**4. Development of a sequential, three-stage dehydration of serpentine in continental rift settings**

Results of 15 years of published research in serpentinite terrains, mostly in the Alpine orogen of northern Italy and Switzerland and the Beltic orogen of southern Spain, are summarized in **Figure 20**. This research has identified three episodes of dehydration of serpentinite, as variously presented in [38–42].

#### **Figure 20.**

*Pressure-temperature constraints for oceanic lizardite serpentine versus orogenic high-temperature high-pressure antigorite serpentine, higher temperature-pressure chlorite-harzburgite, and highest temperature garnet peridotite showing the first, second, and third dehydration episodes (modified from [38]).*

The three dehydration events are now well documented in serpentinite basements and they can be correlated with the three fluid influxes that built the Kupferschiefer-Zechstein sequence. These papers present a wealth of geochemical data that allowed construction of qualitative mass balance constraints for the chemistry that entered the Kupferschiefer-Zechstein brine during the dehydration episodes and, ultimately, resulted in the brine pulses to the surface.

Four stages of serpentosphere evolution are apparent on **Figure 20** that pertain to evolution of the Kupferschiefer-Zechstein mineralization. The first stage involves hydration of mantle peridotite in oceanic settings within a few km of spreading centers to create low-temperature serpentine. This serpentine ultimately contains the entire anomalous metal suite that characterizes carbonaceous black shales in the Kupferschiefer [39]. The highly anomalous nature of the combined Cu-Ag-Pb-Zn-Mo-Au-PGE-Ni-V-Cr-HC-S kerogeno-metallic system strongly suggests a deep-seated, ultra-deep hydrothermal (UDH), serpentinized source in the basement. The kerogeno-metallic system correlates with plumes that traveled upward to the seafloor interface via a network of deeply penetrating basement cracks. Various metals are released in a sequential manner through a series of dehydrations.

**Figure 20** also shows the three main dehydration events/processes that correlate with three main depositional events in the Kupferschiefer mineralization:


#### *Generation of Mud Volcanic Systems Sourced in Dehydrated Serpentospheric Mantle DOI: http://dx.doi.org/10.5772/intechopen.105689*

The overall metal bias and concentration amounts in the serpentosphere are the same as those elements deposited in the Kupferschiefer black shales (for example, Cu, Ag, Hg, Mo, Co, Ni, V, Sb, U, As). In contrast, elements that are not enriched in the Kupferschiefer and that are close to the average detrital shale composition, typically are not enriched in serpentinites (for example, Ba, Sr, Rb, Sc, Nd, Yb, Lu and Sc, as shown on **Figure 21**).

The apparent correlation of three dehydration events with three metallization events in the Kupferschiefer-Zechstein motivated us to do a more detailed investigation. A major question arising from the above observations is the extent to which element partitioning to the brine phase fits the chemistry of metallization and mineralization patterns in the three stages of Kupferschiefer-Zechstein depositional events.

During this literature study, geochemical information (**Table 3**) was examined that pertained to the initial hydration of mantle peridotite by seawater to serpentine in reaction chambers adjacent to the oceanic ridge system [43–47]. Based on the data, we constructed a series of tables (**Tables 4**–**7**), from which qualitative mass balance constraints could be determined for sequential brine evolution via dehydration of serpentine in serpentosphere basements.

The elemental data in **Table 4** through **Table 7** are presented as averages. Given the different sources of information, differing numbers of samples, differing analytical procedures, and differing analytical precisions in the various references, the data should be considered qualitative. Nevertheless, there is enough consistency within the classes and enough numerical differences between the classes that we believe that the average results taken from peer-reviewed literature are qualitatively valid.

Many of the anomalous metals from the Kupferschiefer shown in **Figure 21** occur at elevated concentrations in the postulated serpentinite source region in the lower crust. Many of these metals were found during the compilation and were available to be inventoried for their partition into the rock phases or into the brine phases during the various dehydration events. The elements were grouped by their relevance to the sequential brine expulsion model.

