International Journal of Earth Sciences

https://doi.org/10.1007/s00531-022-02219-9

ORIGINAL PAPER

Dating of polyhalite: a difficult 40Ar/39Ar dating tool of diagenetic

to very low‑grade metamorphic processes
C. Leitner1   · F. Neubauer1 · J. Genser1 · M. Bernroider1

Received: 20 December 2021 / Accepted: 7 June 2022

© The Author(s) 2022

Abstract
Halite already deforms at surface temperatures. A valuable universal dating tool to constrain the timing of sedimentary, dia-
genetic, or deformational structures is still missing. The evaporite mineral polyhalite can be dated by the 40Ar/39Ar method.
On the example of the extremely deformed halite deposits of the Eastern Alps, polyhalite was tested to date early diagenetic
stages of the deposits. The sedimentological investigation of the present study indicates that some of the macrostructures
of polyhalite had a syn-depositional origin during the late Permian. It is supposed that polyhalite originated during reflux
of brines. All samples selected for age dating represent characteristic microfabric types of euhedral to subhedral polyhalite
crystals. Intact macro- and non-recrystallized looking microstructures of polyhalite can be expected to give plateau ages.
However, nearly all measurements produced overdispersed data that do not define an age. The oldest age steps thus represent
only minimum ages. A closer look revealed grain boundary migration, subgrain rotation recrystallization, twinning, and
fluid-supported grain size increase. These recovery processes obscured the original ages and/or reflect the origin of new
polyhalite in place of the original individuals. Based on these microstructures, the age data are supposed to reflect the circu-
lation of aqueous fluids. Just extremely careful separation of individual crystals or in situ age dating under the microscope
will be successful in dating polyhalite. Nevertheless, polyhalite can potentially serve to date deformational events of halite
deposits due to its easy recrystallization property.

Keywords  Polyhalite · Ar–Ar age dating · Microfabrics · Fluids · Haselgebirge Formation

Introduction diagenetic conditions, which makes this mineral interesting

as a potential geochronometer for sedimentary, diagenetic or
Major evaporite successions are generally mobilized dur- low-temperature tectonic processes.
ing post-depositional processes and form complex assem- Synthetic polyhalite becomes unstable by dehydration
blages in compressional and extensional structures, e.g., between 250 and 350 °C under laboratory conditions (Freyer
in diapirs, raft tectonics in epicontinental settings, or col- and Voigt 2003; Wollmann et al. 2008; Wollmann 2010). A
lisional mountain belts (Warren 2016). These structures are natural polyhalite from the Permian Salado Formation, USA,
often of high economic importance, e.g., for salt mining, started to dehydrate already at 507°K = 233 °C. Using in situ
hydrocarbon deposits and repository for nuclear wastes. synchrotron X-ray diffraction, the polyhalite decomposed
However, there is still no tool available to allow dating into vapor, anhydrite and two langbeinite-type phases with
of the structural evolution, besides stratigraphic relation- different Ca/Mg ratios (Guo and Xu 2017; Xu et al. 2017).
ships to country rocks. Potassium containing minerals are The closure temperature has been attempted to determine
commonly used to date geological events by the 40Ar/39Ar only recently in the year 2021. It was assessed by regressing
method. Polyhalite ­[K2Ca2Mg(SO4)4·2H2O] is a K-bearing five steps of degassing between ca. 400 and 600 °C. This
mineral of evaporite deposits. It forms under sedimentary or temperature range is however far beyond the dehydration
temperature of polyhalite (ca. 230 to 350 °C). After dehydra-
* C. Leitner tion, likely melting of langbeinite solid solutions occurred at
christoph.leitner@plus.ac.at 1143.7 K (≈ 870 °C) (Xu et al. 2017). Therefore, not the clo-
sure temperature of polyhalite was determined. The closure
1
Department of Geography and Geology, University temperature of the langbeinite-type phases was calculated
of Salzburg, Hellbrunner Strasse 34, 5020 Salzburg, Austria

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 International Journal of Earth Sciences

between 250 and 280 °C for a cooling rate of 10 °C/Ma 2005) (Fig. 1). The sedimentological investigation of the
(Richards et al. 2021). late Permian Haselgebirge indicates that some of the mac-
Polyhalite age dating by Rb/Sr and K/Ar methods was rostructures of polyhalite have a syn-depositional origin.
tried out in the past, but most ages were difficult to inter- Such structures have never been described before but put
pret within their geological frame (Pilot and Blank 1967; the Alpine deposits in a row with other polyhalite deposits
Brookins et al. 1980; Brookins 1986; Halas et al. 1996; Wój- worldwide (Warren 2018 and references therein).
towicz et al. 2003). Onstott et al. (1995a) used 40Ar/39Ar In this study, only polyhalite of intact syn-sedimentary
dating of polyhalite, however, only total gas ages were men- macrostructures and non-recrystallized looking microfab-
tioned in this abstract, and no full data table was attached. rics of polyhalite were selected and the results of this age
Analytically advanced dating, including 40Ar/39Ar step dating are presented in this paper. However, it turned out
heating, has been performed only on the chemically related that what looked undisturbed at a cursory glance is, in fact,
mineral langbeinite [­ K2Mg2(SO4)3]. Some samples yielded no longer pristine when examined in sufficient detail. The
plateau ages, and discordant age spectra were interpreted as nomenclature of representative polyhalite fabrics was further
mixtures of two or more generations (Lippolt and Oesterle improved relative to Leitner et al. (2014). The samples were
1977; Lippolt et al. 1993; Léost et al. 2001; Renne et al. measured multiply to test the reproducibility of age dating
2001). Recently, alunite ­[KAl3(SO4)2(OH)6] was success- and the robustness of the data on polyhalite. The new data
fully dated (Ren and Vasconcelos 2019). Both sulfates were should be seen in complement to earlier results presented
stable to temperatures high enough to determine their clo- by the authors.
sure temperature. Some polyhalite from the Eastern Alps
was already dated by the authors, however with similar dif-
ficulties to interpret as in the other regions (Leitner et al. Geological setting
2013, 2014, 2020).
All measurements of this study were conducted on sam- The Northern Calcareous Alps represent a cover fold-and-
ples from the evaporitic Alpine Haselgebirge mélange of thrust belt (Fig. 1) with uppermost Pennsylvanian to Eocene
the Northern Calcareous Alps, which is the type region sedimentary successions. In their central and eastern sec-
of polyhalite (Stromeyer 1818; Schlatti et al. 1970; Bindi tors, evaporites of the Haselgebirge Formation were dated

