The Cenozoic Arabia-Iran continental collision was associated with emplacement of a large variety of magmatic rock types. This aspect is particularly evident in the Bijar-Qorveh area of NW Iran, where Miocene andesitic to rhyolitic rocks and Quaternary basic alkaline rocks crop out. The Miocene intermediate to acid compositions show radiogenic Sr and Pb isotopic compositions (87Sr/86Sri 0.70531-0.71109, 206Pb/204Pb 18.71-19.01, 207Pb/204Pb 15.66-15.73, 208Pb/204Pb 38.76-39.14), coupled with unradiogenic Nd isotopic ratios (143Nd/144Ndi 0.51223-0.51265). These characteristics, together with primitive mantle-normalized multielemental patterns resembling “subduction-related” geochemical fingerprints, are considered ultimately derived from the Iranian plate mantle wedge, metasomatised during previous NE-directed Neothetyan Ocean subduction. The alkali-rich andesitic and dacitic rocks evidence both closed- and open-system differentiation, as typically observed for collisional settings in general. Both rock types display a high Sr/Y (37-100) and La/Yb (29-74) “adakitic” signature that it is interpreted here with plagioclase (± amphibole) accumulation or melting of local mafic crustal rocks. Open-system processes involve recycling of crustal cumulates for pyroxene-rich andesite and biotite-rich dacite varieties, and low-degree partial melting of the local crust for leuco-rhyolites. A radical change occurred during the Quaternary, when SiO2-understaturated to SiO2-saturated poorly evolved rocks (basanites, tephrites, alkaline and subalkaline basalts) were emplaced. The complete change of mantle sources suggests a phase of local extensional tectonics related with WNW-ESE right-transcurrent faults. The major oxide, as well as incompatible trace element and Sr-Nd-Pb isotopic fingerprint of these younger rocks is more akin to that of intraplate magmas, but still bearing some evidences for a variable contribution from a “subduction-modified” mantle source. The NW-trend of increasing involvement of this subduction component, is indicative of the strong tectonic control on magmatism. Additional lithotypes indicate the presence of open-system differentiation and remelting processes in the youngest phase of magmatic activity.
The small Quaternary volcanic district of Nowbaran (NW Iran) belongs to the Urumieh-Dokhtar Magmatic Arc, a ∼1800 km long NW-SE striking Cenozoic belt characterized by the irregular but abundant presence of subduction-related igneous products. Nowbaran rocks are characterized by absence of feldspars coupled with abundance of clinopyroxene and olivine plus nepheline, melilite and other rarer phases. All the rocks show extremely low SiO2 (35.4-41.4 wt%), very high CaO (13.1-18.3 wt%) and low Al2O3 (8.6-11.6 wt%), leading to ultracalcic compositions (i.e., CaO/Al2O3 >1). Other less peculiar, but still noteworthy, characteristics are the high MgO (8.7-13.3 wt%) and Mg# (0.70-0.75), coupled with a variable alkali content with sodic affinity (Na2O = 1.8-5.4 wt%; K2O = 0.2-2.3 wt%) and variably high LOI (1.9-10.4 wt%; average 4.4 wt%). Measured isotopic ratios (87Sr/86Sr = 0.7052-0.7056; 143Nd/144Nd = 0.51263-0.51266; 206Pb/204Pb = 18.54-18.66; 207Pb/204Pb = 15.66-15.68; 208Pb/204Pb = 38.66-38.79) show small variations and plot within the literature field for the Cenozoic volcanic rocks of western Iran but tend to be displaced towards slightly higher 207Pb/204Pb. Primitive mantle-normalized multielemental patterns are intermediate between typical subduction-related melts and nephelinitic/melilititic melts emplaced in intraplate tectonic settings. The enrichment in Th, coupled with high Ba/Nb and La/Nb, troughs at Ti in primitive mantle-normalised patterns, radiogenic 87Sr/86Sr and positive Δ7/4 anomalies (from +15.2 to + 17.0) are consistent with the presence of (old) recycled crustal lithologies in the sources. The origin of Nowbaran magmas cannot be related to partial melting of C-H-free peridotitic mantle, nor to digestion of limestones and marls by “normal” basaltic melts. Rather, we favour an origin from carbonated lithologies. Carbonated eclogite-derived melts or supercritical fluids, derived from a subducted slab, reacting with peridotite matrix, could have produced peritectic orthopyroxene- and garnet-rich metasomes at the expenses of mantle olivine and clinopyroxene. The residual melt compositions could evolve towards SiO2-undersaturated, CaO- and MgO-rich and Al2O3-poor alkaline melts. During their percolation upwards, these melts can partially freeze reacting chromatographically with portions of the upper mantle wedge, but can also mix with melts from shallower carbonated peridotite. The T-P equilibration estimates for Nowbaran magmas based on recent models on ultrabasic melt compositions are compatible with provenance from the lithosphere-asthenosphere boundary at average temperature (∼1200 °C ± 50 °C). Mixing of melts derived from subduction-modified mantle sources with liquids devoid of any subduction imprint, passively upwelling from slab break-off tears could generate magmas with compositions recorded in Nowbaran.
