METEORITE IMPACT EFFECTS IN THE LIBYAN GLASS AREA, SOUTHWESTERN EGYPT

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METEORITE IMPACT EFFECTS IN THE LIBYAN GLASS AREA, SOUTHWESTERN EGYPT

via METEORITE IMPACT EFFECTS IN THE LIBYAN GLASS AREA, SOUTHWESTERN EGYPT

دلائل صدمة نيزكية بمنطقة الزجاج الليبى، جنوب غرب مصر

دلائل صدمة نيزكية بمنطقة الزجاج الليبى، جنوب غرب مصر

الملخــص العربى

تقع منطقة الزجاج الليبى، جنوب غرب مصر، بين خطى عرض ´02 º25 – ´13  º26 شمالا ، وبين خطى طول ´24 º25- ´55 º25º شرقا، وعلى بعد حوالى 50 كم من خط الحدود بين مصر وليبيا. وقد نالت المنطقة شهرة واسعة، بعد أن أعلن المستر باتريك كليتون، عن اكتشاف الزجاج الليبى بها، فى ديسمبر من عام 1932م.

ومنطقة الزجاج الليبى تتكون من صخور الأحجار الرملية، التى تغطيها الرواسب المفككة، والكثبان الرملية الطولية، التى تمتد عشرات الكيلومترات، من الشمال إلى الجنوب، فى شبه انتظام، وترتفع إلى حوالى 100 متر عن مستوى سطح الأرض، وتنفصل عن بعضها البعض، بمناطق صخرية، مغطاة جزئيا، بالرمال والحصى وسائر الرواسب المفككة، الناشئة أصلا من فعل الرياح على الصخور المكونة للمنطقة.

والزجاج الليبى، مادة طبيعية فريدة من نوعها، إذ تتكون من حوالى 98 % ثانى أكسيد السيليكون. وهى مادة شفافة إلى نصف شفافة، تتباين ألوانها تباينا كبيرا، فمنها الأبيض ومنها الأسود. إلا أن اللون الغالب هو الأخضر الغامق أو الأخضر المائل إلى الاصفرار. وتبلغ صلادتها حوالى 6 درجات على مقياس موه للصلادة. وتوجد على هيئة قطع صغيرة. وأكبر قطعة معروفة، يزيد وزنها قليلا عن25 كيلوجرام. وتوجد قطع الزجاج الليبى متناثرة على سطح الأرض، ومغمورة كلية أو جزئيا بالرمال السافية، التى تغطى المنطقة.

وقد بينت الدراسات المختلفة، التى أجريت لتحديد عمر مادة الزجاج الليبى، أنها تكونت منذ قرابة 28,5 مليون سنة.

ويعتبر أصل الزجاج الليبى، من أهم المشاكل العلمية بالصحراء الغربية. وقد سيقت بهذا الخصوص عدة فروض، تعزو تكونه لعمليات متباينة، بعضها أرضى تماما، أو سماوى تماما، أو عوامل أرضية-سماوية. والاتجاه العام الآن، اعتبار الزجاج الليبى، مادة تكونت من تأثير صدمة نيزكية ضخمة بالمنطقة،  منذ 28,5 مليون سنة تقريبا. نتج عنها صهر سريع لصخور الحجر الرملى، ثم التصلب السريع للمصهور.

لكن عدم التعرف على شواهد تدل على وجود صدمة نيزكية بالمنطقة- قبل البدء فى هذه الدراسة- كان يعوق قبول هذه الفرضية.

لقد تناولت هذه الدراسة جيولوجية، وبتروجرافية، وجيوكيميائية منطقة الزجاج الليبى. وعرضت الدراسات التى تمت على المنطقة من قبل، والدراسات التى عنيت بمادة الزجاج الليبى، خاصة تلك التى تركزت على مشكلة أصله. ومن الدراسات السابقة، يتضح أن القليل منها عنى بالمنطقة، والكثير اهتم بمادة الزجاج الليبى ذاتها. كما أن الدراسات التى عنيت بالمنطقة، تركزت على نوع واحد من صخور المنطقة، وهو الحجر الرملى الصلد (كوارتز أرينيت)، الذى تظهر مكاشفة على نطاق واسع بالمنطقة.

لقد كشفت هذه الدراسة عن وجود أنواع أخرى من الصخور بالمنطقة. فمكون سعد(طباشيرى علوى)، الذى يشكل أغلب مكاشف صخور المنطقة، يتكون من تتابع من الحجر الرملى الصلد (كوارتز أرينيت)، يعلوه تتابع من الحجر الرملى السيلتى، ويأتى فوقها تتابع من الحجر الرملى الحديدى. ويبلغ سمك هذا المكون حوالى 40 مترا، على الجانب الجنوبى الغربى من المنطقة.

وفى وسط المنطقة تقريبا، يشاهد الكوارتز أرينيت، يعلو الصخور الأخرى، وهذا يشكل، ما يعرف بالإندفاع المركزى. وهذه الظاهرة تميز مواقع الصدمات النيزكية المعقدة.