#### **Figure 21.**

*Composition of Kupferschiefer shale samples (red dots) showing serpentine-affinity metals and oceanic brine components (pink field) as contrasted with average detrital shale (blue plus signs) (modified from [7]).*

## *Soil Science - Emerging Technologies, Global Perspectives and Applications*


#### **Table 3.**

*Sources of chemical data used to construct the following tables.*


#### **Table 4.**

*Brine components - bulk chemical data for serpentinite-related rocks arranged by increasing metamorphic grade (data from [6]).*


*Generation of Mud Volcanic Systems Sourced in Dehydrated Serpentospheric Mantle DOI: http://dx.doi.org/10.5772/intechopen.105689*

#### **Table 5.**

*Base metal components - bulk chemical data for serpentinite-related rocks arranged by increasing metamorphic grade.*


#### **Table 6.**

*Bulk chemical data for peridotite-related and serpentinite-related rocks arranged by increasing metamorphic grade.*


**Table 7.**

*Selected whole rock oxide elements, bulk chemical data for serpentinite-related rocks arranged by increasing metamorphic grade.*

Details of locations, rock types, number of samples and sources are presented in **Table 3**. Samples of fresh oceanic peridotite lithosphere were particularly hard to find, which attests to the pervasive nature of serpentinization at the oceanic crust/ Moho contact. Fortunately, field work by Keith located fresh mantle dunites in the Kizaldag ophiolite complex in southwest Turkey. Full petrochemical data for these samples are available from the lead author [46].

#### **5. Results**

Elements partitioned to the brine, which includes Cl, Li, B, S, C, and volatiles (mainly H2O), are presented in **Table 4**. Base metals (Cu, Pb, and Zn) and related elements (As, Sb, and U) that are variously enriched in the Kupferschiefer-Zechstein plume events [1] are presented in **Table 5**. **Table 6** presents data for elements that are enriched in peridotite (Mg, Sc, Ni, Cr, V) that might be partitioned to brines that deposit these metals in the Kupferschiefer muds. Also, **Table 6** presents additional information for Rb, Ba and Sr that would be strongly partitioned to the brine component. **Table 7** presents selected whole rock oxide data (SiO2, TiO2, Al2O3, Cr2O3, MgO, MnO, and CaO) that variously reflect elements that may be distributed to the brines that deposited the Kupferschiefer-Zechstein system.

#### **5.1 Element gains and losses during formation of serpentosphere**

Chlorine and water show the most obvious pattern for evolution from (1) hydration of peridotite to serpentinite, then brine evolution steps (2), (3), and (4) of the sequential dehydration of serpentinite as documented in **Table 4**. The data show that the peridotitic oceanic lithosphere beneath the oceanic crust contains

#### *Generation of Mud Volcanic Systems Sourced in Dehydrated Serpentospheric Mantle DOI: http://dx.doi.org/10.5772/intechopen.105689*

little water and is hydrated to lizardite serpentinite by the addition of copious amounts of seawater. Chemically, serpentinite is the most hydrated rock on Earth. In oceanic ridge systems, seawater is the only candidate to supply the abundant water, carbon, and chlorine that reside in lizardite serpentine.

The large chemical increases that take place during the conversion of peridotite to serpentinite involve additions of huge amounts of water, halogen, and carbon. Chlorine contents are increased ten-fold, making lizardite serpentinite an excellent candidate for chlorine-enriched brines to supply overlying saline basins. Carbon is augmented twenty-fold, which makes serpentinite an excellent source for hydrocarbon deposits under reduced conditions. In such cases, the carbon travels as reduced, dissolved kerogen (DOC) and is converted to liquid-state hydrocarbons by decompression of the heavy brine fluid in the reservoirs. Water is increased by nine times, making serpentinite the most water-rich major rock type and an excellent source for massive amounts of brine during the dehydration of serpentinite.

In addition, boron and sulfur are added to the rock in abundance and lithium is tripled. Relative to seawater, lithium is at least 15 times higher in serpentinite than in seawater. Hence, serpentinite can provide an abundant source of lithium in brine. Simple evaporation of seawater in the evaporite model does not supply enough lithium as shown in **Figure 22**. The lithium-enriched brines in the Zechstein diapirs clearly contain much more lithium than would be expected to be evaporative products of normal seawater. A metamorphic source for the lithium in Zechstein salines is suggested in [6]. We suggest that serpentinites in the underlying serpentosphere might provide that source.

#### **Figure 22.**

*Li and Mg concentrations in brine from Gorleben and Morsleben. For comparison, the Li content of the groundwater-monitoring network from Morsleben and the Li content of the rocks from Gorleben are displayed. In addition, the development of the Li content in evaporating seawater (blue line) and the first precipitates from seawater are shown (modified from [6]).*

The strong distribution of boron into lizardite serpentinite from boron-poor, fresh peridotite materials indicates that the boron was contributed from the seawater. Hence, the boron in the Zechstein brines is likely to have originated in the seawater that originally made the deep serpentines, and is probably not related to any seawater that might have attended the surface deposition of Zechstein salines.

