METEORITES AND LIQUID WATER Roberto Bartali

ABSTRACT Meteorites formed in the same nebula where planets and satellites of the Solar System grown, so they contain the same chemical elements of rocky planets. It is very important, and interesting, the analysis of meteorites, to understand planet formation because meteorites are preserved from alteration due to tectonics and weathering, so they are in the same original state as when they formed. Some meteorites contains water in the form of microscopic bubble and in the form of hydrated minerals. In this article we will first show how the special type of meteorites called Chondrites are classified and then which are the evidences of the presence of water inside them. CHONDRITE CLASSES

Before speaking about the evidence of water content in chondrites, it is better to know something about them, so the follow is a very brief description of such meteorites.

Figure 1 Example of an ordinary chondrite. This is a polished portion of the Mezo-Madaras meteorite found in Romania. Source: http://meteorites.asu.edu/images/education/mezo-madaras-big.jpg

Chondrite meteorites (figure 1) are so called because they have little spherules called chondrules with size varying from

micrometers to centimetres. Their chemical composition is similar to the composition of the solar nebula, so

they represent the older rock known (4.58 billion years old). They never completely melted; this is

because they formed in a cool zone of the solar nebula, 2.5 to 4 astronomical units from the proto-Sun.

Figure 2 Semacona chondrite showing several chondrules. Source: http://www.psrd.hawaii.edu/Mar00/flashHeating.html

Chondrules (figure 2, figure 9 and figure 10) formed after a high and rapid pressure shock (Boss 2005), some meteorites also suffered from more than one alteration, so we can find older

broken chondrules inside new ones. Chondrites are classified depending on their morphology and chemical composition into different classes, but they share a common origin and they are all depleted of volatile elements like oxygen, nitrogen, hydrogen and helium.

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Depending on their water content and mineral alteration by water and metamorphism they are classified into 6 different petrologic types. This way we have many types of chondrites, and a group of specimens that are unclassified due to some specific features that make them unique and are not included into a specific class. Due to their rocky (mostly silicates) minerals, they have a low quantity of metals

like iron and nickel. Now we will summarize all type of chondrites classified depending on their chemical characteristics (chemical groups):

• Ordinary Chondrites. • Carbonaceous Chondrites. • R chondrites. • Enstatite Chondrites.

Each type is divided into several sub-types, depending on the authors may be several differences with some more types:

• Ordinary Chondrites: H, L, LL. Where H stand for high iron content, L for low iron content and LL for very low iron and metals content (figure 5).

Figure 3 Allende Meteorite: it is a carbonaceous chondrite classified as CV3. http://www.meteorlab.com/METEORLAB2001dev/allende.htm

• Carbonaceous Chondrites: CI, CM, CR, CO, CV, CK, CB.

The capital letter after C represent the initial of the place where the representative meteorite

came from: I = Ivuna, V=Vigarano, O=Ornans, M=Mighei, R=Renazzo, K=Karoonda, B=Bencubbin (Figure 3, figure 6)

• Enstatite Chondrites: EH, EL. Where H stand for high and L stand for low iron content (figure 4).

Figure 4 Abee Meteorite: it is an enstatite chondrite classified as EH4. http://www.meteorlab.com/METEORLAB2001dev/abee.htm

Depending on the water content or aqueous alteration of minerals they are included in petrologic type 1 or petrologic type 2; where

type 1 mean that they are more water altered. If they show some moderate high temperature metamorphism, they are included into petrologic type 3, 4, 5, 6; where type 6 shows the highest temperature metamorphism. Ordinary Chondrites belongs to petrologic type 3,4,5,6. CO and CV carbonaceous chondrites belong to petrologic type 3. CK carbonaceous chondrites and R chondrites (figure 7) belongs to petrologic type 3,4,5,6. EH Enstatite chondrites belong to petrologic type 3,4,5,6.

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Figure 6 Murchison Meteorite: it is a chondrite classified as CM2. http://www.meteorlab.com/METEORLAB2001dev/murchy.htm

Figure 5 Richfield Meteorite: it is a chondrite classified as LL3. http://www.meteorlab.com/METEORLAB2001dev/images/rchfld2.gif

Figure 7

NWA753 Meteorite: it is a chondrite classified as R. http://meteorites4sale.net/I_O_IMAGES/NWA753_25g.jpg

Figure 8 Sioux County Meteorite: it is an achondrite type. http://meteorites.asu.edu/images/education/sioux-big.jpg

EL enstatite chondrites belong to petrologic type 3,5,6. Some authors include another type (7). The nomenclature of chondrites is the combination of the chemical group followed by the petrologic type (also called petrologic grade). For example: H5 is an ordinary Chondrite of type 5.

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Petrologic type 3 is the border line that divides water altered meteorites from those that suffered metamorphism due to temperature. So a type 3 is the one that show neither water nor temperature alteration.

Figure 9 Example of chondrules in the meteorite NWA2892. http://www.meteorites.com.au/collection/NWA%202892%20H-L3%209.84g%20(4%20of%204)-700.jpg

Figure 10 Example of chondrules in a meteorite. http://www.meteorlab.com/METEORLAB2001dev/glossary.htm

