Earth and Planetary Science Letters 392 (2014) 39–49

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Earth and Planetary Science Letters

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F, Cl, and S concentrations in olivine-hosted melt inclusions frommafic dikes in NW Namibia and implications for the environmentalimpact of the Paraná–Etendeka Large Igneous Province

Linda Marks a,∗, Jakob Keiding b, Thomas Wenzel a, Robert B. Trumbull b, Ilya Veksler b,Michael Wiedenbeck b, Gregor Markl a

a Universität Tübingen, Matematisch-Naturwissenschaftliche Fakultät, FB Geowissenschaften, Wilhelmstrasse 56, D-72074 Tübingen, Germanyb GFZ German Research Center for Geosciences, Telegrafenberg, D-14473 Potsdam, Germany

a r t i c l e i n f o a b s t r a c t

Article history:Received 8 January 2013Received in revised form 12 November 2013Accepted 31 January 2014Available online 25 February 2014Editor: T. Elliott

Keywords:Large Igneous ProvincesEtendekamelt inclusionshalogensmass-extinction

Large Igneous Provinces (LIPs) have been proposed to trigger mass-extinction events by the releaseof large quantities of volcanic gases which results in major climatic perturbations causing worldwideecological stress and collapse. A prerequisite for understanding the proposed link between LIP volcanismand biological crisis is reliable information about the total gas emissions during these events. In thispaper we present the first estimations of total F, Cl and S emissions from the Paraná–Etendeka LIP inthe South Atlantic. Data from this province are of special interest because it is among the world’s largestLIPs but is not associated with a mass extinction event. We have determined pre-eruption concentrationsof F, Cl and S by in situ analysis of melt inclusions preserved in olivine phenocrysts from basaltic dikesin the Etendeka province of NW Namibia. The melt inclusions have Mg-rich basaltic bulk compositionswith about 8 to 18 wt.% MgO, overlapping the compositional range of the host rocks. A major featureof the melt inclusions is their wide variation in major and minor element concentrations, including F,Cl and S. This is attributed to trapping of variably-mixed melt fractions during crystallization of olivinein the roots of the dike system. Fluorine concentrations vary from about 190 to 450 μg/g, Cl from <10to 125 μg/g and S from <30 to 1100 μg/g. All inclusions were re-homogenized in heating experimentsand the lowest concentrations may be due to partial leakage of S and halogens. Therefore, the maximumvalues are considered best estimates of the true melt concentrations.These melt inclusion data are combined with the volume of extruded magma in the province (2.2 to2.35 × 106 km3) and with published degassing efficiencies to calculate total emissions from the Paraná–Etendeka LIP of 600–1200 Gt fluorine, 70–470 Gt chlorine and 3100–5400 Gt sulfur. The estimated sulfuremissions are similar to those from the similar-sized Deccan and Siberian LIPs, both of which are relatedto mass extinctions, but the Paraná–Etendeka LIP produced much lower emission of halogens. This mayhelp explaining the smaller ecological impact of the Paraná–Etendeka magmatism. These results supportthe proposal that massive halogen emissions related to LIP volcanism may be an important factor forextinction scenarios because of global destruction of the ozone layer.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Large Igneous Provinces (LIPs) represent vast surface outpour-ing of magma derived from the Earth’s mantle over a very shortgeological time. Improved radiometric dating of LIPs has drawngrowing attention to their synchronicity with major mass ex-tinctions (e.g. Campbell et al., 1992; Courtillot and Renne, 2003;Olsen, 1999; Saunders and Reichow, 2009; Sobolev et al., 2011;

* Corresponding author.E-mail address: marks.linda@web.de (L. Marks).

http://dx.doi.org/10.1016/j.epsl.2014.01.0570012-821X/© 2014 Elsevier B.V. All rights reserved.

Stothers, 1993; Wignall 2001, 2005) and the causal connectionbetween the eruption of LIPs and extinction events is of greatcurrent interest. However, the details of how LIP volcanism is re-lated to global biological impact are still poorly understood (Bryanand Ferrari, 2013; Knoll et al., 2007; White and Saunders, 2005).Monitoring present-day volcanic eruptions by ground- and air-based methods as well as remote sensing provides direct dataon the masses of gas, aerosol and particle emissions from activevolcanoes, as well as on their dispersal in the atmosphere. Esti-mates of atmospheric impact from fossil volcanic systems basedon study of the volcanic rocks suggest that the main culprit for

40 L. Marks et al. / Earth and Planetary Science Letters 392 (2014) 39–49

Fig. 1. (a) Map showing the present day extent of the Paraná–Etendeka province placed in its position during the early break-up phase of west Gondwana (adapted fromPeate, 1997). (b) Schematic geological map of the Henties Bay–Outjo dike swarm (HOD) in the Damara Belt of Namibia based on high-resolution aeromagnetic data LandsatETM+ and Google Earth images. Sample locations are indicated by stars.

climatic perturbations are volcanic gases such as CO2, F, Cl andS, but their relative effects on climate-forcing and environmen-tal impact, including mass extinction, are strongly debated (e.g.Kutterolf et al., 2013; Self et al., 2005; Svensen et al., 2007;Saunders and Reichow, 2009; Thordarson and Self, 1996; Wignall,2001).

Comparing the timing of LIPs with mass extinction revealsa close correspondence in five cases (Wignall, 2001), the mostcompelling and dramatic of which are the end-Permian SiberianTraps and the Deccan traps in India which coincide with theCretaceous–Tertiary boundary (Black et al., 2012; Sobolev et al.2009, 2012; Self et al. 2006, 2008; Tang et al., 2013; Vogt, 1972).Comparatively little is known about volcanic gas fluxes from otherLIPs of similar size, but without correlation with a major extinctionevent, such as the Paraná–Etendeka and the Ontong-Java Plateau.It would seem crucial to also examine these possibly ‘benign’ LIPsto improve our understanding of the causal relationship betweenvolcanic emissions and mass extinctions. This paper presents thefirst constraints on volcanic emissions from the Paraná–Etendekaprovince based on measured S, Cl and F concentrations in olivine-hosted melt inclusions from basaltic dikes in NW Namibia.

