the temperature of the initial melt was 1100 –1180°C. Large harzburgitephenocrysts in picro basalt are xenocrysts formed as a result of melting and disintegration of mantle xenoliths.


Introduction
The presence of basic magmatism is characteristic of various geotectonic sittings [1]. For volcanic islands, mainmagmatism is predominant (over 95% of the volume), whereas the generation of melts occurs at mantle levels under the influence of thermal anomaly (plume). The presence of xenoliths in basalts makes it possible to identify the composition of the mantle beneath the region and estimate the contribution of mantle rocks to the formation of basaltic melts.
The intense magnetic activity on the Canary Islands, as well as the young age of magmatism ,i.e., during the past 500 years according to [2] makes the Canary archipelago one of the crucial objects for studying oceanic island magmatism. The islands of the Canary archipelago have a highly dissected relief and are often eroded and brought to the surface near coastal deposits, which makes these sites attractive for geological study.
Over the past two decades, deposits of the islands of the Canary archipelago have been extensively studied and characterized by a complex of geological and geophysical methods [2,3,4,5,6,7,8]. Thus, today, the Canary archipelago is one of the best-studied volcanic chains in the world. The origin of melts from the islands of the Canary archipelago is widely discussed in the world literature [5,9,14,16,18,19].
This article is devoted to the study of the evolution of melts of the third phase of the eruption of Lanzarote Island, represented by picrobasalts containing xenoliths of mantle rocks.

Geological position
The Canary Archipelago is formed of 7 islands and several smaller erosive remains. Tectonically, the islands are located near the margin of the northwestern African continental shelf. Moreover, the composition of xenoliths found on all islands suggests the presence of an oceanic type of crust below the archipelago, whose age is estimated as 60 Ma [20]. The above circumstances indicate its formation due to the mantle thermal anomaly located inside the lithosphere plate, which is a source of magmas that feed the active volcanoes of the islands.
Lanzarote is the easternmost island ( Fig. 1). Its tectonic position is an intra-plate area located on the North African plate that encompasses both the oceanic and the continental crust.
The island stretches in a northeasterly direction and is parallel to the continental marginof Africa. It rises about 2500 m from the seabed, whereas most of the volcanic structure is submerged. It is connected in its underwater part with the island of Fuerteventura, as indicated by the same type of volcanism and identical equalage composition of sedimentary rocks, and the thickness of the water between the islands does not exceed 40 m [11]. The base of the island was formed during the Oligocene over oceanic deposits with an age of approximately 65-55 Ma and is composed of material from underwater volcanic eruptions and plutonic rocks. Recent volcanoes are grouped in a central rift zone from northeast to southwest, with 1730-1736 vents and deposits that are an expression of the "active" volcanic status of the island [2]. The 1730-1736 eruption is divided into 5 phases (Fig. 1). The composition of magma during the initial phase gradually changed from melanepheline sites through basanites to alkaline basalts. In the remaining 4 phases of the eruption, magma evolved from basanites to olivine tholeiites.
The object of the study was picrobasalt, attributed [12] to the third phase of the volcanic eruption (July 1731 -January 1732). The studied samples were selected from recent, wellpreserved lava flows with primary textural, structural, and mineralogical features in the south-west of the island near the El Golfo village. These rocks contain xenoliths of various compositions: (1) mantle xenoliths according to [21,22] may be fragments of the old suboceanic mantle subjected to metasomatism and partial melting; (2) xenoliths of sedimentary rocks (calcareous-silicate and siliceous rocks, subordinate to limestones, clay, and sandstones), most of which are metamorphosed and metasomatized [10].

Methods of research
For petrological study, 8 rock samples were taken from different points on Lanzarote island. Of particular interest were rocks with large and numerous mantle xenoliths. Thus, 5 samples of picrobasalts were taken near the El Golfo village. To compare the composition of rocks, 2 samples of basalts in the south of the island at Playa Papagayo and 1 sample in the north at Caleta de Famarawere taken.
The mineralogical composition of the rocks was studied in 8thin sections. The composition of rock-forming minerals was determined in the laboratory for mineral matter analysis at IGEM RAS using a JEOL (JXA-8200) electron probe analyzer equipped with 5 wave spectrometers at an accelerating voltage of 20 kV, a Faraday cup current of 20 nA, and a beam diameter of 1 μm. Exposure times for all elements measured in olivine, pyroxene, and plagioclase were10 s at the peak and 5 s on the background on both sides.Forty mineral composition analyses were performed.
The content of major and trace elements on 3 samples was determined by X-ray fluorescence analysis (XRF) using an S4 Pioneer spectrometer (Bruker, Germany) with a 4 kW rhodium tube and a 75 μm beryllium window. Sample preparation for analysis was carried out according to the method described in [23]. The microelement composition of rocks was determined by inductively coupled plasma mass spectrometry (ICP-MS) on a Thermo Fisher Scientific 2 apparatus at the GIN RAS.Thesamples were prepared for analysis by acid decomposition according to the procedure described in. The accuracy of the analysis was monitored by measuring samples of the international standard MUH-1 and WG-1.A geothermobarometry [24] was used to determine the crystallization temperature of the melt. Calculation of temperature parameters was carried out according to equation 32.

