Mineral data (SEM, electron microprobe, Raman spectroscopy) from epithermal hydrothermal alteration of the Miocene Sigri Petrified Forest and host pyroclastic rocks, Western Lesbos, Greece

Data available from a detailed mineralogical investigation of the Petrified Forest of Lesbos and its host pyroclastic rocks [1] are summarized and a link is provided to the full data at https://data.mendeley.com/datasets/dxwfd32zms/1. Samples were taken from petrified wood, fresh and devitrified tuffs, and from epithermal veins and epithermally altered tuffs. Backscattered electron (BSE) images were made by scanning electron microscope (SEM) from polished thin sections of 16 samples to show textural relationships between minerals. Minerals were identified by energy dispersive spectroscopy (EDS). Further chemical analysis by electron microprobe (EMP) were made of trace elements in the petrified wood and of Mn-oxide minerals. Polymorphs of silica were investigated by Raman spectroscopy. SEM X-Ray maps were made of selected sites with manganese oxide minerals. In this contribution, the general character of each analyzed sample is summarized and a brief inventory of available data is presented, with specific reference to features in the on-line data. The significance of these data for the origin of the petrification of the wood and the epithermal veining of the host pyroclastic rocks is provided in “Nature of the hydrothermal alteration of the Miocene Sigri Petrified Forest and host pyroclastic rocks, Lesbos, Greece” [1] https://doi.org/10.1016/j.jvolgeores.2018.11.018. The data will be of comparative value to those investigating petrification of wood, devitrification of tuffs, and epithermal Mn-Fe mineralization in other areas.

summarized and a brief inventory of available data is presented, with specific reference to features in the on-line data. The significance of these data for the origin of the petrification of the wood and the epithermal veining of the host pyroclastic rocks is provided in "Nature of the hydrothermal alteration of the Miocene Sigri Petrified Forest and host pyroclastic rocks, Lesbos, Greece" [1] https://doi.org/10.1016/j.jvolgeores.2018.11.018. The data will be of comparative value to those investigating petrification of wood, devitrification of tuffs, and epithermal Mn-Fe mineralization in other areas.  SEM-EDS has raw analytical totals and individual element oxides recalculated to stoichiometric mineral totals without volatiles (so raw data can be back-calculated). Backscattered electron images are raw data with annotations of mineral types. EMP-WDS analyses have raw oxide data. Raw Raman spectroscopy intensity plots are presented (standard method for mineral analyses).

Experimental factors
Polished thin section (carbon coated)

Experimental features
Polished thin sections were analyzed by SEM using backscattered electron images to locate minerals and energy dispersive spectroscopy to determine their composition; composition of selected minerals was determined using wavelength dispersive spectroscopy on an EMP. Selected minerals were also analyzed by Laser Raman Microspectroscopy.

Data source location
Sigri Pyroclastic Formation, western Lesbos, Greece, details in Value of the data These data on detailed mineralogy of petrified wood samples, devitrified tuffs, and regional epithermal veins are useful for comparison with other areas of similar epithermal mineralogy. Data on the silica polymorphs can be compared with other volcanic successions and petrified wood.
The data will be of value to others working on epithermal veins and petrified wood, particularly with abundant manganese and iron (hydr)oxides.
Manganese and iron (hydr)oxides and devitrified volcanic glass have quite variable chemical compositions. The presentation of a few representative analyses in the summary paper [1] does not fully represent the range of data. Further investigations elsewhere may reveal new meaning to our data variability.

Data
Mineral data are presented for 16 strategically selected samples. Each sample is briefly summarized below, highlighting the mineralogical significance of the sample. Figure references refer to Appendices 1-4 in https://data.mendeley.com/datasets/dxwfd32zms/1. The content of these appendices is as follows: Appendix 1: Scanning Electron Microscope (SEM)-Back Scattered Electron (BSE) images and Energy Dispersive Spectroscopy (EDS) mineral analyses; Appendix 2: Raman Spectroscopy; Appendix 3: SEM X-ray maps; Appendix 4: Electron Microprobe (EMP) mineral analyses. For convenience, we refer to fine-grained mixtures of amorphous silica and iron (hydr)oxides as FeSi mixture, and of amorphous silica and manganese (hydr)oxides as MnSi mixture. The terms "Fe-oxide" and "Mn-oxide" are used for mixtures of various oxide and hydroxide minerals, both in thin section where mineral crystallites can be seen, and in the field.

800a: Yellow zoned pipe from a fault zone
The sample is from a mineralized pipe in a fault zone that shows three concentric zones ( Fig. 1-1.1c). The outer zone is tuff with feldspar and quartz, the middle zone is FeSi ± MnSi mixtures, and the core of the sample consists of two types of silica: amorphous and microcrystalline (e.g. Fig. 1-1.3). Amorphous silica also forms veinlets ( Fig. 1-1.2). The FeSi mixture is very fine grained in the middle zone ( Fig. 1-1.1c), but more crystalline in the outer zone.

800b: brown fragments of vein from a fault zone
The sample consists of fragments of a complex vein in a fault zone cutting tuff ( Fig. 1-2.1). It is predominantly a fine grained FeSi mixture cut by silica veinlets (Fig. 1-2.2, 2.6). Small grains of magnetite and feldspar are present.
Kaolinite has elevated contents of one or more of the elements SiO 2 , FeO, MgO, and K 2 O probably depending on what it is replacing. Both biotite and K-feldspar alter to kaolinite (Fig. 1-10.15). Zoned amygdules (Fig. 1-10.7) consist of an outer rim of kaolinite and tiny particles of ilmenite (6,7) and an inner core made up mostly of kaolinite (1). The amygdule show fractures and probable dissolution. Secondary apatite and probably ilmenite veinlets postdate the kaolinite.