وقد سجلت الدراسة لأول مرة، وجود موقعين لصخور البريشا. الأول فى الجزء الغربى من المنطقة، والثانى على الحدود الجنوبية الشرقية للمنطقة، عند ما يعرف بقارة الحنش. ويمثل هذان الصخران، تأثير الصدمة على صخور الحجر الرملى بالمنطقة. وكشفت الدراسة عن وجود راسب للحديد فى الجزء الغربى من المنطقة. وأقرب احتمالات تكونه أنه نشأ عن سريان المحاليل المحملة بأكاسيد الحديد، خلال صخور البريشيا.

إن أهم نتأئج هذه الدراسة التعرف على الإلمنيت الماجنيزى، داخل صخر بريشيا قارة الحنش. وهذا الطور بتركيبه الكيميائى يمثل معدنا غير أرضى، مما يعنى أنه إدخل إلى هذا الصخر من الجسم السماوى الذى ضرب المنطقة، وهو ما يتوافق مع وجود حبيبات دقيقة تتكون من الحديد-كروم-نيكل، تتشابه تماما مع ما هو مكتشف من فوهة ريس بألمانيا. وهو ما اعتبر من قبل الباحثين على أنه دليل على صدمة بنيزك كربونى.

وأثبتت الدراسة وجود تشوهات مجهرية ببنية معدن الكوارتز المكون للحجر الرملى بالمنطقة. وان هذه التشوهات تتبع مستويات بنيوية فى بلورات الكوارتز مميزة لتلك التى سجلت فى مواقع صدمات نيزكية عديدة.

وأثبتت الدراسة وجود حبيبات دقيقة (مجهرية) من الماس مصاحبة للجرافيت، بعينة صغيرة ألماسية جمعت مصادفة من المنطقة ، فى عام 1996 . ووجود حبيبات ماس مجهرية مع الجرافيت ، يدل بصورة قوية ، على صدمة نيزكية بالمنطقة ، حيث الضغط العالى يسهم فى تحول الكربون إلى جرافيت أو ماس أو كليهما معا .

كما سجلت الدراسة لأول مرة وجود تركيزات عالية نسبيا من عنصر النيكل والكروم والكوبلت والإيريديم يالبريشيا، خاصة بريشيا قارة الحنش. ويعتبر تركيز الإيريديم  ( 61,-22, جزء فى البليون)، دليلا قاطعا على وجود مواد نيزكية مع هذه الصخور.

وتتوقع هذه الدراسة أن منطقة  الزجاج الليبى، تمثل بقايا فوهة نيزكية مركبة، قطرها حوالى 40 كم. ولا تعتبر هذه الفرضية نهائية، وإنما تحتاج إلى مزيد من الدراسات الحقلية لتحقيقها.

كما تناولت الدراسة قطعة نيزكية يبلغ وزنها حوالى 48, جرام، عثر عليها فى مارس 1996 م بالمنطقة. وهذه القطعة النيزكية الصغيرة، ، تشبه إلى حد كبير فى مظهرها حصى الحجر الرملى، ذات الألوان البنية الداكنة، المتناثر بالمنطقة. ويبدو من غياب غلاف الصهر، أنها ذات عمر أرضى كبير نسبيا. وقد كان يظن أنها تمثل قطعة من نيزك بحر الرمال العظيم 1.. الذى عثر عليه فى عام 1991 م بالمنطقة.

وبالدراسة تبين أن هذا النيزك يتكون أساسا من الأوليفين والبيروكسين المعينى، إلى جانب بعض المعادن الأخرى.  وأن تركيب الأوليفين والبيروكسين المعينى يتوافقان وطائفة الأوليفين برونزيت كوندريت. وهذا ما أكدته التحاليل الكيميائية. وبذلك تمثل هذه القطعة النيزكية نيزكا جديدا سمى بحر الرمال العظيم 5..

دلائل صدمة نيزكية بمنطقة الزجاج الليبى، جنوب غرب مصر

رسالة مقدمة

إلى

كلية العلوم-جامعة القاهرة

من

علـى عبد اللـه بركـات

بكالوريوس العلوم ، ماجستير العلوم

للحصول على درجة الدكتوراة فى فلسفة العلوم

(جيولوجيا)

كلية العلوم –

2004

METEORITE IMPACT EFFECTS IN THE LIBYAN GLASS AREA, SOUTHWESTERN EGYPT

METEORITE IMPACT EFFECTS IN THE LIBYAN GLASS AREA, SOUTHWESTERN EGYPT

Ph.D. Thesis

By
Aly A. Barakat Sc., M.Sc.
Cairo University 2004
Approval Sheet:
Title of the Ph.D. Thesis:
Meteorite Impact Effects In The Libyan Glass Area, Southwestern Egypt
Name of the Candidate:
Aly Abd Alla Barakat
Sc., M. Sc.
Submitted To:
Geology Department, Faculty of Science
Cairo University
Supervision Committee:-
 Prof. Dr. Ahmed A. El Kammar
 Geology Department, Faculty of Science Cairo University.
 Dr. Ibtsam Arafa
Geology Department, Faculty of Science Cairo University.
 Mr. Khiert A. Soliman
The Egyptian Geological Survey and Mining Authority.
Prof. Dr. Ahmed A. El Kammar