#### **5.2 Gains and losses during dehydration of serpentine**

Once the serpentine source is hydrated and loaded with potential brine elements, it undergoes a series of dehydrations whereby the brine elements (Cl, Li, B, S, and C) are distributed to the brine reaction products (**Table 4**). The main volume of saline brines in the Zechstein was produced during the second dehydration event, which is associated with the antigorite to chlorite-harzburgite dehydration. There are five cycles of saline deposition in the second phase of Zechstein chemical sedimentation process.

Similar saline sequences appear in other saline basins, such as the Permian Basin in Texas and Michigan Basin in the USA. Considered on a global scale, based on chlorine data in **Table 4**, it is likely that the second dehydration event of antigorite to chlorite-harzburgite is the most important causal factor in the formation of giant saline deposits.

As the system cooled and collected in mud chambers above the deep source, precipitation of sulfides, such as chalcocite, would have released copious amounts of hydrogen and chlorine, as per the equations in **Figure 2**.

#### *5.2.1 Brine element partitioning*

Chlorine, on a mass basis, appears to be largely lost from the rock during the dehydration of lizardite to antigorite. However, another approximately 5 times (5x) loss occurs during the dehydration of antigorite to harzburgite. These dramatic differences, originally observed by Scambelluri and others [40], are inferred to relate to fluid loss from serpentine dehydration in normally dipping subduction zones. Flatly subducted serpentosphere has not previously been examined for its contribution to volatile regimes that might be emplaced in the overlying crust above the flatly emplaced serpentinites. Dehydration of these previously flatly subducted serpentinites can also lead to extensive saline releases that are deposited at the Earth's surface in saline basins. The Kupferschiefer-Zechstein sequence is an excellent example of such a process.

In the Kupferschiefer case, the first dehydration provided highly saline brines where any metals that were present would likely have been complexed as metal chlorides. It is also apparent that sulfur is strongly partitioned to the brine component and would have been present in the early Kupferschiefer brines as H2S.

Boron appears to be strongly sequestered in the brine component. It is thus not surprising that boron minerals appear in the overlying Zechstein saline sequences, especially in the later cycles. The major loss of boron in the rock occurs in the second dehydration, which helps to explain the occurrence of boron minerals in the later cycles of Zechstein deposition.

Boron and its δ11B isotopes can be used to track the serpentine dehydration reaction in normal subduction zones [48] as shown in **Figures 23**–**25**. Lizardite begins to break down to antigorite at about 300°C and the reaction is completed by about 400°C at depths of about 40 km under blueschist metamorphic facies conditions. This reaction coincides with a large release of the boron component to the brine (**Figure 23**) and a distinct lightening of the δ11B isotope signature (**Figure 24**) *Generation of Mud Volcanic Systems Sourced in Dehydrated Serpentospheric Mantle DOI: http://dx.doi.org/10.5772/intechopen.105689*

**Figure 23.**

*Boron concentrations in ppm of Zechstein saline deposits (modified from [49]) and California serpentinites (modified from [48]) and arranged by increasing depth and metamorphic grade.*

**Figure 24.**

*δ11Boron stable isotopes of Zechstein saline deposits (modified from [49]) and California serpentinites (modified from [48]) and arranged by increasing depth and metamorphic grade.*

in dehydrated blueschist-associated serpentinite terranes as established for California serpentinites in the Franciscan assemblage [48].

The δ11B isotope signature also strongly overlaps with boron isotopic data reported from brackish to briny water in the Gorleben diapir by [49]. This overlap suggests the saline brines in the Zechstein saline deposits may have been derived from low temperature lizardite sources (below about 300°C) that dehydrated between circa 265 and 240 Ma using the timing presented in [1]. This event would correspond to the 1st and 2nd dehydration events enumerated in this paper. There is a strong overlap between about +24 and + 10‰ of δ11B isotopes between unmetamorphosed oceanic lizardite and Zechstein salines. There is also a strong

#### **Figure 25.**

*Schematic cross section of a normally developed subduction zone showing a dehydration sequence inferred from boron isotope trends of dehydrating, deep-slab fluids (adapted from [48]). The relative size of '+' and '-' symbols indicates change of the δ11B values; a bold arrow indicates buoyancy-induced flows of serpentinites from deeper portion. Also shown is the inferred position of boron-rich saline lakes, such as those in the California Coast Ranges that may be dehydrational products of lizardite that is dehydrating above the descending slab.*

lightening of the boron isotope data in blueschist-associated lizardite and antigorite serpentinites.

