Data on unveiling the occurrence of transient, multi-contaminated mafic magmas inside a rhyolitic reservoir feeding an explosive eruption (Nisyros, Greece)

This data article includes the description and the geochemical and mineralogical dataset of 67 pyroclastic rock samples from the Upper Pumice (UP) explosive activity of Nisyros volcano (eastern South Aegean Active Volcanic Arc). A detailed field and petrographic description of the studied outcrops and samples are reported, including representative photomicrographs and SEM images, whole-rock major and trace elements compositions of 31 representative samples and Sr-Nd isotope ratios on 22 selected samples. Analytical methods and conditions used for data acquisition are also reported. The UP eruption produced a stratigraphic sequence constituted by a basal fallout deposit, gradually substituted by pyroclastic density current (PDC) deposits; these are overlaid by a lag-breccia unit, and the sequence is closed by a grey ash flow level. The juvenile is mainly constituted by white-yellow, moderately crystalline pumice with rhyolitic composition and homogenous Sr-Nd isotope values. Variable amounts of dense, grey, crystalline juvenile lapilli clasts (CRC, Crystal-Rich Clast), with rounded shape and less evolved composition (andesite to dacite) are also present in the deposit. These mafic CRCs are peculiar due to their large variability in textures (from distinctive diktytaxitic to strongly fragmented structure without a defined fabric) and in the geochemical and isotopic composition. The data acquired were fundamental to reconstruct the complex and peculiar history of ascent, storage and differentiation/assimilation processes of these mafic melts before their intrusion into the shallow, rhyolitic magma chamber, with important implication on the possible eruption trigger during the more recent explosive phase of activity at Nisyros volcano. Moreover, the geochemical and isotopic analyses provide new original data to the general knowledge of the Aegean volcanics. All the data reported in this paper are related to the research article Braschi et al. (2022)

white-yellow, moderately crystalline pumice with rhyolitic composition and homogenous Sr-Nd isotope values. Variable amounts of dense, grey, crystalline juvenile lapilli clasts (CRC, Crystal-Rich Clast), with rounded shape and less evolved composition (andesite to dacite) are also present in the deposit. These mafic CRCs are peculiar due to their large variability in textures (from distinctive diktytaxitic to strongly fragmented structure without a defined fabric) and in the geochemical and isotopic composition. The data acquired were fundamental to reconstruct the complex and peculiar history of ascent, storage and differentiation/assimilation processes of these mafic melts before their intrusion into the shallow, rhyolitic magma chamber, with important implication on the possible eruption trigger during the more recent explosive phase of activity at Nisyros volcano. Moreover, the geochemical and isotopic analyses provide new original data to the general knowledge of the Aegean volcanics. All the data reported in this paper are related to the research article Braschi

Value of the Data
• These data are crucial for the reconstruction of the plumbing system dynamic of Nisyros Volcano before the Upper Pumice eruption. • The data, including mineral chemistry and Sr-Nd isotope ratios, expand and integrate the existent database of volcanic products of the South Aegean Active Volcanic Arc. • Crystal-rich clasts (CRC) show the lowest 143Nd/144Nd values recorded for the Nisyros-Kos-Yali volcanic field. • The data will contribute to a better understanding of the involvement of different crustal components and ascent pathways of mafic magmas below active volcanoes in subduction zones.

Data and Images
Data, images and figures here reported were interpreted and discussed in Braschi et al. [1] to unravel the origin and evolution of the mafic components erupted by the UP activity, and their interaction with the main rhyolitic host magma. The full dataset of major, trace elements and Sr-Nd isotopes on whole rocks, together with glass composition and mineral chemistry is available in the EarthChem Library repository at https://doi.org/10.26022/IEDA/112230 . Table 1 is a list of the samples collected from the Upper Pumice (UP) deposit. The table reports detailed information of the sampling locations for the different outcrops (see also Fig. 1 ), including the type of depositional unit. A schematic petrographic description of each sample is also reported including their structure, paragenesis and texture features. Some samples have been subdivided into different portions according to their characteristics and labelled with different letters.