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

CHAPTER VIII

 

DISCUSSIONS AND CONCLUSIONS

 

8.1-GENERAL CONCLUSIONS:

The geological study carried out on the Libyan glass area suggests the following significant features:

1- The country rocks in many sites in the area, particularly around the prominent hillocks of latitude 25° 17′ N and longitude 25° 36′ E are observed as scattered highly polished blocks of several meters in dimension and standing 1-m above the general desert surface between the dunes. This feature has been recorded by Weeks, et al., (1984). The present study considers these blocks as remnants of the parautocthonous rocks, which are indicator of meteorite impact effects on the area.  The term “parautocthonous’ has been introduced to define rocks in the displaced zone below the zone of excavation in the impact structures. According to the data gained from studying the meteorite impact craters (e.g. Dence, 1965; 1968; 1971; 1973; Dence, et al., 1977; Kieffer and Simonds, 1980; Grieve, et al., 1981; Stöffler, et al., 1988; French, 1998), these rocks are driven downward and outward, more or less coherently but they are not completely broken up or excavated. Instead, they thinned and moved relative to each other. French (op. cit.) stated that: “during these movements, the subcrater rocks are generally displaced as large individual blocks typically tens to hundreds of meters (or even larger) in size. However, adjacent regions within this zone may display little displacement relative to each other and original stratigraphy and structural features may be well preserved in individual blocks, p. 62”.

2- The rock of the hillocks at latitude 25° 17′ N and longitude 25° 36′ E represents central uplift. The central uplift areas characterise larger meteorite impact craters (Fig. 8.1), (see, e.g. Koeberl, 1994). In such craters the cavity floor is unstable and rises rapidly to form a central uplift. The central peak contains severely shocked material. It is often more resistant to erosion than the rest of the crater. In older eroded craters it may be the only preserved feature from the crater morphology (Koeberl, op. cit.).

3- The presence of two sites of megascopic breccia, at latitude 25° 22′ 51″ N and longitude 25° 27′ 34″ E and the second on the northwestern side of Qaret el Hanash which occurs on the southeastern border of the Libyan glass area.

 

 

 

 

 

 

 

 

 

Fig. 8.1: Genaralised cross section through; a- simple meteoritic crater and b- complex impact crater (after, Koeberl, 1994).

 

4- The presence of an iron deposit in the area. Field observations indicate that it is of post-Cretaceous age. The deposit occurs to the north of the exposure of the breccia of latitude 25° 22′           51″ N and longitude 25° 27′ 34″ E and replaces its lower part. It also replaces clastic dykes of the sandstone. This clue comes from observing the iron deposit in the form of dykes intruding the country rocks and as veins. The iron rich strip may mark a fault trending roughly WNW-ESE. Otherwise, it may represent a concentration of clastic dykes trending in the same direction.

 

The petrographical and mineralogical studies on the country rocks indicate that they consist of quartz arenite, silty sandstone and ferruginous sandstone. The limited silicification of the country rocks, in particular the quartz arenite (quartzite) in some sites in the area may be considered as a positive indicator of a meteorite impact event. According to Du Toit, (1954) quartzites (silicified quartz arenites) are simply the result of deposition of silica in the interstices of sandstones at shallow depths in the zones of weathering and /or diagenesis. The simplest sedimentary silicified quartz arenites (orthoquartzites) developed by the addition of interstitial siliceous cement to porous and permeable sandstones in which the quartz grains had mutual tangential, relatively pressure free contacts. Cementation results from the precipitation of quartz on the sand grains where open pore space permits and can continue until all available spaces are filed (Skolnick, 1965).

In the light of the established data on the meteorite impact sites, one can find interpretation for this feature. The shock waves generated from the impact process could originate microbrecciated sandstone zone at deeper level. This zone can be invaded by solution containing silica. Later pressure-solution acts on the rock at normal temperature and pressure. In this case adjustment to stress was by removal in solution of the external quartz material along grain borders and the result was an increase in the area of potential stress bearing surfaces. The mechanism was non-metamorphic, but probably it needs mechanical deformation. Maxwell, (1960) stated that much fracturing and rotation of grains was, in part, the method by which adjustments to the pressure occurred within the material.

The petrographical and mineralogical studies carried out on the breccia of latitude 25° 22′ 51″ N and longitude 25° 27′ 34″ E indicate that it represents impact breccia. In addition to the brecciation of the quartz grains some of these grains show PFs and PDFs. The zircon of this breccia shows evidence of deformation as represented by the fragmented nature of some the studied grains and by non-stoichiometric contents of ZrO2 and SiO2. The detection of glass and graphite is accounted also for the impact origin of this breccia.