The hydrogen release is important because hydrogen released from sulfide precipitation is then available to hydrogenate any pre-existing kerogen that might be traveling as a micro-flocculent or dissolved kerogen (DOC) in the brine. Hydrogenation of the probably Polycyclic Aromatic Hydrocarbon (PAH)-enriched kerogen could lead to alkylation and the formation of alkane hydrocarbons and ultimately lead to generation of oil under hydrothermal conditions.

The above observations are consistent with the tenfold decrease in carbon abundance from the lizardite to antigorite dehydration step. This decrease shows that early Kupferschiefer brines would have been very carbonaceous and very hydrogen-rich due to various sulfide precipitation reactions. Not surprisingly, the Kupferschiefer horizon that coincides with the early-stage brine release is the most carbonaceous unit in the Kupferschiefer-Zechstein sequence.

The likely presence of a dissolved kerogen and kerogen flocculent in early Kupferschiefer carbonaceous brine is also supported by the strong partitioning of bulk carbon to the brine component shown in **Table 4**. The presence of the kerogen is also supported by the transfer of bulk carbon from seawater to fresh peridotite during lizardite serpentinite formation (Eqs. (1) and (2)). The presence of reduced carbon, probably as kerogen carbon (the TOC term in chemical analyses) in oceanic serpentinite, also supports the likelihood of a carbonaceous brine source. The presence of carbon is documented by Früh-Green and others [18] and shown in **Figure 26**. In general, the altered peridotites contain up to five times higher total C-concentrations compared to the oceanic gabbros.

Bulk carbon is non-CO2 carbon [18] and is likely to be reduced kerogen (HC carbon), because graphite carbon is rare in lizardite serpentinite. The higher reduced carbon content is probably supplied by seawater, where hydrogen is created by formation of magnetite in the serpentinite reaction (Eqs. (1) and (2)). Thus, a significant amount of the bulk carbon released to the brine component, as shown by the data in **Table 4**, is likely to be as kerogen carbon. However, it is also probable that much of this carbon is distributed into carbonate carbon as bicarbonate or dissolved CO2. These carbon compounds are available to precipitate extensive amounts of calcitic and dolomitic carbonate in the overlying, more oxidative, Zechstein saline sequences.

*Generation of Mud Volcanic Systems Sourced in Dehydrated Serpentospheric Mantle DOI: http://dx.doi.org/10.5772/intechopen.105689*

#### **Figure 26.**

*Bulk carbon content vs. C-isotope ratios of oceanic gabbros and serpentinites (modified from [18]).*

The release of reduced carbon to the brine component during lizardite dehydration to antigorite is consistent with ferric:ferrous ratios of lizardite versus antigorite, as compiled by Page [25] and Coleman [27]. Ferric:ferrous ratios determined for lizardite are 9.8:1 (**Table 2**). Ferric:ferrous ratios for antigorite are much more reduced with average ferric:ferrous ratio of 0.31:1, which is about 32 times more ferroan. Such reduced ferric:ferrous ratios for antigorite indicate the lizardite to antigorite dehydration occurred under very reduced, hydrocarbon-stable conditions. The brines created under these conditions were very reduced and carried a large component of reduced kerogen capable of reacting to liquid state oil in upper crustal reservoirs. The reduced context of the lizardite to antigorite dehydration helps explain the light sulfur isotopes documented above.

The Kupferschiefer black carbonaceous shales are only one example of a metalliferous, hydrocarbon-rich black shale, and many black shales may have formed this way. These black shales may be chemically distinguished from more aluminum-rich, detrital shales derived from continental granitic sources. Thus, it is an important possibility that carbonaceous black shales in general may have a deep-sourced serpentospheric component.

#### *5.2.2 Base metal partitioning*

Since 2014, abundant data for copper (for example in Scambelluri and others [40]) now exists throughout all four stages (one hydration stage and three dehydration stages) of the brine generation process (**Table 5**). Copper is slightly added to oceanic lizardite serpentosphere from average harzburgite-dunite. Harzburgite, which constitutes the main volume of oceanic peridotite, contains an average between 20 and 34 ppm Cu, which indicates that the formation of lizardite serpentine from mainly harzburgitic peridotite was largely isochemical. However, average copper is lost to the brine by about two times from the lizardite precursor during the first dehydration to antigorite. During the antigorite to chlorite-harzburgite dehydration, average copper is lost to the brine by 13x during the second stage of dehydration. Significantly, copper appears to be retained in the garnet-peridotite

rock (perhaps by garnet) during the third dehydration, which would explain the relative absence of copper in the Rote Fäule.

