June 2012 LIP of the Month

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Thermal metamorphism of wall-rocks provides an explanation for the causal link between LIP emplacement and mass extinctions

Clément Ganino, GEOAZUR, Université de Nice - Sophia Antipolis, CNRS/IRD, France; Clement.Ganino@unice.fr

The convincing temporal coincidence between LIP (large igneous province) emplacement and biotic crisis (Courtillot and Renne, 2003) faces the problem of determining a simple causal link. Self et al. (2006) demonstrated that during Deccan Trap emplacement, the emission of carbon dioxide is between ~220 and 1100 Gt par year resulting in a limited contribution to the environmental impact of the eruption. However, by measuring sulfur and chlorine concentrations in rare glass inclusions inside crystals and on glassy selvages preserved within lavas Self et al. (2008) shows that Deccan lavas contained 0.07 weight % of S and 0.04 weight % of Cl, yielding 3.5 teragrams of SO2 and 1 teragram of HCl for every cubic kilometer of lava erupted. They concluded that the environmental impact from even individual eruptions during past flood basalt activity was probably severe due to these components. LIP lavas are mostly flood basalts with similar volatile contents, and would be predicted to contribute to climate and biotic impact relative to their volume. However, this is clearly not the case, as shown in figure 1. The absence of a correlation between the LIP lava volume and severity of the associated impact on biosphere (Wignall, 2001 ; Ganino and Arndt, 2009), has led to a growing interest in the subvolcanic processes.


Figure 1 : Volume of LIP vs Genera extinction (Wignall, 2001).

During the emplacement of LIPs, widespread sub-volcanic plumbing systems are developed - commonly in sedimentary basins. The Karoo basin (Fig. 2a) exposed probably one of the world's largest sill and dike complexes covering an area of about two thirds of South Africa. The complex consists of interconnected and coalescing sill and dikes, some of them having the geometry of a saucer (e.g. Chevallier and Woodford, 1999). The volume of the Karoo sill and dike complex approximates 50,000 km3 (Hughes, 1982) and the magma was mainly emplaced during less than 1 m.y. about 183 Ma ago (e.g. Duncan et al., 1997). Another example is from the Siberian traps (Fig. 2) which shows the structure of the plumbing system in a cross section after Czamanske et al. (2002). This section, in the region of the Noril’sk Mine shows the mineralized intrusions and underlines the extensive area of contact between the magma and its wall-rock.


Figure 2 : (a) Simplified geological map of the Karoo LIP (Svensen et al., 2007). (b) Synthetic geological section of Siberian traps (Czamanske et al., 2002).

The emplacement of a large volume of magma with a temperature about 1200°C is responsible for two main processes: (1) the contamination of the magma by assimilated wall rock, and (2) the warming of the wall-rock resulting in the formation of contact metamorphic aureoles and the generation and emission of fluids.

The generation of fluids strongly depends on the nature of the wall-rock and can explain the severity of biotic crisis. A first distinction has to be done between a wall-rock composed of igneous rock (and their metamorphic derivates, gneiss and metavolcanic rocks) and sedimentary rocks (and their metamorphic derivates, marbles, slates, etc.). Igneous rocks do not contain abundant volatiles and do not generate large volume of volatiles - except water - when heated.  On the other hand, sedimentary rocks are often rich in volatiles (Fig. 3) and can be responsible for generation of significant volumes of gases. Carbonates contains about 50 wt% CO2 and evaporites contains about 50 wt% Cl or SO2. For this reason, the emission of volatiles during heating depends on the nature of the sedimentary wall-rock.


Figure 3 : Contents of volatiles of common rocks

Organic carbon-rich shales and coal

Organic matter may buffer the oxygen fugacity resulting in the formation of reduced fluids like CH4 (e.g., Connolly and Cesare, 1993). Heating of rocks with petroleum bearing pore fluids will generate excess methane. By computing mass balance calculation on aureole data, Svensen et al. (2007) showed that up to 1800 Gt of carbon dioxide was formed from organic material in the western Karoo Basin and about 27,400 Gt may have formed in the entire basin during the intrusive event. They conclude that this immense degassing resulted in the formation of numerous vent complexes and breccia pipes (Figure 4) and supports a causal relationship between the intrusive volcanism, the gas venting, and the Toarcian global warming.


Figure 4 : Breccia pipes in the Karoo Basin (Svensen et al., 2007)

Carbonates

Limestones and marlstones are characterized by calc‐silicate formation (Figure 5) during heating and release of CO2‐dominated and H2O‐bearing fluids.