The petrographical and mineralogical studies of Qaret el Hanash indicate that the matrix contains several interesting mineral phases in addition to quartz. The reported glass within this breccia may be created as a result of heat generated by the impact process. The detection of wollastonite in the matrix of this breccia is also a prominent feature. The shape of the grain (see, Fig. 4.26) gives the impression that it belongs to the orthorhombic system. It can be considered as pseudowollastonite, which is known in the literature as the high temperature form of wollastonite (Deer, et al., 1992). Wollastonite in general can be found in carbonaceous meteorites as well as in the terrestrial rocks as a result of metamorphism. Accordingly, the detection of wollastonite within the matrix of Qaret el Hanash breccia can indicate incorporation of a meteoritic material or the effect of impact metamorphism.  The pseudowollastonite in particular has a definite insight on the impact process. The natural occurrence of this phase is very rare, but it has been reported in pyrometamorphosed rocks in southwest Iran, where sediments were backed by burning of hydrocarbon seepage in prehistoric times. It occurs frequently in glass and slag (Deer, et al., op. cit.).

The studied titanium-iron oxide minerals within the matrix of Qaret el Hanash breccia provide interesting observations. The shape of some of the grains indicates that they were introduced to the rock from a close source, i.e. no transportation for long distance. This is due to the irregular outlines and the shattered and fragmented nature of most of the grains. The variable compositions of these grains may indicate that some of them were derived from former titanomagnetites, hemo-ilmenites and Ti-bearing silicates such as biotite or sphene (terrestrial minerals) and hibonite (extraterrestrial mineral). Hibonite (CaAl12O19) contains a considerable amount of Ti and it is the candidate source for the ilmenite with high Al and Ca contents. Hibonite is a prominent phase in the carbonaceous chondrites (Brearley and Jones, 1998).

The Mg-ilmenite is a very interesting feature. The terrestrial Mg-ilmenite is a prominent constituent of kimberlites and of the xenoliths contained with them. Its occurrence has been proposed as an exploration tool in the search for kimberlites (Deer, et al. op. cit.). However, the composition of the present phase differs from the established compositions of basaltic and kimberlitic Mg-ilmenites (see, e. g. Deer, et al., op. cit.; Barnes and Kunilov, 2000). The present phase has a relatively high concentration of Ti than that of the terrestrial Mg-ilmenites. On the other hand it’s composition falls within the established composition of Mg-ilmenites from carbonaceous chondrites, (Steele, 1995) and that from Lunar soil brought by Luna 20, (Haggerty, 1973). The nature of the grains (see Fig. 4.31), indicates that they had not transported for long distances. On these grounds, one can expect that the material was incorporated to the present breccia by the impactor. Such view has been supported by the detection of Fe-Cr-Ni phase within the matrix of this breccia. The detection of such phase provides the solid and unambiguous evidence of the meteoritic material within this breccia, because the occurrence of this phase within target rocks has been considered as indicator of a carbonaceous meteorite impacting bodies (El Goresy and Chao, 1976b).

From these observations, it is clear that this breccia consists of local sandstone rock fragments within a matrix of finer quartz grains with glass and several other phases. Some of these phases are of meteoritic origin. On these grounds, Qaret el Hanash breccia may represent fall back material ejected by the impact process and deposited through the wide rock fractures within the outer rim of the crater or represents direct impact breccia. In this later case, Qaret el Hanash itself could represent part of the eroded crater. Later the material was subjected to hydrothermal alteration represented by secondary goethite and clay minerals.

The question naturally arises: why the study does not detect kamacite and taenite within the matrix of this breccia? Such minerals are more precisely characteristic of extraterrestrial bodies and their occurrence means direct evidence of a contamination with meteoritic material. The answer comes from the fact that these minerals are highly sensitive to terrestrial weathering. The dissolved iron from the oxidation of these minerals is commonly precipitated in the immediate vicinity in the form of the mineral limonite. The released nickel is not incorporated in limonite (Ramdohr, 1963), but may be leached away.

A note worth mentioning here is the small expected amount of the projectile that is incorporated in the target rocks. The established data indicate that impactites contain very small (at the parts per million level) impactor materials (Palme, 1982). The lunar regolith which is the product of repeated impact events contain only about 2 % of the meteoritic materials (Anders et al., 1973). The only exceptions in this regards are the Wabar Crater Arabia and the Meteor Crater Arizona USA. Some workers (e.g., Hörz, et al. 2002) considered the relatively high content of the projectile within the melts of the Meteor Crater as unusual feature and indicates low velocity impact processes.

The petrographical and mineralogical studies of the iron deposit indicate that it is composed of iron-oxyhydroxides cement and diffuse through angular to subrounded sandstone rock fragments of the sandstone country rocks. The quartz grains of these sandstone rock fragments are shattered and fragmented. This feature indicates that the deposit formed as a result of circulating iron-rich solutions through fractured rock bodies. Accordingly, the deposit formed post to the event that led to fracturing the country rocks. Iron may be related to leaching from the area and its near vicinity and /or related to hydrothermal solutions.