The dehydration sequence for copper explains the copper distribution in the three-fold Kupferschiefer-Zechstein metallized brine sequence. The first two dehydrations produce the copper enrichments observed in the Weissliegend-Kupferschiefer and overlying lower Zechstein (Werra cycle). The third sequence (Rote Fäule) has long been observed to be barren of copper.

Recent literature [1, 2, 50–52] has shown that the Rote Fäule event is a late, overprinting, cross-cutting, copper-poor event. This highly oxidative, hematitestable, highly acidic event is also copper destructive with respect to the earlier Weissliegend-Kupferschiefer copper mineralization. However, a minor amount of copper might be destroyed and then reprecipitated near the contact with the earlier Kupferschiefer (the so-called 'transition zone').

The copper-poor nature of the late third-stage brine is predicted by the dehydration data. Copper contents change from nearly absent (1 ppm) in the chloriteharzburgite to much richer (25 ppm) in the garnet peridotite. The combination of strong copper partitioning to the second dehydration event and the distribution of whatever copper might be left to the garnet peridotite leads to the expulsion of a copper-poor brine in the third stage dehydration event. Thus, it is no surprise that the Rote Fäule brine is copper-poor.

Whereas the first two stages of the Kupferschiefer-Zechstein depositions were reduced to highly reduced, the third Rote Fäule stage is highly oxidized and hematite stable. This completely different alteration and metal overprint suggests the appearance of a dramatically more oxidizing brine that overprinted the earlier, more reduced stages. The massive volume of Rote Fäule alteration cannot be explained by a simple change in oxidation state of the pre-existing, more reduced brines that had been previously deposited. The appearance of a third independent, more sulfur-poor, oxidative brine event that was independent of the first two brine events appears to be a simpler alternative than a single hydrothermal event that became oxidized in its later history. The source of this third event would be the third dehydration event induced by chlorite-harzburgite to garnet-peridotite dehydration. Unfortunately, no ferric/ferrous data is yet available for the later dehydration event.

Lead, zinc, arsenic, and antimony display similar patterns to that of copper. They are present in more or less equal levels in the early harzburgite precursor and its hydrated lizardite serpentine product, but are strongly lost to the fluid in the first two stages of brine generation. Whatever is left, however, seems to be captured by the garnet peridotite during the third dehydration, which explains the relative lack of enrichment of these elements in the late-stage Rote Fäule.

Another strong characteristic of the Rote Fäule third dehydration overprint is its overall lack of sulfur (**Table 4**). Sulfur depletion, combined with high oxidation state, explains hematite stability in this sulfur-depleted event. The lack of sulfur in the Rote Fäule coincides with the strong partitioning of sulfur into the garnetperidotite rock during the third dehydration. The withdrawal of sulfur from participating in third stage brine deposition can largely explain the sulfur depletion that characterizes the oxidized, Rote Fäule hydrothermal plumes.

Whereas the Rote Fäule is barren with respect to familiar Kupferschiefer chemicals such as Cu-S-Pb-Zn-Ag, the Rote Fäule is not barren with respect to other elements. As Pieczonka and Piestrzyňski [53] have shown, significant gold resources have been discovered in and immediately adjacent to Rote Fäule (**Figure 27**). The gold mineralization is accompanied by significant platinum group elements (PGE) and uranium. Historically, Rote Fäule was considered the 'death' of copper mineralization and was avoided wherever it was encountered. However, the *Generation of Mud Volcanic Systems Sourced in Dehydrated Serpentospheric Mantle DOI: http://dx.doi.org/10.5772/intechopen.105689*

#### **Figure 27.**

*Late, noble metal overprint in the Rote Fäule in the Sieroszowice-Polkowice copper mining district, southwestern Poland (modified from [50–52]).*

discovery of gold, PGE, and U in the Polish Kupferschiefer points to the potential of Rote Fäule as an economic target in existing Kupferschiefer deposits where mining infrastructure exists or can be rehabilitated (e.g., the Mansfeld-Sangerhausen area in Germany). Unfortunately, no data was uncovered for PGE or Au-Ag during this literature survey.

The uranium enrichment of the Rote Fäule, as well as other parts of the Kupferschiefer, can be explained as resulting from the strong distribution of uranium to the fluids throughout all three dehydrations. Also, uranium was strongly enriched during the initial serpentinization of the harzburgite step. This uranium enrichment implies that seawater was the primary source of uranium in the serpentosphere as peridotites have little or no uranium enrichment. Seawater was likely also the source of uranium for the uranium expelled during the various dehydrations that were deposited in the overlying Kupferschiefer deposits.

