Unusual Impact Melt Rocks

8/8/2019 Unusual Impact Melt Rocks

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Unusual melt rocks from meteorite impact

(first published on www.impact-structures.com in 2004)

by Kord Ernstson1, Uli Schssler2, Ferran Claudin3 and Michael Hiltl4

Abstract. - We show and discuss unusual impact melt rocks from the sedimentary target of the

Azuara/Rubielos de la Crida multiple impact in Spain: a silicate melt rock originating from the

melting of shale, a carbonate-phosphate melt rock showing liquid immiscibility of carbonate

and phosphate melt, carbonate and sulfate melt rocks, a carbonate-psilomelane melt rock, and

amorphous carbon particles in a microbreccia probably being carbon glass that has originated

from the shock melting of Cretaceous coal.

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1.Fakultt fr Geowissenschaften der Universitt Wrzburg, Pleicherwall 1, D-97070 Wrzburg,

Germany. [email protected]. Institut fr Mineralogie der Universitt Wrzburg, Am Hubland, D-97074 Wrzburg, [email protected]. IES Giola, Llinars del Valls. Barcelona-08450, Spain. [email protected]. Carl Zeiss NTS GmbH, Carl-Zeiss-Str. 56, D-73447 Oberkochen, Germany. [email protected]

Introduction

By definition [1], an impact melt rock is a crystalline rock that, in a meteorite impact

event, has solidified from shock-produced impact melt and that contains variableamounts of rock fragments. On the other hand, shocked impact rocks that contain

impact melt particles in a clastic matrix (clastic matrix = matrix of fragmental rock) are

termed suevites or suevite breccias. Both impact melt rocks and suevites are well

known from many impact structures which were formed in targets of crystalline

basement rocks or in mixed targets of sedimentary rocks overlying crystalline

basement rocks. In these craters, melt rocks and suevites may form more or less

thick layers (melt sheets, suevite layers) that can normally be easily recognized (e.g.,

in the Mistastin, Vredefort, Lappajrvi, Sksjrvi, Ries, Mien, Rochechouart impact

structures, and in many others) and that are the source for collectors and collections

of impactites. Impact structures in purely sedimentary targets, to the contrary, in

general lack these impact melt rock and suevite layers, which has especially been

pointed out by Kieffer & Simmonds [2]. They suggest that in sedimentary targets the

high amount of volatiles (water vapor from shocked porous rocks, carbon dioxide

from shocked carbonate rocks) prevents the formation of coherent melt masses.

Instead, the shock-produced impact melt is finely dispersed by the volatiles to form

microscopic glass particles only. In our opinion, there may be an additional reason for


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the apparent lack of melt rocks and suevites in impact structures in sedimentary

targets. The rocks do exist, but they are not recognized as such. This may especially

happen when the sedimentary target is composed to a large extent of carbonate

rocks (limestones, dolomites) or/and evaporites (gypsum, anhydrite, chlorides).

Carbonate rocks, e.g., are known to melt like other rocks, but the produced carbonate

melt cannot be chilled to form glass, because it very rapidly crystallizes to form again

a carbonate rock. Since the textures may show rough similarities, these crystallized

carbonate melts may on cursory inspection be easily confused with normal

limestones or soil formations as, e.g., calcrete (caliche). Gypsum, exposed to high

shock-related temperatures, is transferred to anhydrite by loss of crystal water.

Anhydrite may melt under complex conditions [3], but probably the melt upon cooling

and in contact with water will crystallize again to gypsum. Likewise, during impacts in

sedimentary, i.e. volatile-rich targets with variable lithology, high shock pressures and

temperatures may lead to further complex melting and cooling processes and, in the

end, to melt rocks of very peculiar composition and texture.

Here, we report on some unusual melt-bearing rocks and impact melt rocks that were

produced in the Mid-Tertiary multiple impact on the Iberian Peninsula [, 4, 5, 6, 7].

The multiple impact comprises the 35-40 km-diameter Azuara structure, the Rubielos

de la Crida 40 km x 80 km impact crater chain, and suspected additional craters of

smaller size. Exceptional with regard to the large crater diameters, the cosmic

projectiles impacted a purely sedimentary target of roughly 10 km thickness. The

unusual melt rocks to be described are related with the sedimentary target and partly

with the contribution of carbonate and evaporite rocks. The suevite breccias

abundantly exposed in the Azuara/Rubielos de la Crida impact region [4, 6] will not

be considered here.