 

The petrographical study of the microstructures within the quartz grains from the studied samples indicates that these microstructures are characteristic of meteorite impact effects. Most of the quartz grains show a mottled pattern of extinction “mosaicism”. This feature is very interesting and considered to be indicative of meteorite impact process. The dominant size of these mosaics is > 500 micron, but it is distinctly heterogeneous. However, it decreases with increasing pressure. Extensive studies have been directed to establish a relationship between the degree of mosaicism and shock pressure (Dachille, et al., 1968; Hörz and Quaide, 1973; Hanss, et al., 1978; Schneider, et al., 1984; Ashworth and Schneider, 1985; Langenhorst and Deutsch, 1994).

Concussion fractures are much obvious in the studied samples from the area, in particular at the hillocks of the central part. These fractures radiate from the point of contact between the quartz grains. It is well developed within the quartz of the Coconino sandstone from Arizona Crater, U.S.A (Bunch and Cohen, 1964) and in the sandstone of the BP and Oasis impact structures in Libya (French, et. al., 1974). Concussion fractures are attributed to the sudden grain motion and contact during the passage of a shock wave, (Kieffer, 1971).

The planar fractures (PFs) and the planar deformation features (PDFs) are characterised for meteorite impact event. According to the well-established data, (e.g., Chao, 1967; Kieffer, et al., 1976; Grieve, et al., 1996), the planar fractures (PFs) and planar deformation features (PDFs) in quartz are good indicators of the hypervelocity meteorite impact. The crystallographic orientations of the planar deformation features are concentrated around the planes (0001) and {10`11}. These orientations have been considered as an indicator of a shock pressure less than 20 GPa (e.g., Grieve, et al., op. cit.). The question naturally arises in such case: were these fractures in the quartz were originally from the source rocks or as a result of event latter after their depositions? The answer to this question comes from the fact that the fractured quartz cannot survive long transportation, because it is susceptible of pulverisation of the fractures (Lambert, 1979). Accordingly, the fractures in the quartz are related to an event in the area itself.  The irregular fractures in the quartz of the sandstone country rocks may be attributed to tectonic movements. The tectonic movements are also related to the meteorite impact.

 

The mineralogical study on the diamondiferous material indicates that it consists of diamond and graphite with other phases. The detection of diamond from the area in the form of relatively large mass (30 grams) bears definite insights on the impact event on the area. This material may be formed from the effects of the impact process on the carbonaceous material within the target rocks or from the meteorite itself. According to the present state of knowledge, there is no conclusive evidence on the formation of this diamond from a definite source. Recommended isotopic study is needed to determine the source of the carbonaceous material of this diamond. However, the site at which this mass was found is located just few hundred meters southwest of the mixed breccia (see, Fig. 3.11). As it has been shown in chapter three, the mixed breccia contains graphitised coal and graphite. This may suggest that the diamond formed from the impact process of the carbonaceous material that was originally present in the target rocks. This is concicident with established studies (see, Koeberl, et al., 1995) that “the distribution of impact diamonds in impactites is a function of the initial distribution of coal or graphite bearing rocks among the target rocks, as well as the shock donation. No impact diamonds are recovered from the central part of the crater (e.g., a hot melt body), because temperatures were too high and led to a combustion of the carbon. However, in the zone immediately following the temperatures were low and the pressure was high enough to form abundant impact diamonds” p. 777. Accordingly, abundant impact diamonds have been formed through the crater rims. On the other hand, Nininger (1950) on his account on the structure and composition of the fragments of the Canyon Diablo iron meteorite which is responsible on the formation of the Meteor Crater, Arizona, USA, showed that shock produced diamonds are present mainly in the meteorite fragments found at the crater rims.

Accordingly, the site of the diamond can help in the determination of the crater boundary regardless the source of the carbonaceous material if it is from the meteorite or from the target rocks.

The chemical analysis carried out on samples representing the country rocks from the area indicates that they are silica-rich sediments with relatively slight to clear variation in their contents of the other major oxides. The quartz arenite and the silty sandstone contain SiO2 around 95 wt % and the ferruginous sandstone is iron rich rock (7.48-13.82 wt %Fe2O3). The previous studies did not show the variation in the chemical composition of the country rocks. They were concentrated on only one type, which is very pure sandstone (quartz arenite). The chemical analysis for the samples from the country rocks of the area indicates that they do not show incorporation of meteoritic material. This for the very low concentration of some of the trace elements; such as Ni, Co, Cr and Ir, in agreement with all the previous chemical studies on the area of the Libyan glass. This feature indicates that the country rocks in their present stratigraphical situation, were much deeper from the point of the impact. This is in agreement with Weeks, et al., (1984) that the impact may be on a much higher stratigraphic level than the present level. A worthy mentioned feature in this regard is the clear depletion in the Na2O and K2O within the country rocks of the Libyan glass area. This feature reflects the absence of any mineral bearing Na and K, such as; feldspar from these rocks.

The country rocks contain lower concentration of REE averaging 33.46, 71.78 and 43.70 ppm for the quartz arenite, silty sandstone and ferruginous sandstone, respectively. The pattern of the chondritic normalised REE of the country rocks shows LREE enrichment over HREE, with clear depletion in Eu.