#### *5.2.3 Peridotite-related element partitioning*

As shown in **Figure 21** and **Table 6**, Kupferschiefer deposits are notable for containing elements common in peridotites, such as Mg, Ni, Cr, and others, which are especially enriched in Kupferschiefer black shale facies. Typical peridotite elements that are enriched and the amount they are increased in Kupferschiefer black shales relative to detrital shale include cobalt (100x), chromium (2x), vanadium (10x), and nickel (5x).

Examination of dehydration data in **Table 6** shows that nickel is lost during the lizardite to antigorite dehydration. Whereas there is little change in chrome in terms of brine enrichment, the brines nevertheless may replicate the relative abundance of elements in the peridotite precursor to the Kupferschiefer-Zechstein mineral deposits. A similar pattern is present for magnesium, whereby magnesium remains relatively unchanged through the dehydration process.

Magnesium enrichments observed in the Kupferschiefer-Zechstein sequence may be related to the process of steatization, where talc is created during dehydration of both lizardite to antigorite and antigorite to chlorite-harzburgite. Steatization can be described by the chemical reaction of Eq. (3). Steatization

releases water and extra magnesium to a brine component and potentially PGE elements, possibly due to volume changes during steatization from larger volumes of serpentine and destruction of PGE-bearing minerals, such as magnetite and awaruite. It is significant that talc has frequently been observed in the overlying Zechstein carbonates (**Figure 1**) [1].

Steatization: serpentine plus carbonic acid goes to talc plus Mg-brine (Eq. (3)).

$$\begin{aligned} &2\mathbf{M}\mathbf{g}\_3\mathbf{Si}\_2\mathbf{O}\_5\left(\mathbf{OH}\right)\_4 + \mathbf{H}\_2\mathbf{CO}\_3 + \text{heat} \\ &\rightarrow \begin{pmatrix} \mathbf{M}\mathbf{g}\_3\mathbf{Si}\_4\mathbf{O}\_{10}\left(\mathbf{OH}\right)\_2 + \mathbf{M}\mathbf{g}\mathbf{CO}\_3 \end{pmatrix} + 2\mathbf{M}\mathbf{g}\_0 + 2\mathbf{H}\_2\mathbf{O} + 2\mathbf{H}\_2 \\ &\text{Serventile} + \text{Carbonic Acid} + \text{heat} \\ &\rightarrow \left(\mathbf{Talc} + \mathbf{M}\mathbf{g}\text{nesite}\right)\text{ steatite} + \mathbf{M}\mathbf{g} - \text{charged brine.} \end{aligned} \tag{3}$$

Zechstein carbonates also show a chemical trend that leads to the magnesium corner on a MgO-KAlO2-Al2O3 ternary diagram (**Figure 28**). Much of the data for the chemical muds is derived from magnesium-chloritic muds that are interfingered with salts in the Zechstein sequence as inventoried by Bodine [54].

From the perspective of the deep-sourced, hydrothermal, mud volcanic-brine model, chemical muds derived from deep ultramafic sources contain magnesiumrich minerals like serpentine, clinochlore, talc and tri-octahedral clays (saponite) that were formed in high-density chemical brines. Detrital mud contains continentally derived, aluminum-rich minerals, such as kaolinite, pyrophyllite, and dioctahedral smectite (montmorillonite-beidellite series) clays deposited by sedimentary processes, possibly derived from granitic, continental sources.

Data are also presented in **Table 6** for rubidium, barium, and strontium. These elements are also typical of Zechstein brines [55], as is rubidium enrichment following potassium in muscovite in the lower Kupferschiefer (T-1) unit. In particular, barium and strontium show strong enrichments in the lizardite product of mantle peridotite hydration by seawater. Seawater is probably the source of the barium and strontium. Strontium is then strongly partitioned to the brine component during the lizardite to antigorite dehydration in step 2. Strontium is then also strongly partitioned into the garnet peridotite rock component in step 4. This pattern explains strontium enrichment as strontianite-celestine in the

#### **Figure 28.**

*Ternary diagram (MgO-KAlO2-Al2O3) showing the contrast between chemical mud from the deep ultramafic mud vs. shallow detrital mud. Green ellipse includes black shale muds from various black shale basins in the continental United States (modified from [54]).*

#### *Generation of Mud Volcanic Systems Sourced in Dehydrated Serpentospheric Mantle DOI: http://dx.doi.org/10.5772/intechopen.105689*

Zechstein saline sequence, where it occurs as celestine that is closely associated with anhydrite mainly in the upper anhydrite unit and in local, crosscutting veins that one-third of the time are associated with talc. Hryniv and Peryt [55] interpreted the veining as derived from brine introduction from a source outside of the saline section. The talc-celestine association is consistent with a possible ultra-deep brine source.