Petrogeochemical characteristics of picrobasalt
On the west coast of Lanzarote, outcrops are extended;almost black streams of chilled magma, in the roof of which the structures of quenching zones with pahoehoe and numerous lava breccias are often preserved. Clumps of basalts of various sizes containing large xenoliths of mantle rocks of 10 cm in size were observed everywhere along the coast. Powerful pyroclastic flows are located in close proximity to the outcrops and are composed of tuffs with numerous volcanic bombs and lapilli.
According to petrochemical characteristics (Table 1), picrobasalts correspond to low potassium tholeiitic basalts with thepredominance of Na over According to the TAS classification, the volcanic compositions are located in the field of basalts, less frequently,basanites. (MORB) of normal (N-) and enriched (E-) types, island arcs (IAB) and oceanic islands (OIB). The rocks are normalized to a primitive mantle according to [29].  and Ti (Fig. 2). The REE trace of picrobasalts is similar to those for basalts of oceanic islands (OIB, Fig. 2).
Olivine phenocrysts (Ol-Ph) form large (0.3-0.5 mm) resorbed grains of angular and irregular shape (Fig. 4). The central parts of the grains are homogeneous and characterized by high Mg # 89-91 (Mg # = Mg / (Mg + Fe) * 100), Ni and Cr concentrations and low concentrations of incompatible elements (Al, Ti, Mn) ( Table 2). The core sections are surrounded by a narrow (less than 0.005 mm) rim of less magnesian composition (Mg # 85), characterized by a low Ni content and higher contents of incompatible elements. The Ol-Ph rims contain spinel inclusions with chromium number Cr # 25-45 (Cr#= Cr / (Cr + Al + Fe) * 100).
According to the content of trace elements, xenolithicolivines are comparable with the central parts of Ol-Ph on picrobasalt.

Discussion
The porphyritic texture of picrobasalt indicates that the melt which formed lava flows contained crystalline mineral phases. This can oftenindicate the existence of an intermediated magmatic chamber in which crystallization of phenocryst minerals could occur [18,26]. At the same time, phenocrysts can be xenocrysts, melt fragments of the disintegration of rocks trappedbythe melt during ascent (Larrea et al. 2013). In any case, phenocrysts are an indicator of events in the formation of the initial melt for volcanic rocks and their study allows us to draw conclusions about the previous evolutionary stages of the initial magma.
Recent studies [14,16,23] have shown that olivine is an important petrogenetic indicator. Reliable data on the distribution of trace elements have been established for some elements (e.g., Ni, Co, Mn, Cr) of olivine phenocrysts from basalts formed in different tectonic settings [27,28], separated by trace elements and dispersed through olivine into three groups, whose distribution depends on (1) the ionic radius close to Mg (Ni, Mn, Co, Cu, Zn, Li); (2) melt composition (Ti, Zr, Nb, Y, P); (3) controlled by temperature during equilibrium crystallization of the mineral (Cr, Al, V, Sc, Ca, Na), which revealed systematic differences between mantle substrate compositions,, element distribution mechanisms in olivine, and their potential for use in geobarometry and interpretation of mantle processes.Generalized information on the concentrations of trace elements in olivine from different basaltic melts revealed patterns of their distribution depending on the source of melting.
In picrobasalts, Ol-Ph has a sharp and clear boundary between the homogeneous central part and the narrow marginal zone. Assuming the distribution coefficient K D Fe-Mg olivinemelt = 0.3 ± 0.03 [25], Mg# of the central parts of Ol-Ph is excessive with respect to the contained melt, which may indicate its crystallization from a more primitive melt. However, in terms of magnetism and the content of trace elements, such as Ni, Ca, and Ti, the central parts of Ol-Ph differ from the marginal parts and are comparable with the olivine of harzburgitexenoliths.On the variation diagrams (Fig. 5), the compositional points of the central parts of Ol-Ph and olivine from harzburgite xenolith form a local area of points with Mg # from 89 to 92 located in the region of olivine compositions of dunite and harzburgite xenoliths.
Mg# of the olivine in the bulk corresponds to Mg# of the surrounding melt. According to the content of trace elements, the olivine of the groundmassis similar to the compositions of the Ol-Ph rims. This olivine enriched of incompatible elements Al, Ca, and This likely to indicate an enriched source of the melts.
Given The significant differences in the content of trace elements between Ol-Ph and groundmass olivine , as well as the geochemical similarity of olivine in harzburgite xenoliths and central parts of the Ol-Ph, wecan assume that Ol-Ph is most likely a product of harzburgite xenoliths disintegration, which was accompanied by the resorption of xenolithic olivine which affected its morphology. During crystallization from a melt of liquidus olivine (groundmass olivine), the central parts of Ol-Ph were surrounded by narrow rims. This mechanism explains the geochemical similarity of the bulk olivine and the Ol-Ph rim.
The presence of rare inclusions of low-chromium spinel and magnetite in the Ol-Ph and groundmass olivine boundaries and at the same time their more frequent occurrence in inclusions in pyroxene indicate that crystallization of spinel and magnetite occurred at the final stage of olivine fractionation, probably synchronously with the crystallization of large grains of groundmass pyroxene .Plagioclase was the last to crystallize from the melt.  (Neumann et al, 1995).
The temperature of crystallization of clinopyroxene obtained using a geothermometer [24] varies in the range of 1100-1180°C. established that the equilibrium temperature of mineral paragenesis in xenoliths of harzburgite composition varies from 1170 to 870 °C. The higher temperature of the initial melt compared to mantle rocks trapped during its ascentcould could leadto melting and disintegration of the mantle xenoliths. (Aparicio et al. 2006) found in their work that the initial melt could have a basanite composition and temperature ~ 1200°C, at which the calcium-siliceous and siliceous sedimentary rocks trapped at a depth of 4-5 km acquired tholeiitic structure. Thus, the calculated temperature is in good agreement with the data of previous researchers.

Conclusion
Based on the study of picrobasaltic lavas on Lanzarote and the peridotite xenoliths contained in them it was found that (1) the rocks studied belong to the third phase of the eruption (1731 -1732); their geochemical composition supplements the existing data on the