Non-Magmatic, Volcanic Ash Fragments and Condensates From the Passively Degassing Plume of Popocatepetl Volcano, Mexico - Indicators of Active Contact Metamorphism and Related Degassing ?
Obenholzner, J.H. (+), Schroettner, H. (*), Golob, P. (*), Delgado, H. (**)
(+) Naturhistorisches Museum/Mineralogie, Postfach 417, A-1014 Vienna, Austria (+)
(*) ZFE, TU-Graz, Steyrerg. 17, A-8010 Austria ()
(**) Instituto de Geofisica, UNAM, Coyoacan 04510, Mexico D.F., Mexico ()
Abstract: Magma-wall rock interaction can contribute gases to the magmatic system as well as absorb volatiles to the country rock. These processes are happening at depth being far away from direct observation. Besides isotopic composition of gases, if it is possible to sample gases, the micro-analytical investigation of xenoliths is a potential instrument to study the mineralogical products of related chemical reactions. Plume-derived aerosols can be another source to obtain information about these processes. At Popocatepetl volcano FESEM/EDS could analyse contact metamorphosed particles as part of the volcanic ash and aerosol particles from the passively degassing plume. Wollastonite, hercynite and glass of contact metamorphism-related origin is present in the ash. Condensates from the passively degassing plume are rich in phosphorus, indicating a possible non-magmatic source for this element.
Keywords: SEM, FESEM; EDS, contact metamorphosed particles, volcanic ash, wollastonite, ferrobustamite, hercynite, buchites, volcanic aerosol, gas, recent activity of Popocatepetl volcano, Mexico; carbonate platforms, evaporites.
Micro- and nanometer-sized particles of volcanic origin are transporting various chemical elements into different layers of the atmosphere, according to the energy of an eruption. These particles can act as a nucleus for further condensation processes or can trigger heterogeneous chemical reactions in the atmosphere. The abundance of chlorine in the studied particles, especially in crystallites on spherical particles, can be an underevaluated contribution to volcanogenic ozone destruction and climate change. A review on observed and possible interdependence of volcanism and climate change is summarized by Robock (2000).
This study presents new data of plume-derived aerosols, a chemically very distinct environment. The analytical possibilities of state-of-the-art SEM techniques could provide a unique opportunity to cooperate between the analyst and the modeler. Results could also be fertile for better modeling of stratospheric chemistry.
The size of volcanic fragments, associated wall rock fragments and volcanic aerosols varies from meter-sized blocks to smaller 1/16 mm for fine volcanic ash (1/16 mm equals 62.5 µm). Volcanic aerosol particles (fluids and solids) are micro- to nanometer sized. Aerosol sciences classify solid particles from fume (ca. 0.001-1µm), clay (ca.0.01-2µm), silt (2-20µm), fine sand (20-200µm) to dust (1-10000 µm). Size ranges are according to Hinds (1982).
All particles smaller than several millimeters are candidates for scanning electron microscope (SEM) analysis. Recent developments in SEM techniques, like the very high resolution field emission gun SEM (FESEM) have improved morphological, mineralogical and textural examinations. Modern SEM systems are equipped with an energy dispersive X-ray detector (EDS) for chemical analysis.
A variety of contact metamorphism (CM) and related metasomatic minerals indicate the interaction of magma with carbonate bearing rocks or other sediments. Contact metamorphic events might occur in the region of a magma chamber, within magma storage zones at higher levels or as wall rock-magma interaction during magma ascent. These processes can contribute CO2 (even SO2, F, Cl and other volatiles) to the original gas content of the magmatic system.
Fluffy and spherical aerosols are interpreted to be the product of complex condensation processes, leading to the formation of fluffy, maybe semi-solid or spherical, solid particles. The latter ones are sometimes coated by crystallites of an S-bearing Mg-chloride. The high contents of Mg are unusual as the average row of abundant cations in a volcanic gas from a calc-alkaline magma is Na>K>Al>Fe>Ca>Zn>Mg. Usually Mg is 10-20 times lower than Na or K. Spherical particles occur as single spheres or as coagulated spheres.