856-2 Petrified wood
The sample is petrified wood with layers of different colors (Fig. 1-15.1). Features are similar to 856-1. The observed colors are controlled by the amount of Fe present (red) and the relics of plant tissues (black) (Figs. 1-15.6, 28e32, 21).

Field collection of samples
In the field, rock samples were collected from the Jithra ignimbrite, the sediments underlying the ignimbrite, hydrothermal yellow veins cutting across pyroclastic rocks or tree horizons, and small samples from fossilized trees from the Sigri Petrified Forest (Table 1). Samples were selected based on the presence of alteration during field observation, in order to identify hydrothermal mineral assemblages present in these rocks by comparison to unaltered rocks.

Sample Preparation
Sample preparation was mostly carried out in the Department of Geology at Saint Mary's University. All field samples were first visually examined to determine significant areas appropriate for cutting. The samples were cut using water and oil-cooled diamond blade rock saws and were sent to Vancouver Petrographics Ltd to be made into polished thin sections.
The polished thin sections of samples analyzed, using the scanning electron microscope (SEM) and the Electron Microprobe (EMP), were carbon coated using a Leica EM CED-030 carbon coater. In contrast, the carbon coating was cleaned off polished thin sections analyzed using the Laser Raman spectroscopy microscope (LRM), by gently rubbing methanol soaked Kim Wipes on the surface of the polished thin sections.
Samples analyzed for X-ray diffraction were prepared at Bedford Institute of Oceanography. The rock samples were crushed separately by placing them inside an iron crusher and by using a hammer to apply force until the materials were completely pulverized. Glass slides were cut into one inch squares and labeled on the underside. A smear slide was produced by spreading each sample mixed with methanol thinly and evenly across the one inch glass slide. The slides were air-dried until the methanol completely evaporated.

Analytical techniques
The samples were analyzed using a variety of techniques: (1) Scanning electron microscope (SEM) for chemical analyses, mineral identification and for textural relationships; (2) Electron microprobe (EMP) for quantitative chemical analyses of minerals; (3) Laser Raman microspectroscopy (LRM) for crystal structure determination; and (4) X-ray diffraction (XRD) analysis for mineral identification. The various analytical techniques used to analyze each sample is summarized in Table 1. Mineral abbreviations used are after Whitney and Evans [2].

Electron microprobe (EMP)
A petrified wood sample and another sample containing "Mn-oxide" were analyzed at the Regional Electron Microprobe Centre at Dalhousie University, Halifax. Analysis was performed using a JEOL-8200 electron microprobe (EMP) which is equipped with a Noran 133 eV dispersive spectrometer and five wavelength spectrometers. This provides a more careful identification of the "Mn-oxide" minerals and a more accurate trace element identification of the petrified wood sample. Quartz and hematite standards were used for the wood; the same standards plus pyrolusite and chromite were used for the "Mn-oxide". The equipment was operated at 15 kV.

Scanning electron microscope (SEM)
SEM analysis was performed in the Regional Analytical Centre at Saint Mary's University using a TESCAN MIRA 3 LMU Variable Pressure Schottky Field Emission Scanning Microscope. The SEM is equipped with both a backscattered electron detector (BSE) and an INCA X-max 80 mm 2 silicon drift detector (SDD) energy dispersive spectrometer (EDS) which are able to provide qualitative elemental/ phase information and semi-quantitative elemental information respectively about the sample being analyzed. The EDS system has a detection limit of <0.1%. A pure cobalt plate was used as a standard to calibrate the beam for analysis. The beam has an average diameter of approximately <10 nm and has an X-ray production volume of~10 mm (depending on the minerals). The SEM has a maximum resolution of 1.2 nm at 30 kV. All analyses were acquired at 20 kV. Minerals were identified from EDS analyses using criteria summarized by Ref. [3] (their Table 2).
The Oxford Instruments INCA software is equipped with a QuantMap package which maps elements based on quantitative elemental compositional data. QuantMap data are presented as oxides.

Laser Raman microspectroscopy (LRM)
Laser Raman spectroscopy analyses for samples containing SiO 2 minerals were performed at the Regional Analytical Centre at Saint Mary's University using a Horiba Jobin-Yvon LabRam HR confocal instrument. The LRM uses a 100 mW 532 nm Nd-YAG diode laser from Toptica Phonotics and a Synapse charge-coupled device form Horiba Jobin-Yvon. The LRM uses a maximum of 100Â Olympus MPlanN objective for image capture and analysis. The analyses were done in order to determine the silica polymorphs that are present. Raman spectra from Kingman and Henley [4] (their figure 1) and Ilieva et al. [5] (their figure 5) were used for comparison.

X-ray diffraction analysis (XRD)
X-ray diffraction was carried out on random powder mounts prepared by crushing the sample and stirring it onto a glass slide with a little methanol, which was allowed to evaporate. Samples were run on a Siemens Kristaloflex diffractometer using Co Ka radiation. The air-dried samples were scanned from 2 to 52 2q, with a 0.2 step. Diffractograms were processed using EVA software. Minerals were identified from their characteristic d-spacing.