Melt inclusions have become a popular tool for studying mag-matic processes since they often provide valuable information thatare hidden or lost in bulk rocks. For example, melt inclusions inearly crystallizing phenocrysts like Mg-rich olivine have been usedas a proxy for the parental melts in a basaltic system (Hanski et al.,2010; Kamenetsky et al., 2010; Keiding et al., 2011; Sobolev, 1996;Sobolev et al., 2011; Yaxley et al., 2004). For the purpose of thisstudy, the greatest advantage of melt inclusions is that they aretypically trapped at higher pressures than the erupted magma.Being contained within relatively incompressible phenocrysts, theinclusions preserve information on the dissolved volatile concen-trations that are degassed and lost from the magma during shallowemplacement and eruption. Olivine is the best such container formelt inclusions in basalt because of its high mechanical strengthand lack of cleavage, optical transparency and relatively simplechemical composition. Although erupted basalts arguably providethe most direct information on volcanic emissions, olivines are rareor absent in the Paraná–Etendeka lavas (e.g., Marsh et al., 2001;Peate, 1997) and where present, the crystals are altered and they

contain few if any melt inclusions. Therefore we focus on basalticdikes, which are less fractionated and better preserved than thelavas. Previous studies of olivine-bearing dikes from the HentiesBay–Outjo dike swarm in NW Namibia demonstrated that theyrepresent the magma feeder system for the Etendeka flood basalts(see below) and that they commonly contain olivine-hosted meltinclusions of suitable size for analysis (Keiding et al., 2011). Meltinclusions from these dikes provide the opportunity to investigatethe volatile characteristics of the Etendeka basaltic magmas and toput first-order constraints on the atmospheric loading of volatileelements from the Paraná–Etendeka LIP.

2. Geological background and sample details

The Etendeka volcanic province in NW Namibia is part of theEarly Cretaceous Paraná–Etendeka LIP (Peate, 1997), which is con-sidered to be closely related to the Tristan da Cunha mantleplume in space and time (e.g. Ewart et al., 2004a; Hawkesworthet al., 1992; Marsh et al., 2001; O’Connor and Duncan, 1990; Peateet al., 1990; Renne et al., 1996). The Etendeka province is onlyabout 5% (Fig. 1a) of the total Paraná–Etendeka province (Milneret al., 1995), but is disproportionately well studied because ofexcellent exposures. The province consists of voluminous basalticand silicic volcanic rocks, contemporaneous intrusive ring com-plexes, and extensive dike swarms (e.g., Ewart et al., 1998, 2004a,2004b; Marsh et al., 2001; Miller, 2008; Trumbull et al., 2003,2007). Geochemical comparison of the eruptive sequences and in-dividual lava flows reveal that the Paraná and Etendeka Provincescan be correlated across the ocean basin (Marsh et al., 2001;Peate, 1997) and were erupted predominately at 130–135 Ma (Mil-ner et al., 1992, 1995; Peate et al., 1992; Renne et al., 1992, 1996;Turner et al., 1994).

Post-breakup erosion in NW Namibia has removed much of theEtendeka volcanic succession and exposed extensive dike swarmsin underlying rocks of the Neoproterozoic Damara Belt. The largestof these is the Henties Bay–Outjo dike swarm (HOD), whose ex-tent is shown in Fig. 1 based on satellite images and aeromagneticmapping. The dikes strike mainly NE–SW following major struc-tures of the Damara Belt. However, coast-parallel dikes are alsoprominent, especially in coastal areas in the northern Etendeka

L. Marks et al. / Earth and Planetary Science Letters 392 (2014) 39–49 41

Fig. 2. Variations diagrams of TiO2 versus Zr and Ti/Zr versus Zr/Y showing the close chemical correspondence between the HOD dikes and the Tafelberg (Etendeka) andGramado (Paraná) basalt types. Data fields after Ewart et al. (1998).

province where many dikes have been shown to be syn-tectonicwith listric faults offsetting the Etendeka lavas (e.g. Ewart et al.,2004a). The compositional range of HOD dikes is from basalt torhyolite, but by far the most common constituents are tholeiiticlow-Ti basalts equivalent in composition to the main ‘Tafelberg-type’ Etendeka lavas (Trumbull et al., 2004). The basaltic dikes havereceived most attention and despite some compositional diversity,most can be confidently assigned to known compositional sub-types of the Paraná–Etendeka flood basalts (e.g. Ewart et al. 2004;Marsh et al., 2001; Keiding et al., 2013; Thompson et al., 2001;Trumbull et al., 2004, 2007).

Of particular interest for this study are the picritic, olivine-richdikes described by Thompson and Gibson (2000) and Keiding et al.(2011). Eight dike samples from the southern Etendeka province(Fig. 1) were chosen for this study based on previous work thatdemonstrated the abundance of pristine crystals of high-Mg olivinewith abundant primary melt inclusions. Descriptions of the dikesetting, melt inclusion occurrence and crystallization conditionscan be found in Thompson et al. (2001), Keiding et al. (2011) andKeiding et al. (2013). Of particular interest for this study are the pi-critic, olivine-rich dikes described by Thompson and Gibson (2000)and Keiding et al. (2011). Eight dike samples from the southernEtendeka province (Fig. 1) were chosen for this study based onprevious work that demonstrated the abundance of pristine crys-tals of high-Mg olivine with abundant primary melt inclusions(Thompson et al., 2001; Keiding et al., 2011, 2013). Logically, theseolivine-bearing samples are geochemically less evolved than theEtendeka lavas, which lack olivine, but the dikes are regarded bymost workers as genetically related to the main lava series. Fig. 2shows an example of whole-rock compositions of the dike samplesfrom this study overlapping the primitive end of compositionalfields for the main Southern Etendeka lava type (Tafelberg) andits equivalent in Brazil (Gramado), taken from Ewart et al. (1998).

