4. DISCUSSION
4.1 M INERAL GEOCHEMISTRY
4.1.4 Alteration of mineral chemistry due to serpentinisation and hydration
The most basic serpentinization reactions were presented in the introduction page 23. Below follows a more detailed description of the serpentinization process with respect to the
minerals observed in the studied peridotite.
Most of the samples analysed from the two peridotite bodies in Stølsheimen show a high degree of serpentinization, some magnetite, diopside and at one locality some brucite. This fit really well with peridotites metamorphosed in the greenschist facies (Winter, 2010).
When brucite is present it has lower concentration than serpentine and will be consumed when reaction with antigorite at approximately 400oC;
20Brc + Atg= 34 Fo + 51H2O
The olivine (forsterite) created at this point is more rich in Mg than the original olivine before the serpentinization process began. The typical peridotite at this point has the
minerals diopside, antigorite and forsterite. It should be mentioned that the diopside minerals should not be taken for primary minerals, this is a metamorphic mineral. If primary CPX is present it is in the form as augite (Bucher and Grapes, 2010). When the temperature reaches approximately 530oC;
8 Di + Atg = 18 Fo + 4 Tr + 7 H2O
This point includes the loss of diopside and the creation of tremolite.
The main role of Al is to stabilize Al rich phase often chlorite, this mineral has been seen in the talcified regions of the peridotite bodies, but is not very abundant.
Some of the metamorphic minerals from the studied bodies differ from the usual geochemistry seen in such minerals. Below follows a discussion of theese minerals.
92 4.1.5 Rodingitization
The diopsides from the Raudberget dunitic body show a high content of CaO (27 Wt. %) and low content of Al2O3 (0-1.63 Wt. %)( see previous chapter). The Al2O3 content in cpx varies from 2-8 Wt.% in peridotite bodies, with average 5 Wt.% in subcontinental lithosphere and 6-7 Wt.% in sub-oceanic lithosphere (Seyler and Bonatti, 1994). The low Al2O3-content implies that the diopsides are not primary minerals, the classification of subcontinental vs sub-oceanic lithosphere made in the previous section may for this reason be debateable since it may not a primary mineral. It can also be seen that the high Ca-content in the diopsides makes the diopside plots at the top or above diopside field in the classification scheme (Fig.
49).
In the Oman ophiolites the diopside occurs as diopisidte dykes in the peridotite section of the ophiolite. The diopside is characterized by a high CaO-content (Python et al.,, 2007) that is very similar to that found in the Raudberget body. The diopside has been suggested to be Ca enriched from seawater that may penetrate into the mantle. The seawater gets enriched in Ca when travelling through the crust (Python et al., 2007). In hyperextended passive margins it are known that seawater travels along detachment faults and serpentinize the mantle (e.g.
Escartin et al., 1997).
Figure 56. Model of how seawater penetrates into the crust and get enriched in Ca, The Ca rich fluid then starts to metasomatize the mantle and recrystallize the diopsides rich in Ca.
93 Diopside is commonly observed in contact with serpentine and secondary olivine ( Fig. 31).
This also points towards serpentinization as a process that can create the diopside. From mafic rocks it is known that rodingites form during serpentinization. The term rodingite was first used by Bell et al., (1911) to described grossular rich veins crosscutting serpentinite.
This has later been defined to be a process that happens when the primary CPX is broken down, when this happens a Ca rich fluid is created (Bilgrami and Howie, 1960, Capedri et al.,, 1978, Dubinska, 1997, Frost et al.,, 2008). This fluid starts to metasomatically alter the mantle. This can explain why the diopsides have such a high Ca-content and also why the diopsides have quite a high content of Si. The high content of Mg and the low content of Al on the other hand cannot be explained by rodingitization. This could be due to this being a ultramafic and not mafic rock (Python et al., 2007). Rodingites normally form by Ca- metasomatism in mafic rocks (e.g. Austrheim and Prestvik, 2008), whereas the diopside recrystallizes in ultramafic rocks. Python et al. (2007) has called the process in ultramafic rocks for “diopsidization”. It is also important to mention that the diopsidization has only been seen to occur in dunitic or harzburgitic rocks. In the Oman ophiolites it can be seen that this take place in the upper dunitic transition zone. It is likely that since the process takes place in an ultramafic rock, that the Al-content is very low and the Mg-content is
correspondingly high.
This process is also known from the Melange unit at Kopparholmen (Fig. 11) structurally below to the Lindås Nappe (Bøe, 1985). Here two amphibolite dykes consist of garnet, diopside, sphene, epidote, calcite and some serpentine occur. The diopside mineral is seen to have a very high content of CaO (47-50 Wt.%). Metasomatic alterations have destroyed every primary minerals and only secondary is left. This represent a complete rodignitization process (Bøe, 1985).
