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Hydrothermal alteration of kimberlite by convective flows of external water.

Afanasyev AA, Melnik O, Porritt L, Schumacher JC, Sparks RS - Contrib Mineral Petrol (2014)

Bottom Line: Excess volume of serpentine results in filling of pore spaces, eventually inhibiting fluid flow.Fast reactions and increases in the volume of solid products compared to the reactants result in self-sealing and low water-rock ratios (estimated at <0.2).Such low water-rock ratios result in only small changes in stable isotope compositions; for example, δO(18) is predicted only to change slightly from mantle values.

View Article: PubMed Central - PubMed

Affiliation: Institute of Mechanics, Moscow State University, Moscow, Russia.

ABSTRACT

Kimberlite volcanism involves the emplacement of olivine-rich volcaniclastic deposits into volcanic vents or pipes. Kimberlite deposits are typically pervasively serpentinised as a result of the reaction of olivine and water within a temperature range of 130-400 °C or less. We present a model for the influx of ground water into hot kimberlite deposits coupled with progressive cooling and serpentisation. Large-pressure gradients cause influx and heating of water within the pipe with horizontal convergent flow in the host rock and along pipe margins, and upward flow within the pipe centre. Complete serpentisation is predicted for wide ranges of permeability of the host rocks and kimberlite deposits. For typical pipe dimensions, cooling times are centuries to a few millennia. Excess volume of serpentine results in filling of pore spaces, eventually inhibiting fluid flow. Fresh olivine is preserved in lithofacies with initial low porosity, and at the base of the pipe where deeper-level host rocks have low permeability, and the pipe is narrower leading to faster cooling. These predictions are consistent with fresh olivine and serpentine distribution in the Diavik A418 kimberlite pipe, (NWT, Canada) and with features of kimberlites of the Yakutian province in Russia affected by influx of ground water brines. Fast reactions and increases in the volume of solid products compared to the reactants result in self-sealing and low water-rock ratios (estimated at <0.2). Such low water-rock ratios result in only small changes in stable isotope compositions; for example, δO(18) is predicted only to change slightly from mantle values. The model supports alteration of kimberlites predominantly by interactions with external non-magmatic fluids.

No MeSH data available.


Graphs showing the results of XRD analysis of samples from the three lithologies at the A418 pipe. a Plot of olivine abundance against serpentine abundance, b plot of total abundance of fresh olivine against the fraction of fresh olivine in the olivine + serpentine component of the rock (a measure of how altered the rock is 1 being fresh and 0 being totally altered), and based on our modelling results, we can translate degree of alteration into a measure of original porosity (φ) with complete alteration occurring when the original porosity exceeded 40 %
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Fig10: Graphs showing the results of XRD analysis of samples from the three lithologies at the A418 pipe. a Plot of olivine abundance against serpentine abundance, b plot of total abundance of fresh olivine against the fraction of fresh olivine in the olivine + serpentine component of the rock (a measure of how altered the rock is 1 being fresh and 0 being totally altered), and based on our modelling results, we can translate degree of alteration into a measure of original porosity (φ) with complete alteration occurring when the original porosity exceeded 40 %

Mentions: Here, we give an example of serpentisation observed at the Diavik A418 pipe, NWT, Canada, where deposit-scale variations in the abundance of serpentine and olivine were examined in detail. Over a hundred samples were taken from the 290 m bench in the open pit and analysed for mineral abundance using quantitative X-Ray Diffraction (XRD) by the Saskatchewan Research Council in Saskatoon. Sampling was carried out by Diavik geologists on a grid (Fig. 9a). The three main lithologies within the pipe were included in the sampling as follows: (1) MUD, a massive kimberlitic silt to fine sand with low abundances of olivine macrocrysts (>1 mm, <10 %); (2) MK, a massive, poorly sorted, mudstone clast- and olivine-rich volcaniclastic kimberlite with 20–50 % olivine macrocrysts (which includes altered and unaltered olivine); (3) FBK, a diffusely to well-bedded (mm-to-cm scale bedding) ash-aggregate, and olivine-rich volcaniclastic kimberlite with highly variable olivine macrocryst abundance (20–90 % in individual beds) (Porritt et al. 2012). The abundance of olivine and serpentine measured by XRD in the samples is shown in Fig. 9b, c, and the abundance of serpentine and smectite is seen in Fig. 9d. Smectite is a product of advanced alteration of other components of the kimberlite. The same data are plotted on diagrams of olivine versus serpentine content and olivine versus the ratio of olivine to serpentine plus olivine (Fig. 10)Fig. 9


Hydrothermal alteration of kimberlite by convective flows of external water.