The dike samples are holocrystalline, with variable grain sizesdepending on thickness of the host dike. The olivine phenocrystsset in a fine-grained matrix of plagioclase, clinopyroxene and mi-nor amounts of oxide phases and apatite. No primary hydrousminerals occur in these rocks. Secondary minerals include sericitein altered plagioclase and local biotite rims around oxide grains.The olivine grains are equant, subhedral and vary in size fromabout 1 to 6 mm. They contain euhedral to subhedral inclusionsof dark brown Cr–Al spinel and melt inclusions, the latter rang-ing from about 5 to 100 μm in diameter. The melt inclusions aretypically rounded and partly to completely re-crystallized, withdaughter minerals of Al-rich clinopyroxene, Al-rich orthopyroxene(Fig. 3) and spinel. Spinel may be of mixed origin. It is ubiquitousin all melt inclusions and probably a daughter phase in many, butconsidering the abundance of spinel solid inclusions in the hostolivine, and the variable sizes of inclusion-hosted grains, it is likely

that some inclusions represent mixtures formed by heterogeneousentrapment of melt trapped along solid spinel crystals.

3. Analytical techniques and methods

In order to re-homogenize the crystallized melt inclusions,olivine grains containing inclusions were first handpicked andloaded into a Pt-capsule, which was put into an oxygen fugacity-controlled gas mixing furnace at 1-atm pressure. The olivine grainswere heated to 1350◦C for 15 minutes only and then rapidlyquenched in air to minimize potential loss of volatile contents dueto diffusion (e.g., Buchholz et al., 2013; Hauri, 2002). The oxygenfugacity in the furnace was kept at 3–4 log units below the quartz–fayalite–magnetite (QFM) buffer to avoid oxidation of the meltinclusions and host olivine during heating (Buchholz et al., 2013).This treatment resulted in homogeneous glassy inclusions (Fig. 3d),some containing shrinkage bubbles and remnant spinel. Individualolivine grains were then mounted in epoxy and hand-polished un-til the desired melt inclusion was exposed at the surface.

The major element composition of host olivines and their re-spective melt inclusions (electronic supplement) were determinedon a JEOL 8900 Superprobe at the University of Tübingen. Naturaland synthetic standards where used for the calibration. Operat-ing conditions where 15 kV accelerating voltage and 10 nA beamcurrent. A focused beam was used for all olivine analyses anda 5–10 μm defocused beam was used for all inclusion glasses, de-pending on the size of the melt inclusion. Sodium was analyzedfirst in the sequence to minimize loss by volatilization under theelectron beam. Where size permitted, melt inclusions were ana-lyzed at up to 2–3 points to check for homogeneity and the resultswere generally indistinguishable within error. Well-characterizedstandard reference materials were used to assess the electron mi-croprobe data quality. Analytical uncertainty (as assessed by repli-cate analyses) was typically within 5% for Na, Mg, Ca, Ti, Al, Si andFe and within 20% for K, P and Mn (1 s.d.).

The possibility of F, Cl and S analysis was tested by electronmicroprobe, but the concentrations were found to commonly betoo low for meaningful interpretation. Therefore, the concentra-tions of these elements were measured by secondary ion massspectrometry (SIMS) using a Cameca IMS 6f instrument at the GFZPotsdam. The sample surface was first cleaned in an ultrasonicbath with ethanol and then gold-coated prior the SIMS analyses.The melt inclusions were bombarded with a 1 nA Cs+, 10 kV pri-mary probe that had a 5 μm diameter. Our setup used a massresolving power M/�M ∼ 4500. To minimize the possible effect ofsurface contamination, particularly for Cl, each analysis consistedof a 2 min raster preburn (15 μm, 5 nA) followed by a 5 min spotpreburn (5 nA) before collection of 10 cycles of the masses 18O,19F, 30Si, 32S and 37Cl. The total analysis time was 15 min per spot.

42 L. Marks et al. / Earth and Planetary Science Letters 392 (2014) 39–49

Fig. 3. Photographs of olivine-hosted melt inclusions before (a–c) and after homogenization (d). (a) Olivine with abundant melt inclusion and (spinel) mineral inclusions.(b) Image of melt inclusion probably formed by heterogeneous entrapment of spinel crystal (seen to the left) and melt droplet that is now partially crystallized to orthopyrox-ene (opx.). (c) Backscattered electron (BSE) image of crystallized melt inclusion consisting of glass and clinopyroxene (cpx.) and spinel daughter minerals. (d) Microphotographof two homogenized melt inclusion after heating experiments.

The ion counts were normalized to both 18O and 30Si masses. Re-peated measurements of the reference basalt glasses VG-2 (USNM111240/52) and VG-A99 (USNM113498/1) were used to determineanalytical reproducibility and to calculate concentrations from theion ratio data. Reference values used for these glasses, from compi-lations in GeoReM (http://georem.mpch-mainz.gwdg.de/) are listedin Supplemental Table S1. The stability of the 30Si signal was foundto be slightly inferior to that of 18O over the period of the analyses,leading to a poor repeatability of about 10% for normalizing againstthe 30Si signal vs. <5% (1 s.d.) for the 18O signal. Therefore, we cal-culated the concentrations of F, Cl and S in melt inclusions basedon a normalization using 18O as the reference isotope, and oxygenvalues calculated from major element compositions determined byelectron microprobe. Accuracy of analyses was assessed using theVGA-99 as a quality control material, yielding relative uncertaintyof 6% for S and 12% for Cl. The accuracy of F concentration couldnot be rigorously assessed due to the lack of independent data forVGA-99. As the 19F− count rates were as high or higher than for32S and 37Cl, we expect the accuracy will be similar to or betterthan those for the other two elements.