Fig. 1  Tectonic map of the

Northern Calcareous Alps
as part of the Eastern Alps,
location of the Bad Dürrnberg-
Berchtesgaden and Altaussee
salt deposits

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International Journal of Earth Sciences

to late Permian (~  250 Ma, Piller et al. 2004) by paleonto- revealed the composition in oxide weight percent. The cal-
logical and geochemical methods (Klaus 1965; Spötl and culation method after Love/Scott1 revealed the formula units
Pak 1996). The evaporites were deposited in an aborted of polyhalite.
rift setting (Spötl 1989). Stratigraphically younger, up to
40
3.5 km of shallow water carbonates were deposited on the Ar/39Ar technique
passive margin facing towards the Neotethyan ocean (Mandl
2000). During convergent tectonics, the stratigraphic level Polyhalite samples were manually reduced to small pieces
of these evaporites acted as a décollement level. Evaporite- with a hammer. They were washed with distilled water and
decorated thrusts are known in all structural levels of the dried with isopropanol to free them from dust and Cl-ions
Northern Calcareous Alps since a long time (e.g., Weber of halite. Chlorine produces Ar isotopes during irradiation,
1958, 1997a; Schauberger et al. 1976; Tollmann 1976; Wes- which may tamper the proportion of Ar isotopes from poly-
sely 2006; Lobitzer 2011; Mandl et al. 2012; Granado et al. halite. Grains of 200–250 µm size were selected under the
2019; Fernandez et al. 2020). The thrusting and propaga- microscope. Enough grains of each sample were packed into
tion direction of thrusting was from east/southeast to west/ aluminum-foil and put into quartz vials.
northwest (Linzer et al. 1995; Mandl 2000; Schorn et al. Details of the analytical 40Ar/39Ar technique is described
2013a). The main thrusting event between late Jurassic and in Leitner et al. (2014) and Cao et al. (2017). Irradiation
early Cretaceous was associated with high-grade diagenetic was conducted for 16 h in the Magyar Tudományos Aka-
conditions (~ 200 °C) in the southern and central parts of démia (MTA) Központi Fizakai Kutato Intézet (KFKI)
the Northern Calcareous Alps (Bojar et al. 2016 and refer- reactor (Debrecen, Hungary). No Cd shielding was applied.
ences therein). Finally, the NCA fold-and-thrust belt was Flux-monitors were placed between the samples for cal-
transported over the stable European lithosphere during late culation of the J-values. The distance between adjacent
Eocene and superimposed by Oligocene to Miocene strike- flux-monitors was c. 5  mm. Corrections for interfering
slip faulting. isotopes were the same as described earlier: Correction
factors were calculated from 45 analyses of co-irradiated
Ca-glass samples and 70 analyses of K-glass samples, and
Materials and methods are: 36Ar/ 37Ar (Ca) = 0.000225, 37Ar/ 39Ar (Ca) = 0.000614,
38
Ar/39Ar(K) = 0.0117, and 40Ar/39Ar(K) = 0.0266. Varia-
Locations and material tion in the flux of neutrons were monitored with the DRA1
sanidine monitor for which a 40Ar/ 39Ar plateau age of
Samples were taken from two salt bodies. The salt body 25.26 ± 0.05 Ma has been reported (van Hinsbergen et al.
of Altaussee (UTM 33 T 405316 5278325) has a vertical 2008).
40
thickness of > 800 m. The salt body of Bad Dürrnberg-Ber- Ar/39Ar analyses were carried out at the Department of
chtesgaden (UTM 33 T 351091 5278007) is at least 1000 m Geography and Geology at the University of Salzburg. The
thick and crops out c. 60 km west of the other deposit. The equipment used was the same as described earlier: 40Ar/39Ar
dominant rock type in both salt bodies is a protocataclasite analyses are carried out using a ultrahigh vacuum Ar-extrac-
of halite and mudstone (Leitner et al. 2011). Samples were tion line equipped with a combined MERCHANTEK™ UV/
selected from the Altaussee (ALT), Bad Dürrnberg (DÜ) and IR laser system, and a VG-ISOTECH™ VG-3600 noble
Berchtesgaden (BGD) salt mines (Fig. 1). gas mass spectrometer. Stepwise heating analyses of sam-
ples are performed using a defocused (~ 1.5 mm diameter)
Electron microprobe analysis 25 W ­CO2-IR laser operating in T ­ em00 mode at wavelengths
between 10.57 and 10.63 µm. The laser is controlled from a
Measurements were performed on a JEOL electron micro- PC, and the position of the laser on the sample is monitored
probe (JXA-8600), equipped with a wave-length dispersive on the computer screen via a video camera in the optical
system. An acceleration voltage of 15 kV and a low sample axis of the laser beam through a double-vacuum window
current of 20 nA were applied to prevent decomposition of on the sample chamber. Gas clean-up is performed using
polyhalite under the electron beam. Additionally, the spot one hot and one cold Zr-Al SAES™ getter. Gas admittance
was defocused to a diameter of 15 µm and after measurement and pumping of the mass spectrometer and the Ar-extraction
of sulfur, the sample was moved one beam diameter to start line are computer controlled using pneumatic valves. The
measurement of potassium and calcium. Sulfur, potassium VG-3600 is an 18 cm radius 60° extended geometry sector
and calcium were all measured with the same analyzing field mass analyzer instrument, equipped with a bright Nier-
crystal. Synthetic and natural mineral standards were used type source operated at 4.5 kV. Measurements are performed
to analyze the emitted wave lengths of the sample and to on an axial electron multiplier in static mode, peak-jumping
quantify their amount. Standard ZAF correction calculation and stability of the magnet is controlled by a Hall-probe.