Afanasyev AA, Melnik O, Porritt L, Schumacher JC, Sparks RS - Contrib Mineral Petrol (2014)

Graphs showing the results of XRD analysis of samples from the three lithologies at the A418 pipe. a Plot of olivine abundance against serpentine abundance, b plot of total abundance of fresh olivine against the fraction of fresh olivine in the olivine + serpentine component of the rock (a measure of how altered the rock is 1 being fresh and 0 being totally altered), and based on our modelling results, we can translate degree of alteration into a measure of original porosity (φ) with complete alteration occurring when the original porosity exceeded 40 %
© Copyright Policy - OpenAccess
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC4459436&req=5

Fig10: Graphs showing the results of XRD analysis of samples from the three lithologies at the A418 pipe. a Plot of olivine abundance against serpentine abundance, b plot of total abundance of fresh olivine against the fraction of fresh olivine in the olivine + serpentine component of the rock (a measure of how altered the rock is 1 being fresh and 0 being totally altered), and based on our modelling results, we can translate degree of alteration into a measure of original porosity (φ) with complete alteration occurring when the original porosity exceeded 40 %
Mentions: Here, we give an example of serpentisation observed at the Diavik A418 pipe, NWT, Canada, where deposit-scale variations in the abundance of serpentine and olivine were examined in detail. Over a hundred samples were taken from the 290 m bench in the open pit and analysed for mineral abundance using quantitative X-Ray Diffraction (XRD) by the Saskatchewan Research Council in Saskatoon. Sampling was carried out by Diavik geologists on a grid (Fig. 9a). The three main lithologies within the pipe were included in the sampling as follows: (1) MUD, a massive kimberlitic silt to fine sand with low abundances of olivine macrocrysts (>1 mm, <10 %); (2) MK, a massive, poorly sorted, mudstone clast- and olivine-rich volcaniclastic kimberlite with 20–50 % olivine macrocrysts (which includes altered and unaltered olivine); (3) FBK, a diffusely to well-bedded (mm-to-cm scale bedding) ash-aggregate, and olivine-rich volcaniclastic kimberlite with highly variable olivine macrocryst abundance (20–90 % in individual beds) (Porritt et al. 2012). The abundance of olivine and serpentine measured by XRD in the samples is shown in Fig. 9b, c, and the abundance of serpentine and smectite is seen in Fig. 9d. Smectite is a product of advanced alteration of other components of the kimberlite. The same data are plotted on diagrams of olivine versus serpentine content and olivine versus the ratio of olivine to serpentine plus olivine (Fig. 10)Fig. 9

Bottom Line: Excess volume of serpentine results in filling of pore spaces, eventually inhibiting fluid flow.Fast reactions and increases in the volume of solid products compared to the reactants result in self-sealing and low water-rock ratios (estimated at <0.2).Such low water-rock ratios result in only small changes in stable isotope compositions; for example, δO(18) is predicted only to change slightly from mantle values.

View Article: PubMed Central - PubMed

Affiliation: Institute of Mechanics, Moscow State University, Moscow, Russia.

ABSTRACT

Kimberlite volcanism involves the emplacement of olivine-rich volcaniclastic deposits into volcanic vents or pipes. Kimberlite deposits are typically pervasively serpentinised as a result of the reaction of olivine and water within a temperature range of 130-400 °C or less. We present a model for the influx of ground water into hot kimberlite deposits coupled with progressive cooling and serpentisation. Large-pressure gradients cause influx and heating of water within the pipe with horizontal convergent flow in the host rock and along pipe margins, and upward flow within the pipe centre. Complete serpentisation is predicted for wide ranges of permeability of the host rocks and kimberlite deposits. For typical pipe dimensions, cooling times are centuries to a few millennia. Excess volume of serpentine results in filling of pore spaces, eventually inhibiting fluid flow. Fresh olivine is preserved in lithofacies with initial low porosity, and at the base of the pipe where deeper-level host rocks have low permeability, and the pipe is narrower leading to faster cooling. These predictions are consistent with fresh olivine and serpentine distribution in the Diavik A418 kimberlite pipe, (NWT, Canada) and with features of kimberlites of the Yakutian province in Russia affected by influx of ground water brines. Fast reactions and increases in the volume of solid products compared to the reactants result in self-sealing and low water-rock ratios (estimated at <0.2). Such low water-rock ratios result in only small changes in stable isotope compositions; for example, δO(18) is predicted only to change slightly from mantle values. The model supports alteration of kimberlites predominantly by interactions with external non-magmatic fluids.

No MeSH data available.