Examples are the following reactions :

4 qtz + 3 dol + H2O = 3 cal + tlc + 3 CO2    (~325°C)

3 cal + 2 tlc = dol + tr + CO2 + H2O                         (~380°C)

2 qtz + dol = di + 2 CO2                               (~400°C)

2 qtz + 3 cal + tr = 5 di + 3 CO2 + H2O        (~420°C)

3 dol + di = 4 cal + 2 fo + 2 CO2                  (~500°C)

Brucite is often described from contact aureoles developed from dolomitic protoliths. It is not clear if it derived from the hydration of periclase (produced by temperatures as high as 700°C) or if it form directly in presence of water (at ~400°C).

dol = cal + per + CO2                                                (~700°C)

dol + H2O = cal + brc + CO2                                 (~400°C)


Figure 5 : Mineralogy of skarns showing the diversity of calc-silicate minerals whose formation emits carbon dioxide. (a) diopside marble from Aguablanca contact aureole, Spain, (b) garnet and scapolite marble from Aguablanca contact aureole, Spain , (c) garnet –rich skarn from Aguablanca contact aureole, Spain ,(d) serpentinized olivine and calcite “zebra-rock” from Panzhihua contact aureole, China, (e) brucite marble from Panzhihua contact aureole, China (f) olivine marble from Panzhihua contact aureole, China.

For the reasons developed above, pure limestones/marble is sterile whereas there are many reactions that emit carbon dioxide from dolostones and silica- or clay-rich limestones. The influence of the composition of the carbonates on their maximum potential release is illustrated in Fig. 6. Up to more than 30 wt% CO2 can theoretically be released by heating of impure dolostone.


Figure 6 : Triangular diagram showing high-temperature (850°C) metamorphic degassing of carbonates based on the studies of Aguablanca (Spain) and Panzhihua (China) contact aureoles

Evaporites

Evaporites recrystallize without significant volatile release unless the temperature is extremely high and SO2 is released from anhydrite breakdown. Evaporites with anhydrite and rock salt can generate SO2 and HCl at high temperatures. If organic matter or petroleum is present, CH4 and halocarbons (e.g. CH3Cl or CH3Br) can also form. Svensen et al (2009) recently confirmed halocarbon generation by experiments in which natural rock salt from the Tunguska Basin in eastern Siberia was heated to 275°C, to simulate contact metamorphism.

A synthesis of the fluid generation potential calculated for different lithologies has been proposed by Aarnes et al. (2011) (Figure 7). This study does not consider dolostones and the importance of the carbonate composition, but reveals the immense fluid generation potential for coal and halite.


Figure 7 : Total fluid generation potential calculated per unit area of selected host-rocks intruded by a 100 m thick sill (Aarnes et al. 2011). The numbers are given in wt% fluid and the pies are scaled to the total weight of the fluids released in the unit column. The initial fluid bulk compositions of the rocks are given in the boxes.

To quantify the volume of the emitted volatiles and evaluate their impact on environment, we have to estimate the intensity of the metamorphism - by studying the petrography of the aureole - and the volume of the aureole which is a fundamental parameter but  hard to determine. Usually contact aureole are thicker around felsic intrusion than around mafic intrusions and this is often explained by the very limited volume of fluids in mafic magma that is responsible for a thin “dry” aureole.

Here I refer to three documented case studies :

a)      Panzhihua intrusion part of the Emeishan LIP: This 20 km-long 5 km-thick intrusion has a particularly well exposed contact aureole due to an active open-pit extracting Fe-Ti-V ore (Figure 7).


Figure 8 : One of the open-pits of Panzhihua mining company with good exposure of the contact between the intrusive body (dark rock) and the metamorphosed dolomitic wall-rock (light rock).

Ganino et al. (2008) estimated the thickness (~300 m) of the metamorphic aureole in a mostly (>80%) dolomitic wall-rock. We then estimated the ratio between the magmatic degassing (from the intrusion) and the metamorphic degassing (from the heating of the wall-rock) as following : During the heating, the reaction

MgCa(CO3)2 = MgO +   CaCO3 + CO2

   (dolomite)          (periclase)     (calcite)

formed 240 grams of carbon dioxide for each kilogram of dolostone (Ganino and Arndt, 2009). That is to say ~22 Gt carbon dioxide were generated considering the volume of the aureole (estimated as ~17 km3). This quantity is much more than the 2 Gt CO2 produced by the magmatic body assuming the usual CO2 contents of mafic magmas. At the intrusion scale, metamorphic degassing is 11 times more important than magmatic degassing. The next step is to estimate the amount of degassing at the LIP scale. It is very difficult to evaluate the relative abundance of intrusions and lava flow in the case of Emeishan Large Igneous Province which is affected by regional tectonics due to its proximity to Himalaya. However, in other LIPs, an order of magnitude estimate is a volumic ratio of 1/3 for intrusions and 2/3 for the volcanic pile.

