LZS-1, Lanzarote (Canary Islands, Spain) lunar (Apollo 14) simulating basalt soil

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material and methods

During the field campaign, 3,000 g of basalt rocks were collected from the Tao quarry. Among them, to carry out the analyses, “fresh” basalt samples of 1500 g which did not show any alteration were selected, trying to represent their different textural characteristics (eg massive, vesicular). For sampling, we had the appropriate permits to access certain areas of exploration, thanks to the support of the UNESCO Global Geopark of Lanzarote and the Chinijo Archipelago17.

In accordance with the results obtained in the previous articles18,19,20,21, it was decided to sieve the “fresh” 1500 g basalt samples into two main fractions in order to represent the main grain size distribution intervals of the lunar soil, in accordance with the most frequent intervals (Table 1). Figure 4 shows two regolithic material canisters. One belongs to the fraction between 63 and 125 µm (1000 g) and the other represents the fraction 22 on the characteristics of the shape of the grains of the lunar soil.

Table 1 Mean and median values ​​of lunar regolith particle size at Apollo 11-17 landing sites and CAS-1, JSC-1 regolith simulants5and LZS-1.
Figure 4

Basalt simulant LZS-1 Lanzarote.

Figure 5
number 5

Scanning electron microscope image showing grain size and shape of LZS-1.

For mineralogical and geochemical characterization, we perform X-ray diffraction techniques (Bruker D8 Advance diffractometer), X-ray fluorescence (Bruker S2 Ranger spectrometer), Energy Dispersive X-ray scanning electron microscopy (JEOL JSM-820), Microanalysis electronic probe (JEOL Superprobe JXA-8900 M) and mass spectrometry with inductively coupled plasma (mass spectrometer, with ICP ionization source, Bruker Aurora Elite). For the petrophysical characterization, we performed tests to determine the chromatic parameters (MINOLTA CM-700d/600d spectrophotometer). Determination of hardness according to the LEEB rebound method (EQUOTIP 3 Hardness Tester). Determination of surface roughness (TRACEIT optical surface roughness meter). Determination of the speed of propagation of ultrasounds (PUNDIT of CNS ELECTRONICS, Unit LPF-04-US). Determination of the porosity accessible to water, of the real and apparent density, and of the compactness. Determination of mercury accessible porosity, pore size distribution and tortuosity. Estimation of uniaxial compressive strength (UCS). For the development of LZS-1, basalt rock samples were first cut into regular shapes and placed in ovens for 48 h at temperatures of 60°C until a constant mass was achieved. A third of the samples were reserved for petrophysical tests (surface hardness, ultrasonic pulse velocity, mercury saturation and Hg porosimetry). The rest (2/3) was conveyed to a ball mill where 1500 g of regolith simulant were obtained. The sieving of basalt was standardized in the ASTM standard in which the numbers 10 (2 mm), 18 (1 mm), 35 (0.5 mm), 60 (0.250 mm), 125 (0.125 mm) were used. To obtain fractions below 65 µm, 500 g of the ground material was pulverized in an agate mortar and then passed through a No. 230 (0.065 mm) sieve. Finally, it was stored in two different containers depending on the fractions reached (Fig. 4).

Petrography and Mineralogy

Textually, the samples are heavily vesiculated, in which the percentage of vacuoles can reach 48% of the total in some cases. Its size is variable. Tao samples have vesicle diameters of 150 µm to 2.5 mm. Other textures that we can differentiate are glassy (hypocrystalline), aphanitic and microcrystalline textures.

Mineralogically, three subclasses of silicates are represented (Fig. 6); nesosilicates: with olivine characterized by their marked relief, their straight extinction, their high birefringence and their rounded shape; (clino-ortho)pyroxene inosilicates, tectosilicates of the plagioclase (calcium) group and glass consisting mainly of pyroxene and plagioclase. On the other hand, there are metal oxides mainly of iron, titanium and chromium.

Figure 6
number 6

Image of samples analyzed at the macroscopic and microscopic scale with reflected light in parallel and crossed nickels and electron microprobe image. The blue and brown colors correspond to the minerals of the olivine group. The black color corresponds to the vitreous mass which has a predominantly pyroxenic composition. The data obtained with the electron microprobe and with the SEM-EDX made it possible to identify the main mineral phases.

Before proceeding with any analysis, the samples were cleaned and dried in ovens at 70°C until a constant mass was reached. After that, 10 g of basalt was crushed and ground for XRD (Fig. 7) and XRF. The rest of the chemical analyzes were carried out on the thin sheets made at the Center for Research Support of the Complutense University of Madrid (CAI-UCM).

Picture 7
number 7

XRD data from LZS-1 and a detailed image from the SEM-EDX where the main mineral phases are depicted.

The composition of the main elements is shown in Table 2, comparing it with that of CAS-1, JSC-1, FJS-1 and MKS-1. LZS-1 is similar to CAS-1, JSC-1 and Apollo 14 agglutinate sample 141635.

Table 2 Major element composition of LZS-1 and comparison with CAS-1, JSC-1, FJS-1, lunar soil simulant MKS-1 and Apollo 14 lunar regolith5.