4. Results

4.1. Olivine compositions

Olivine compositions range from Fo84 to Fo94 (Fig. 4; Supple-mental Table S2), with variations in Fo content within samplesof up to 8 mol% (Fig. 4). Individual grains are mostly unzoned(within-grain variations typically less than 1 mol% Fo), except forlocal 5–10 μm thick rims on crystals with the highest Fo con-tent. These Fe-enriched rims indicate re-equilibration with themore evolved magma during emplacement (Thompson et al., 2001;Keiding et al., 2011). The olivine Fo content and bulk rock Mg-number (Mg# = 100×MgO/[MgO+0.9FeOT], on a molar basis) for

Fig. 4. Bulk rock Mg# of the Etendeka picritic dike rocks versus Fo content of theolivine crystals found within them. The dashed lines represent olivine-melt equilib-rium compositions for KD = 0.31–0.35 (Ulmer, 1989). The three olivine types aredefined by the location compared to the equilibrium lines; Type 1 (squares) plotsbetween the KD lines, type 2 (triangles) below and type 3 (circles) above the equi-librium lines.

our samples are shown in Fig. 4 with respect to olivine-melt equi-librium lines for KD of 0.31 and 0.35, representing low- and high-pressure conditions, respectively (1 and 20 kbar; Ulmer, 1989). Thisplot allows distinguishing three types of olivine: Type 1 olivines(square symbols) plot within the equilibrium range, type 2 olivines(triangle symbols) plot to the right of this range and type 3 olivines(circles) plot to the left. Petrographically, the different olivine typescan generally not be distinguished by crystal shape or internal fea-tures. The simplest explanation for off-equilibrium points is thatthe bulk rock Mg# has shifted due to variable amounts of olivineaccumulation (type 2) or removal (type 3), as indicated by the

L. Marks et al. / Earth and Planetary Science Letters 392 (2014) 39–49 43

Fig. 5. Major and minor elements versus K2O showing the compositional evolution of melt inclusions from the HOD dikes. Whole rock compositions are shown as gray fieldfor comparison and all analyses are normalized to 100%.

arrows in Fig. 4. The possibility that some of the high-Fo olivinesare entrained xenocrysts from the mantle is ruled out becausetheir CaO contents (about 0.25 to 0.35 wt.%) much higher thantypical values for mantle xenoliths (<0.1 wt.% CaO; Hervig et al.,1986). Note that several samples contain more than one of thesetypes of olivine, indicating some complexity in the crystallizationhistory of the dike magmas (for more discussions see Keiding etal., 2011, 2013).

4.2. Melt inclusion compositions

Olivine-hosted melt inclusions commonly show evidence forpost-entrapment crystallization of olivine on inclusion walls dur-ing cooling. This process can be corrected for by incrementallyadding olivine to the inclusion composition until it matches Mg/Feequilibrium with the olivine host (e.g. Danyushevsky et al., 2002;Kent, 2008). There can also be a post-entrapment loss of Fe frominclusions due to re-equilibration of Fe and Mg between meltinclusions and host olivine during cooling (Danyushevsky et al.,

2000, 2002; Gaetani and Watson, 2000). Following Keiding et al.(2011), we can discount significant Fe-loss in the present study be-cause (1) compositional gradients in host olivine are absent or op-posite to what iron loss would produce, (2) none of the inclusion–host pairs show high KD(Fe–Mg)liq–olv values and correlations be-tween FeO and KD(Fe–Mg)liq–olv, which Fe-loss would produce;and (3) there is no correlation of FeO with inclusion size as ex-pected for the re-equilibration process (see Danyushevsky et al.,2002). However, there is a mismatch between the observed KD(Fe–Mg) values for inclusions and host olivine (0.15–0.35; Supplemen-tal Table S4), and the expected equilibrium value of 0.32 ± 0.03.The low-KD compositions can be adjusted by olivine addition asdescribed above, but for some inclusions, the correction requiresup to 25 wt.% olivine to be added, which is unrealistic for sim-ple post-entrapment crystallization. The implication is that someinclusions were affected by another process, and a possible clueis their higher than typical Fe contents (up to 17.1 wt.% FeOT).Anomalously high Fe concentrations in melt inclusions have beenattributed by others to accidental entrapment of Fe-rich spinel or

44 L. Marks et al. / Earth and Planetary Science Letters 392 (2014) 39–49

Table 1Estimated emissions of S, Cl and F from three different LIPs.

