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End-Permian Extinction amplified by Plume-Induced Release of Recycled Lithospheric Volatiles

Magmatic volatiles can be released to the atmosphere that can lead to climatic changes and environmental degradation that can be substantial, such as the production of acid rain, acidification of the ocean and depletion of ozone, the combination of which can result in the collapse of the biosphere. In the entire history of the Earth the largest of all the recorded mass extinction that have occurred took place at the end of the Permian, and it coincided with the emplacement of the Siberian large igneous province (Siberian Traps), which suggest that a key driver of global environmental change is large scale magmatism. The source and nature of volatiles in the Siberian large igneous province (SLIP) has remained contentious. In this paper Broadly et al. present halogen compositions of the subcontinental lithospheric mantle xenoliths that were emplaced prior to and after the Siberian flood basalts. In this study they found that the Siberian lithosphere is massively enriched in halogens from the infiltration of subducted volatiles that were derived from seawater and that a considerable amount, up to 70% of lithospheric halogens are assimilated into the plume and then released to the atmosphere during emplacement. The interaction of the plume and the lithosphere is therefore a key process that controlled the content of volatiles in large igneous provinces and, therefore, the extent of environmental crises, the led to mass extinctions during their emplacement.

Large igneous provinces (LIPs) result from the rapid eruptions of large volumes of magma over short geological timescales. The Siberian flood basalts (SFB) that were emplaced at the boundary of the Permian-Triassic comprised about 4 x10⁶ km³ of basalt in less than 1 My (Burgess et al., 2017). The eruption of the SFB took place contemporaneously with the main stage of the end-Permian crisis, and it has been hypothesised to have contributed to environmental changes that resulted in the loss of >90% of the all marine, and >70% of all terrestrial species (Wignall, 2001; Erwin et al., 2002). The mass extinction that occurred at the close of the Permian has been attributed to sharp fluctuations in global temperatures and/or increased levels of UV radiation that was the result of extensive depletion of the ozone, both of which are known to be associated with the magmatic release of volatiles to the atmosphere (Wignall, 2001; Beerling et al., 2007; Svensen et al., 20090; Grard et al., 2005; Guex et al., 2016), The amount of volatiles that are expected to have been released to the atmosphere from the SFB, assuming conventional plume source magmatism, is insufficient to account for the extent of environmental degradation and climatic fluctuations that occurred during the end-Permian crisis, which required an additional source of volatiles to be released during emplacement of the Siberian flood basalts (Self et al., 2006; Sobolev et al., 2009). In order to reconcile the volatiles missing from the SFB, it has been argued vigorously that large quantities of volatiles were released by contact metamorphism of a sedimentary sequence (Burgess et al., 2017; Svensen et al., 2009), by melting of recycled eclogite within the mantle plume (Sobolev et al., 2011) or by melting of the cratonic lithosphere (Guex et al., 2016). It is not known, however, the source of volatiles species that were responsible for climatic fluctuations and ozone depletion during the end-Permian crisis.

In this paper Broadley et al. report the first detailed halogen data (Cl, Br and I) data for peridotite xenoliths from 2 Siberian kimberlites:

i)                   Udachnaya at 360 Ma, that was emplaced before, and

ii)                Obnazhennaya at 160 Ma, after the eruption of the SFB about 250 Ma (Reichow et al., 2002; Ivanov et al., 2013).

The Udachnaya (n = 9) represent extraction of melt from the depleted cratonic mantle, whereas Obnazhennaya xenoliths (n = 6) contain cratonic lithosphere from the Archaean as well as melt residues that were generated from the SFB plume (Pernet-Fisher et al., 2015). A means to estimate the composition of the SFB eruption before eruption, and to quantify the contribution of the lithospheric mantle to the halogen budget of the SFB, is provided by determining the halogen composition of the cratonic subcontinental lithospheric mantle (SCLM; Udachnaya) and the plume residues (Obnazhennaya).

Lithospheric mantle – a reservoir for halogens

The SCLM retains geochemical homogeneities that were introduced through interactions with the mantle, crustal and subduction-related sources (Walker et al., 1989; McDonough, 1990), because of its isolation and non-conductive nature. Metasomatic components that are infiltrating the SCLM are sampled by mantle xenoliths. These xenoliths, which are transported rapidly to the surface during kimberlite volcanism, provide a window into the composition and origin of SCLM volatiles (Taylor et al., 1998; Howarth et al., 2014; Parry et al., 2015).