4.1.6 Metamorphism of magnetite and ferian chromite
The content of Al and Cr in primary magnetite is quite distinct from the content in secondary magnetite. The ratio between Cr and Al are characterized by a high content of Al vs Cr, where the Cr-content is commonly below 100 ppm (Barnes and Roeder, 2001). During metamorphism of spinels the Al-content is reduced in comparison to Cr. The Al rich fluids react with silicates and form secondary minerals like chlorite and amphiboles. The resulting
94 Al poor chromites that recrystallize commonly get classified as ferrian-chromite. They are commonly surrounded by magnetite rims as can be seen in the Raudberget and Vetle Raudberget bodies in Stølsheimen. The magnetite rims are often pure magnetite in greenschist facies (Barnes and Roeder, 2001)
4.1.7 Classification based on the minerals from Raudberget and Vetle Raudberget
From the description above it is clear that the minerals from the meta-peridotites in Stølsheimen are not primary, but secondary. Knowing this makes it difficult to get valid results when using the mineral chemistry to classify the tectonic environment and sub-oceanic versus subcontinental lithosphere. This is evident from Fig.54-56. As already shown the Na versus Cr plots in diopside, these suggest that this is oceanic mantle lithosphere. We need, however, to be very careful when using this information when we know that the diopside is not a primary mineral, but has been affected by Ca-metasomatism, and has a much lower Al content than what is common in CPX.
When calculating the Al in teraheder vs octaheder coordination, it was seen due to the low Al-content, all were in tetraeder coordination and therefore this classification cannot be used (see Fig. 54).
The same problems are encountered when trying to plot the Al-content in the magnetite and ferrian-chromite, as can be seen by Fig. 52 The content of Al is very low in this mineral.
When calculating the Cr/(Cr+Al) we get a value that is between 0,7-1 and when plotting it is observed that they plot as either fore-arc, subcontinental or Alpine peridotites (see Fig. 55) for description of the fore-arc environment see pages 3 in chapter 1. Barnes and Roeder (2001) pointed out that during metamorphism the spinels commonly loose Al, but the Cr does not get removed as easy. They also point out that metamorphic spinels are classified as ferrian-chromite, which is the case with the spinels from Stølsheimen. For this reason spinels cannot be used to classify the peridotites.
All the olivine observed has an extremely high content of Mg and those have Fo-content between 90-97. From the olivines studied by Bonatti and Michael (1989) it is seen that the Fo-content varies between 88-92 %. Most of the results from Stølsheimen plot outside this
95 range. It is also important to state that in their description of olivine and spinels they
disregarded the serpentinized samples to get a best result of the different environments.
Unfortunately unaltered peridotite has not been found in Stølsheimen.
4.2 Major and minor elements
4.2.1 How to use major and minor elements in classification
The content of Al2O3 is seen to decrease from the continental undepleated lithosphere towards the oceanic trench (Fig. 57). An opposite trend can be seen when looking at the Mg#; an increase in the number when going from the undepleted continental lithosphere to the oceanic trench (Fig.58) (Bonatti and Michael, 1989).
Figure 57. The distribution of Al2O3 from the oceanic trench to continental un-depleted lithosphere. The open circles represent the Al2O3 whole rock content from Raudberget and Vetle Raudberget. Modified from Bonatti and Michael (1989).
96 Figure 58. The distribution of the 100* Mg# from the oceanic trench to the undepleted
continental lithosphere. The whole rock Mg# form Stølsheimen varies between 75-87 and thus do not plot inside this plot. When using this information carefulness is needed since Mg and Fe are easily altered during hydration. Modified from Bonatti and Michael (1989).
4.2.2 Mobility of major and minor elements during hydration
Some of the major element data may be easily remobilized during serpentinization.
Especially Ca, Mg, Fe and Na (Seyfried and Dibble, 1980). The Al-content on the other hand commonly does not get strongly affected by the serpentinization process. The Si-content has also seen to be strongly affected by serpentinization reactions (e.g. Beinlich et al., 2010). It was observed during mass-balance calculation, that during serpentinization of ultramafic clasts Mg was removed into the water and the content decreased. The Si-content on the other hand increased in spite of some being lost to water (Beinlich et al., 2010). This fact is
explained by Le Chatelier’s principle: when a system in equilibrium is affected by an external factor (pressure change, temperature change or concentration change). It will adjust itself so that the alteration is as small as possible and a new equilibrium will be created.