Silicate melt rock

Many of the unusual melt rocks are intermixed in a polymictic megabreccia exposed

along the road between the junction to Cutanda and the village of Barrachina in the

Rubielos de la Crida impact basin [6]. The polymictic megabreccia is assumed to

have been formed in the impact cratering process by the deformation and intermixingof large rock complexes of different lithology (Fig. 1)


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Fig. 1. Two aspects of the megabreccia in the Rubielos de la Crida impact basin. To the left:several blocks of different lithology in contact. A microbreccia exhibiting apophyses has beeninjected into the middle block. To the right: A body of silicate melt rock (the light ribbon)embedded in the megabreccia.

The silicate melt rocks occur as porous, fine-grained, whitish to yellowish blocks of

variable size in a range of decimeters up to 1 - 2 meters (Fig. 1B), and they consist

mainly of a milky white glass which forms tiny spheroids and lens-shaped bodies

(Figs. 2, 3).

Fig. 2. The glass of the silicate impact melt rock under the microscope. The field is 15 mmwide.


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Fig. 3. SEM images of the impact glass.

The glass is estimated to make up more than 90% of the rock. This is typically shown

by a distinct amorphous glass hump occurring in x-ray powder diffractograms.

Some relics of plagioclase, quartz and mica within the glass masses are indicated by

respective reflection peaks. Grains of quartz, twinned plagioclase and occasional

mica are also found in thin sections of the glass matrix. In rare cases, the quartz

fragments show planar deformation features (PDFs) and, more frequently, multiple

sets of planar fractures (PFs), both indicative of impact shock. Feldspar grains show

isotropization in the form of multiple sets of isotropic twinning lamellae and isotropic

spots (diaplectic crystals), and they have sometimes become almost completely

isotropic (diaplectic glass), indicating shock peak pressures of the order of 30 GPa

(300 kbar). From a microprobe geochemical analysis we conclude that the glass has

originated from the melting of shales very common in the sedimentary target. An

interpretation of the glass to be volcanic ash as claimed by local geologists and, e.g.,

M. R. Rampino (written communication), can basically be excluded.

Carbonate-phosphate melt rock

A very special kind of former melt was found also within the Barrachina megabreccia

of the Rubielos de la Crida impact basin [5]. The whitish melt rock (Fig. 4) is

composed of irregular spheroids up to 4 mm in size, which are embedded within an


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Fig. 4. The white carbonate-phosphate melt rock embedded in a breccia composed of marl andlimestone.

Fig. 5. The carbonate-phosphate melt rock in close-up. Calcitic amoebic bodies are floating inphosphate glass (white). The field is 30 mm wide.

extremely fine-grained glass matrix (Fig. 5). In thin section (Fig. 6) and SEM image

(Fig. 7), the spheroids turn out to be globular to amoeba-like calcite particles. They

are coarse-grained in their centers and display decreasing grain size towards the

rims. Regularly, a perpendicular grain orientation towards the rims is observed. The

contact with the matrix is extremely fine-grained.

Fig. 6. Photomicrograph of the carbonate-phosphate melt rock. The amoebae-like bodies

composed of calcite crystals are surrounded by Ca-P glass that under crossed polarizersproves to be optically isotropic. Note the increasing size of the crystals towards the center ofthe calcitic bodies. Width of the field is 6 mm.

Fig. 7. SEM image of the contact between amoebic calcite and phosphate glass in thecarbonate-phosphate melt rock.

The glass matrix mainly consists of CaO and P2O5 with minor contents of F, S, Cl

and NaO. Locally, a strong enrichment of Ba and S at the expense of the CaO and

P2O5 content is observed. In part, the Ca-P glass is recrystallized to form apatite, as

verified also by x-ray powder diffraction analysis. A similar melt rock has been

reported for the suevite of the Ries crater. In the suevite, the calcite particles have


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identical structure and composition compared with the melt rocks of Barrachina and

are interpreted as quench products of a carbonate melt [8]. Different from the

Barrachina melt rocks, the matrix in the Ries samples is silicate glass as a result of

carbonate-silicate liquid immiscibility. In our case, the melt rock displays a small-

scaled immiscibility of coexisting former carbonate melt and phosphate melt.