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For each increment the intensities of 36Ar, 37Ar, 38Ar, 39Ar, Samples used for age dating
and 40Ar are measured, the baseline readings on mass 35.5
are automatically subtracted. Intensities of the peaks are Type 1, polyhalite replacing halite
back-extrapolated over 16 measured intensities to the time
of gas admittance either by a straight line or a monotonically Polyhalite partly replaced halite of a displacive halite crystal
decreasing exponential, depending on intensity and type of in mudstone in sample ALT-28 (Fig. 2g). The hoppered hal-
pattern of the evolving gas. ite crystal displays an edge length of c. 1.2 cm. Substitution
Inspection of intensities was applied regarding back- was from outside towards the center by forming a polyhal-
ground, system blanks, interfering isotopes, and post-irradia- ite margin. The polyhalite fibers are oriented normal to the
tion decay of 37Ar. Calculations of isotope ratios, errors, ages surface of the crystal. The average grain size of polyhalite
and plateau ages followed suggestions of McDougall and is 1–2 mm, however single fibers reach 4–5 mm in length.
Harrison (1999), Scaillet (2000), Steiger and Jäger (1977),
Ludwig (2012), and Schaen et al. (2020). Type 2, polyhalite veins in mudstone

Polyhalite veins in mudstone are typically arranged subpar-

allel to the sedimentary layering. Location ALT-87 shows
Results
polyhalite types 1 and 2 next to each other: Polyhalite sub-
stituted displacive halite crystals, but additionally, veins of
Polyhalite types—macrostructures and microfabrics
polyhalite developed above and below this layer (Fig. 3a).
Sample ALT-53D displays several connected polyhalite
Polyhalite occurs in a wide variety of structures. In the fol-
veins. Most of them are oriented subparallel to the layering
lowing, polyhalite macrotypes are distinguished, which were
of the mudstone. The vertically oriented fibers are several
already recognized in Leitner et al. (2013). Thereby, the
millimeters in length (Fig. 3b). Sample BGD-37D was taken
nomenclature of polyhalite fabrics is based on four differ-
from a fibrous polyhalite vein from the Berchtesgaden mine.
ent macrofabric types. In the present paper, the further dif-
The vein in mudstone is c. 3 cm in thickness. The polyhalite
ferentiation of types is based on a subdivision of the fourth
fibers are up to 2.5 cm large (Fig. 3c). Sample ALT-42A is a
type, bedded polyhalite. The microfabric subdivision uses
polyhalite vein of around 2 cm in thickness. It shows at least
crystal shapes, i.e., (1) blocky, (2) large lath-shaped and (3)
three stages of antitaxial fibrous polyhalite growth. Mud-
tiny lath-shaped.
stone particles exist in the median line from the initial crack.
All polyhalites tested by electron microprobe were
In the inner zone, the fibrous grains are 2–3 mm in size. The
chemically pure. The content of subordinate cations was
outer parts show fan-shaped fibers, up to 7 mm in length.
low and similar, with oxide percentages below 0.3 percent
The growth direction was from inside towards the wall rock.
(Mn < Na < Sr < Fe) (Supplementary Information 1).
The fibers show undulatory extinction, a serrated habit, and
grain boundary migration. Small anhydrites (0.2 mm) are
Halite‑anhydrite‑polyhalite nodules distributed randomly over the fibers. They include polyhalite
and are themselves partly corroded (Fig. 3d).
Some enterolithic folded layers and nodules in mudstone
consist of halite, anhydrite and polyhalite (Fig. 2a–b). Halite Type 3, polyhalite porphyroblasts in anhydrite
is always present in the center of such nodules (Fig. 2c–d).
Large anhydrite crystals of 0.5–2 cm are arranged along the Porphyroblastic polyhalite grains grew within an anhydrite
rim of the nodule. The anhydrite crystals can be prismatic matrix. The polyhalite grains of sample ALT-11 are up to
or lath-shaped (Fig. 2e–f). Polyhalite is located between the 8–10 mm in diameter. The central part is dark red, whereas
large anhydrite crystals and halite. Polyhalite forms seams of the rim is bright red in color. Core and rim are part of the
fibers around the anhydrite crystals. Larger polyhalite some- same crystal under the microscope. Polyhalite displays a
times also forms euhedral lenticular crystals in contact to patchy structure from undulatory extinction (Fig. 4a–b).
halite. Nodules of this type were found in the Altaussee, Bad The surrounding groundmass consists of anhydrite with an
Dürrnberg, and Berchtesgaden mines (Fig. 2a–f). average size of 0.1–0.2 mm. The equigranular, polygonal
The nodules display the typical form of sabkha nodules, fabric shows a slightly preferred orientation of anhydrite.
which form by evaporation of a mudflat. It is supposed The polyhalite grains contain numerous anhydrite inclu-
that sulfate was replaced by halite first (see “Discussion”). sions. The bright red margins relate to an increased Fe con-
Anhydrite and polyhalite substituted halite within the nodule tent (Fig. 4c).
shape. Anhydrite and polyhalite replaced halite in the same Sample ALT-18 consists of anhydrite with polyhalite
relative age relation as in displacive halite crystals (Fig. 2g). porphyroblasts (Fig. 4d). Under the microscope polyhalite