The Emeishan LIP intruded a dolomitic basin and making the hypothesis that all the intrusions developed similar contact aureoles in dolostones, the total volume of metamorphic degassing at the basin scale can be deduced: Following Self et al.’s (2006) estimation for the Deccan, 11,200 Gt CO2 were released by the Emeishan basalts, 5,600 Gt CO2 were produced by the intrusive bodies and 11 times more, that is to say 62,500 Gt CO2 were produced by contact metamorphism in the dolomitic wall-rock of the intrusive bodies. Globally, in the case of Emeishan LIP, the contact aureole released 3.7 times more carbon dioxide than the magmas.

b)      Karoo: From Total Organic Carbon analyses (Figure 9) in organic-rich shales around Karoo conduits, Svensen et al. (2007) demonstrated that nearly 2,000 Gt CO2 could have been produced from the Ecca Group in the western Karoo Basin. Following the idea the authors consider that the production potential for the total area with sill intrusions in organic rich shale is at least 27,400 Gt CO2.


Figure 9 : Carbon loss in metamorphic shale. Three dolerite sills (red) are present in the borehole, and the vitrinite reflectance and the total organic carbon (TOC) values document the heating effect on the sediments. The Whitehill Formation shows a significant reduction in TOC immediately above the sill intrusion. Part of the TOC reduction in the Whitehill Formation contact aureole can be explained by transformation to graphite.

c) Siberia

Svensen et al. (2009) explained that heating of organic-rich shale and petroleum bearing evaporites around sill intrusions led to greenhouse gas and halocarbon generation in sufficient volumes to cause global warming and atmospheric ozone depletion. At the sedimentary basin scale, they estimate that metamorphism of organic matter and petroleum could have generated more than 100,000 Gt CO2.


Figure 10 : Schematic evolution of the Tunguska Basin pipes and the venting of carbon gases and halocarbons to the atmosphere (Svensen et al., 2009). The pipe evolution is partly based on Von der Flaass and Naumov (1995) and Von der Flaass (1997): 1) Emplacement of sills into organic rich sediments and evaporites with petroleum accumulations (P). 2) Contact metamorphism of shale, evaporite, and petroleum, leading to gas generation and overpressure (shown as stippled lines). Melt is accumulating within evaporite sequences in the source region of the pipe. 3) Pipe formation and eruption. Glass in the breccias show that the magma was disrupted and fragmented in the source region before vertical transport and phreatomagmatism. Powerful eruptions led to wide craters and subsidence. Gases generated in contact aureoles are now released to the atmosphere. 4) Continued degassing from both magma and sediments through the pipes and the crater-lakes. Contact metamorphism of shallow organic-rich sequences (coal) along dikes, and appearance of the first lava flows further to the north in the basin. The inferred gas composition is shown in the frame, alongside the estimated carbon gas and halocarbon production potential for the pipe degassing alone.

Conclusion

Sedimentary rocks have to be considered as vast reservoirs for fluids that can be released during contact metamorphism following the emplacement of igneous intrusions. Devolatilization of vast volumes of sedimentary rocks may be responsible for massive release of these fluids and impose perturbations in the global climate. Numerous processes come into play during the emplacement of large volumes of hot magma into heterogeneous crustal rocks, and any attempt to quantify rigorously the total impact must take all of these into account. Other aspects as the variations in magma flux through the duration of the eruption (Chenet et al., 2009), and the variations in sea-level, or the composition and temperature of the atmosphere, and the configuration of the continents may enhance or mitigate the impact of the eruptions. However, for all the reasons developed here, the absence of a correlation between the volume of LIPs and the intensity of biotic crisis is possibly explained by the multiplication of the volatiles flux depending on the nature of the wall-rocks.


Figure 11 : Synthesis from Ganino and Arndt (2009). Percentage of generic extinctions (from Rhode and Muller, 2005) versus volume of erupted basalt from Courtillot and Renne (2003) for major LIPs. Two main populations of LIPs are evident, one for which the associated rate of extinction is close to the background rate (Columbia River to Ontong-Java plateau) and another for which the rate is far higher. Those in the latter group intrude sedimentary rocks that released abundant greenhouse or toxic gases.

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