The size of studied spherical particles varies about 1 µm in diameter, coagulated spheres can show diameters of up to 10 µm, fluffy particles vary from 1.5 to 10 µm. Smaller particles are abundant but have not been analysed yet, according to limited FESEM time.
According to the limits of EDS (interaction volume depending on acceleration voltage and the density of the sample), the C- and F-content of the teflon filter and the C-coating) we could characterize the chemical composition of individual aerosols. Elements detected in fluffy aerosols are Si, Al, Ca, Na, Mg, K, Fe, Ti, P, Cu, Zn, Bi, Pb, Mo, Sn, S, Cl, O. Most of the spherical particles do not show internal mixing (particle inside a particle) as the surfaces appear smooth and none of the studied particles showed breakage. Only one particle contains Ti-, Pb- and Cr-rich crystallites inside the sphere. Many of the coagulated or aggregated particles are internally mixed according to different chemical/mineralogical components.
Elements detected in spherical particles are Si, Al, Ca, Na, K, Mg, Fe, Ti, P, Mn, Cu, Zn, S, V, Ni, O, (Pb, Cr). Individual spheres show also individual chemical composition. The variety of chemical composition indicate different micro-environments in the plume and/or at the zone of mixing with the surrounding atmosphere.
Part 1: Indicators of contact metamorphism asssociated with the eruptions at Popocatepetl volcano.
Stratigraphy:
According to Fries (1965a+b) the basement of Popocatepetl volcano comprises Cretaceous carbonates (ca. 3 km) and Tertiary sediments with intercalated evaporites (ca. 500 m). The stratigraphy of the younger history of Popocatepetl volcano is described in Siebe et al. (1996). In relation to the archeological time scale for central Mexico the major Holocene and younger Pleistocene volcanic events at Popocatepetl are:
Present Eruption (1994-?) [CM]
Upper Ceramic Plinian Eruptive Sequence (675-1095 AD)
Intermediate Ceramic Eruptive Sequence (125-255 AD)
Nealtican andesite flow (ca. 2300 BP) [CM]
Lower Ceramic Plinian Eruptive Sequence (215-800 BC)
Upper Pre-Ceramic Plinian Eruptive Sequence (2830-3195 BC)
Plinian eruption (ca. 14.000 BP) [CM]
All age data, except for present eruptions, are bracketed by 14C analysis. [CM] refers to contact metamorphosed xenoliths or particles observed in lava flows, pyroclastic deposits or recent volcanic ash.
Contact metamorphism indicated in past eruptions:
Past eruptions of Popocatepetl (plinian eruption, ca. 14.000 years B.P.) contain blocks of Ca-silicate rocks with diopside, grossularite as the predominant mineral, or dolomite containing clinohumite ((Mg, Fe++)9(SiO4)4(OH, F)2). As the availability of F in most sediments is rather low, clinohumite probably indicates the migration of F from the magma into the wall rock. Major explosive eruptions caused the fragmentation of the host or country rock and the sedimentation of polymict fall and flow deposits in proximal facies .
Mm-sized inclusions in pumice of the Tocuila lahar (plinian eruption of ca. 14.000 BP, redeposited 11.000-12.000 years B.P.) contain grossularite with droplet-shaped haüyne (Na, Ca)4-8[Al6Si6 O24](SO4,S)1-2, diopside, K-feldspar, quartz and sylvite. The inclusions document a homogeneous distribution of CM-altered wall rock fragments within the vesiculating magma.
Only one sample (P 97-02) from the “ocre surge” beneath the “pink pumice” (Upper Ceramic Plinian Eruptive Sequence, ca. 800 A.D.) contains abundant euhedral to anhedral anhydrite. Anhydrite could be formed by hydrothermal alteration of limestone/dolomite or hydrothermal deposition inside the crater. However, a magmatic origin of the anhydrite is possible but not proven yet.