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Fig. 2  Halite-anhydrite-polyhalite nodules and polyhalite type 1 (pol- replaced by halite first (!), and later by anhydrite and polyhalite. g
yhalite substituting halite). a–c Enterolithic folds and nodular struc- Halite in displacive halite crystals was replaced by polyhalite. Images
tures. d–f Some of the nodules consist of halite + anhydrite + poly- from Leitner et al. (2013) are marked with a black dot. Abbreviations:
halite. Chart gives relative ages of minerals within nodules: Sulfate Hl halite, An anhydrite, Po polyhalite, effl. efflorescent halite
nodules most likely formed in a sabkha environment. Sulfate was

prisms are locally circumfluent by elongate anhydrite crys- red polyhalite. The large halite patch is interpreted to be
tals. The polyhalite prisms are blocky to elongate. The a former displacive halite crystal, which persisted during
internal structure of the polyhalite porphyroblasts looks the transformation of anhydrite into polyhalite. Euhedral
decomposed but is of similar birefringence color. These polyhalite crystals are located at the contact to halite.
structures are interpreted as subgrain rotation recrystal- Tiny-grained patches of probably decomposed earlier large
lization (Fig. 4e). A halite patch is surrounded by light polyhalite crystals surround them (Fig. 4f).

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Fig. 3  Polyhalite type 2 (polyhalite veins). a–c Antitaxial polyhalite latory extinction. Images from Leitner et al. (2014) are marked with a
veins subparallel to the layering in mudstone. d Fan-shaped fibers black dot. Crossed polarizers. Abbreviation: Po polyhalite
normal to layering, several growth stages. Serrated fibers with undu-

Fig. 4  Polyhalite type 3 (polyhalite porphyroblasts in anhydrite). a–c increase towards halite. Images from Leitner et  al. (2014, 2020) are
Patchy structure from undulatory extinction. The iron content varies marked with a black dot. Crossed polarizers. Dashed line indicates
significantly from core to rim. d–e. Decomposed looking porphyro- approximate position. Abbreviations: An anhydrite, Hl halite, Po pol-
blasts, interpreted as subgrain rotation recrystallization. f Grain size yhalite

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Type 4, bedded polyhalite (1) Microfabrics of blocky polyhalite: Sample DÜ-3B

was taken from a bedded polyhalite. The color of the
The most common type of polyhalite is bedded polyhalite. polyhalite was unusually reddish gray. Only under the
Bands of varying red and gray massive polyhalite mark microscope, a blocky shape and a grain size of 0.1–
the sedimentary layering or foliated layering, respectively. 0.5 mm was recognized. Towards mudstone the grain
A basis to separate the many different microfabrics under size becomes smaller. The grains exhibit the common
the microscope is the shape and grain size of the polyhal- symmetric polyhalite twinning, but multiple twinning
ite crystals: (1) blocky-shaped polyhalite crystals, (2) large and even chessboard-like twinning structures are pre-
lath-shaped polyhalite crystals, and (3) tiny lath-shaped sent, too. The grains show a slightly shape preferred
polyhalite crystals. The large lath-shaped polyhalite crys- orientation parallel to the sedimentary layering. Large
tals are usually oriented subparallel to the bedding. The anhydrite crystals of several centimeters in size grew
tiny lath-shaped polyhalite crystals sometimes form a felted over the polyhalite and included the polyhalite grains
microstructure (Fig. 5). Microfabrics of bedded polyhalite in a poikilotopic manner (Fig. 5a–c).
can show a combination of these polyhalite crystal types and (2) Microfabrics of large, lath-shaped polyhalite: The mac-
show additional structures. Deformational structures com- roscale sample ALT-81O represents a boudin neck,
prise shear zones, broken large anhydrite, strain shadows, which formed during foliation of competent polyhalite
or the alignment of grains. For more detailed information layers. The blurry red and black bands show thinning
about deformational structures see Schorn et al. (2013b) and and convergence at one end of the sample. Interest-
Leitner et al. (2014, Fig. 3). ingly, the rock looks widely intact under the micro-

Fig. 5  Polyhalite type 4 (bedded polyhalite). a–c Blocky polyhal- halite. g–h Schlieren on macroscale, foliation, and domains of differ-
ite with twins, locally overgrown in a poikilotopic manner by large ent grain size under the microscope. i Nodular structures outlined by
anhydrite crystals. d–f Macroscopically foliated polyhalite, which mudstone; felted, but non-aligned tiny lath-shaped polyhalite. Abbre-
displays large lath-shaped crystals under the microscope, interpreted viations: An anhydrite, Po polyhalite
as recrystallization and grain size increase. g–i Tiny lath-shaped poly-