Field observation
Representative photos of the sampled outcrops of the UP deposit are shown in Figs. 2-8 and illustrate in detail the different depositional units emplaced by the UP eruption and their juvenile components.
Figs. 9-12 report selected representative images of cut blocks of samples and hand-specimen highlighting the difference between the two main lithotypes (pumice and crystal-rich clasts, hereafter CRCs) and within the CRC population itself. The CRCs show wide variation in their vesiculation and colour; the latter is due to the different proportion of crystal content (both for phases and size) and groundmass, varying from grey to white. Opx Samples with mingling features are doubled to describe crystal-rich portions and pumiceous portions; "-" thin secrtion not available CRC: Crystal-rich Clast; Plg: plagioclase; Cpx: clinopyroxene; Opx: orthopyroxene; Amph: amphibole; Ol: olivine; Ox: oxides; ph: phenocrysts (crystals > 0.5 mm); mph: microphenocrysts (crystals > 0.3 mm). Reaction rims: presence of olivines and/or opx with reaction rims to amphiboles; ph + mph in CRC (%): estimated abundance of crystals coarser than the average size of the microcrystalline groundmass in the CRCs.    Pumices are the prevalent juvenile components whereas CRCs constitute about 5% of the deposit. The lithic content is less than 2%, there are very small quantities of fine ash and loose crystals as matrix. This unit has been interpreted as pyroclastic fall deposit, emplaced from the column of a Plinian or sub-Plinian eruption [2 , 3 , 5] . The schematic map of the UP distribution and outcrops location is also reported (B).  Representative image of the second unit (Unit-B) at the contact with the overlying Unit-C, close to outcrop 7, along the main road to Cape Katsouni, in the north-east part of Nisyros. Unit-B is a succession of several layers of fully diluted pyroclastic current (according to [6] ) alternated with fallout levels. The flow levels are formed by a matrix of ash and loose crystals where sub-to well-rounded pumice lapilli are immersed, alternating with layers of coarser ash. Unit-C is a massive deposit of unconsolidated material, composed of coarse ash, fine lapilli and loose crystals, with well rounded, slightly vesiculated pumice and dispersed lithic clasts [2] , interpreted as a granular fluid-based current (according to [6] ).

Fig. 7.
Representative image of the principal outcrop of unit-D exposed on the main street south of Cape Katsouni (outcrop 8). Unit-D is a dense pyroclastic current, gradually interlayered toward the top with lithic-rich lenses. This unit is constituted by a breccia deposit composed of rounded pumices and abundant (up to 15%) dense juvenile clasts with crenulated or "bread crust" surfaces, up to few tens of centimetres in diameter, and angular lithic clasts within an unconsolidated ash matrix including. Lithics mainly consist of fresh and hydrothermalised lava clasts; fragments of hypoabyssal igneous rocks, skarn and limestone with hydrothermal alteration are also present [as also reported by 2, 3]. Unit-D is interpreted as a lag-breccia deposit [2] , emplaced from a dense PDC formed by the collapse of the eruptive column as a consequence of the caldera collapse [7] .  8. Image of the top unit of the UP sequence (Unit-E) composed by a massive or weakly laminated deposit formed by grey ash with loose crystals, rounded centimetre-sized pumice and lithic lava and limestone clasts (about 20%, [2] ). This unit have been interpreted as a deposit from diluted pyroclastic density currents [2] or due to a phreatomagmatic eruptive event [5] .   Table 1 for details.  Table 1 for details.
Pumices from all the deposits have similar features. They are porphyritic, mainly composed by a glassy matrix and a crystal content up to 5-10%; vesicularity vary between 30% and 50%; the matrix appears often fibrous or fluidal. Paragenesis is mainly composed by plagioclase (always more than 75%), orthopyroxene and amphibole; clinopyroxene and olivine are rare; accessory phases are oxides and apatite, often included in orthopyroxene. Crystals are often found as glomeroporphyritic aggregates. Plagioclase phenocrysts sometimes show disequilibrium textures, with sieved cores, resorbed zones, resorbed or overgrowth rims.
Crystal-rich clasts are highly heterogeneous in textures and were subdivided in three groups, named Type-A, Type-B and Type-C. The paragenesis is similar between the three groups: plagioclase represents more than 50% of the mineral assemblage, followed by amphibole and pyroxenes; olivine is rare; accessories phases are oxides and apatite. Amphibole can be found associated with plagioclase to form the microcrystalline groundmass network, either with acicular or tabular habitus, or as reaction rims on pyroxenes. Rare amphibole and pyroxene phenocrysts can be up to 2 mm, while plagioclase phenocrysts can reach 6 mm.
Type-A clasts are mostly found in fallout deposits and are characterised by microcrystalline texture with almost equigranular crystal size (0.1-0.5 mm), constituted by tabular plagioclases, amphiboles and pyroxenes (mainly orthopyroxene), with variable oxides content. Crystals are dispersed in a glassy, highly vesiculated groundmass, without a defined fabric.
Type-B clasts are the more variable in terms of crystal content and size; they show microcrystalline, inequigranular, low porphyritic texture, with variable crystal orientation defining at places a sort of network, likely the Type-C textures. They are present both in the fallout and lag-breccia deposits.
Type-C clasts are mostly found in the lag-breccia deposits, and they are characterized by a equigranular, low porphyritic textures with diktytaxitic voids, formed by a network of acicular plagioclases and amphiboles, with interstitial pyroxene. They show interstitial glass and variable vesicle abundance, generally lower than the other two types.