Province V melt (km3) Pmelt (kg/km3) Φmelt DS DCl DF ES ECl EF

Melt volatile loss (μg/g) Emission (Gt)

Paraná–Etendeka Min 2.18 × 106 2.69 × 1012 0.8 707 45 137 3.3 × 103 0.7 × 102 0.6 × 103

Max 2.35 × 106 2.77 × 1012 0.9 927 80 205 5.4 × 103 4.7 × 102 1.2 × 103

Mean 2.27 × 106 2.73 × 1012 0. 85 817 62 171 4.2 × 103 2.5 × 102 0.9 × 103

Deccan Traps Min 1.3 × 106

Max 2.5 × 106

Mean 1.9 × 106 3.3 × 103 1.9 × 103

Siberian Traps Min 1.0 × 106 6.3 × 103 3.4 × 103 7.1 × 103

Max 4.0 × 106 7.8 × 103 8.7 × 103 13.6 × 103

Mean 2.5 × 106 7.1 × 103 6.1 × 103 10.4 × 103

V melt is the volume of erupted magma. Volumes for the Paraná–Etendeka are from Gladczenko et al. (1997) and from calculation in this paper (see details in text). Volumesfor the Deccan and Siberian Traps are from Jay and Widdowson (2008) and Wignall (2001). P melt is the rock density calculated from melt inclusion compositions (Supple-mental Table S4) and the model of Lange and Carmichael (1990). Φmelt is the melt fraction without crystals based on visual estimates of modal proportions of phenocryststo matrix in thin sections. DS, DCl and DF are the melt volatile losses and are estimated from the maximum melt inclusion volatile contents in Supplemental Table S4 andgas emission efficiencies from Black et al. (2012). ES, ECl and EF are total emission values calculated for Paraná–Etendeka from text Eq. (1). Emission values for Deccan andSiberian Traps are based on estimates from Self et al. (2008) scaled to the size of the province (Vmelt) and from Black et al. (2012), respectively.

other solid phases, which were partially dissolved during high-temperature homogenization runs (e.g. Kent and Elliott, 2002;Rowe et al., 2006). As described above, spinel crystals in someinclusions are not true daughter phases, and we suggest that Fe en-richment from partial melting of accidental spinel inclusions dur-ing homogenization is a good explanation for inclusions far fromFe–Mg equilibrium. To avoid the uncertain validity of the olivine-correction method for some of the inclusions, we choose to simplyreport the measured compositions for all inclusions in this paper.For the volatile and incompatible elements of interest in this paper,the olivine correction would dilute their concentrations propor-tional the amount of olivine added, so the true melt concentrationsmay be slightly lower than the measured values. An example ofthe minor difference between corrected and uncorrected compo-sitions is sample JVT-09-32, where melt inclusions were analyzedin this study and independently by Keiding et al. (2011). The av-erage measured value for MgO from Table 1 is 14.6 wt.%, whereasthe olivine-corrected average for this sample from Keiding et al.(2011) is 15.1 wt.%.

The uncorrected melt inclusion compositions correspond topicro-basalt to basalt, with MgO contents from 8 to 18 wt.% andMg# from 53 to 83 (Table 1). One inclusion (Supplemental Ta-ble S4, ID# JVT-32e3-1) represents an outlier with anomalouslylow Na2O and TiO2 as well as an extremely high CaO/Al2O3 ratioand extremely low volatile contents and this analysis is not con-sidered further. Selected geochemical features of the inclusions areshown in Figs. 5 and 6, with symbols distinguishing the types ofhost olivine (see Fig. 4). The range of whole-rock compositions forthe 8 samples is shown in gray fields in Fig. 5 (for data see Sup-plemental Table S4). The first-order feature of both the host rocksand included melts is the wide range of compositions present anda general lack of strong inter-element correlations. Incompatible el-ements in the melt inclusions (K, Ti, P) do show a weak positivecorrelation (Figs. 5d, 5f) but systematic relationships with othermajor components are lacking. The melt diversity displayed bythese inclusions cannot be explained by fractional crystallizationfrom a common source magma, and must reflect a more complexmagmatic history. Other evidence of complexity is the heteroge-neous olivine populations in the dikes, as illustrated in Fig. 4. Theinclusion data (Fig. 5) show that all three types of olivine con-tain a similar range of melt compositions, with overlaps for mostmajor and minor elements, and a similar degree of scatter. It isworth noting that Keiding et al. (2011) reported a limited set oftrace-element data, including REE concentrations, for melt inclu-sions from some of the samples included in this study. From thecombined major and trace-element data, Keiding et al. (2011) at-tributed the melt inclusion diversity to trapping of variably mixed

melt fractions during olivine crystallization in the roots of the HODdike system. See also Thompson et al. (2007) for discussions ofmagma diversity in dikes from the central part of the HOD to theSE of our study area.

The variability of melt inclusion compositions also holds for thevolatile element concentrations (Fig. 6). The sulfur concentrationsrange from <30 to 1100 μg/g and halogen contents are 190 to450 μg/g F and <10 to 125 μg/g Cl (Fig. 6). The correlations ofthese volatile elements with incompatible major elements in themelt inclusions are weak (Fig. 6), and this is also true for inter-element correlations like Cl/F and S/F. Overall, the S, Cl and Fconcentrations in HOD melt inclusions are low relative to com-parable inclusion data from basaltic olivines in other LIPs (e.g.Thordarson and Self, 1996; Self et al., 2008; Black et al., 2012).Implications of this difference for a comparison of LIP environmen-tal impacts are discussed below, but a first question is if the lowvolatile concentrations are secondary effects of melt degassing orpost-entrapment losses. Degassing of the melts before olivine crys-tallization is unlikely because a thermobarometry study by Keidinget al. (2013) showed that clinopyroxene in most HOD dikes crys-tallized at 4–6 kbar pressure and olivine is an earlier phase inthese rocks. By contrast, exsolution of halogens and sulfur frombasaltic magma typically takes place at less than 1 kbar (Wilsonand Head, 1981). However, volatile losses from inclusions aftertrapping are possible, either in nature during cooling or in theheating runs used for re-homogenization. For example, there hasbeen much discussion of water losses from melt inclusions dueto the rapid diffusion of hydrogen (Chen et al., 2011; Gaetani etal., 2012; Hauri, 2002; Hauri et al., 2002; Portnyagin et al., 2008;Qin et al., 1992). Baker and Balcone-Boissard (2009) raised con-cerns that halogens and sulfur might also be affected, although thediffusion rates of those elements in olivine are much slower thanfor hydrogen and Koleszar et al. (2009) reported olivine-hostedmelt inclusions that we interpreted to have not remained closedfor of F (and H2O), suggesting diffusive re-equilibration throughthe olivine host. However, Buchholz et al. (2013) showed that lossof halogens and sulfur from inclusions during re-homogenizationruns is low. A more serious concern is degassing from breachedinclusions. Cooling contraction of the dikes may result in micro-fractures developing in olivine at the margins of melt inclusions,and these enhance volatile loss during reheating (e.g., Kent, 2008;Tait, 1992). The melt inclusions in this study were checked for ev-idence of breaching before analysis and none was found, but wedo not rule out leakage completely and indeed, examples of Cland S contents below detection limit of SIMS (Figs. 6a, 6c) maywell reflect this process. Nielsen et al. (1998) proposed that S con-tents can test melt inclusion integrity since most erupted basalts