4.2.3 Classification based on major and minor elements from Raudberget and Vetle Raudberget
When looking at the Al2O3-content in the XRF analyses it is clear that they are very low in comparison to other peridotites. The Al2O3 in peridotites have a mean of 1,5-2,0 Wt.%
97 (Winter, 2010), the content of the Stølsheimen peridotites are much lower than this reported values. The Almklovdalen peridotite in WGR is another example of a dunitic body that has an extremely low content of Al2O3 (0-0,54 Wt.%). This dunite has been proposed to be a primary dunite originating from a depleted magma (Beyer et al.,, 2006). If this is the case with the dunites from Stølsheimen, the mentioned classification based on whole rock Al2O3
is not valid since the discrimination has not looked at a primary depleted origin for the peridotites. When using Bonatti and Michael (1989) criteria for classification based on the Al2O3-content the peridotites from Stølsheimen are plotting in the trench environment. This is probably not correct since the Al2O3 probably was very low in the first place.
When calculating the MgO-number it is observed that it is varies between 76-86 Wt.%, which is below the classification by Bonatti and Michael (1989). It has however been seen that the Mg and Fe-content in peridotites easily gets affected by hydration and this can then explain why the MgO-number is so low.
4.3 Trace elements
4.3.1 How to use trace elements in classification
Enrichments and depletion of trace elements are an important factor when discriminating tectonic environment of peridotites.
Mid ocean ridges are characterized by much depleted trace element composition. The content of B, Cl and U are commonly high. There are also observed an enrichment in U, Pb, Sb, Sr and Li (Kodolányi et al.,, 2012).
Magma poor passive margins commonly show the highest content of incompatible trace elements when compared to mantle wedges and mid ocean ridges. There is also observed the highest values of boron, which may imply that the fluids have affected the passive margins more than the MOR and the fore-arc (Kodolányi et al., 2012). It is also seen that the passive margins have lower content of Sr than observed at MOR. The reason for this
observation could be that passive margins have calcite as the dominant carbonate, whereas MOR has aragonite as the dominant carbonate. Aragonite contains higher levels of Sr than calcite (Milliken and Morgan, 1996).
98 Figure 59. Primitiv mantle plot showing how the primitive mantle values varies between
MOR, passive margin and Mantle wedge peridotites. Most of the trace elements from the peridotites in Stølsheimen are below the detection limit and therefore it is hard to use when trying to distinguish between different tectonic environments. From Barnes et al. (2014).
The serpentinites from fore arc environments commonly have lower concentrations of trace elements than the mentioned environments. This could be due to more melting and partial melting in the fore arc than in MOR and passive margins. However, the commonly observed enrichment in Cs, Rb, Sr, Sb, and Li together with high concentrations of LILE could imply that the fluids affecting the serpentine has another composition in fore-arcs than what is observed in the other mentioned environments (Kodolányi et al., 2012).
Two plots (Fig. 59 and Fig. 60), the primitive mantle plot and the chondrite plot are commonly used when looking at trace elements from peridotites. Fig. 60 can be used distinguishing between MOR, mantle wedge and passive margins (for description of the MOR and the passive margin see chapter 1) (Kodolányi and Pettke, 2011, Barnes et al., 2014) . The shape observed in the chondrite plot (Fig. 61) can also been used when classifying the peridotite protolith. Observations suggest that concave shaped chondrite patterns fit with lherzolitic affinity while the U/V shaped pattern fit with harzburgitic and dunitic protolith (e.g. Prinzhofer and Allègre, 1985).
99 The chondrite plot (Fig. 60) can also be used to distinguishing between ophiolites and Alpine peridotites. The ophiolitic peridotites generally have lower contents of trace elements than the Alpine peridotites (Bodinier and Godard, 2003). The Alpine peridotites can also be divided into unmetamsomatized fertile Alpine lherzolite, metasomatized Alpine lherzolite and unmetasomtized refractory Alpine peridotites (Fig. 60) (Bodinier and Godard, 2003).
The first is commonly seen with a small depletion in light rare earth elements (LREE) and an almost flat heavy rare earth elements (HREE) trend. The second is characterized by
enrichment in LREE when compared to HREE. The third is characterized by low content of rare earth elements (REE) with HREE fractured pattern, U-shaped REE pattern, N-morb pattern and flat REE pattern (Bodinier and Godard, 2003).