Geochemistry
The following table ( Table 2 ) reports the complete dataset of major and trace element data on 31 whole-rock samples of pumice and CRCs of the Upper Pumice activity. Selected incompatible trace elements and Rare Earth Elements (REE) together with Sr-Nd isotope ratios were also determined on a further selection of 22 samples.
The pumices are rhyolites (SiO 2 > 70 wt.%) belonging to the high-K calc-alkaline series, whereas the CRC show an affinity with the calc-alkaline series, ranging from basaltic andesite/andesite to dacite (SiO 2 between 56 and 64 wt.%). Loss on ignition (LOI) is always lower than 2% in the CRCs, while it is up to 5.3% in the pumices.
REE and incompatible element patterns ( Figs. 23 and 24 ) are typical for subduction-related calc-alkaline rocks. REE values are normalised to the chondrite data, while incompatible elements are normalised to the primordial mantle values [9] . Symbols used in the graphs are the same used in Braschi et al. [1] : purple symbols represent samples from the fallout deposit, the green ones are samples from the lag-breccia deposit and those from PDC units are black; open diamonds represent pumices, CRCs have different symbols for each texture typology (circles for Type-A, triangles for Type-B and squares for Type-C).   Table 2 Major and trace element composition and Sr-Nd isotopic ratios of the studied samples from the Upper Pumice deposit (Nisyros, Greece).          Cu  bdl  bdl  bdl  bdl  bdl  10  10  bdl  bdl  10  bdl  bdl  bdl  bdl  bdl  Zn  70  50  60  40  50  30  40  50  50  50  50  50  50  40  40  Ga  12  14  15  13  15  13  13  15  14  15  14  14  14  13  13  Rb  80  37  38  86  24  88  88  40  29  30  24  20  25  87  90  Sr  303  467  419  280  607  276  277  519  552  639  598  595  504  277   Major and trace elements data were performed at the Actalbs Laboratory (Ancaster-Ontario, Canada). Sr Isotope ratios were determined by TIMS Thermo-Finnigan Triton-Ti at the Radiogenic Isotope Laboratory of the Department of Earth Sciences, University of Florence. Nd isotope data were performed at the Radiogenic Isotope Laboratory of the IGG-CNR of Pisa by MC-ICPMS Thermo-Finnigan Neptune-Ti. * Major elements were analysed at the Department of Earth Sciences of the University Florence by XRF and trace elements were analysed at the Department of Earth Sciences of the University of Perugia by ICP-MS (see [8] for analytical details). Italic labels: trace elements analysed by XRF at the Department of Eearth Sciences of the University of Florence (see [6] for analytical details). La/Sm and Tb/Yb ratios are normalised to chondritic values. nd = not determined; bdl = below detection limit; 2se: 2 standard error of the mean.

Mineral chemistry
In situ investigation of crystal chemistry was also performed on 10 selected samples of pumices and CRCs to explore the minerals and glass compositional variability. The following tables ( Tables 3-7 ) report a representative selection of the mineral chemistry composition for plagioclases, pyroxenes, amphiboles and oxides, as well as the composition of glasses. Table 3 Representative major and minor element composition of glasses on selected samples from the Upper Pumice deposit (Nisyros, Greece).    The composition of glassy groundmasses were obtained with a Jeol JXA 8600 superprobe at the CNR-IGG in Florence. bdl = below detection limit Table 4 Representative plagioclase composition (wt.%) on selected samples from the Upper Pumice deposit (Nisyros, Greece). Plagioclase crystals analysed in the pumice samples are generally more albitic (ca. 30 % An) than those in CRCs ( > 60 % An).       Footnotes: gdm = crystal size < 0.5 mm. Ab = Albite, An = Anorthite; Or = Orthoclase.

Table 5
Representative pyroxene composition (wt.%) on selected samples from the Upper Pumice deposit (Nisyros, Greece). The pyroxenes are mostly orthopyroxenes; clinopyroxenes are less common and are generally found as microcrystals or in aggregates.