L. Marks et al. / Earth and Planetary Science Letters 392 (2014) 39–49 45

Fig. 6. K2O versus volatile contents in olivine-hosted melt inclusion from the studiedHOD dikes.

are close to sulfur saturation, and concentrations much below thesaturation limits suggest leakage. This criterion does not necessar-ily hold for picritic, near-primitive compositions like our inclusions,which may not reach sulfide saturation (Keays and Lightfoot, 2007;Momme et al., 2003). The interpretation of intermediate S valuesof 400–600 μg/g from the melt inclusions is uncertain, but in anycase, the maximum sulfur contents from our study (∼1100 μg/g)are in same range as those for undegassed inclusions from theEmeishan LIP (1100–1300 μg/g in Zhang et al., 2013) and we re-gard them as reliable estimates for the melt concentration. Cor-responding maximum values for Cl and F are 126 and 445 μg/g,respectively.

4.3. Estimation of eruptive volatile release

The established method to estimate the total release of volatilesfrom past volcanic eruptions (the ‘petrologic method’, e.g. Devineet al., 1984) is based on the difference in volatile concentrations inmelt inclusions (assumed undegassed) relative to the rock matrix(assumed degassed), scaled to the volume of the eruption or erup-

tions in question. Volatile emissions quantified using this approachare calculated by Eq. (1) after Gerlash et al. (1994):

E = 10−18 × Δvolatile × ρmelt × φmelt × V melt (1)

where E is total emission mass in gigatons; Δvolatile is the con-centration difference between melt inclusions and rock matrix inμg/g; ρmelt is the average density of the melt in kg/km3; φmeltis the melt volume fraction without crystals; and V melt is theeruption volume in km3. Application of the petrological methoddepends on a number of conditions and assumptions (see Blakeet al., 2011), the most troublesome of which for our study isthe absence of fresh glassy matrix in the dike rocks. All samplesof HOD dikes are holocrystalline or contain patches of devitri-fied and partly altered volcanic glass whose volatile contents areuseless for constraining the degassed melt concentrations at thetime of eruption. To circumvent this problem, we use values ofdegassing efficiency from Black et al. (2012) for well-constrainedanalogue systems including the Laki fissure eruption in Iceland,the Deccan Traps, and the Columbia River Flood basalt. The Lakiexample has often been taken as a historical equivalent of floodbasalt eruptions, with similar melt compositions and eruptionstyle to other continental flood basalt settings (Self et al., 2008;Thordarson and Self, 1996). The compilation of Black et al. (2012)shows that estimates of degassing efficiency are quite constant forindividual volatiles (e.g., 30–45% for F and 64–84% for S), withmore variability for Cl (36–63%). Because some melt inclusionsmay have undergone variable volatile loss due to breaching duringthe homogenization runs (see above), we use the maximum inclu-sion concentrations for the Δvolatile calculation. These values are1104 μg/g for S, 455 μg/g and 126 μg/g for Cl (Table 1, Fig. 5). Thehighest Cl value is an outlier whose concentration is more than3 times higher than any others. This could reflect excess Cl froman alteration product, so Table 1 also lists a more conservative es-timate for total Cl release based on a value of 43 μg/g.

The concentrations of halogens and S in melts need to be scaledto total erupted volumes in the Paraná–Etendeka province. Thisinvolves large uncertainties due to factors of limited exposure, ad-vanced state of erosion and submergence of former coastal parts ofthe province. The Paraná basin accounts for the main onshore partof the erupted LIP, with an estimated surface area of 1.2×106 km2

(Peate et al., 1992). Remnants of the Etendeka volcanic rocks inNamibia total 0.08 × 106 km2 (Erlank et al., 1984), to which weadd the equivalent amount based on the area covered by the HODdikes (Fig. 1). The maximum volcanic thickness recorded in theParaná and Etendeka provinces are 1700 m and 900 m, respectively(Peate et al., 1992). We apply that these maxima as constant thick-nesses for the respective provinces to compensate for the unknownamounts of volcanic section removed by erosion, since the top oflava successions are not exposed and thermochronology estimatesfor denudation since 130 Ma are on the order of 3 km in Namibia(Raab et al., 2002). The resulting values for total erupted volumessum to 2.18 × 106 km3 for the Paraná–Etendeka province. This issimilar but smaller than the volume estimate of 2.35 × 106 km3

by Gladczenko et al. (1997), who also included the contribution ofseaward-dipping reflectors offshore which are interpreted as for-mer subaerial flows.