Achondrites are rocks that share almost the same properties of Chondrites, but lacks of chondrules, so they are classified as a different group (figure 8). WATER CONTENT Meteorites of petrologic type 1,2 and 3 are called unequilibrated because they lack of thermal metamorphism. Instead, petrologic type 4,5,6 and 7 are called equilibrated because of their extended thermal alterations, but it is important to understand that they never melted, they suffered only partial melting due to large impact, the temperature for the highest metamorphism only reached about 900ºC. Type 1 are those meteorites that do not show any chondrules, but they certain have chondrules during some period of their early history, and formed at lowest temperature (<150ºC), water content from 18% to 20%; but they must not be confused with Achondrites. Type 2 meteorites show very low quantity of chondrules but they are very sharp visible, the water content is about 16%; and they present some degree of water alteration; they formed at slight high temperature (<200ºC). Aqueous alteration may be occurred in the parent body (asteroid) at very low temperature (20 to 50 ºC) in a water rich environment, this type of chondrites may contain up to 20% (of their weight) of water. It is possible that up to 50% of this water is terrestrial contamination. Determination of the quantity of water present in meteorite minerals is very hard, because depending on the time of residence on Earth, the meteorite may absorb a large quantity, another issue is that some minerals can be altered by terrestrial water after the meteorite falls. One form to know haw much water is indigenous, is to measure the D/H (Deuterium-Hydrogen) (Robert 2001; Robert, Deloule 2002) ratio, if this ratio is different from any known on Earth, we can conclude that it is of extraterrestrial origin. If the ratio is similar to Earth water, a careful examination must be made, but if

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the water content in meteorite is greater than 0.8% in weight, water contamination is almost invisible. This contamination became clear in minerals like olivine and Pyroxene if their water content is less than 0.8%. Baker et al. (Baker et al 2001) developed a method to measure the amount in oxygen isotopes (17O and 18O). They found that water at low temperature has isotopic oxygen similar to Earth water, but at high temperature (300ºC), water is clearly of extraterrestrial origin. CM samples of chondrites showed a simple release pattern of the isotope 18O, but that of the isotope 17O of CI chondrites pattern was more complex. This value is positive for CI meteorites and negative for CM like that measured for the matrix and silicates minerals. They conclude that carbonaceous chondrites retain an

isotopic record of the water they contain and hence of their parent body. Similarity in 17O for carbonates with water and phyllosilicate minerals suggest that they formed at the same time or that water isotopic composition do not changed during the alteration process. Here is better to open a parenthesis about phyllosilicate minerals because they are present in chondrithes and they are composed of hydrated minerals, they often contain water molecule trapped between their sheet structures. Weathering and erosion of phyllosilicate minerals is very low and they are also tolerant to high pressure and temperature, because of this, they are well preserved even in older meteorites and represent the composition of their parent body. They are found on sedimentary and metamorphic rocks. Some minerals of this subclass that are found in meteorites are (phyllosilicate minerals, 2007; Lewis 1995): Serpentine, Chamosite, Montmorillonite, Kaesurtite, Muscovite mica, Sodalite and Richterite (figure 11).

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CONCLUSIONS It is clear that CI and CM chondrites mineralogy is highly altered by liquid water

present in the parent body (Wilkening 1981) or even before, in the solar nebula where the parent body accreted. Phyllosilicates may be formed by reaction of high temperature silicates with water vapour in the solar nebula at a moderate temperature of 350K, then, they accreted to form asteroids. Matrix material observed in CI and CM chondrites is similar to interstellar matter and to synthetic vapour deposited on crystalline silicates. If this is the case, water in CI and CM chondrites originally formed during accretion of the solar nebula, so these meteorites are the oldest rocks we can find. Another possibility is that, if silicates in the solar nebula were anhydrous, they have reacted with water vapour in the nebula and produced phyllosilicates. Bunch and Chung (Bunch 1980) proposed that pyroxene and olivine minerals in the parent body reacted with liquid water to form phyllosilicate minerals. Chondrite meteorites are the oldest rocks we can find, because they formed during the condensation of the Solar nebula, from where all planets came from. They contain pristine materials and if they can be collected as soon as they fall on Earth, we can also known better how planet formed from accretion of small rocks. Chondrites also contain water, so most of the water on our planet could be of meteoritical origin, when the Earth was in the accretion phase and during the heavy bombardment phase some 4.5 to 4 billion years ago. REFERENCES Beatty J.K., Collins Petersen C, Chaikin A, The new solar system, Cambridge, 1999. Lewis J.S., Physics and Chemistry of the Solar System, Academic Press, 1995. All about meteorites, 2007: http://www.meteorite.fr/en/classification/ Wittke J, Meteorite book, 2007. Boss A.P., Durisen R.H., Chondrule-forming shock fronts in the solar nebula: a possible unified scenario for planet and chondrite formation, ApJ 621:L137–L140, 2005. Robert F., Deloule E., Using the D/H ratio to estimate the terrestrial water contamination in chondrites, Lunar and Planetary Science XXXIII, 2002. Robert F., Isotope geochemistry:the origin of water on earth, Science 2001, Vol. 293. no. 5532, 1056 - 1058 Baker L, Franchi I.A., Wright I.P., Pillinger C.T., Oxygen Isotopes in Water Extracted from Carbonaceous Chondrites, 64th Annual Meteoritical Society Meeting, 2001. Baker L., Franchi I.A., Maynard J.M., Wright I.P., Pillinger C.T., Measurement of oxygen isotopes in water from CI and CM chondrites, Lunar and Planetary Science XXIX, 1998.

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Wilkening L.L., On the source of liquid H2O in the carbonaceous chondrites parent bodies, Lunar and planetary Institute, 1981LPI..12..1185W, 1981. Grimm R., McSween H, Water and the thermal evolution of thew carbonaceous chondrites parent bodies, 1989Metic..24R..272G, 1989. Phyllosilicate minerals: http://www.galleries.com/minerals/silicate/phyllosi.htm Bunch T.E., Chang S., Carbonaceous chondrites—II. Carbonaceous chondrite phyllosilicates and light element geochemistry as indicators of parent body processes and surface conditions, Geochimica and Cosmochimica Acta, Vol. 44 Issue 10, 1980.

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