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scope (Fig. 5d). The laths are oriented subparallel to decay of 37Ar was applied (09s0098, 09s0099, 09s0698,
each other and subparallel to the layering. The laths 09s0663, 09s0699, 09s0716, 15s0693) (Fig. 6, Supplemental
display twinning parallel to the long axis of the crys- Information 2). The 37Ar/39Ar (Ca/K) ratio of all measured
tal. The grain size ranges from 1 to 50 µm. Sample steps was plotted against age and indicates some variation in
ALT-81O was dated with the Ar–Ar method, display- the chemical composition of the polyhalite grains (see “Dis-
ing only age steps between 140 and 190 Ma that do not cussion”). For the other measurements, the time between
define an age (Leitner et al. 2020). Sample ALT-60B irradiation and measurement was more than ten times the
displays a coarse grain size with laths up to 1 mm in half-life of 37Ar and therefore the 37Ar/39Ar (Ca/K) ratio was
size (Fig. 5e). Nevertheless, also small grain sizes of much lower than the typical value, between 0.10 and 0.01
100 µm are present in the upper left corner. Twinning is (14s0398, 07s0560, 14s0433, 14s0976, 14s0977, 14s0432,
parallel to the long axis of the grains. The grains show 14s0705, 07s0562, 14s0431 and 14s0430). Because 37Ar
a preferred E-W orientation on the photograph. The was nearly the same as background, corrections for Ca-
clay particles were squeezed by the growing polyhalite. derived 36Ar and 39Ar could not be applied. The error on the
(Fig. 5f, arrow). age based on the post-irradiation decay of 37Ar will be not
(3) Microfabrics of small to tiny lath-shaped polyhalite: more than 0.6 Ma and, therefore, lies within the error of all
Sample ALT-37B consists of the typical massive red presented ages. 38Ar/39Ar-ratios like that of K-rich silicate
shining polyhalite (Fig. 5g). It shows macroscopically glass (= 0.0117) indicated that no significant portions of Cl-
schlieren, which contain some mudstone and blurry ions were in the measured sample.
halite. Parallel layers of different grain size between Sample ALT-28 (Fig. 6a), type 1, from the Altaussee
0.05 and 0.1 mm are visible under the microscope. mine yielded a plateau age of 235 ± 2  Ma for measure-
The crystals possess a slightly shape preferred orien- ment 09s0098 (98.3 percent 39Ar released; Leitner et al.
tation and display lobate grain boundaries. In certain 2013), and measurement 09s0099 yielded a plateau age
domains, coarser polyhalite of 0.1–0.3 mm size exhibits of 236 ± 2 Ma (71.8 percent 39Ar released; Leitner et al.
no preferred orientation, but straight grain boundaries. 2013). Measurement 14s0398 yielded an age of around
Black lines of mudstone are not aligned straight to the 233 Ma for the last ca. 60 percent 39Ar released. However,
orientation of the foliation layers (Fig. 5h). Sample because of an MSWD of 3.9, this age was not considered a
ALT-70 displays a similar tiny grain size. However plateau age. The age is interpreted to represent a minimum
different, the microstructure is felted, and not aligned. age (“ ≥ 233 Ma”). Disturbed age patterns are considered
A nodular structure is recognizable by arrangement of to relate to overprint (see “Discussion”), and therefore, all
the clay particles. The nodular structure suggests a sub- oldest age steps are interpreted to represent only minimum
stitution of former sulfate nodules (Fig. 5i). ages. This is indicated by the sign “ ≥ ” on the plots in Fig. 6
for all samples discussed in the following.
40
Ar/39Ar age dating Three samples of the vein type 2 were measured. Only
three steps were measured for sample ALT-42A (Fig. 6b)
Fragments of individual crystals of the 0.20–0.25 mm frac- from a single grain during measurement 07s0560. Ninety
tion were separated from samples ALT-28, ALT-42A, ALT- percent of 39Ar were released in one step with an age of
53D, BDG-37D, ALT-11 and DÜ-3B under the binocular. 232 ± 2 Ma. The Ca/K ratio as seen in 37Ar/39Ar-ratios of
However, no confirmation was possible that the single grains the core is higher than the typical value of 0.49, which
were really individual crystals. The single grain of ALT-37B possibly relates to occasional small anhydrite crystals
was a polycrystalline aggregate due to the tiny grain size as observed in thin section. Five grains were measured
of this sample. During multiple grain measurements, 4–7 in nine steps in experiment 14s0433. They gave a heav-
grains were measured together. All reported errors in the ily disturbed age pattern. The oldest age of 221 ± 1 Ma
text correspond to the 2σ-level (95.5% confidence level). was measured, when 31.5% of 39Ar were released. Simi-
Full results are given in Supplementary Information 2 and lar disturbed age patters were measured for ALT-53D
are graphically shown in Fig. 6. The patterns of three already (Fig. 6c). Measurement 09s0698 revealed an oldest age
published measurements in Leitner et al. (2013, 2014) are step of 224 ± 2  Ma (five grains; Leitner et  al. 2014),
shown for comparison. measurement 14s0976 an oldest age step of 215 ± 1 Ma
Degassing of argon occurred within a narrow range of (five grains), and measurement 14s0977 an oldest age
laser power. Due to the unknown Ar retention behavior step of 219 ± 1 Ma (single grain). The youngest ages were
uneven degassing occurred, and between 3 and 13 steps measured during each of the three measurements, when
were measured. When sufficient steps were measured, the between 30 and 50% of 39Ar were released. Measurement
37
Ar/39Ar (Ca/K) = 0.49 ratio was relatively constant for 09s0663 of seven grains of sample BGB-37D (Fig. 6d)
those samples, where correction for the post-irradiation from the Berchtesgaden mine yielded a slightly disturbed