Neutron-irradiated noble gas mass spectrometry was used to determine the halogen and noble gas composition of Udachnaya and Obnazhennaya. It was found that the range of concentrations of Cl, Br and I within the Udachnaya and Obnazhennaya xenoliths are distinct, with average concentrations from crushing and stepped-heating consistently higher in the Udachnaya samples, which indicates that they originate from different domains within the SCLM. As was indicated during crushing experiments, halogen-bearing fluids present in the samples, have a similar range of Br/Cl and I/Cl values in both Udachnaya and Obnazhennaya xenoliths. The xenoliths show evidence for distinct endmember halogen compositions during stepped-heating of crushed residues. Similar Br/Cl and I/Cl values of more mantle-like values are retained by Udachnaya to those that were measured during crushing.

Helium isotope data that had been published previously also vary between xenolith suites, with the average ³He/⁴He value of Udachnaya (0.4 ± 0.3 R_A, R = ³He/⁴He of atmosphere = 1.38 x 10^-6) consistently lower than Obnazhennaya (4.2 ± 0.9 R_A), which has a maximum ³He/⁴He value (8.4 ± 0.3 R_A) that is similar to mid-ocean ridge basalt (MORB) (Barry et al., 2015). ³He/⁴He, Br/Cl and I/Cl seem to be coupled, which shows that fluids within the Obnazhennaya xenoliths represent a mixture that is rich in ⁴He, Br and I, and a component that has mantle-like ³He/⁴He and halogen composition. The lower ³He/⁴He and the elevated Br/Cl, I/Cl values that are characteristic of Udachnaya xenoliths are considered to be representative of the ancient metasomatised section of the SCLM (metasomes) that was present prior to the major influence of the SFB mantle plume. Contrasting with this, it is suggested by Obnazhennaya helium and halogen data a plume-like origin that mixed substantially with volatiles that were SCLM-derived. Similar rare earth element (REE) patterns and ages of melt extraction between Obnazhennaya xenoliths and the SFB (Pernet-Fisher et al., 2015) are supportive of the observations of a plume-like volatile source in Obnazhennaya xenoliths.

Within the metasomatised section of the SCLM the initial inventory of halogens prior to the impingement of the plume can be estimated by using the composition of the Udachnaya xenoliths. The Siberian SCLM transitions from depleted harzburgite and Iherzolites to peridotites that were predominantly metasomatised at a depth of 180-190 km (Griffin et al., 2002). The metasomatised portion of the Siberian SCLM contains about 0.6-1.5 x 10¹⁹, 1.6-2.7 x 10¹⁷ and 0.5-1.1 x 10¹⁴ kg of Cl, Br and I, respectively, if it is assumed that the Udachnaya xenoliths are representative of metasomatised peridotite xenoliths in the lower 30 km of the SCLM, and taking the surficial area of the Siberian Craton (4 x 10⁶ km²). Therefore, the metasomatised Siberian SCLM is enriched in Cl, Br and I by factors of up to 125, 675 and 100 times, respectively, relative to the depleted MORB mantle (DMM) (Kendrick et al., 2017). Therefore, the SCLM is notably larger and more heterogeneous halogen reservoir than has been previously considered, and may impart a major influence on global volatile cycles (Burgess et al., 2002).

LIP emplacement – release of lithospheric halogens

According to Broadley et al. even small proportions of the halogens present in the base of the Siberian SCLM were released to the surface it would have important consequences for the halogen cycle, because of the comparatively large quantity of halogens present in the base. When halogens are erupted to the stratosphere they catalyse reactions that destroy ozone, which raises the levels of UV radiation that reaches the surface of the Earth that are biologically damaging (Johnston, 1980; Daniel et al., 1999). Potentially, the transit of the SFB through the SCLM could have liberated major amounts of halogens, as well as other volatiles, to the atmosphere, which would have contributed to the decline of species and extinction during the end-Permian crisis.