These above enrichments are observed in so-called 'carbonate reef' environments in the middle Zechstein that are associated with hydrocarbon deposition, mainly as gas. The deep-sourced serpentinite model would suggest that both the strontium and hydrocarbons may have a deep source. The enrichment observations also correlate with the mantle helium anomaly documented by Karnkowski [56].

### *5.2.4 Whole rock oxide partitioning*

**Table 7** shows several percent of silica loss and about 25% aluminum loss to the fluid component during the first dehydration from lizardite to antigorite. This observation may help explain the early abundance of silica in the Weissliegend silica extrudite sand unit, as recently reinterpreted by Keith and others [1] and Spieth [2].

The analogous pattern for aluminum helps to explain the presence of early clays in the Weissliegend and especially the muscovitic clays (illite) in the lower Kupferschiefer black shales.

As with sulfides, precipitation of illite clay produces hydrogen. The electrostatic effects at clay layer boundaries also help in the catalyzation of alkane hydrocarbons from more hydrogen-poor, Polycyclic Aromatic Hydrocarbon (PAH)-kerogens that initially enter the system in its early stages. Little change happens during the lizardite to antigorite dehydration (dehydration 1). Aluminum is partitioned into the garnet during the chlorite-harzburgite to garnet peridotite dehydration (dehydration 3). This progressive sequestering of the aluminum component aids in explaining the transition from aluminum-rich materials in the lower part of the Kupferschiefer sequence to the more carbonate-rich materials in the overlying Zechstein cycles.

Calcium shows a dramatic loss to the fluid during the lizardite to antigorite dehydration, which implies that the brines are strongly charged with a calcium component. However as with aluminum, calcium shows little change during the antigorite to chlorite-harzburgite dehydration and is probably partitioned to the garnet peridotite rock during the third dehydration (dehydration 3).

The data suggest that calcium is progressively available throughout the late Kupferschiefer and early stages of Zechstein deposition, and, along with the sulfur change discussed above, calcium is available to make abundant anhydrite in the lower part of the Zechstein in the Werra cycle. As sulfur is continuously partitioned to the brine component during the first and second dehydration, sulfur abundance appears diminished in the upper Zechstein cycles and late Rote Fäule.

## **6. Summary of three dehydration events**

Details of the corresponding serpentosphere dehydrations and mineralization stages in the Kupferschiefer are summarized in **Table 8**. The three sequential dehydration events are inferred to have been driven by the input of progressively higher amounts of mantle heat that were focused on deep serpentosphere crust near the base of deep-seated fault conduits, such as the Odra fault system. Based on


#### **Table 8.**

*Stages in formation of Weissliegend-Kupferschiefer to Rote Fäule correlated with corresponding dehydrational stages of the underlying serpentosphere.*

extensive studies of dehydrated serpentinites in the Alpine and Beltic orogens, the earlier releases from the serpentines to the brines feature Na, Ca, and Cl, whereas the later releases contain more K, Rb, and Ba. This geochemistry is consistent with the chemo-stratigraphy of the Zechstein, which features more K- and Mg-rich saline brines in the upper cycles.

This study has shown that a mineralogical and geochemical connection can be drawn between the chemical stratigraphy of the Kupferschiefer-Zechstein and the chemistry and mineralogy of the underlying serpentosphere basement that occurs in structurally uplifted blocks between Zechstein 'basinal' lows.

*Generation of Mud Volcanic Systems Sourced in Dehydrated Serpentospheric Mantle DOI: http://dx.doi.org/10.5772/intechopen.105689*

The basins are likely created by withdrawal of mud and brine from the underlying mud-volcanic chambers. The connection is further reinforced by a tri-part, pulsed chemical stratigraphy that includes:


This pulsed chemical sequence, at least in part, can be matched with a tri-partite, pulsed dehydrational sequence that may have affected the underlying serpentosphere during Permo-Triassic time. Each pulse reflects a progressive heating and dehydration of the serpentinite basement that released various chemical components that reflect the increased thermal heating. In this mud-volcanic model, the Kupferschiefer-Zechstein sequence represents brine products formed during the first and second dehydration events in the serpentinite basement. In contrast, the Rote Fäule reflects oxidized Fe-Au-PGE (U), high salinity brines driven off during later thermalism associated with the third dehydration event described above.