Contact metamorphism indicated in on-going eruptions:
Ash from the eruptions of winter 1995 (01-02-1995), spring 1995 (03-20-1995 and winter 1996 (11-28-1996) accidentially contains a variety of contact-metamorphosed rock fragments. The 11-28-1996 ash is characterized by dome fragments and contains pseudo-vesiculated or mossy shaped aggregates of a Ca-silicate whose composition is close to CaSiO3. The Ca-silicate is probably wollastonite. High-temperature wollastonite (pseudowollastonite: b-CaSiO3) is known in nature only in pyrometamorphosed rocks. There are two low-temperature modifications of wollastonite (a-CaSiO3 ,wo-Tc and wo-2M). A wollastonite-2M is described from highly metamorphosed ejecta of volcanoes like Monte Somma, Vesuvius. Modern crystallographic studies could demonstrate that Fe-rich wollastonite is identical with ferrobustamite. EDS data of analysed wollastonites document traces of manganese, which is present in minor amounts in ferrobustamite. Only wo-15 has a MnO content (1.21 %) similar to typical ferrobustamite (see table 1.1).
Most of the observed Ca-silicates are highly herterogeneous at very small scale. Electrom microprobe analysis would barely produce better results than EDS. For these reasons we try to model the P-T-t conditions of Ca-silicate formation following the well studied reaction quartz + calcite <=> wollastonite and carbon dioxide. Vesicles might be indicative for fast mineral reactions and CO2-release. High pressure experiments (1GPa) performed at the GeoForschungsZentrum Potsdam (GDR) document CO2-filled porespace around wollastonite rims grown on quartz (Lauterjung et al., 2000).The wollastonite particles are unusually rich in Fe; the BSE images do not show the typical fibrous features known from light microscopy. There is no volcanic glass associated with the wollastonite particles, it therefore might have formed without direct interaction with the melt. One microclast appears like a vesiculated glassy fragment, but mineralogically it is SiO2, Ca-feldspar and Ca-silicate. Wollastonite aggregates contain randomly distributed pyrite (FeS) and rare molybdenite (MoS2) occurs.
Textural and compositional differences between wollastonite types
Micro-clasts are texturally very different. Type 1 appears like an aureole-derived wollastonite (fig. 1). Veinlets (width ca. 20µm) of massive, chemically homogeneous, non-vesiculated wollastonite are intergrown with K-feldspar which has inclusions of quartz. This assemblage indicates sanidinite facies of contact metamorphism. Sanidinite facies is defined by low pressure and increased temperature. Water escapes at these temperatures and is not available for facilitating reactions or crystallizations. The mineralogical consequences of this combination of conditions are:
“1. Chemical and thermal equilibrium are rarely attained. The number of associated minerals therefore is likely to exceed that demand by the mineralogical phase rule.
2. High-temperature minerals appear in the mineral assemblages of the sanidinite facies.
3. Sanidine, often with a high content of soda, is a critical mineral of this facies. Whether stable or metastable at the time of crystallization, its presence in a mineral assemblage indicates rapid cooling from an unusual high temperature of metamorphism.
4. As a result of partial or complete fusion, glass is sometimes present in rocks of the sanidinite facies.” (Turner et al. 1960).
Insert tab. 1.1
Insert fig.1
One micro clast appears like a vesiculated glassy fragment. Mineralogically it is Si-rich, glassy(?) material, Ca-feldspar and Ca-silicate (type 2). The wollastonite is massive, with some inclusions of quartz and rare sulfides. Maximal width of the wollastonite is ca. 40 µm (fig. 2). One vesicle-like vug (diameter is ca. 5 µm) is surrounded by massive, chemically homogeneous wollastonite. Other vesicle-like vugs of the glassy (?) part of the microclast are open or filled with sulfide.