Sulfate melt rock

In the Barrachina megabreccia in the Rubielos de la Crida impact basin, white clasts

are embedded (Fig. 8) that consist of highly porous material (dry-rock densities of

only 1.4 g/cm! were measured). Only a few rock fragments are interspersed (Fig. 9).

Chemically, the white material is nearly pure CaSO4. In thin section, the matrix may

show flow texture but is otherwise not resolved by the optical microscope. SEM

images (Fig. 10) show a distinct vesicular texture obviously related with the high

porosity. Mineral fragments, mostly quartz and feldspar, are partly strongly shocked

(PDFs, diaplectic glass). Shock effects occur also in minerals of the interspersed rock

fragments.

Fig. 8. Block of probable sulfate melt rock embedded in the megabreccia.

Fig. 9. Clast of highly porous calcium sulfate probably crystallized from a sulfate melt. Note theenclosed quartzite fragments.


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Fig. 10. SEM images of the sawed surface of probable sulfate melt rock. Note the vesicular

texture.

Obviously, the CaSO4

material is not a chemical sediment (gypsum, anhydrite), and a

pedogenic origin can clearly be excluded. With respect to the high porosity, the flow

texture and the strong shock effects, we suggest the clasts to have formed by

crystallization from a shock-produced sulfate melt. The melting point of anhydrite is

1,750 K, a temperature which must have been exceeded to produce the silicate melt

in the Barrachina megabreccia (see above, and [6]). Crystallization from an anhydrite

melt is also discussed for material in suevite breccias from the Chicxulub impact

structure [9] and in impactites from the Haughton impact structure [10].

Carbonate melt rocks

Abundant relics of former carbonate melt are proposed to occur in the Azuara impact

structure and the Rubielos de la Crida companion impact basin [6]. They are found

in the form of decimeter and meter-sized blocks, as dikes cutting sharply through the

country rock (Fig. 10), and as highly porous, foamy and feathery material within

brecciated rocks. As already mentioned, carbonate melt cannot be chilled to form

glass, but rapidly crystallizes to carbonate again Therefore, the origin from a melt can

only indirectly be suggested by the occurrence of skeletal, dendritic crystallites,

vesicular texture and related features. Typical carbonate rocks which we interpret to

have crystallized from a carbonate melt are shown in Figs. 11, 12 and 13.


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Fig. 14. A dike of carbonate-psilomelane melt rock cutting through Muschelkalk dolomite.

Fig. 15. Section across the dike of carbonate-psilomelane melt rock (m) . d = dolomite hostrock, c = seam of calcite grown perpendicular to the wall of the dike.

Fig. 16. Photomicrograph of the carbonate-phosphate melt rock. Note the many gas vesicles

(gv). The field is 1 mm wide.

As shown by thin sections under the microscope, the dike consists of a light matrix of

carbonate minerals, hosting a high amount of black spherical to amoebic particles

(Fig. 16) which show gel-like layered structure in the reflective light. The rim (c in Fig.

15) towards the dolomite country rock (d in Fig. 15) is characterized by pure

carbonate crystals having grown perpendicular to the wall of the dike. A typical

feature of the dike is the high amount of gas vesicles (Fig. 16). The gel-structured

black particles turned out to be ore minerals containing Mn and Ba as major elements


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beside highly variable amounts of light components like H2O. After HCl treatment and

removal of the carbonate matrix of a black vein sample, the remnant in an x-ray

powder diffractogram was identified as a very imperfectly crystallized psilomelane (a

hydrated manganese oxide). At first glance the calcite-psilomelane composition ofthe dike may be explained by hydrothermal or weathering fluids having circulated

through the rocks. Two features, however, do not really match this point of view:

-- The very high amount of gas vesicles is totally uncommon for hydrothermal dikes

and can only be explained by rapid melting-cooling processes with incomplete

degassing of the melt.

-- The growth of the calcite crystals (c in Fig. 15) perpendicular to the wall of the dike

again clearly points to crystal growth from a melt, perpendicular to the cooling front.