Udachnaya xenoliths formed at greater depths, >50 km difference (Howarth et al., 2014), in the lithosphere compared to Obnazhennaya xenoliths. The identification of metasomatic signatures that were Udachnaya-like in Obnazhennaya is an indication that the volatiles that are present in the metasomatised basal SCLM were mobilised and ascended to shallower levels of the SCLM. There are trace element signatures in Obnazhennaya xenoliths that fall within the values reported for the SFB (Howarth et al., 2014), strong depletions of P-platinum-group elements that are not characteristic of cratonic lithosphere and Os isotopic composition that are consistent with an age of formation that is similar to the time of plume impingement (Pernet-Fisher et al., 2015). It is indicated by these characteristics that the part of the lithosphere sampled by Obnazhennaya kimberlite represent the melt residue of the SFB plume (Pernet-Fisher et al., 2015). The identification of signatures of metasomatised SCLM within the Obnazhennaya xenoliths suggests, therefore, that the SFB plume impacted the base of the lithosphere, with the results that the melts incorporated volatiles that had been mobilised from the deeper metasomatised SCLM, after which it was erupted at the surface or stalled in the lithosphere. The contribution of volatiles derived from the SCLM to the SFB plume can therefore be estimated by using differences in the halogen and noble gas signatures between the Udachnaya (metasomatised SCLM) and Obnazhennaya (SFB + metasomatised SCLM) xenoliths.

The amount of assimilation from the SCLM (Udachnaya) can be estimated by the extent of mixing between the 2 sources, if it assumed that the melt residues in Obnazhennaya lithosphere had a starting composition that was similar to the SFB plume (12.7 R_A and mantle-like Br/Cl and I/Cl values (Kendrick et al., 2012; Basu et al., 1995). In order to reconcile He, Br and I systematics between the SFB plume and the SCLM component that is represented by Udachnaya requires that up to 70% of volatiles in Obnazhennaya are derive d from SCLM. Also, any potential overprinting that is related to crustal assimilation which affects the halogen composition of the melt can be excluded because the rapid transport of xenoliths to the surface  via kimberlite volcanism limits interaction with the surrounding crust (Kelly & warthog, 2000; Alexeev et al., 2007).

It is estimated that the total fluxes of Br and I to the atmosphere are 2.3 c 10¹³ kg and 9.6 x 10¹⁰ kg, respectively, taking the volume of Cl that was degassed as calculated from the SFB melt inclusions (8.7 x 10¹⁵ kg) (Aiuppa et al., 2005). It is assumed by this calculation that the melt had Br/Cl and I/Cl values that are similar to Obnazhennaya and that the halogens are not fractionated during degassing (Ross et al.,2005). Reactive gases HCl and Br are injected into the lower stratosphere by explosive eruptions (~12-25 km) which deplete ozone levels; whereas in effusive eruptions soluble HCl is washed out before it reaches the stratosphere (Kendrick et al., 2012). When only explosive events are considered (20-30 % of the SFB (Millard et al., 2006), a rate of injection into the stratosphere is about 75% (Black et al., 2012) and the amount of Cl that is measured within the SFB (Aiuppa et al., 2005), (⅔ of the total eruptive volume over 300 ky) (Burgess & Bowring, 2015) is equivalent to 0.5-1.0 Pinatubo (1991-1992) eruptions, which caused a 15-20% reduction in global ozone (Westrich & Gerlach, 1992) every year for 300 Ky. It is predicted by models of ozone depletion during the eruption of the SFB that use estimated stratospheric HCl fluxes predict a 30-55% reduction in ozone over the same eruptive timeframe. These stratospheric HCl flux estimates are 5 times lower than that predicted from the SFB melt inclusions (8.7 x 10¹⁵ kg) (Aiuppa et al., 2005). Also, the consequences of Br degassing on the depletion of ozone is not taken into account in these estimates. During the emplacement of the SFB the large release of Br to the stratosphere, as indicated by the high Br/Cl value of the Siberian SCLM, probably exacerbated further the depletion of the ozone. The capacity of Br to deplete ozone is much greater, about 45 times more effective (Daniel et al., 1999), and could have reduced the level of ozone by a further 20% during the SFB eruption. The scale of the halogen degassing fluxes is sufficient to incur a near to total loss of global ozone during the end-Permian crisis, though there are several uncertainties in the rate and magnitude of volatile degassing during the magmatism of the SFB. Melt inclusions within the SFB contain 0.01-0.33 wt% Cl (Sobolev et al., 2011; Aiuppa et al., 2005), which is an order of magnitude higher than the maximum Cl concentrations in other LIPS. It has been found that inclusions with high concentrations of Cl were equally enriched in other volatile species such as fluorine (1.95 wt%) and sulphur (0.51 wt%) (Aiuppa et al., 2005). A very low degree of partial melting, or the assimilation of volatiles from another unknown reservoir would be required to form such high volatile content within these melts, from an initial DMM-like composition. Volatiles can be concentrated in the melt fraction by low degrees of partial melting, however, the high contents of Mg measured within the melt inclusions precludes low degrees of partial melting, which suggests that the assimilation of volatile-rich material is the most likely cause of the high volatile contents in the SFB.