Insert fig. 2
Type 3 wollastonite is chemically heterogeneous according to Fe-content (fig. 3, 4). This type shows patchy intergrowth of Fe-rich (FeO ca. 12%) and Fe-poor (FeO ca. 3%) wollastonite which contains randomly distributed pyrite (FeS) and rare molybdenite (MoS2). The size of patches ranges from 1 to 10 µm in diameter. Type 3 wollastonites are pseudo-vesiculated indicating fast kinetics of the wollastonite-forming reaction with vigorous gas release. Vesicle diameters range from 1 to 10 µm. The composition of the gas could have been CO2 or SO3 according to the reaction calcite + quartz = wollastonite + CO2 or anhydrite + quartz = wollastonite + SO3 (Wood 1994). Both reactions do not explain the high Fe-content of wollastonite. An Fe-Ca-carbonate component of the Cretaceous carbonate protolith is more likely than a Fe-/Ca-sulfate component of an evaporitic succession or of hydrothermally altered volcanic rocks.
Insert fig. 3
Insert fig, 4
Another type of wollastonite (type 4) is Fe-poor and occurs in a paragenesis with quartz (fig. 5). These patches are strongly intergrown with SiO2, are chemically homogeneous, non-vesiculated and have diameters of 10 to 20 µm.
Insert fig. 5
Adjectives (A, B, C) of wollastonite refer to grain size of Nov. 96 volcanic ash: A: <65µm,>32µm; B: >20 µm; C:>20 µm (floats on water). Wollastonite (B1) is vesiculated and rounded, like the grain was tumbled. The grain shows a reaction rim at the outermost edge, parallel to the grain boundary. Chemically this zone is slightly Mg-rich relative to the core, maybe indicating an interaction with a fluid while tumbled. Size of the grain is 20x40 µm, vesicles are heterogeneously distributed, max. length is 10µm. One vesicle intruding the grain from the outer rim is partially filled with a glassy material rich in P (15-28% P2O5). Wollastonite (B2) is a vesiculated fragment with a glassy rim (SiO2: 80-95%). At the interface between wo and glass, sphene crystals are abundant. The size of the fragment is 20x50 µm, the maximal. length of vesicles is 5 µm. Wollastonite (B3) is a subangular clast comprising vesiculated wo with inclusions of Cl-rich glass (SiO2: 75-80%, Cl: ca. 3%), plagioclase-like inclusions and irregularly distributed Fe-oxides, and a glass (SiO2: 60-65%) vesiculated towards the boundary with wo and a pyx inclusion. Vesicles sometimes resemble the shape of crystals, maybe remnants of dissolved minerals. The particles rich in Ca-silicates (11-28-96 ash:) can also contain fluorite crystals (diameter: 1µm), documenting another F-phase except of clinohumite found in dolostone clasts of the ca. 14.000 BP eruption.
Other particles from the 11-28.96 event are Si-rich glasses of contact metamorphic origin („buchites“) containing anhydrite crystals and inherited zircons. The vesiculation of this glass is typical bimodal with coalescing vesicles (~10µm) and a sponge-type vesiculation (~1µm). EDS analysis of the glass is Na2O=1.93, MgO=0.19, Al2O3=4.37, SiO2=85.55, K2O=0.30, CaO=0.35, TiO2=0.48, MnO=0.16, FeO=0.53, P2O5=0.95, SO3=4.41, Cl=0.13.
None of the different wollastonite types show alteration (exception: B1) or indications of incipient retrograde metamorphism like growth of hydrous lime silicates. This observation is interpretated as an indicator for the young age of wollastonite. Rounded grains refer to tumbling in the conduit. We could detect rounded wollastonite particles (B1) with alteration rims due to interaction with a liquid or gas phase and rounded grains of microliths-containing volcanic glass, also with alteration rims.
Temperatur indication for wollastonite formation
If Fe-wollastonite occurs with hedenbergite it can serve as a geothermometer indicating 500-600° C. The formation temperature for MoS2 is reported as 480 to 570 °C from fumarole experiments (Bernard 1985) and from fumarole-wall rock alteration at about 700 °C (Getahun et al. 1996). Molybdenite from fumarole incrustations can contain minor amounts of Fe and Re (Bernard 1985). These data are in accordance with the data reported by Tilley (1948) for the formation of Fe-wollastonite. Wollastonite is also reported as a sublimate phase from Merapi volcano (Symonds et al. 1987).