3. RESULTS
3.4 M INERALOGY AND MINERAL CHEMISTRY
3.4.2 Diopside
The diopside (Fig. 49) show high content of SiO2 (52,91-56,20Wt.% with average
54,65Wt.%) and high levels of CaO (24,41-26,74Wr.% with average 25,61Wt.%) (Fig. 49) and MgO (17,49-18,77Wt.% with average 18,00Wt.%). The content of Na2O
(0,00-0,36Wt.%), Al2O3 (0,00-1,63Wt.%), K2O (0,00-0,02Wt.%), TiO2 (0,00-0,03Wt.%), FeO (0,59-1,82Wt.%), MnO(0,00-0,19Wt.%), Cr2O3 (0,03-0,68Wt.%) and NiO (0,00-0,10Wt.%) show very low levels.
The lowest values of the Al2O3-content are seen in samples with severe serpentinization, while the highest values are observed in samples with less serpentinization and in contact with the most FeO rich olivines.
83 The content of Na2O and Cr2O3 from the diopside has been used in a plot to classify if the dunite has a continental lithosphere origin or oceanic lithosphere origin (Kornprobst et al.,, 1981). The plot shows an oceanic origin as seen in Fig. 50 This will be further discussed in the discussion.
Figure 49. Classification scheme of Clinopyroxene. As can be seen from the plot the Pyroxene plots on top or above the diopside field due to the high content of CaO. See chapter 4 for further discussion of their formation and metamorphism.
Figure 50. Plot of the Cr*1000 vs Na*1000 to classify the peridotite as oceanic lithosphere or continental lithosphere. After (Kornprobst et al., 1981).
84 3.4.3 Magnetite/Ferrian chromite
The magnetite rims (Fig. 31B) are characterized by a very high content of FeO (68, 66-87,84Wt.%) and varies a bit. The content of SiO2 (0,00-0,33Wt.%), Na2O (0,00-0,06Wt.%), Al2O3 (0,00-0,03Wt.%), K2O (0,00-0,03Wt.%), CaO (0,00-0,04Wt.%), TiO2
(0,00-0,09Wt.%), MnO (0,05-0,51Wt.%), MgO (0,51-3,63Wt.%), Cr2O3 (1,02-12,16Wt.%) and NiO (0,61-1,72Wt.%) are quite low.
The FeO-content and Cr2O3 content varies a bit in the rims, this imply that the magnetite is not a pure magnetite, but do have a little Cr2O3 content in the rims as well (Fig. 51).
The ferrian-chromite (Fig 31B) has high content of Cr2O3 (34,84-51,07Wt%) and high levels of FeO (21,11-55,49Wt.%). The Al2O3-content (0,19-21,61Wt.%) and MgO
(1,63-14,32Wt.%) varies a bit. The SiO2 (0,03-0,40Wt.%), Na2O, K2O , CaO and TiO2-content are low, below 0,05Wt.%. The MnO (0,42-1,24Wt.%) and NiO (0,02-0,35Wt.%) are quite low.
Figure 51. Classification scheme of the different spinel minerals. It can be shown that the most abundant is magnetite, which in most cases represents the outer rims of zoned minerals with ferrian-chromite in the cores. The Al2O3, Cr2O3 and FeO content is calculated by dividing the respective element on the sum of Al2O3+FeO+Cr2O3 . Notice also that one point has been classified as Cr-magnetite, this imply that the rims of magnetite is not always pure magnetite.
85 3.4.4 Carbonates
In this work there has been observed two kinds of carbonates in the rocks from Raudberget and Vetle Raudberget; calcite and dolomite. In NGU reports there have been seen in addition magnesite and breunnerite (Bakke, 1985).
The calcite is characterized by a CaO (52,83-55,56Wt.%) content with very little variation.
The SiO2 (0,0038-0,034Wt.%), FeO (0,00-0,11Wt.%), MnO (0,04-0,11Wt.%) and MgO (0,00-1,57Wt.%) are low in the calcite minerals
The dolomite is characterized by a lower content of CaO (30,50-31,22Wt.%) than the calcite minerals and moderate level of MgO (20,02-23,74Wt.%). The content of SiO2
(0,008-0,025Wt.%), FeO (0,22-0.52Wt.%) and MnO (0.14-0.19Wt.%) are very low.