The composition of the Obnazhennaya xenoliths, which were assumed to represent plume melt residues, can be used to estimate whether the SCLM is the potential source of volatile enrichment in the SFB. In order to account for the elevated olivine Fo >92 (forsterite content, Fo%: molar Mg/(Mg + Fe) x 100) (Pernet-Fisher et al., 2015) and the Chlorine composition of this melt can therefore be estimated by using a batch melting model (Shaw, 2006). Partition coefficients for Cl between olivine (D_Cl^Ol/Melt =1.9 x 10^-2) and pyroxene (D_Cl^Pyx/Melt = 1.5 x 10^-2) that were determined experimentally were used to calculate the Cl concentration of the melt at 1,500^oC, prior to eruption (Joachim et al., 2015). Cl concentrations of 0.1-0.2 wt% in the melt were yielded by using the range of Cl concentrations in the olivine and pyroxene minerals from Obnazhennaya xenoliths. These estimates are considered to be upper limits, given the potential for an unknown proportion of intact fluid inclusions to remain after crushing, therefore Cl data that is based on stepped heating are likely to overestimate the abundance of Cl within the minerals. It is notable, however, that melt Cl estimates are consistent with values that have been published for the eruptive melt composition (0.01-0.33 wt% Cl) (Sobolev et al., 2011; Basu et al., 1995), which provides comfort in the assumptions that were made and confirming that the SFB melt was already Cl enriched prior to eruption. According to Broadley et al. these arguments therefore constitute further, albeit indirect, support for the SCLM origin for the majority of halogens in the SFB melts.

End-Permian extinction – implications

It was considered that the depletion of ozone during the end-Permian crisis led to the decline of the dominant terrestrial plant species at the time, which was followed by the rapid expansion of opportunistic lycopsids (Visscher et al., 2004). The global distribution of the microspores that have been preserved from these emerging lycopsids exhibit features that are indicative of a failure in the normal development process of the spores. It is suggested by the global dispersion of these mutagenic spores that this was a reaction to global stress factors that are not likely to be related to global temperature changes from the release of gasses such as SO₂ and CO₂ during the emplacement of the SFB (Beerling et al., 2007). A 5-fold increase in the occurrence of mutagenic malformations, as well as complete sterilisation (Benca et al., 2018) has resulted from experiments on the effects of end-Permian UVB regimes on modern conifers. The result of this would have been widespread deforestation and the collapse of the terrestrial biosphere, which indicates that the depletion of ozone was a major contributing factor in the mass extinction event in the end-Permian (Beerling et al., 2007; Benca et al., 2018).

The peak of mutagenic spores occurred prior to the rapid negative shift in δ¹³C in end-Permian carbonates that has been attributed to the extinction of calcified marine life (Visscher et al., 2004). The δ¹³C excursions coincide with a change from eruptions that were predominantly extrusive to eruptions that were predominantly intrusive of the Siberian LIP (Burgess et al., 2017). It was considered that the emplacement of sills into sediments that were rich in volatiles released vast quantities of volatiles, which included CO₂ and halocarbon gasses to the atmosphere, which led to climate change and depletion of the ozone (Burgess et al., 2017; Svensen et al., 2009). It is clear from the palynological evidence (Visscher et al., 2004), however, that prior to the onset of the marine extinction, a reduction in terrestrial biodiversity was occurring. Also, there is evidence of reduced rates of sedimentation prior to the Permian-Triassic boundary which indicates there was a global regression of eustatic sea level, which was potentially caused by a lowering of global temperatures and the onset of glaciation (Baresel et al., 2017). The rapid decrease in temperatures has been linked to SO₂ emission to the atmosphere during the eruptive phase of the SFB.

