Editorial for Minerals Special Issue “From Diagenesis to Low-Grade metamorphism”

Rocks affected by pressure–temperature conditions in the transitional field between diagenesis and low-grade metamorphism make up large domains of the Earth’s upper continental and oceanic crust [1]. Due to its borderline character, the conventional approaches of metamorphic petrography or those of sedimentary petrology are not completely adequate to unravel the physical–chemical genetic conditions of these rocks [2]. In general, the system does not reach equilibrium, or this is limited to the nanoscale, thus the advance of mineral reactions is controlled by kinetics and results in common defective mineral phases commonly occur [3]. In this context, the concept of reaction progress [4] has been consolidated as more adequate than that of grade [5]. This explains why the study of these rocks has developed during decades of specific methods of study [6] and consequently has its own nomenclature [1]. Powerful tools for the study of defective phases, with special emphasis on clay minerals, are electron microscopy and X-ray diffraction, which have led to the development of specific criteria of grade and pressure gradient [5]. In addition to clay minerals, other materials such as organic matter [7,8], index minerals of basic rocks [9,10], and fluid inclusions [11] have also been applied to these diagenetic/metamorphic rocks. Even though initial apparent contradictions arise, further studies have demonstrated the complementariness of the different approaches and encourage their joint use [12,13].

In the last few decades, the general scenario, basic processes, and fundamental of methods have been established [14,15,16] and may be efficiently applied to provide information about geotectonic contexts [17], as some of the contributions to this Special Issue show. Nevertheless, specific aspects, such as the effect of low-[16,18] or high-[19,20] pressure gradients or the role of the original material, different to those traditionally considered, together with the effect of fluids [21,22,23], are still poorly known and open to debate. Additionally, numerous studies ([24,25,26] and references therein) have emphasized the significance of the retrograde processes on the mineral assemblages of these materials. The term “retrograde diagenesis” was coined [27] to designate “fluid mediated retrograde processes occurring under diagenetic conditions” [24].

Nowadays, most of the traditional tools (mainly applied to clastic rocks, rich in clay minerals, especially white mica) remain as powerful criteria to establish the reaction progress in the diagenetic to low-grade metamorphism path. Namely:

(1)

Type of order (R parameter) and % illitic layers in the smectite-illite system ([28] and references therein) to define the advance of the reaction progress during the diagenetic stage.

(2)

Illite polytype, which basically differentiates the diagenetic (1M_d polytype) from higher grades (2M₁ polytype) [11].

(3)

The Kübler Index (KI) [29], which provides a scale for the diagenesis-anchizone-epizone grade definition [30]; to be valid, it must be correctly standardized according to the newest recommendations [31]. Originally, this parameter was broadly considered as a measure of the “illite crystallinity”, and lately, it was effectively correlated with the crystalline domain size of the illites, directly measured on transmission electron microscopy (TEM) images [32,33,34].

(4)

b parameter of white mica [35], which allows a qualitative characterization of the pressure gradient.

(5)

Vitrinite reflectance [36], which gives a scale for the maturation of organic matter of plant origin. It has been widely correlated with the KI, being this correlation dependent on, and hence, informative of, the type of thermal gradient in the basin [12,16]. It equilibrates quickly with the temperature and is not affected by retrogradation.

(6)

The geothermometer based on Raman spectra of carbonaceous material [7].

(7)

The metamorphic facies based on mineral paragenesis of basic rocks [9,10].

(8)

Index minerals in clastic rocks having particular compositions [11,37], such as pyrophyllite [38], paragonite [39], mixed Pg/Ms [39], the kaolinite group polytypes [40,41,42], sudoite [43,44], epidote [45], chloritoid [46], stilpnomelane [47].

(9)

The color of conodonts [23,48,49], which gradually change from amber to brown to black, as their small amounts of organic matter pass through the temperature range 50 °C to 300 °C and to gray, white, and finally hyaline, at higher temperatures. This reaction of organic matter is not affected by retrogradation.