The magnesite is characterized by a very low content of CaO (0-0.41Wt.%), MnO (0.08-0.64Wt.%). The FeO-content is fairly low (1.40-8.28Wt.%) and the MgO
(40.40-48.33Wt).% is high (Bakke, 1985).
The breunnerite is characterized by a low CaO (0.12-0.41 Wt.%) and MnO (0.32-0.83 Wt.%). The FeO-content is fairly low (10.83-13.32 Wt.%) and the MgO (38.58-42.72 Wt.%) (Bakke, 1985).
3.4.5 Chlorite
The chlorite (Fig. 42) is characterized by a SiO2 -content of (28.99-34.67Wt.% with average 31.56Wt.%) and MgO (26.77-34.58Wt.% with average 31.38Wt%). It can also be seen that the levels of MgO varies a bit. The content of Al2O3 (12.11-18.90Wt.% with average 15.13Wt.%) is moderate to low and this value also varies a bit. The content of K2O (0.00-0.013Wt.%), CaO (0.014-0.017Wt.%), TiO2 (0.00-0.08Wt.%), MnO (0.00-0.16Wt.%), Cr2O3 (0.05-2.92Wt.%) and NiO (0.16-0.37Wt.%) have very low concentrations.
3.4.6 Serpentine
The serpentine is characterized by a high content of SiO2 (32.70-44.75Wt.% with average 41.82Wt.%) and MgO (22.73-44.56Wt.% with average 39.08Wt.%). It can also be seen that
86 the content of this elements varies a bit. The content of Na2O (0.00-0.036Wt.%), K2O (0.00-0.03Wt.%), CaO (0.00-0.10Wt.%), TiO2 (0.00-0.04Wt.%), MnO (0.00-0.23Wt.%), Cr2O3 (0.00-1.13Wt.%) and NiO (0,00-1.12Wt.%) content are also very low. The FeO (1.25-8.41Wt. %) and Al2O3 (0.00-10.53Wt. %)
There could not be seen any differences between the different locaTiO2ons of serpenTiO2ne e.g. serpentine in veins and serpentine in matrixes.
Any X-ray diffraction (XRD) analyses on the serpentine minerals have not been run; this is the only way to differ between the different serpentine minerals since it is impossible to distinguish between them on the basis of geochemistry.
3.4.7 Talc
The talc (Fig. 26) is distinguished by a high content of SiO2 (58.55-64.75 Wt. % with average 61.07 Wt. %). The MgO (26.44-29.13 Wt. % with average 27.81) is fairly high the content of FeO (2.31-4.20 Wt. % with average 3.36 Wt. %) is low. The Na2O (0-0.05 Wt.%), K2O (0-0.14 Wt.%), Al2O3 (0.05-1.26 Wt.% with average 0.26 Wt.%), CaO (0-0.11 Wt.%), TiO2 (0-0.03 Wt.%), MnO (0-0.09 Wt.%), Cr2O3 (0-0.04 Wt.%) and NiO (0.04-0.25 Wt.%) contents are very low.
3.4.8 Amphiboles
The tremolite is characterized by a high content of SiO2 (51.67-63.24 Wt.% with average 57.74 Wt.%). The MgO (21.72-31.71Wt. % with average 23.53 Wt.%) is fairly high, while the CaO (0.02-14.09Wt.% with average 13.36Wt.%) varies a bit. The FeO (1.58-3.93Wt.%
with average 2.94Wt.%) is low. The Na2O (0-0.03 Wt.%), Al2O3 (0.01-0.42Wt.%), K2 O (0-0.04Wt.%), TiO2 (0-0.03Wt.%), MnO (0.0-0.21Wt.%), Cr2O3 (0-0.10Wt.%) and NiO (0.06-0.20Wt.%)
The actinolite is characterized by a high content of SiO2 (54.53-58.22 Wt.% with average 56.60Wt.%). The MgO (19.61-22.32Wt. % with average 20.97 Wt.%) is fairly low and the CaO (12.64-13.83Wt.% with average 13.01Wt.%) varies a bit. The FeO (4.42-6.64Wt.%
with average 5.39Wt.%) is fairly low. The Na2O (0.02-0.35Wt. %), Al2O3 (0.02-2.69Wt. %),
87 K2O (0.01-0.07Wt.%), TiO2 (0-0.04Wt.%), MnO (0.08-0.41Wt.%), Cr2O3 (0.-0.67Wt.%) and NiO (0-0.17Wt.%) content are very low.