The chemical analysis of the mixed breccia shows some interesting features. The contents of Na2O, MgO, P2O5 and TiO2 are relatively higher than that recorded for the country rocks (quartz arenite and silty sandstone).

Moreover, the ratio K2O/Na2O of this breccia is relatively higher than that for the country rocks. It ranges from 1.5 to 3.5, while it ranges from 0.5 to 1.2 in the country rocks. There is a reliable explanation for these features in the light of the meteorite impact effects. According to Bouška, et al. (1993), the K2O/Na2O ratio in impact glasses is generally higher than that in the target rocks, for the lower mobility of potassium compared to sodium in the acidic and medium acidic melts. The process of selective vaporisation and condensation of the vapours formed on impact leads to an increase in the K2O/Na2O ratio. At higher temperatures above 2273-2373 K, potassium is ready vaporised but condenses on a decrease in the temperature much faster than sodium. The study of Jakovlev, et al,. (1978, in Bouška, et al., 1993) indicates that during the melting of the silicate rocks, the K2O/Na2O increases in acidic rocks in dependence on the selective vaporisation, while it decreases in mafic rocks (SiO2 < 50 %). The study of Grieve (1978) indicates that the K2O content in the impact glass of the Brent Crater increased by up 5 % compared to the target rocks when the shock wave was greater than 20 GPa.

The concentrations of the trace and REE are relatively much higher in the mixed breccia than that of the country rocks. At the meantime, it contains much higher concentrations of Zr and Hf than that of Qaret el Hanash breccia. The concentrations of Ni, Co, Cr and V in this breccia may indicate incorporation of meteoritic material because it contains lower content of Fe2O3. Such clue has been supported by the relatively high Ir content (1.6-1.9 ppb).

The relative enrichment of REE within this breccia relative to the country rocks agrees with the concept indicating that impact processes can involve the concentration of some lithophilic elements in the melt. Their volatility is low and thus their contents in the impact glass are relatively higher than in the target rocks of the craters. These elements include Zr, Ba, Sr, and REE. These changes have been detected from many meteoritic craters such as Clearwater Lake West, Carswell, Manicouagan and Henbury (Bouska, et al. 1993). However, the enrichment of REE in the mixed breccia may be related to the mineral zircon which was detected petrographically. According to Taylor and McLennan (1985), the influence of zircon on the total REE content is possible, especially when Zr content exceeds 300 ppm. The average content of Zr in this breccia is higher than that limit, which agrees well with the concept of Taylor and McLennan (1985). The chondrite normalised REE pattern of the mixed breccia shows LREE over HREE (LREE/ HREE ranges from 3.89 to 4.27) with slight depletion in Eu and slight positive anomaly in Yb and Lu.

 

The chemical analysis of Qaret el Hanash breccia indicates that it contains higher concentration of some of the trace elements and REE than other rocks in the area, except the iron deposit. For example it contains higher concentrations of U, Th, Ba, Sr, Ni, Cr, Co, V and Sc. Barium and Sr may be related to the relative enrichment of Ca within this breccia, while V may be due to the relatively high contents of ilmenite. The EDS analysis of some ilmenite grains confirms the presence of V. There is no reliable explanation for the relatively high concentration of the other elements except the incorporation of a meteoritic material. However, Ni has been reported within Fe-Cr-Ni particles in the matrix of this breccia and glass. The existence of Fe-Cr-Ni particles within this breccia indicates that the target was near the impact point in order to receive relatively heavy vapour. More definitely the material of Qaret el Hanash breccia was closer to the impact point than the mixed breccia. This agrees with the topographic situation of Qaret el Hanash which is 96 m higher than the present level of the area of the glass distribution. The detection of relatively high Ir content (2.0-2.2 ppb) in Qaret el Hanash supports the view that this breccia has incorporation of the meteoritic material.

REE (205.66 ppm, in average) in Qaret el Hanash breccia is higher than any other rock from the area. The chondrite-normalised pattern of the REE shows that this breccia is strongly enriched in the LREE relative to the HREE, with strong depletion in Eu. Such pattern may indicate more contribuition from crustal material.

Incorporation of the meteoritic material could not account for the enrichment of the REE in this breccia. This is because the meteorites have low concentration of the REE. However, the relatively higher contents of Ni and Cr, Co and Ir can be held as an evidence of possible contribution of an extraterrestrial material.

 

The iron deposit is one of the interesting features in the area of the Libyan glass. Decomposition of iron bearing minerals and direct precipitation from solution are the two processes that lead to the formation of iron oxy-hydroxide deposits such as; goethite, hematite, etc (Pichler and Veizer, 1999). Direct precipitation from solution occurs either via the slow hydrolysis of Fe3+ or due to oxidation of Fe2+ bearing solutions (e.g., Murray, 1979). The former leads to the formation of goethite and akageneite. The later mineral forms when the solution contains Cl(Murray, op. cit.).