A summary of these results and calculation parameters is listedin Table 1 together with published estimates of volcanic emissionsfrom the Siberian and Deccan LIPs. Our calculations of total vol-canic emissions from the Paraná–Etendeka event are 3100–5400Gt sulfur, 70–470 Gt chlorine and 600–1200 Gt fluorine. The rangesare based on minimum and maximum volume estimates, degassingefficiencies and other parameters on Table 1. We applied the max-imum concentration values from the melt inclusions for all (ex-cept Cl as described above), and the estimates are still conserva-tive because the melt compositions are more primitive than the

46 L. Marks et al. / Earth and Planetary Science Letters 392 (2014) 39–49

associated lavas and some enrichment of the incompatible halo-gens and S can be expected before eruption. To constrain thepossible magnitude of this effect we can compare average con-centrations of similarly-incompatible elements in the melt inclu-sions and erupted lavas that are not affected by degassing. Usingthe sum of (Na + K) as well as P for this purpose, the meltinclusion averages are 2.2 wt.% for Na2O + K2O and 0.10 wt.%for P2O5. The corresponding values from the main basaltic unitsof the Paraná–Etendeka province (Tafelberg–Gramado type), fromthe GeoRoc database (www.georoc.org), are 3.9 and 0.25 wt.%, re-spectively. Thus, assuming a closed-system fractionation withoutsaturation or degassing, the pre-eruption concentrations of Cl, Fand S could be 2–3 times higher than the melt inclusion values.

5. Discussion

5.1. Comparison with other LIPs

To our knowledge, this study is the first to constrain total vol-canic emissions of S, F and Cl from the Paraná–Etendeka LIP. Com-pared with the Deccan and Siberian Traps LIPs, where the sameapproach was applied (Table 1), the Etendeka-Paraná event re-leased comparable masses of sulfur but much less fluorine andchlorine. This contrast in halogen emissions from these provincesmay be significant because the Paraná–Etendeka LIP is the onlyone which is not correlated in time with major global extinctions.Erba et al. (2004) proposed a link of the Paraná–Etendeka LIPwith the Valanginian Wessert oceanic anoxic event (OAE), whichis manifested by widespread occurrences of black shales and aδ13C excursion in pelagic sediments of that age. However, there aredoubts whether the OAE caused major extinctions, the age corre-lation is tentative (Courtillot and Renne, 2003; Gröcke et al., 2005;Wignall, 2001), and in any case the environmental impact was noton the same devastating scale as the Deccan and Siberian exam-ples. It has been suggested that the moderate impact of Paraná–Etendeka magmatism relates to lower eruption rates, but recentgeochronology studies reduce the duration of the main phase ofvolcanism to <1.2 Ma, implying eruption rates on the order of 2km3/yr, which are similar to those for LIPs correlated with massextinctions (Thiede and Vasconcelos, 2010). Our geochemical studysuggests that a much lower emission of halogens could be a con-tributing factor to the lower environmental impact of the Paraná–Etendeka LIP.

5.2. Environmental effects and potential kill mechanisms of LIP volcanicemissions

A close temporal association of LIP events with global massextinctions has been demonstrated but the factor or factors link-ing large-scale volcanic eruptions with the collapse of ecosystemsare not well understood. We suggest that important insights maycome from considering why the Paraná–Etendeka event did notcause mass extinctions whereas other LIP events of similar magni-tude apparently did. Most scenarios of volcanic triggering of massextinctions consider the environmental effects of S and CO2 emis-sions. Self et al. (2005, 2006) questioned whether the scale of CO2release by volcanism is sufficient as a kill mechanism given the lowsolubility of CO2 in basaltic magma (but see Wignall 2001, 2005),and they suggested that S emissions could play a more significantrole. The environmental effects of volcanic S emissions can includecooling by formation of H2SO4 aerosols that reduce insolation (orcause acid rain if removed by precipitation), and warming by thebuildup of the greenhouse-active SO2. However, the estimates oftotal sulfur release from ‘lethal’ LIP events like the Siberian andDeccan examples are not significantly larger than for the ‘benign’Paraná–Etendeka LIP, suggesting that sulfur emissions alone may

not be the controlling factor for global mass extinction. The mag-matic CO2 concentrations in these LIPs are not available for com-parison, but following Self et al. (2005), they are unlikely to differmuch because of the low solubility in basaltic melt. On the otherhand, Ganino and Arndt (2009) and Svensen et al. (2007, 2009)pointed out that CO2 emission from LIPs may be greatly boostedby thermal decarbonization of local sedimentary rocks. They citedthe Siberian LIP as an example where the appropriate lithologiesexist in abundance (black shales, carbonates), and the associatedextinction event was uniquely devastating. Such rocks are not sig-nificant in the Paraná–Etendeka province.