This deep-sourced, chemical mud volcanic-brine model satisfactorily explains most of the major, often strongly contradictory, observations on the Kupferschiefer-Zechstein. Some of these contradictory juxtapositions include different age dates for different minerals in the same rock and the juxtaposition of high temperature and low temperature mineral assemblages in the same rock. These apparently conflicting observations are ultimately explained by a 'deep-to-seep' model originating in the hot, deep serpentosphere and extruding into a cooler, shallow, seep environment on the shallow sea or lake bottom.

This model of deep-sourced mud-brine volcanism not only explains the Kupferschiefer conundrums, but also explains many other geologic puzzles, for example the origin of oil and other Kupferschiefer analogs, such as the Zambian copper belt. The dehydration model also explains the mass balance problem for salines in salt basins. The evaporative model typically requires too much seawater with a chemical composition different from that observed in many saline basins, especially the Kupferschiefer-Zechstein.

#### **7. Conclusions**

The main goal of this paper was to investigate the chemical correlation between the three-fold dehydration sequence of serpentine in the lower crust and the threefold mineralization sequence in the Kupferschiefer-Zechstein in the uppermost crust. Another goal was to examine evidence for a continental serpentosphere layer beneath Poland and Germany. A final goal was to examine additional evidence, such as carbon and sulfur isotopes in the Kupferschiefer descriptions, for additional evidence of a deep source.

Abundant evidence was found in the geologic and geophysical literature that a continental serpentosphere layer exists as a several km thick layer that has P seismic wave velocities (Vp) of 6.8–7.8. Serpentinite is also a common rock in the pre-Carboniferous basement of Caledonide age (380–450 Ma) that exists in the basement massifs adjacent to the Kupferschiefer occurrences.

Regional-scale, deep-seated fault systems, such as the Odra fault, provide a plumbing system through which fluids can ascend from any dehydrational events that occurred in the lower crust. These dehydration events acted on the 135-millionyears earlier, low-angle, tectonic emplacement of Caledonide ultramafic basement beneath northern Europe.

During the late Paleozoic assembly of the Pangea continent, mantle heat flow focused in the basement and started to dehydrate the underlying ultramafic serpentosphere. The dehydrational, high-density, hot, hydrothermal, mud-brine products were then focused into the deep-seated fracture system. The mud-brine products accumulated as numerous, low-relief, mud-volcanic fields and shallow basins developed on the Permian unconformity above the Rotliegend.

The three-fold dehydration sequence of serpentinite and resulting depositional sequence (**Table 8**) occurred in the following stages:


This sequence, which was hypothesized as a product of dehydration of the basement serpentinite, was examined in more detail by compiling chemical information from a three-fold, dehydrational sequence of serpentinite found in Alpine orogens. Chemistry compiled from the literature, as well as from unpublished MagmaChem data, shows that element distribution into the various brine systems correlates with that found in the three-fold Kupferschiefer depositional sequence.

The first two stages in the sequence contain a high percentage of high-density mud that accumulated as mud volcanoes on the Rotliegend unconformity. The third dehydration stage (Rote Fäule) was much more water-dominated and had lower pH. The Rote Fäule was emplaced as a late-stage overprint that destroyed the pre-existing Weissliegend-Kupferschiefer-lower Zechstein mineralization and replaced it with a hematite-stable Au-PGE-U-enriched mineralization that is not yet fully explored.

The specificity of the deep-seated, hot, hydrothermal, mud-volcanic model provides explanatory power that does not exist in previous, more compartmentalized models. The mud-volcanic model presented here embraces not only the narrow data set of the Kupferschiefer, but also places it in a broader perspective that includes the entire Weissliegend-Kupferschiefer to Zechstein to Rote Fäule sequences.

Beyond its implications for the Kupferschiefer-Zechstein, the ultra-deep hydrothermal (UDH), mud-volcanic model has implications for the origins of the socalled 'red bed copper' model. The red-bed copper deposits can also be interpreted as deep-sourced, chemical, exhalative sediments, with an ultra-deep serpentospheric source for hydrocarbons in general and oil in particular.

*Generation of Mud Volcanic Systems Sourced in Dehydrated Serpentospheric Mantle DOI: http://dx.doi.org/10.5772/intechopen.105689*