The magnesio-hornblende is characterized by a high SiO2 (45.14-53.04 Wt. % with average 49.95). The MgO (18.01-23.01Wt. % with average 20.65 Wt.%) is fairly low and the CaO (10.93-12.86 Wt.% with average 12.03 Wt.%) is also low. The Na2O (1.22-2.02 Wt. %) and Al2O3 (4.08-9.91 Wt.% with average 6.63 Wt.%) are quite low. K2O (0.13-0.67 Wt.%), TiO2
(0.11-0.66 Wt.%), MnO (0.10-0.34 Wt.%), Cr2O3 (0-0.56Wt.%) and NiO (0.05-0.17 Wt.%) are very low.
Figure 52. Classification of amphiboles as can be seen the two lithologies with amphiboles consist of tremolite, actinolite and magnesio hornblende. For a detailed description of their chemistry see section above.
3.4.9 Brucite
The brucite has a high MgO (46.46-47-42 Wt.%) and low levels of FeO (1.85-2.58 Wt.%).
The content of SiO2 (0-0.06 Wt.%), Al2O3 (0-0.03 Wt.%), K2O (0-0.01 Wt.%), CaO (0-0.04 Wt.%), TiO2 (0.007-0.03 Wt.%), MnO (0.38-0.47 Wt.%) and NiO (0.02-0.05 Wt.%) are very low.
88
4. Discussion
The first part of the discussion will present on how to use mineral chemistry, major, minor and trace element geochemistry in classifying peridotites and the problems with the classification due to hydration reactions. Based on this knowledge it will be given an interpretation of the data collected from the studied peridotite. Lastly the origin of the peridotite, as remnants of hyperextended exhumed mantle versus ophiolite will be discussed with respect to the peridotite and the country rock association.
4.1 Mineral geochemistry
4.1.1 Clinopyroxene
The Al, Ti and Na content of CPX can be used when looking at differences between a suboceanic or a subcontinental origin of peridotites (Seyler and Bonatti, 1994). The Al content in CPX varies between 2-8Wt.%, where the average subcontinental CPX has 6-7 Wt.% while the subocceanic has 5 Wt.% (Seyler and Bonatti, 1994). It has also been observed that CPX from suboceanic
mantle has a higher ratio of Al in octahedral position than tetrahedral position while the opposite is shown for the subcontinental lithosphere (Fig. 53).
The Ti-content can also be used in discriminating between the two types of origin (Seyler and Bonatti, 1994).
Kornprobst et al. (1981) showed that the Na-content in subcontinental lithosphere was higher than in the subocceanic lithosphere (Fig. 50). He further suggested that the ratio between Cr and Na in CPX could be used as a
89 and suboceanic mantle. This observation was criticised by Sen (1982) who argued that data from some oceanic and continental xenolithes overlap and that Kornprobst et al. (1981) used both spinel and plagioclase peridotite oceanic samples. The observations concerning Na was confirmed by Seyler and Bonatti (1994).
4.1.2 Spinels
The concentration of Al, Cr, Mg and Fe in spinels can be used when differentiating between abyssal versus continental peridotites and, which peridotite field present (Dick and Bullen, 1984). The Cr# (Cr/ (Cr+Al)) and Mg# (Mg/ (Mg+Fe)) in spinels can also be used when dividing the peridotite into different environments (Fig. 54). It is seen that peridotites from fore-arc environments have higher Cr# and low Mg# while the abyssal peridotite has lower Cr# and higher Mg# (e.g. Okamura et al., 2006).
Figure 54. Showing the distribution of Cr# from mid-ocean ridges to Alpine peridotites. The green area shows the Cr# from the ferrian-chromites from Stølsheimen. Modified from Lee (1999)
4.1.3 Olivine
The Fo-content in olivine can also be used as an indicator for, which environment the peridotite body did form. Bonatti and Michael (1989) looked at the whole rock chemistry
90 and mineral chemistry from peridotites from subduction zones to undepleted continental lithosphere. From their interpretation it can be seen a linear trend from 88 % in the
undepleted continental lithosphere increasing towards 92% Fo in oceanic trenches (Fig 55).
Figure 55. Distribution of fo % from continental undepleted lithosphere to oceanic trench.
An increasing trend from continental to oceanic trench can be observed. The green area mark the most abundant fo-content from Stølsheimen plot, as can be observed those result
91 plot outside the area defined for the different tectonic environments. Modified from Bonatti
and Michael (1989).
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.