Similar calcite growth has been observed in amoebic carbonate particles embedded

in phosphate or silicate glass matrix of suevite samples from the Rubielos de la

Crida and the Nrdlinger Ries impact structures, respectively (see above,

Carbonate-phosphate melt rock). This texture has been interpreted as the result of a

quench crystallization from a carbonate melt. As a conclusion, the psilomelane-

calcite dike turns out to be a former manganese-bearing carbonate melt which was

injected into cracks at the crater floor and then rapidly cooled. Probably the

psilomelane is secondary and formed by replacing a primary Mn mineral. Mn, partly

in minable quantities, is not uncommon in the target rocks of the impact structure.

Melt rock from Almonacid de la Cuba

The peculiar rock shown in Fig. 17 is exposed near the village of Almonacid de la

Cuba at the NE ring of the Azuara impact structure (UTM coordinates 6 84 300, 45

73 900) and is so far unique with respect to occurrence, composition and texture. A

comparable rock is completely unknown in the stratigraphic record of northern Spain.

Within a dense to porous and even foamy greyish carbonate matrix, components of

snow-white color are embedded, which may be extremely vesicular as


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Fig. 17. Sawed surface of the Almonacid de la Cuba impactite.

shown in the photograph (Fig. 17). The rock forms an extensive deposit, which has indetail been investigated by Tanja Katschorek [12]. From field work and petrographic

analyses, she concludes that the deposit has resulted from an expanded, turbulent,

and dilute flow by inclusion of considerable amounts of carbonate melt, similar to

volcanic surges. The matrix may have solidified completely from a carbonate melt

giving the rock the character of an impact melt rock [1].

Amorphous carbon in a microbreccia - solidified from a carbon melt?

In the Barrachina megabreccia referred to above, blocks of a fine-grained

microbreccia (Fig. 1) are locally intercalated. The microbreccia consists of loosely

cemented carbonate and, subordinate, quartz particles. Small black clasts

measuring between 0.5 and 2 mm are widespread and striking. Two kinds of these

black components can be distinguished. The first kind is ordinary charcoal which

under the microscope shows the typical charcoal structure. The second kind of black

particles is very hard, occurs in very irregular forms and has a surface gleaming like

glass (Fig. 18). Qualitative microprobe element scans show the particles to be

composed of carbon and oxygen as the only major components. Additional elements

are Ca and S in varying concentrations. X-ray powder diffraction analysis of the

particles resulted in diffractograms without any reflections, but showing a typical

amorphous glass hump.


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Fig. 18. Particles of amorphous carbon in an unknown compound with oxygen. Scale bar 1 mm.

Fig. 19. SEM image of an amorphous carbon particle from the Rubielos de la Crida

microbreccia.

Fig. 20. SEM image of a fracture surface of an amorphous carbon particle. Note the lancet

markings typical of glass fracture.

Carbon in elemental form has repeatedly been described from impact structures

[e.g., 13, 14]. As has been shown by hypervelocity impact experiments [13], a

possible source of elemental carbon is carbonate rocks, which could also apply to the

carbonate-rich target of the Rubielos de la Crida structure. As a further possibility

and taking into account the glass-like appearance (Figs. 18, 19, 20) and the irregular

shapes of the particles, the amorphous carbon may be quenched carbon melt from

extremely shocked coal of the Cretaceous Utrillas lignite deposits in the target.

Carbon melts at temperatures of roughly 3,800 K which, in a hypervelocity meteorite

impact, is exceeded at highest shock levels. From stratigraphical considerations, the

lignite layers could have been deposited in the very center of the Rubielos de laCrida structure. The role of remarkable contents of oxygen detected in the carbon


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particles is still unclear. Compounds of carbon and oxygen do not occur in solid state.

We propose the possibility that the carbon may occur as fullerenes which are able to

trap gases within their cages. Fullerenes have been reported in relation with the

Sudbury impact structure and the Permian-Triassic boundary [15, 16].

Conclusions

We have presented and discussed seven different rocks from the area of the

Azuara/Rubielos de la Crida impact. They have either been solidified from a melt, or

they contain glass that has been quenched from a melt. However, not all aspects of

their formation are completely understood which is related with the generally poor

knowledge of the formation of carbonate and sulfate melts in the impact cratering

process. The rocks prove to be rather inconspicuous and unprepossessing, and for

rock hunters they may be the Ugly Ducklings among the impactites. That is why they

probably have never been recognized by geologists previous to the discovery of the

large impact event. Without closer inspection, the mostly whitish, yellowish and

greyish rocks have either not been given further consideration, or they have been

confused with common carbonate rocks or soil formations like calcrete (caliche), and

dike-like occurrences of melt rocks and melt-bearing rocks were regarded as karst

features, e.g., by local geologists. From our experiences in the Azuara and Rubielos

de la Crida impact area with rocks originating from the melting of sediments we

suggest that in impact structures in sedimentary i.e. volatile-rich targets, there may

generally be more impact melt rocks than previously assumed. In impact research,

this idea gains acceptance only hesitantly (see e.g., [17].