Experimental Design, Materials and Methods
The field work was carried out with a special care in sampling all of the different juveniles characterising each outcrop of the Upper Pumice deposits. A total of 67 samples ( Table 1 ) was collected during two field campaign in 2006 and 2014.
Pumices were sampled from each location with the aim to investigate the possible variability within the evolved juvenile component for a total of 16 samples ( Fig. 1 ). The CRCs were also sampled in detail, collecting 51 samples, according to their evident textural and physical variability (i.e., density, colour, crystal content) to explore their recurrence and distribution among the different outcrops.
During preparation all specimens were cut in order to remove altered portions, then grinded and powdered in an agate mill.
Major and trace elements were analysed by Actalabs Laboratories (Ontario, Canada) using the Lithogeochemistry-4Lithoresearch analytical package. The procedure consists in a lithium metaborate/tetraborate fusion digestion and analyses are carried out using ICP-OES for major elements and ICP-MS for trace elements (see www.actlabs.com ). Accuracy and precision for major elements are estimated as better than 3% for Si, Ti, Fe, Ca, and K, and 7% for Mg, Al, Mn, Na; for trace elements (above 10 ppm) they are better than 10%. REE, Rb, Sr, Y, Zr, Hf, Nb, Th, and U were analysed.
Selected powdered samples were processed in an ultraclean laboratory environment (class 10 0 0) at the Department of Earth Sciences of the University Florence. They were preliminarily treated with 2 mL diluted 1 N HCl in an ultrasonic bath for 15 , twice, then rinsed three times with Milli-Q water to minimise isotopic variation induced by supergene processes that could overprint the magmatic signature (e.g. [12] and references therein). After that they were processed using the standard digestion technique described in [13] consisting in a sequential addition of concentrated HF and HNO 3 (in proportion of 4:1) of suprapure quality, followed by a double addition of concentrated HNO 3 , and subsequently by some 10 mL of diluted 6 N HCl and placed on a hot plate at 140 °. Cation-exchange AGW and Ln-spec reusable resins were used for Sr, REE and Nd purification respectively, by sequential addition of properly diluted HCl suprapure acid, as described in Avanzinelli et al. [13] .
Sr isotope ratios were determined at the Department of Earth Sciences of the University Florence using the Thermal Ionization Thermo-Finnigan Triton-TI mass spectrometer (TIMS), equipped with nine collectors coupled with nine exchangeable amplifiers. For measurements with the thermal ionization mass spectrometer, 100-150 ng of sample were loaded on single Re-filament as nitrate form, with TaCl 5 and H 3 PO 4 as activator and to keep the signal stable during the analyses. 87 Sr/ 86 Sr were measured dynamically using the amplifier rotation method and corrected using an exponential mass fractionation law to 87 Sr/ 86 Sr = 0.1194. Each ratio is the average of 120 measurements, to reach good precision (2se) of the data. Within run, replicate measurements of international NIST SRM 987 standard (0.710251 ± 0.0 0 0 011 [13] ) gave mean values of 87 Sr/ 86 Sr = 0.710252 ± 0.0 0 0 011 (2sd, n = 5) well comparable with those reported in literature [14] . All errors reported are 2se (2 standard error of the mean) for single data precisions and 2sd (2 standard deviation) for standards reproducibility ( Table 8 ). The Sr analytical blank, measured during the course of the analytical session, is 60 pg, which is in agreement with blank reproducibility of the lab.
Nd isotope ratios were measured in the Laboratory of Radiogenic Isotopes at the IGG-CNR of Pisa using the new Thermo-Finnigan multicollector inductively coupled plasma mass spectrometer (MC-ICP-MS) Neptune-Plus, equipped with a combined cyclonic and Scott-type quartz spray chamber, Ni-cones, a MicroFlow PFA 100 μl/min self-aspiring nebuliser and a Teledyne Cetac ASX-560 Autosampler. All samples were diluted in ultrapure 2% HNO3 solution after digestion and elemental separation. During Nd analyses, instrumental mass fractionation was corrected using the 146 Nd/ 144 Nd ratio (0.7219). Mass interference correction was performed using the ratios 147 Sm/ 144 Sm (4.838710), and 147 Sm/ 148 Sm (1.327400). The analytical accuracy and reproducibility for the within run internal standard NdFi is 0.511460 ±14 (2sd, n = 4), well comparable to the Table 8 Accuracy and reproducibility of Sr-Nd isotopes measurements on international and internal reference standards.

Method
Multi-dynamic collection mode Isotope Sr ISOTOPE Within run standard 87Sr/86Sr 2se SRM987 0 average value reported in [13] measured by TIMS. Long-term external reproducibility of the laboratory for 143 Nd/ 144 Nd on international reference material J-Ndi-1 was 0.512098 ± 5 (average of 17 replicates), which match well the reference values of [15] , ( Table 8 ).
A number of 10 samples were selected, on the basis of their textural and compositional representativeness, among the different juvenile types (pumices and CRCs) for mineral chemistry investigations on minerals and glasses. Analyses were performed by electron microprobe JEOL Superprobe JXA-8600 at the IGG-CNR of Florence. Working conditions were 15 kV of accelerating voltage and 10 μA of beam current. Beam diameter varied from 2 to 5 μm for mineral phases and 10 for glasses. Peak counting time was 15 sec for major elements, except for Na that is counted for 10 sec to minimize the alkali loss effect, and 40 sec for minor elements. Backgrounds were counted at specific positions for 5 and 20 sec on major and minor elements, respectively.
Scanning Electron Microprobe (SEM) images were achieved at the MEMA laboratory of the University of Florence using 20 kV of acceleration voltage and 2 nA of probe current.