The use of geothermobarometry based on thermodynamic equilibrium among mineral components in the diagenesis–low-grade metamorphism transition has been traditionally precluded by the small scale at which mineral equilibrium is reached and the tiny grain-size of minerals, usually under a few microns and frequently at the nanoscale. Nonetheless, during recent years, the use of electron microscopy techniques has allowed some advance in this field. Geothermobarometry based on local equilibria of chlorite-phengite pairs is very adequate, due to the ubiquitous presence of these minerals in very low-grade pelites; even finding pairs in equilibrium is not an easy task, which can be partly solved, however, by the use of energy dispersive spectroscopy (EDS) on scanning electron microscopes (SEM). In this technique, the area to be analyzed is routinely selected on backscattered electron (BSE) images, allowing the easy selection of non-contaminated grains in apparent textural equilibrium. The use of the same analytical routine as in the electron microprobe, in terms of preparation of samples and standardization, yields sufficiently accurate results ([20] and references therein).

In the same way, chlorite geothermometry has experienced a remarkable advance with the development of the pure thermodynamic thermometers [50,51] and the semi-empirical thermometers [52,53,54,55,56]. These last ones are easy to apply, particularly after the publication of the Verdecchia spreadsheet [57], and especially useful in the diagenetic-low grade metamorphic conditions. At low-temperature, low-pressure conditions, the cited thermometers tend to produce similar results, within the range of the error of the method (around 50 °C) and are not affected by the intrinsic limitation of the method, related with high sums (near 6 afu) of octahedral cations, which is limited to higher temperatures [55,58]. An important caution to be contemplated is the complete absence of contamination or mixed-layering with other phases, which should be checked according to the maximum of interlayer cations in the chlorite analysis explicitly allowed by the method.

This Special Issue of Minerals presents six original contributions that cover a broad range of topics concerning low-grade rocks from Japan, Brazil, Morocco, Spain, the Iberian marine continental platform, and the Greater Caucasus in the Russian Federation. One of the papers focuses on inclusions remaining from a high P metamorphism in rocks affected by a complex metamorphic history, which includes prograde as well as retrograde processes [59]; another [60], with garnet nucleation and growth, which record the initial steps of dehydration within the subduction zone. Two of them present mineral transformations in the smectite-illite system during diagenesis, either of chemical [61] or burial [28] origins, and a third the incorporation of B, coming from the transformation of organic matter, into the illitic tetrahedral layers [62]. The last one [63] uses the clay transformations and related parameters to establish the pressure/temperature conditions in a key region for the interpretation of the Eurasia–Africa collision in Cenozoic times.

The contribution from Vladimir Kamzolkin and co-authors studied metamorphic rocks (gneisses, amphibolites, and blastomylonites) from the Fore Range Zone (Greater Caucasus) that are the host rock of eclogites [59]. These metamorphic rocks, belonging to the Armovka Formation, underwent low-grade retrograde metamorphism. Consequently, index minerals that are traditionally employed to acquire information about the P-T conditions during the prograde path, and particularly those corresponding to the metamorphic peak, were altered. They describe high-pressure mineral inclusions, mainly composed of omphacite, phengite, garnet, and paragonite enclosed by pyrite and chalcopyrite. They analyze these inclusions by EDS and applied the geobarometer based in the Si-content of phengite to constrain the P attained during prograde metamorphism. The pressure ranges from 1.7 ± 0.2 to 1.9 ± 0.2 GPa for temperature of 600 ± 40 °C, which allows the authors to conclude that the metamorphic rocks of the Armovka Formation were buried in the subduction zone attaining the P conditions of the eclogite facies, forming a coherent subduction complex with the eclogites.

The contribution from Mutsuko Inui and co-authors studies pelitic schists of the Nagatoro area, Kanto Mountains, Japan, to analyze garnet nucleation and growth during metamorphism of the Sanbagawa metamorphic belt [60], one of the world’s most comprehensively studied subduction complexes. The study of garnets in rocks belonging to subduction complex is a key point because garnet-forming reactions are one of the most common types of dehydration reactions taken place in subducting crustal rocks. Based on the study of morphology, chemical composition, and crystal size distribution of garnets, this contribution constrains the conditions of garnet nucleation and growth in pelitic rocks during subduction. The smaller garnet grains show log-normal size distribution, hence, they formed in the same event during metamorphism, after overstepping condition to form the core; such conditions were slightly different for the various grains, as they were controlled by the local chemistry of the rock.