The Fe/Mn ratio ranges from 487.53 to 622.92. The low content of MnO within this deposit may be interpreted, as the conditions were not oxidising enough to precipitate Mn (Pichler and Veizer, op. cit.). This may be valid when it is sure that the original solutions contain high concentration of Mn. However, the depletion of Mn, in the present case, can be attributed to the original lack of Mn. This is clear from the low concentration of MnO in the country rocks and the breccias. Other important feature supports such clue is the relative low content of FeO within the iron deposit, which indicates that the conditions were more oxidising.

Unfortunately, the relatively low concentration of the trace elements such as Ni, Cr and Co does not allow a reliable connection between this deposit and the hydrolysis of meteoritic iron minerals. As it is well established, all meteorites contain appreciable amounts of iron as a free phase or as compound with other elements. Assuming that the iron deposit was formed from the decomposition of meteoritic minerals, the question naturally arises, where are the other meteoritic elements, such as, Ni, Cr and Co?

The more reliable interpretation for the source of iron in this case is the decomposition of previous iron minerals such as goethite and hematite (from the country rocks) and the formation of an iron-rich solution and re-deposition of the iron hydrooxides in more concentrated form giving the present deposit. The contribution of the impact process is necessary and more needed here. The impact process opened the way to the mineralised solution. Without this process the occurrence of the iron deposit in this area could be questioned and represents a mystery. According to many writers (see, e.g., Koeberl, 1994), hypervelocity meteorite impact with the ground causes a definite crater. A thick sheet of clasts from crushing the country rock covers the bottom of such crater.  Cracks, fissures and even faults are well developed in the country rock particularly, the bottom of the crater. Accordingly, the area is more suitable for mineralization, Grieve and Masaitis, (1994). For example, the Sudbury impact structure, Ontario Canada, (Dietz, 1966) holds the largest Ni-Cu mineralisation. Grieve, et al., (1991) consider that the mineralisation is a direct product of the impact event.  The Red Wing Greek crater contains significant oil reserves (Donofrio, 1981). Moreover, the hypervelocity meteorite impact event may start hydrothermal activity (Koeberl, et. al., l989, Naumov, 2002).

The REE contents within the iron deposit vary from about 50.91 to 201.48 ppm (106.27 ppm average). The chondrite-normalised REE pattern shows the least degree of LREE enrichment relative to the HREE where the LREE/HREE is less than 3, with moderate Eu deficiency.

 

From these observations it seems quite likely that the formation of the iron deposit in the Libyan glass area may be related to the impact event which happened from about 28.5 million years ago and formed Libyan glass. The presence of PDFs in some of the quartz grains in this deposit is a guide feature for this conclusion. A limited degree of the support for this view is the relatively high content of Ni in some of the analysed specimens of this deposit. Further analyses for Ir and Os, in this iron deposit may shed more lights on this opinion.

 

The studied meteorite represents a weathered stone that is related to the olivine-bronzite chondrites (H-group) as indicated by the chemical and mineralogical compositions. The slight depletion in Ni and Co, relative to the olivine bronzite chondrites, can be attributed to terrestrial weathering. The meteorite shows abnormally high content of U, which is much higher than that of the L-chondrites and H-chondrites. The high U content may be related to the terrestrial weathering. The meteorite shows clear evidence of thermal metamorphism. This is clear from the obliteration of the chondritic structure of the stone as well as from the homogeneity of olivine and orthopyroxene that are representing the main mineral constituents of the meteorite. Such a view has been supported by the presence of tiny grains of plagioclase. Accordingly the meteorite can be classified as H6-chondrite.

Barakat, (1996) was erroneously considered the present stone as one piece from the Great Sand Sea 001 meteorite The thought was depending on examination of the exterior features of the stone. However, the chemical and mineralogical compositions of the Great Sand Sea 001 meteorite indicate that it is related to the olivine-hypersthene chondrites (L-group), whereas, the present stone is one of the olivine-bronzite chondrites (H-group). This is a final proof that the present stone represents an independent meteorite has no obvious connection with the impact process.

 

8.2-SIGNIFICANCE AND IMPORTANCE OF THE IRIDIUM CONTENTS:

The most interesting achievement of the chemical analyses of the present study is the detection of high Ir content within the breccias. This value represents the highest reported values for the area and for the Libyan glass itself. For instance, Koeberl, (1997) reported <0.1 and < 0.2 ppb maximum concentration of Ir in the surface sands from the area. The highest reported concentration of Ir by the previous studies was detected from the black streak portions from the Libyan glass itself is 1.25 ppb and considered as a good indicator on the incorporation of chondritic meteorite (Murali, et al. 1997).

According to the available information, during the hypervelocity meteorite impact, variable amounts of the meteoritic material are incorporated to the target rocks. This leads to a recognisable enrichment of these meteoritic indicator elements to the target rocks. As example, the platinum group elements (e.g., Ir) are several orders of magnitude more abundant in most meteorites (i.e., chondrites and iron meteorites) than the crustal rocks. Chondritic meteorites contain about 400-800 ppb Ir or Os, whereas the average crustal Ir and Os abundance are only 0.02 ppb (e.g. Taylor and McLennan, 1985). Koeberl (1994) showed that the addition of about 0.1 wt % of a chondritic component to terrestrial crustal rocks would result in an addition of 0.4-ppb Ir to the background value. Accordingly, the enrichment of PGEs (usually, Ir) in the impact melt or breccia may provide a good evidence on meteorite impact event (Morgan, et al., 1975; Palme, et al., 1978; 1979 & 1981; Palme, 1982).