5.3. The role of halogen emissions

The high Cl and F emissions from the Siberian and DeccanLIPs compared to the Paraná–Etendeka event (Table 1) suggestthat halogens may be an important factor in the ecosystem im-pact of LIP volcanism. The potential of halogens to trigger en-vironmental disturbances is less well studied than that of sulfurand CO2 but it may be profound. Direct toxicity of fluorine fromvolcanoes, related to skeletal fluorosis, is well known. For exam-ple, the 1783 Laki eruption in Iceland caused a loss of over 50%of grazing livestock (Thordarson and Self, 2003). Chlorine emis-sions contribute to the formation of acid rain, in addition to CO2and S. Acid rain and the acidification of oceans were advocatedas a kill mechanism for extinctions (e.g. Gertsch et al., 2011), butRampino and Self (2000) pointed out that, like fluorosis, acid raincan have a major impact regionally but is unlikely to have beof global significance. A globally more significant effect of vol-canic halogen emissions to the atmosphere could the destructionof the ozone layer (Johnston, 1980). The stratospheric ozone (O3)helps filter the 290–315-nm wavelength of sunlight (UV-B) whichis the most important spectral region for cancer and cell muta-tions (Alpen, 1998). Indeed, the development of an atmosphericozone shield has been considered as one of the prerequisite eventsfor life to exist on land (Lunine, 1999). Reactions with Cl in theatmosphere breaks down ozone to form ClO and O2, and thecontribution of volcanic halogens to this process has been docu-mented in studies of modern eruptions like Mt Pinatubo in 1992(McCormick et al., 1995) and Hekla in 2000 (Millard et al., 2006;Rose et al., 2006). Suggestive evidence that destruction of theozone layer contributed to (terrestrial) mass extinctions related tothe Siberian Traps event is the global prevalence of mutated poly-nomorphs observed in end-Permian rocks (Visscher et al., 2004).Beerling et al. (2007) estimated that 2200 Gt Cl from the SiberianTraps could lead to 33–55% O3 destruction given a main eruptionphase of 200 kyr or less. The more recent estimates of Cl emis-sions from Sobolev et al. (2009) and Black et al. (2012), given inTable 1, are 3 times larger than those of Beerling et al. (2007).Another consideration is the role of Br, which is typically 200times less abundant than Cl (Bureau et al., 2000) but 60–100 timesmore effective for the destruction of ozone (Daniel et al., 1999;Sinnhuber et al., 2009). Bromine is also less soluble in water thanCl, which can be washed out as HCl by rain before reaching thestratosphere (Pinto et al., 1989; Tabazadeh and Turco, 1993). Fi-nally, the concept that non-volcanic gases may contribute to theimpact of a LIP event which was mentioned in the context of CO2also applies to the halogens, which form organic compounds suchas CH3Cl in the soils and sediments (Svensen et al., 2009; Visscheret al., 2004). According to modeling by Beerling et al. (2007) thecombined effect of volcanic HCl release and pyrolysis of sedimen-tary organic compounds could result in up to 80% reduction of theozone layer.

Clearly, the global impact of volcanism depends on delivery ofemissions to the stratosphere (Black et al., 2012; Thordarson etal., 2009) and thus physical parameters such as eruption style, lat-

L. Marks et al. / Earth and Planetary Science Letters 392 (2014) 39–49 47

itude and elevation of volcanic regions, as well as atmosphericconditions including moisture content in the eruption plume maybe as important as magmatic volatile concentrations. Nevertheless,our finding of low Cl and F contents in melts from the Paraná–Etendeka LIP, which is not associated with mass extinction, com-pared with high halogen contents in those that are, should en-courage more research on the environmental impact of halogenemissions from large volcanic eruptions.

6. Conclusions

This study presents the first assessment of total volcanic emis-sion of F, Cl and S from the Paraná–Etendeka Large IgneousProvince based on estimates of extruded lava volumes and fromin situ analyses of re-homogenized melt inclusions in olivine phe-nocrysts from Mg-rich dikes in the Etendeka province of NWNamibia. To obtain reliable values for F, Cl and S at low concen-tration, these elements were determined by secondary ion massspectrometry (SIMS), whereas major and minor elements were an-alyzed by electron microprobe. The melt inclusions are quite vari-able in composition, with 8–18 wt.% MgO, and they overlap withthe host rock composition. The wide diversity in major and mi-nor element contents in the inclusions and the range of forsteritecontents in the host olivine are attributed to trapping of variably-mixed melt increments during protracted olivine crystallization inthe root zone of the dike system. The volatile element concentra-tions in melt inclusions are also variable and only poorly correlatedwith incompatible minor elements like K, Ti and P. The concen-trations range from 190 and 450 μg/g for F, <10 and 126 μg/gfor Cl and <30 to 1105 μg/g for S. We attribute the low end ofthese ranges to a partial loss of volatiles during re-homogenizationruns and use the maximum values for calculating total emissionmasses. The measured F, Cl and S concentrations in non-degassedmelts were combined with estimates of the total volume of ex-truded lavas in the Paraná–Etendeka province, and with empiricalvalues for degassing efficiency from well-studied analogue vol-canic systems, to calculate total volcanic emissions during the LIPevent. The results are 600–1200 Gt fluorine, 70–470 Gt chlorineand 3100–5400 Gt sulfur.

These values are important for the discussions of the environ-mental impact of LIP events and their possible link to global massextinctions. A comparison is made between the Paraná–Etendekaevent, which is not associated in time with a mass extinctionevent, and two similarly-sized LIPs (Deccan and Siberian traps)which do have this distinction. The Paraná–Etendeka LIP producedsimilar total emissions of sulfur as the other LIPs but up to 25times lower masses of Cl and F. This difference suggests that halo-gens may be an important factor in the environmental impact ofmassive volcanic eruptions in addition to the CO2 and S emissionsthat have received the main attention. There are independent sug-gestions that volcanic release of halogens causes destruction of thestratospheric ozone layer, increase UV radiation and cause ecosys-tem stress. The global impact of volcanic halogen emissions fromLIPs has not been extensively studied, but our results suggest thatscenarios linking halogens to mass extinctions should be furtherexplored, especially for surface ecosystems.

Acknowledgements

This study was undertaken within the Priority Program SAM-PLE (SPP-1375) funded by the Deutsche Forschungsgemeinschaft.Jakob Keiding acknowledges support from the Carlsberg Founda-tion. We thank Dougal Jerram and Marc Krienitz for help in fieldwork, discussions of Etendeka geology and provision of samples.The manuscript was considerably improved by comments and dis-cussions on a previous version by Michael Marks, and by construc-

tive journal reviews by Adam Kent and an anonymous reviewer aswell as advice by editor Tim Elliott.

Appendix A. Supplementary material

Supplementary material related to this article can be found on-line at http://dx.doi.org/10.1016/j.epsl.2014.01.057.

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