The work by Arostegui and co-authors deals with a key topic in the study of burial diagenesis of siliciclastic rocks: the process of smectite illitization [28]. This process involves a series of chemical reactions and structural changes that give rise to coherent domains intermediate between smectite and illite, the so-called I/S mixed-layer minerals. Many important studies have been done about this topic; however, the correspondence between the interpretation of I/S mixed-layers as determined by XRD and their physical reality has been a topic for disagreement among researchers during decades. With the aim to analyze the evolution in the 2:1 expandable layer charge and their layer distribution during smectite illitization in burial diagenesis of a marly sequence, the authors apply XRD analysis of the 2:1 expandable layers intercalated with n-alkylammonium ions, which has shown great resolution for calculation of the charge and its distribution between the tetrahedral and octahedral layers, as well as to determine the homogeneity, or not, of the 2:1 layers at the crystallite scale. Moreover, with the goal to correlate the layer-charge properties determined by the n-alkylammonium method with the clay mineralogy and micro/nano textural relationships, they performed a detailed TEM study on samples prepared using LR white resin, in order to facilitate the differentiation of illite and smectite interlayers in TEM images.

Martos-Villa et al. present B-content and its isotopic composition of clay samples [62], together with their detailed mineralogy, in mud-volcano sediments of the Gulf of Cadiz (Atlantic Coast between Europe and Africa). The data demonstrate that B is fixed in the illitic tetrahedral layer during the diagenetic smectite to illite reaction. Molecular modelling of the illite structure by electron density functional theory (DFT) methods inferred that the substitution of B for Si in the tetrahedral site is energetically favorable, more than that in the interlayer sites. According to its abundance and isotopic composition, the B comes from the hydrocarbon-related fluids approaching temperatures of methane generation, around 150 °C, that is, similar to that of the more mature illite-smectite mixed-layers found in the samples.

Rodríguez-Ruiz et al. describe the low-grade metamorphic evolution of the higher units of the Internal Zone of the Rif Cordillera (N Morocco), using clay mineralogy, Kübler index, mica polytypes, and chlorite geothermometry [63]. Several tectonic units are formed by the same stratigraphic sequence of materials, registering all the metamorphic evolution from diagenesis at the top to epizone (greenschist facies) at the bottom. High-resolution transmission electron microscopy and electron diffraction allowed the determination of coexisting 2M and 3T mica polytypes, which together with the wide range of phengitic substitution in micas and temperature in chlorite, indicates that the rocks conserve information from the whole decompression path, including the high-pressure stage. Therefore, these shallowest units of the metamorphic pile followed a clockwise PTt evolution parallel to the other units of the Betic-Rif Chain, widely described in the literature.

The work by Armstrong and co-authors presents a study of the clay mineralogy of sediments from several lakes from the Nhecolândia region [61], the southernmost region of the Pantanal Wetland (Brazil). Sediment samples were analyzed by X-ray diffraction and fluorescence as well as scanning and transmission electron microscopy, together with the geochemical characterization of the lake waters, including in situ determination of pH, EC, and TDS. These data led the authors to classify the lakes as freshwater, oligosaline, and saline. The clay minerals identified in the fine fraction of sediments were illite, kaolinite, smectite, and I/S mixed-layers. According to textural and morphological features, the authors interpret that smectite and I/S mixed-layers are authigenic in origin, produced by the particular chemical conditions of some of the lakes. Smectite is mainly dioctahedral with some mixture of trioctahedral one. The work discusses the link between geochemistry of the lakes and authigenesis of smectite.

To conclude, the above-described methods allow the determination of the reaction progress, and an adequate combination of them offer qualitative information about the pressure regimen of a region. Nevertheless, it will not always be possible to translate this information into an accurate estimation of the pressure–temperature conditions. A number of questions remain open about the complex field of the diagenesis–metamorphism transition, dominated by the lack of mineral equilibrium.