The concurrent timing of the eruptive phase of the SFB and the evidence from palynology for depletion of the ozone is not consistent with the idea that the degassing of sedimentary brines in the late intrusive phase of igneous activity was the primary cause of halogens causing the destruction of the ozone. It was shown in this study that the majority of halogens in the SFB were added during plume-lithosphere interaction, which was followed by their subsequent release to the atmosphere during explosive eruptions. According to Broadley et al. enrichments of sulphur, which co-existed with halogens in SFB (Aiuppa et al., 2005), may also have been derived from the SCLM. It is therefore suggested by evidence of a decline in terrestrial species prior to the Permo-Triassic boundary that the release of halogens and gaseous sulphur species, as well as the subsequent decrease of ozone and global temperatures, respectively, were the predominant factors involved in the initiation of mass extinction of the end-Permian. The change of eruptive phase from explosive to intrusive may have played a role in the extension of the extinction from a phenomenon that was mainly terrestrial to a global event.

Volatiles in the Siberian lithosphere – subducted origin

It is indicated by the high concentrations of halogens in the Udachnaya xenoliths that the Siberian SCLM has been enriched in volatiles by metasomatic processes. The values of Br/Cl and I/Cl that are similar to fluids trapped within minerals in the altered oceanic crust (AOC), which suggests that subduction-derived fluids drove the metasomatism of the Siberian SCLM (Chavrit et al., 2016). When combined with the noble gases, the Udachnaya xenoliths show an evolution from seawater-like ³He/⁴He, Br/Cl and I/Cl values, to values that have increasingly radiogenic ³He/⁴He values that are enriched in Br/Cl and I/cl values, which suggested further that metasomatic fluid originated as seawater that has subsequently evolved, during subduction or within the SCLM, due to fractionation of halogen and the production of ⁴He from the decay of U to Th (Barry et al., 2015). Values of up to 7.7% (Jacob et al., 1994) are exhibited by eclogite xenoliths from the Udachnaya kimberlite, which lie outsider the normal mantle range (+5.4 ± 0.2%) (Eiler, 2001) and indicate that they originated as oceanic crust that had undergone low temperature alteration (Jacob et al., 1994). It has been shown that eclogites that formed from the subduction of oceanic crust retain the halogen and oxygen isotopic signatures of the oceanic crust protolith during metamorphism, which provides a mechanism for the delivery of halogens that were derived from subduction to the Siberian SCLM (Svensen et al., 2001; Philippot et al. 1998).

Up to 70% of volatile content to the Siberian plume has been shown in this study to have originated from assimilation of metasomatised lithospheric material. Therefore, an integral role in controlling the volatile content of LIPS is played by the composition of the SCLM, and as such the overall effect that they have on the global environment. It appears that the Siberian SCLM volatiles originate from the subduction of a component within the AOC that is derived from seawater, based on the evolution of the Br/Cl, I/Cl and ³He/⁴He values of the xenoliths from seawater – to AOC – like values, coupled with the AOC-like δ¹⁸O values within eclogite xenoliths from Udachnaya. Enrichment of volatiles, which are derived from seawater in the Siberian SCLM, provided the plume with an abundant supply of halogens, which had been released to the atmosphere during eruption and resulted in reductions of ozone levels that were extensive globally and the decline of the biosphere. The SCLM is also a major repository for other volatiles species that had been subducted, including sulphur and carbon (Callegaro et al., 2014; Foley & Fischer, 2017), which can also contribute to environmental degradation during plume -lithosphere interaction and emplacement of a LIP. Therefore, the SCLM can store volatiles that have been subducted that can be mobilised periodically and released to the surface of the Earth and the atmosphere during deep-seated melting and volcanism, which lead to devastating impacts on the global environment.

Broadley, M. W., et al. (2018). "End-Permian extinction amplified by plume-induced release of recycled lithospheric volatiles." Nature Geoscience 11(9): 682-687.