References

[1] IUGS Subcommission on the Systematics of Metamorphic Rocks, Study Group forImpactites.[2] Kieffer, S. W. and Simonds, C. S. (1980). The role of volatiles and lithology in the impactcratering process. Review of Geophys. and Space Physics, 18: 143-181.

[3] Ivanov, B.A., Langenhorst, F., Deutsch, A. and Hornemann U. (2003). Theoreticalconstraints on shock melting and decomposition of anhydrite. Bayerisches Geoinstitut,

Bayreuth, Annunal Report 2003.

[4] Ernstson, K. and Fiebag, J. 1992. The Azuara impact structure: New insights fromgeophysical and geological investigations. Geol. Rundschau, 81: 403-427.[5] Ernstson, K., Schssler, U., Claudin, F. & Ernstson, T. (2003). An impact craterchain in northern Spain. Meteorite, 9, no 3, 35-39.


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[6] Ernstson, K., Claudin, F., Schssler, U. & Hradil, K. (2002). The mid-TertiaryAzuara and Rubielos de la Crida paired impact structures (Spain). Treballs delMuseu de Geologia de Barcelona. 11: 5-65.[7] Ernstson, K., Claudin, F., Schssler, U., Anguita, F. and Ernstson, T. (2001).Impact melt rocks, shock metamorphism, and structural features in the Rubielos de

la Crida structure, Spain: evidence for a companion to the Azuara impact structure,in: Impact markers in the stratigraphic record, 6

thESF-IMPACT workshop Granada,

abstract book: 23-24.[8] Graup, G. (1999). Carbonate-silicate liquid immiscibility upon impact melting: RiesCrater, Germany. Meteoritics Planet. Sci., 34: 425-438.[9] Claeys, P., Heuschkel, S, Lounejeva-Baturina, E., Sanchez-Rubio, G., andStffler, D. (2003). The suevite of drill hole Yucatn 6 in the Chicxulub impact crater.Meteor. Planet. Sci., 38, 1299-1317.[10] Osinski, G.R. and Spray, J.G. (2003). Evidence for the shock melting of sulfatesfrom the Haughton impact structure, Canada. Abstract, 6th Annual MeteoriticalSociety Meeting.

[11] Schssler, U., Ernstson, T., and Ernstson, K. (2004). Impact-induced carbonate-psilomelane vein in the Azuara structure of northeastern Spain. www.uni-wuerzburg.de/mineralogie/schuessler/Monforte-vein.pdf.[12] Katschorek, T. (1990). Geological investigations at the northern rim of theAzuara structure (NE Spain). Diploma thesis (in German), University of Wrzburg.[13] Bunch, T. E., Becker, L., Schultz, P. H. and Wolbach, W. S. (1997). Newpotential sources for Black Onaping carbon. Large Meteorite Impacts and PlanetaryEvolution, 30. 8. 5. 9. 1997, Sudbury, abstracts.[14] Heymann, D. and Dressler, B. O. 1997. Origin of carbonaceous matter in rocksfrom the Whitewater Group of the Sudbury structure. Large Meteorite Impacts andPlanetary Evolution, 30. 8. 5. 9. 1997, Sudbury, abstracts.[15]Becker, L., Poreda, R. J., Bada, J. L. (1996). Extraterrestrial Helium Trapped inFullerenes in the Sudbury Impact Structure. Science, 272: 249-252.[16] Becker, L., Poreda, R. J., Hunt, A. G., Bunch, T. E. and Rampino, M. R. (2001).Impact event at the Permian-Triassic boundary: evidence from extraterrestrial noblegases in fullerenes. Science, 291: 1530 1533.[17] G. R. Osinski and H. J. Melosh (2004). Impactites on Mars: What should weexpect and what is the role of volatiles? Second Conference on Early Mars, JacksonHole, abstract.

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Unusual Impact Melt Rocks