<p><strong>Introduction</strong></p> <p>The detection of anomalous bright reflections beneath the Martian SPLD by MARSIS radar has sparked a vast debate on the characterization of the basal material. The source of the reflections has been interpreted by someone as an evidence of basal liquid salty water [1, 2], while by others as clay sediments or volcanic rocks [3, 4]. Though it has been demonstrated that clays cannot be the responsible of the strong reflections [5], it has not yet been well understood if the dielectric permittivity of the volcanic rocks can be so high as to cause the reflectivity seen by MARSIS. Furthermore, although there are in literature several attempts to demonstrate a correlation between the dielectric properties of volcanic rocks and their whole-rock compositions [6], these data can hardly be compared with each other, since every work employs different experimental setup and procedures. Therefore, a definitive dataset of the permittivity of such samples and planetary simulants does not yet exist, especially at the operating frequencies of planetary radar sounders ( 1 MHz - 1 GHz). The present work aims to be a first step towards a reliable dataset of electromagnetic measurements of volcanic rocks, investigating the relation between their geochemical and their electric and magnetic properties.</p> <p><strong>Methodology </strong></p> <p><strong>&#160;</strong></p> <p><strong><em>Sample characterization</em></strong></p> <p>&#160;</p> <p>We studied the granular samples of three different volcanic rocks that are plausibly representative of the overall surface compositions of the terrestrial planets [7]. In the following, the samples are named as: St. Augustine (2006), Etna (1991-1993) and Etna (Holocene). The St. Augustine (2006) sample is a lava rock coming from the 2006 eruption of the St. Augustine stratovolcano in Alaska [8]. Sample Etna (1991-1993) comes from the Piana del Trifoglietto lava field at SE of Etna summit craters generated by the eruption started on December 1991 and stopped on March 1993 [9]; the rock is characterized by a potassic geochemical signature. Sample Etna (Holocene) is a mugearite from Pizzi Deneri Formation and it comes from an ancient eruption (57000-15000 years ago), representing the more sodic volcanic activity [10]. In Fig. 1 and Tab. 1 are shown respectively the TAS diagram of the samples and their geochemical composition.</p> <p>The samples exhibit different grain densities: <em>&#961;<sub>St. Augustine(2006) </sub>= (2.736&#177; 0.001) g/cm3, &#961;<sub>Etna(91-93)</sub>= (2.961 &#177; 0.001) g/cm<sup>3</sup></em> and <em>&#961;<sub>Etna(Holocene)</sub>= (2.787 &#177; 0.001) g/cm<sup>3</sup></em>.</p> <p><em><img src="" alt="" width="689" height="93" /></em></p> <p><em>Table 1.</em> <em>Whole-rock compositions of the samples.</em></p> <p><em><img src="" alt="" width="546" height="396" /></em></p> <p><em>Figure 1</em>. <em>Total Alkali vs. Silica diagram of the samples.</em></p> <p><strong><em>Electromagnetic measurements and mixing formulas</em></strong></p> <p>&#160;</p> <p>The electromagnetic measurements were carried out with a two port Vector Network Analyzer (VNA), employing a cage coaxial cell and using the experimental procedure and setup described in [11]. The complex permittivity &#949; and magnetic permeability &#181; were estimated by using the Nicholson-Ross-Weir algorithm and the equations slightly modified in [12]:</p> <p><img src="" alt="" /></p> <p><img src="" alt="" /></p> <p>where <em>F<sub>g</sub></em>&#160;is a factor related to the geometry of the