Figure 60. Normalized chondrite plot from whole rock trace elements. The plot shows the distribution of trace elements from peridotites from different environments and also how to distinguish between different processes affecting peridotites. Modified from Bodinier and Godard (2003)
4.3.2 Mobility or immobility of trace elements during hydration
The amount of REE in serpentinites is dependent on the fluid rock interaction, which again is dependent on the abundances in the primary phases and secondary phases of the ultramafic protolith (Deschamps et al.,, 2013). From synthetic growth of peridotite it is shown that during serpentinization of harzburgite an increase in the LREE took place (Menzies et al.,, 1993). Early data on the alteration of REE by serpentinization concluded that the
serpentinization enriched the LREE and HREE and depleted the middle rare earth elements
100 (MREE) in dunites and harzburgites (e.g. Frey, 1969, Kay and Senechal, 1976). This is, however, still debated since a depletion in LREE also have been reported (Frey et al.,, 1991).
Tatsumi et al., (1986) described dehydration experiments on synthetic serpentine doped with 11 REE (Cs, Rb, K, Ba, Sr, La, Sm, Tb, Y, Yb and Nb) at 12 kbar and 850oC, that the REE with largest ionic radiuses more easily gets transported by fluids during dehydration. When investigating the serpentinites in the Guatamala fore-arc from the Deep Sea Drilling Project Leg 84 (Site 566) it was observed that the content of REE depends on the serpentine mineral present. It was seen that the transition from chrysotile to antigorite caused an depletion in Cl (90%), B (80%) and Sr (50%) (Kodolányi and Pettke, 2011). Kodolányi and Pettke (2011) suggested that the observed values could be explained by the crystal lattice of antigorite is poorly designed to incorporate trace elements when compared to chrysotile and possibly also lizardite. The low content of REE in peridotites, especially in dunites and harzburgite, have been explained by the low content of CPX minerals. Since most REE are stored in CPX (Menzies et al., 1993). It has also been showed that weathering and aerial alteration can remobilize REE (e.g. Négrel et al.,, 2000). It has, however, been reported that in most cases the hydration only moderately affect the REE composition, and the REE content still can be used in classifying the original protolith. At least for mantle wedge peridotites and abyssal peridotites (Deschamps et al., 2013). The positive correlation by many trace elements and Yb confirms this and may point in the direction that this elements may be immobile (Deschamps et al., 2013).
4.3.3 Classification based on trace elements from Stølsheimen
As can be seen by the trace element data from the studied peridotites they are really low!
Most of them are below the detection limit for the ICP-MS (For description of the process see chapter 2). This fact is, however, not very unusual since peridotites and especially
dunites have very low content of REE (Lipin et al.,, 1989). This fact is also seen in the much depleted isolated Almklovdalen dunitic body. The body consist of depleted dunite and fertilized garnet peridotite. It is proposed that the garnet peridotite was created after re-fertilization of the depleted dunitic body by mantle metasomatism (Beyer et al., 2006). It has also been seen from the literature that there can be some changes in REE content in peridotites, but these changes are only moderate and most likely do not change the original REE pattern from the protholite.
101 We can safely conclude that the trace elements in Raudberget and Vetle Raudberget are very low indeed, probably because an original very low content that may have been amplified during hydration/metamorphism. The content for many of the elements are below detection limit for the method used here.
It should also be noted that REE commonly do not get removed from peridotites, some authors have also given the content of REE in ppb since they are so low in peridotites (e.g.
Melcher et al.,, 2002).
The trace elements available show a high content of LREE, the MREE and HREE are below the detection limit. The same values for LREE can be seen for passive margins (Fig. 60).
The primitive mantle plot also show that the available trace elements plot inside the field for the passive margin. Carefulness is very important when trying to use the data to classifying tectonic environment since many of the REE are below the detection limit and their
concentrations are very low.
4.4 Conclusions from the geochemistry
The conclusion from the geochemistry is that all rocks analysed have been affected by serpentinization. This is, however, not surprising. In the result chapter it was shown that the peridotites were severely brecciated and also some places very foliated. All samples also showed serpentinization and primary mineralogy is not preserved. This fact makes the geochemistry difficult to use if the geochemical data was used uncritically they probably would give false information on the tectonic environment. For information on the tectonic environment a closer look at the country rock is needed.
4.5 Passive margin origin or ophiolitic origin
As seen above the peridotites from Stølsheimen are severely affected by hydration/
metamorphism so the mineral chemistry and the whole rock chemistry cannot be used with confidence when trying to classify, which environment the peridotite originated. In order to do so a closer look at the melange rock surrounding the meta-peridotites in the study area,
metamorphism so the mineral chemistry and the whole rock chemistry cannot be used with confidence when trying to classify, which environment the peridotite originated. In order to do so a closer look at the melange rock surrounding the meta-peridotites in the study area,