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Fig. 6  Age dating results of 40Ar/39Ar step-heating experiments. ages, see discussion about the interpretation of ages. For samples
Measurements taken from earlier publications are marked with a with correction of post-irradiation decay of 37Ar, the 37Ar/39Ar ratio is
black dot: ALT-28, measurements 09s0098 and 09s0099 (Leitner plotted. Abbreviations:  PA plateau age (Ludwig 2012), MSWD mean
et  al. 2013), ALT-53D, measurement 09s0698 (Leitner et  al. 2014). square of weighted deviates
Laser-power increase from left to right. All oldest ages are minimum

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 International Journal of Earth Sciences

argon release pattern with a maximum age of 235 ± 1 Ma Primary macroscale structures
(46% released). An aliquot under laboratory number
14s0432 showed a heavily disturbed age pattern, when Some macrostructures indicate a syn-sedimentary origin of
four grains were measured. Thereby, the last step gave the Alpine polyhalite. Enterolithic folds and nodules in mud-
an oldest age step of 232 ± 1 Ma (9.3% of 39Ar released). stone consist of halite, anhydrite and polyhalite. Enterolithic
Fragments of the core and the rim of sample ALT-11 folds and nodules are usually typical shapes of gypsum and
(Fig. 6e), type 3, from the Altaussee mine were meas- anhydrite, which crystallize in a sabkha environment. The
ured separately. The core yielded a somewhat disturbed sulfate nodules were pseudomorphically replaced by halite.
argon release pattern. The Ca/K ratio is elevated, which Halite replacing sulfate has been described as a substitution
probably relates to the numerous inclusions of anhydrite. during the reflux of brine through deposited evaporites (e.g.,
The pieces from the rim also yielded a disturbed release Lowenstein 1988; Hovorka 1992). In a second step, the large
pattern. However, the ages of core and rim overlap and anhydrite crystals and polyhalite replaced the halite in the
should thus be seen as of the same age within uncertainty. nodules. Also, during this second step, brines of different
The oldest age of the core was a step at 229 ± 1 Ma. chemistry migrated through the deposit and changed the
Bedded polyhalite, type 4, was measured in two sam- mineral composition (Fig. 2).
ples. The coarse-grained sample DÜ-3B (Fig. 6f), from Polyhalite is commonly known to form from reflux brines.
the Bad Dürrnberg mine yielded a somewhat disturbed Polyhalite was observed to form after gypsum in recent
Ar release pattern during measurement 14s0705 of five deposits in Baja California (Holser 1966; Pierre 1983). Poly-
grains. The first age step (12.3% of 39Ar released) and the halite formed diagenetically early in the Messinian Mediter-
last one (12.7% of 39Ar released) gave the same oldest age ranean salt deposit of Sicily (Lugli 1999) and in the Bad-
of 236 ± 1 Ma. Measurement 15s0693 of a single grain enian Carpathian salt deposits (Hryniv et al. 2007; Warren
showed a similar release pattern. The first step (25.3% of 2018). Polyhalite from Polish Permian Zechstein deposits
39
Ar released) and the last significant step (14.9% of 39Ar was explained by reflux brines (Peryt et al. 1998), and so was
released) gave an oldest age of 232 ± 1 Ma. also the origin of polyhalite from the British Permian Zech-
In contrast, the fine-grained polyhalite sample ALT- stein deposit, which contains the world´s largest occurrence
37B (Fig.  6g) gave much younger ages. The measure- of polyhalite (Kemp et al. 2016). The most detailed descrip-
ment 07s0562 of a single grain yielded a staircase pattern. tion of polyhalite origin was given for the Permian Salado
After an initial age step of 44 ± 19 Ma, three large steps Formation in Texas, USA. Typical depositional cycles start
dominate, with 113 ± 2 Ma as the last significant step. with marine sulfates, are followed by halite, and finally end
As sample ALT-37B was proved to be pure polyhalite by by continental muddy halite. All three parts of this typical
EDX, the error on the age will be not more than 0.6 Ma. succession contain polyhalite. A polyhalitization during each
The aliquots of the sample resulted in heavily disturbed cycle was interpreted (Lowenstein, 1988). There are strong
age patterns in measurements 14s0431 (six grains) and similarities of the Permian Haselgebirge Formation to the
14s0430 (single grain). Both measurements started with Permian Salado Formation. Both contain assemblages of
an age step like the last age step. At 20–40% of all 39Ar mudstone, displacive halite and polyhalite. The substitution
released, the youngest ages of roughly around ~ 100 Ma of halite by polyhalite in the Haselgebirge salt, for instance
were measured. in displacive halite crystals, indicates polyhalite crystalliza-
tion by reflux brines like in the other deposits.
Another major indication for an early growth of poly-
halite is the development of antitaxial veins parallel to the
Discussion sedimentary layering. Vertical fibers in subhorizontal veins
were described in terms of crystallization power for various
Macroscopically primary and non-deformed structures minerals (Hilgers and Urai 2005). The vertical fibers must
of polyhalite were identified in the mines and under the lift the overburden for growth, which seems to be only pos-
microscope. Euhedral to subhedral polyhalite can be sible, when the overburden is only some tens to hundreds of
expected to give plateau ages in the best case. In particu- meters at maximum. From a sedimentological point of view,
lar, fibers, porphyroblasts and blocky shapes of polyhal- polyhalite originated syn-depositionally.
ite crystals looked intact and not deformed. Age dating
results of these structures are discussed in the present Disturbed age patterns
paper. It turned out that still much work will have to be
done to distinguish primary grains from recrystallized Vertical fibers of antitaxial veins survived the halite foliation
grains or parts of grains and separate them for dating. events because they were protected from strain mostly by
the surrounding mudstone. However, a closer look showed