cell, <em>&#915;</em> and &#936; are the reflection and transmission coefficients, <em>l<sub>e</sub> = 5 cm</em> is the electrical length of the cell, <em>c</em> is the velocity of the light in a vacuum and &#957; is the frequency. The complex effective permittivity of the two phases granular rock-air was studied using Lichtenecker and Bruggeman mixing formulas:</p> <p><img src="" alt="" /></p> <p><img src="" alt="" /></p> <p>where <em>&#949;<sub>i</sub></em> and <em>&#949;<sub>e</sub> </em>are respectively the permittivity of the environment (the solid grains) and inclusions (air) and <em>f = 1- </em><em>&#934;</em> is the volume fraction of the grainsin the sample, with <em>&#934;</em> the porosity.</p> <p><strong>Results and conclusions</strong></p> <p>&#160;</p> <p>Measurements with the VNA were performed on the granular samples at room temperature, for percentage porosities ranging from 31% to 55%. &#160;The magnetic permeability &#181; is not reported because the three samples do not show a magnetic behavior (and then we can consider <em>&#181; </em><em>&#8771;</em><em> 1</em>). Fig. 2 illustrates the complex dielectric permittivity as a function of frequency and at <em>&#934; = 0.36. </em>Gray areas in the plots show values that are not reliable since at high frequencies the NRW algorithm diverges because of the cell resonances. Data have larger uncertainties at low frequencies due to VNA instrumental limits. The two Etna samples show a similar dielectric behavior, instead the St. Augustine sample has lower values of both real and imaginary part of permittivity. In the end, we fitted at 80 MHz the complex permittivity as a function of porosity using eqs. 3 and 4. In Tab. 1 we show the measurements at <em>&#934; = 0.36 </em>and the values obtained with the mixing formulas at<em> &#934; = 0</em>. The St. Augustine sample has the lower values of the solid complex permittivity, probably due to its lower iron abundances and higher abundances of <em>SiO<sub>2, </sub></em>while the two Etna samples show a similar dielectric behavior. Further measurements will be required in the future in order to identify the relation between the electromagnetic properties and the geochemical compositions of volcanic rocks that can be accounted as good simulants of the surface and subsurface of terrestrial planets, such as Mars and Venus.</p> <p><img src="" alt="" /></p> <p><em>Figure 2. Complex permittivity frequency spectra of the samples.</em></p> <p><img src="" alt="" width="585" height="224" /></p> <p><em>Table 2. Complex permittivity measured at </em><em>&#934; = 0.36 </em><em>and fitted at </em><em>&#934; = 0 </em><em>with eqs. 3 and 4.</em></p> <p><strong>References</strong><strong>&#160;</strong></p> <ul> <li>Orosei R. et al. (2018) Science, 361(6401), 490-493</li> <li>Lauro S. E. et al. (2020) Nat. Astron., 5(1), 63-70</li> <li>Smith I. B. et al. (2021) Geophys. Res. Lett. 48, 15</li> <li>Grima C. et al. (2022) Geophys. Res. Lett. 49.2, e2021GL096518.</li> <li>Mattei, E. et al. (2022). Earth and Plan. Sci. Lett. 579, 117370.</li> <li>Rust A. C. (1999) Jour. of Volc. and Geoth. Res. 91.1: 79-96.</li> <li>McSween Jr et al (2009) Science, 324.5928: 736-739.</li> <li>Larsen J. F. et al. (2010). Rapp. tecn. US Geological Survey.</li> <li>Calvari S. et al. (1994) Acta Vulcan., 4: 1-14.</li> <li>Vona A. et al. (2017) Chem. Geol., 458: 48-67.</li> <li>Brin A. et al. (2021) Icarus 114800.</li> <li>Mattei E. et al. (2013). IEEE trans. on instr. and meas., 62(11), 2938&#8211;2942.</li> </ul>
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