 

The abundant sandstone country rocks and the absence of any magmatic rocks in the area and its near vicinity are helpful in the detection of foreign elements, i.e. they can show any incorporation of meteoritic material. The reported value of Ir in the mixed breccia is around 1.6-1.9 ppb and Qaret el Hanash breccia is 2.0-2.2 ppb. This value indicates that this rock contains about 0.40 to 0.6 wt % of a chondritic material. However, on the other hand it may indicate incorporation of higher percent of a comet or achondritic meteorite (they are originally depleted in Ir than that of the chondrites) and lower percent of iron meteorite (contains higher Ir than that of chondrites).

However, the concentration of the other elements within the studied rocks is useless for the effect of terrestrial weathering, i.e. it is not useful to consider the concentration of definite elements, such as, Ni and Co in this regard as conclusive evidence for the impact event like that of Ir. These mobile elements could be easily changed in response to weathering processes acting over the last 28.5 million years.

In this regard it is important to mention that it is not necessary to detect meteoritic elements in all the established meteoritic impact sites to accept its formation by meteorite impact process. Bazilevskii, et al., (1984), studied the Ir contents in several impact craters and they found that products of shock melting form two groups of structures have different Ir levels. The first group is represented by samples from the Wabar, Meteor Crater, Brent, Saaksjarvi, Wanapitie, Strangeways, Lappajarvi, East Clearwater and Rochechouart craters and is characterized by high Ir levels of 1-1000 ppb, indicating contamination by meteoritic material. The second group of samples represents the aouelloul, lonar, Nicholson, El’gygytgyn, Mistastin, Janisjarvi, Ries, West Clearwater and Manicouagan craters. These samples show low levels of Ir (<0.1 ppb). The accepted interpretation for the latter case is the low Ir contents within the impacting bodies (comets or achondritic meteorites) or mobilisation and loss of the Ir in the gas phase during the impact event.

The reported Ir content in the breccias (1.6-2.2 ppb) places the area within the first group of meteoritic craters that contains high Ir levels. However, the relatively low concentration of Ir in the studied area may be attributed to the high temperatures generated from the impactor. According to Bouška, et al., (1993) there are some impact structures contain impactites without marked meteoritic admixtures. These are mostly the larger structures that had high temperatures leading to the vaporisation of a substantial portion of the rocks in the centre of the crater, including the impacting body itself.

The area of the Libyan glass falls within the large meteoritic structures judging from the present days estimated quantity of glass which according to Weeks, et al,. (1984) is about 1.4*109 grams, with original mass 10,000 times greater than this quantity. However, Barakat, et al. (1997) estimated the present quantity of glass as 1.7*108 grams and 9.7*107 grams. This huge quantity of glass indicates that high temperatures were originally generated as a result of the impact process to produce this mass of glass.

How can Ir be incorporated into the mixed breccia without the presence of meteoritic minerals? The occurrence of meteoritic elements within the target rock does not necessarily mean transportation through meteoritic minerals but may be incorporated via the gases injected though the target rock from the impact process, in particular, when the target material are far way from the centre of the impact point or in much deeper level. In these cases, it receives only gases carrying meteoritic elements. Such elements could be randomly distributed via the fractures of the target material and incorporated to the glassy material, which may be recorded as coating to the mineral constituents.

 

8.3-THE EXPECTED CRATER IN THE AREA:

Seebaugh and Strauss, (1984) suggested that the formation of Libyan glass may be connected with the direct impact of a small icy comet (200-300 m diameter; 1g/cm3 density) into the area excavating a small crater (~3 to 6 km diameter).  However, a weak cometary body is unlikely to have survived passage through the atmosphere (Melosh, 1989). Accordingly, the expected impactor must be larger than that suggested by Seebaugh and Strauss, (op. cit.), which suitable with the formation of a large crater.

Unfortunately, there is no clear crater form in the area to be recognised easily or detected by satellite images. However, based on the field observations, which indicate central uplift, the presence of Qaret el Hanash Hill on the southeastern corner of the area, the main distribution of the glass fragments, the detection of two sites of megascopic breccias and the site of the reported diamond, one can suggest that the area includes relics of an eroded crater (astrobleme). The major axis of this crater trends WNW-ESE from Qaret el Hanash on the southeastern corner of the area to the northwestern corner of the area. Qaret el Hanash may represent remnants of the eroded southeastern rim of the crater. The petrographical and chemical studies carried out in the present study support such view.

However, it is worthy to mention that this is not a final poof on the crater in the area. Further field study is strongly recommended to verify this clue.

Discovery of FeCr-Ni specks within Qaret El-Hanash Breccia of the Libyan Glass area, South Western Egypt.

via Discovery of FeCr-Ni specks within Qaret El-Hanash Breccia of the Libyan Glass area, South Western Egypt.