13
International Journal of Earth Sciences

undulatory extinction of the fibers, grain boundary migra- this study seem to contradict this, because the Ca/K lies
tion between the fan-shaped fibers, and overgrowth by small between 1.00 and 1.21, which is larger than expected for all
anhydrites. 29 measurements (Supplementary Information 1). A pos-
The prismatic porphyroblastic polyhalite crystals in sible explanation could be fractionation: While the ratio of
anhydrite look primary. They originated during migration the atomic masses of 40Ca and 39K from microprobe trends
of potassium and magnesium bearing fluids through anhy- to be Ca/K > 1, the ratio of atomic masses of 37Ar and 39Ar
drite. A closer look under the microscope revealed a patchy from mass spectrometer trends to be Ca/K < 1. However,
structure and a pattern of similar extinction orientation, at the present state of knowledge, there is simply too little
which was interpreted as subgrain rotation recrystallization. knowledge about polyhalite to explain this difference, and
The other samples tested and discussed here, were fabrics more baseline studies are needed.
of euhedral and subhedral polyhalite of bedded polyhalite. When the ratio 37Ar/39Ar of the measured steps of the
The best examples of non-recrystallized looking grains corrected samples is plotted against age (Fig. 7), some inter-
are large polyhalite crystals subparallel to the layering in pretations can be made. The core and the rim of ALT-11,
DÜ-3B, ALT-81, and ALT-37B. They contradict the orienta- as well as the aliquots of ALT-28, which gave plateau ages,
tion of the lowest stress component indicated by the vertical scatter the most in their Ca/K value and expose the largest
fibers in the veins. On the other hand, on the macroscale the error bars. This is a hint to continued or repeated recrys-
rocks clearly show signs of deformation, such as schlieren, tallization. On the other hand, the fibrous sample BDG-37
blurry halite patches or boudins. The orientation subparal- from the Berchtesgaden mine, the fibrous sample ALT-53D
lel to the pseudo-sedimentary layering represents the folia- from the Altaussee mine, and the foliated, well-recrystal-
tion. The grain size increased, and straight grain boundaries lized sample DÜ-3B from the Bad Dürrnberg mine show
developed by fluid-supported recovery processes. similar Ca/K (37Ar/39Ar) values, but also large differences in
The 37Ar/39Ar ratio indicates chemical homogeneity or age. Their geochemical composition is roughly the same but
chemical variations of a material. Onstott et al. (1995b) going into more detail, even more nuances of chemical dif-
derived the theoretical value 0.54 from particle flux of ferences can be recognized (Supplementary Information 3).
39
 K(n,p)39Ar and 40Ca(n,α)37Ar, when irradiated in a reac- However, a systematical analysis would need more data and
tor, i.e., for Ca/K = 1. From the seven samples of this study thus goes beyond the aim of the present study. Most meas-
(09s0098, 09s0099, 09s0698, 09s0663, 09s0699, 09s0716, urements of polyhalite produced disturbed age patterns. The
15s0693), which were corrected for post-irradiation decay overdispersed data do not define an age, as demanded for an
of 37Ar, a mean value of 0.49 was calculated from the isochron age or a plateau age (Supplementary Information
median values of the samples (Fig. 7). The Ca/K ratio is 2). The extent to which the 37Ar/39Ar represents different
lower than expected for nearly all measurements. Is Ca stoi- stages of growth is uncertain. In any way, the measured ages
chiometrically reduced relative to K? The EMPA data of must be seen as minimum and maximum ages only.