The trace element composition of LZS-1 was analyzed by ICP-MS (Table 3) and found to resemble basanite-basalt. Rare earth element (REE) concentrations are shown in Fig. 8 showing slight enrichment in REE although no anomaly in Ce or Eu. LZS-1 has high levels of incompatible elements and is enriched in large lithophilic ions (Fig. 8). This suggests that the material originated in the mantle, with no differentiation into the magma chamber.

Table 3 Abundances in trace elements of the lunar soil simulant LZS-1.
Picture 8
figure 8

Distribution scheme of REEs normalized to chondrites of the lunar soil simulant LZS-1 and compared to 5 groups of basaltic lunar samples with high Al content25 normalized to chondrite according to Evensen. et al.26.

Table 4 shows the average composition of the main mineral phases of the lunar regolithic simulant LZS-1 compared to five groups of aluminum mare basalt samples from the lunar surface.25.

Table 4 Average composition of major mineral phases in five groups of aluminum mare basalt samples from the lunar surface25 and in LZS-1 obtained with SEM-EDX.

There is no evidence of weathering process in the minerals after the eruptive process. No carbonates, quartzes or clays were found that would indicate that the basalts were weathered.

Physical properties

The physical properties of color, hardness, ultrasonic pulse velocity, density, porosity, and uniaxial compressive strength of LZS-1 were determined by petrophysical testing. In fact, values ​​obtained from LZS-1 of density, ultrasonic pulse velocity, porosity and uniaxial compressive strength can be compared to a lunar basalt model from Apollo 1420 and with the data obtained from the Fra-Mauro training27that they also correspond to the Apollo 14 mission.

The color was measured with a Minolta CM 700d spectrophotometer and COLOR DATA SPECTRAMAGICTM NX CM-S100W software, using Standard illuminant D65 (standard illuminant of the CIE -Commission Internationale de l´Eclaraige-, equivalent to daylight with ultraviolet radiation and a color temperature of 6504 ºK) and a viewing angle or viewing angle of 10º (Fig. 9).

Figure 9
number 9

Data from the color test where it can be observed that there is practically no dispersion between the samples analyzed and therefore it can be said that they have not been altered by the weathering processes after the eruption.

The CIE L* a* b* system was used. Where L* is the attribute that determines how light, bright, or dark a color is. Displays values ​​from 0 (pure black) to 100 (pure white). The higher the value, the lighter the color, and the lower the value, the darker the color. a* and b* are the chromatic coordinates: the axis -a*, + a* represents the degree of saturation towards green (-a*) and towards red (+ a*). The -b*, +b* axis represents blue to yellow and C* is the Croma obtained with Eq. (1).

$$ C^{*} = left( {a^{*2 } + b^{*2 } } right)^{{{raise0.7exhbox{$1$} !mathord{left / {vphantom {1 2}}right.kern-nulldelimiterspace} !lower0.7exhbox{$2$}}}} $$

(1)

The hardness (Fig. 10) was measured with the EQUOTIP 3 device according to the LEEB rebound method (EHT, Equotip Hardness Tester).

Picture 10
number 10

Equotip 3 device hardness data.

This measurement is indicative of the surface hardness of the materials used in engineering and was used to estimate the uniaxial compressive strength of the samples from the Tao quarry. The results of uniaxial compressive strength estimation (Fig. 11) were obtained with Eq. (2).

$$ SCU left( {MPa} right) = 2*10^{ – 8} *EHT^{3.3492} $$

(2)

Picture 11
figure 11

UCS estimation data derived from the equation obtained by Yilmaz. & Goktan28. BES Scoriaceous Basalt, BAFV Vesicular Aphanitic Basalt, BOPM Massive Pyrogenic Olivine Basalt, BPLM Massive Plagioclassical Basalt, FON Phonolite, IGNS Unwelded Ignimbrite, TRQ Trachyte, IGS Welded Ignibrite, TRQB Trachybasalt (Modified from20).

The ultrasonic pulse velocity represents the overall anisotropy (Table 5) of the rock and the roughness the anisotropy at the surface (Fig. 12).

Table 5 Ultrasound pulse velocity test and anisotropy results where Vmed is: average velocity, StD Med: average standard deviation, dM: total anisotropy and dm: relative anisotropy.
Picture 12
figure 12

Roughness data from the TRACEIT Optical Surface Roughness Meter.

Rock density and porosity were calculated by mercury intrusion porosimetry (Micromeritics Autopore IV, with a maximum pressure of 400 MPa). Density values ​​ranged from 2.8 g/cm3 at 3.0g/cm3 in all Tao samples. In turn, due to the high penetration of mercury into the pores of the rock, two porosity values ​​were obtained: microporosity ( 5 µm). The results (Table 6) show that the Tao Basalts have a much higher microporosity than the macroporosity.

Table 6 Results of mercury porosimetry tests.

Comparison between these results and data from the Apollo 14 mission27 and a model of lunar basalt20 appear in Table 7. As can be seen, there is a strong correlation not only in terms of mineralogical and geochemical terms, but also in terms of physical properties. For this reason, the Tao basalts are considered analogous to the lunar surface according to the Apollo 14 mission landing site.

Table 7 Comparison of physical properties between a lunar basaltic model, the Fra Mauro formation and LZS-1.

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