Fig. 7  Age plotted against

37
Ar/39Ar. All measured ratios
are significantly lower than the
theoretical value form Onstott
et al. (1995b)

13
 International Journal of Earth Sciences

The measurements face yet another difficulty. The pat- water-soluble mineral. Therefore, the mixture of ages
terns often started with a sudden release of argon, which seems to relate most probably to (2) recovery processes.
showed an age like that at the end of the measurement. In
between, younger ages appeared. Similar patterns were The age dating results in the context of the local
observed earlier during in vacuo experiments of micas geology
(McDougall and Harrison 1999). A differential release of
Ar isotopes could be an explanation for these younger ages The new dating results should be seen in complement to pre-
(Villa 2021). In case of the present study, the younger ages viously measured age data (Leitner et al. 2013, 2014, 2020).
in between could be the result of a breakdown of polyhal- The earlier results dated obviously foliated structures. Dur-
ite ­[K2Ca2Mg(SO4)4·2H2O] to vapor, anhydrite and two ing this study samples of macroscopically syn-depositional
langbeinite-type phases (langbeinite ­[K2Mg2(SO4)3], Xu structures and primary, non-recrystallized looking fabrics
et al. 2017). It is likely that vacancies form during the sud- were chosen. However, it turned out that what looked undis-
den expulsion of water molecules. A differential release of turbed at first glance was no longer pristine when examined
Ar isotopes based on their atomic masses would shift the in sufficient detail.
age towards younger ages. Ages around ~ 235 Ma occur in many samples as the old-
No ages of the depositional age at ca. 250 Ma were est age steps. ALT-28 was the only sample, which gave con-
measured. Only very blurred ages younger than 236 Ma sistent plateau ages in repeated experiments at around ~ 235.
were measured. The Alpine orogeny lasted from upper These plateau results should be seen with caution how-
Jurassic to middle Cretaceous. During this overprint ca. ever, regarding the broad scatter of 37Ar/39Ar (= Ca/K)
200 °C were reached. The overprint could thus potentially ratios. Types 1 and 2 yielded similar ages of ~ 233–236 Ma
result from (1) Ar loss, or (2) recovery processes enhanced and ~ 232–233. Type 4 bedded polyhalite with a blocky fab-
by fluids and grain size increase. A natural polyhalite was ric yielded age steps of ~ 236 Ma and 233 Ma. These oldest
observed to decompose already at 507°K = 233 °C (Xu ages represent only minimum ages but are contemporaneous
et al. 2017). This temperature is close to the measured with the ones that defined plateau ages for sample ALT-
temperatures reached in the salt deposits at ca. 200 °C 28 Ma. The age at early Carnian is well known for extensive
(Leitner et al. 2014, 2020). The closure temperature of fluid flow within the western, central and eastern Northern
polyhalite might coincide with the decomposition tem- Calcareous Alps. Synthetic ore mineralization affected the
perature but might be also lower. Unfortunately, Rich- early Carnian carbonate platforms (Weber 1997a, b; Henjes-
ards et al (2021) determined the closure temperature of Kunst et al. 2014; Prochaska 2016). Between c. 230 and
langbeinite-type phases, and the closure temperature of 240 Ma pelagic deep-water sediments were deposited in
polyhalite remains unknown. Moreover, it is known that the region, which were interpreted to relate to the aborted
muscovite and biotite can grow above and below their Meliata rift (Kozur 1991; Mandl and Ondrejičková 1991;
closure temperature, so consequently, Ar loss by diffu- Neubauer et al. 2000). This rift could have been a poten-
sion cannot be totally excluded. However, diffusion is a tial heat and fluid source. However, from the status quo of
much slower process than recrystallization by circulating knowledge, a sound interpretation of a massive fluid event
fluids (Villa and Hanchar 2017). The term hygrochronom- at 235 Ma is limited at the moment.
eter was proposed to address datable minerals that record Later deformations of the salt relate to salt diapirism and
circulating fluids (e.g., Bosse and Villa 2019). There are to the Alpine orogeny. There are hints to Triassic diapirs,
numerous hints from microstructures for the recrystalli- which were squeezed and extruded to the surface during
zation of polyhalite from the Alpine deposits. Under the the Jurassic (Fernandez et al. 2020). Alpine deformational
microscope many recovery processes were identified, such events in evaporites were dated between ca. 145 Ma and
as subgrain rotation recrystallisation, twinning, and grain 90 Ma by K-feldspar and white mica associated with tem-
boundary migration (Figs. 3, 4, 5). Villa (2021) discussed peratures as high as 250 °C (Spötl et al. 1998; Frank and
the recrystallization of micas on the scale of ≤ 20 µm that Schlager 2006; Dallmeyer et al. 1998; Bojar et al. 2016).
can lead to slightly different phases or compositions, and,
of course, age. Only small amounts of fluid are sufficient
for recrystallization, and fluids can be remobilized many Conclusions
times from grain boundaries during differential tectonic
stress. This is especially the case for highly water-soluble Sedimentologic and diagenetic structures were found, which
materials (Urai et al. 2008; Desbois et al. 2012). Even a prove a syn-depositional origin of polyhalite in the Eastern
complete recrystallization is possible as seen from moving Alps. Such structures have never been described before from
grain boundaries across inclusions during in situ obser- the Alpine deposits but put them in a row with other poly-
vations (Schenk and Urai 2005). Polyhalite is a highly halite deposits worldwide.

13
International Journal of Earth Sciences

Nearly all measurements of primary or euhedral to sub- Acknowledgements  The mining companies Südwestdeutsche Sal-
hedral looking grain shapes of polyhalite produced over- zwerke AG and Salinen Austria AG are thanked for their support and
access to the underground mines. The paper benefited substantially
dispersed data that do not define an age. The data do not from reviews by Fred Jourdan and Igor M. Villa. The final work was
meet the criteria of reproducibility, but nonetheless the data funded by the Austrian Science Fund (FWF), grant number P22,728.
contain some information. Most important, the age data rep-
resent at least maximum and minimum ages. Funding  Open access funding provided by Paris Lodron University
Water-based recrystallization processes can be reactivated of Salzburg.
from fluids along grain boundaries, and recrystallization can
Open Access  This article is licensed under a Creative Commons Attri-
occur on very small scale. Also, other recovery processes bution 4.0 International License, which permits use, sharing, adapta-
enhance the reorganization of the crystal. Recovery pro- tion, distribution and reproduction in any medium or format, as long
cesses such as subgrain rotation recrystallization, twinning, as you give appropriate credit to the original author(s) and the source,
grain boundary migration and grain size increase were seen provide a link to the Creative Commons licence, and indicate if changes
were made. The images or other third party material in this article are
in many examples under the microscope. Because polyhalite included in the article's Creative Commons licence, unless indicated
is a highly water-soluble mineral, these processes, mainly otherwise in a credit line to the material. If material is not included in
based on the presence of water, are the suggested reason for the article's Creative Commons licence and your intended use is not
the virtual scattered ages of polyhalite. permitted by statutory regulation or exceeds the permitted use, you will
need to obtain permission directly from the copyright holder. To view a
One cutback of the present study was the insufficient spa- copy of this licence, visit http://​creat​iveco​mmons.​org/​licen​ses/​by/4.​0/.
tial resolution of the studied samples. Optical microscopy
revealed many different structures. However, element map-
ping down to 1 µm or a systematic analysis of Ca, K and Cl
by the Ar–Ar method would be helpful to identify homoge- References
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