To better characterise the hydrated minerals in chondrites Evolved Gas Analysis of Murchison, Orgueil and selected minerals has been carried out. Meteorite water release profiles show significant differences to expected reference minerals

Baker, L.

Franchi, I.A.

Morris, A.A.

Wright, I.P.

English

Open Research Online (OU)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsEvolved gas analysis of hydrated phases in Murchisonand OrgueilConference or Workshop ItemHow to cite:Morris, A.A.; Baker, L.; Franchi, I.A. and Wright, I.P. (2005). Evolved gas analysis of hydrated phases inMurchison and Orgueil. In: 36th Lunar and Planetary Science Conference, 14-18 Mar 2005, Houston, Texas, USA.For guidance on citations see FAQs.c© [not recorded]Version: [not recorded]Link(s) to article on publisher’s website:http://www.lpi.usra.edu/meetings/lpsc2005/pdf/1925.pdfCopyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukEVOLVED GAS ANALYSIS OF HYDRATED PHASES IN MURCHISON AND ORGUEIL.  A. A. Morris, L.Baker, I.A. Franchi and I. P. Wright. Planetary and Space Science Research Institute, The Open University, Mil-ton Keynes, MK7 6AA (a.a.morris@open.ac.uk).    Introduction:  Chondritic meteorites represent some of the most primitive material in the Solar Sys-temand and thus can provide a central role in under-standing its early history. Many chondrites have ex-perienced secondary processing; aqueous alteration is one of the most widespread. Recognition and charac-terization of such processes offer valuable insight into the early stages of planetesimal formation and evolu-tion. Aqueous alteration can generate large isotopic fractionation of oxygen isotopes [e.g. 1], a key pa-rameter in the study of meteoritic material. Therefore it is important to have detailed knowledge about the minerals present and the isotopic variability within these complex meteorites. However, the carbonaceous chondrites are fine grained, complex mineral assem-blages making unequivocal mineral specific isotopic analyses difficult. Baker et al [2] have attempted to determine the isotopic composition of hydrated phases directly from measurement of the structural OH groups in the phyllosilicates in carbonaceous chondrites using a stepped heating extraction procedure to resolve ter-restrial contamination and characterize the host phases. The aim of this work is to apply Evolved Gas Ana-lyis (EGA) with combined Thermogravimetric (TG) and Differential Scanning Calirometry (DSC) to car-bonaceous chondrite matrices to better characterize the hydrated mineral phases present and their OH thermal release characteristics. This is particularly important when attempting to unravel the isotopic fractionations found in these rocks. This study presents preliminary findings from EGA of OH for Murchison (CM) and Orgueil (CI). Background:  Mineralogical Products of Aqueous Alteration. Secondary matrix phases include Mg-, and Fe-bearing phyllosilicates, chlorites, smectites, hy-droxides, oxides, and carbonates [3]. A recent study by Bland et al. [4] has ascertained the modal mineralogy of individual phases from selected unequilibrated chondrites including Murchison and Orgueil,  summa-rised in Table 1.  Evolved Gas Analysis (EGA). Dehydration of a sample at relatively low temperatures (100-250oC) will remove relatively weakly bonded water from minerals, this includes water that is adsorbed to surfaces, inter-layer water and water of hydration. At higher tempera-tures, above 300oC, dehydroxylation takes place where strongly bonded hydroxyl (OH) groups are removed from the structure (lattice water) [5]. The peak release temperature is related to the nature of the hydroxylated phase, and thus to its structure and chemistry [5]. De-hydroxylation temperatures may then be assigned to specific minerals so allowing a preliminary identifica-tion of the phases present.   Table 1: Summary of the modal distribution of hy-drated phases in two chondritic meteorites [4]  Phase Murchison (wt%) Orgueil  (wt%) Mg-phyllosil (Serpentine) 22.8 7.3 Fe-phyllosil (Cronstedtite) 58.5 - Saponite-Serpentine - 64.2 Ferrihydrite - 5.0  Method:  All samples except the minerals chry-sotile and chamosite were analysed as powders. Sam-ples were placed into platiumum buckets and loaded into a Netzsch STA 449 C Jupiter for TG and DSC analysis; interfaced to a Hiden Analytical HPR20 Re-sidual Gas Analyser for EGA. Samples (6-18mg) were run at a ramp rate of 10oC/minute.   Results The EGA profile of Murchison is shown in Fig 1. The low temperature peak (103˚C) is likely to be adsorbed water. Although Mg-rich serpentine is a major component in Murchison, terrestrial examples of such serpentine polymorphs have peak OH releases at temperatures above 600˚C – much higher than any release observed in Murchison. However, the major release in Murchison (417˚C) does match that of cronstedtite (416oC) (Fig 2) – consistent with previous work (Table 1) [4].  The peak observed at 539oC may be caused by the presence of an Fe-bearing chlorite (chamosite?) which peaks at 540oC, although a second characteristic high temperature release ≈720˚C was not found for Murchison.  Clearly the results from Murchison remain only partially understood. The mis-match between the ter-restrial serpentines and any peaks from Murchison may be the result of subsequent processing or chemical differences. In particular, the Murchison serpentines are particularly Fe-rich [6]. Charcterisation of addi-tional terrestrial standards may improve the identifica-tion the serpentine phases. The possibility that the 539˚C release in Murchison is related at least partially Lunar and Planetary Science XXXVI (2005) 1925.pdfto a chlorite can not be overlooked. One of the main aqueous alteration phases of chondrule mesostasis in CMs is a mixture of Mg, Fe-rich phyllosilicates, spin-ach; with progressive alteration spinach changes from Fe-chlorite to intermediate chlorite, and finaly to mag-nesian serpentine, approaching that in the matrix [7]. Bland et al [4], using XRD/Mossbauer, do not report the presence of any Fe-chlorite; however using EGA this study may indicate the presence of such a phase. The EGA profile of Orgueil is shown in Fig 2. The large low temperature release (doublet at 122˚C and 229˚C) is likely to be a combination of adsorbed water, interlayer water and dehydration of gypsum (120-150˚C) and ferrihydrite (235˚C [8]). Ferrihydrite has been reported [4,6] as being a component of the CI matrix which is intimately associated with fine-grained phyllosilicates. The high temperature release of Orgueil is composed of 3 discrete components – at 450, 553 and 720˚C. Either brucite (451oC) or non-tronite (451oC) could correspond to the 450oC peak in Orgueil. The 553oC peak is not consistent with any expected phases it is however similar to that of chamosite, a phase not previously reported in CIs. The 720oC peak is close to the 714oC release identified from saponite, however the higher temperature com-ponent from saponite is not evident.  The main phase reported by Bland et al [4] is a saponite-serpentine mix identified previously by Tomeoka and Buseck [6]. A number of saponite sam-ples analysed for this study revealed a major release around 836˚C – much higher than any release observed in the meteorites. However, the saponite-serpentine phase in Orgueil is usually described as a disordered interstratified phase where the saponite and serpentine are intergrown and occur as poorly crystallised plate-lets [6]. Such a fine grain size and disordered state could have a marked lowering affect on the release temperature for the OH groups. Summary: While there is some apparent success at matching the release water from OH groups in Mur-chison and Orgueil with terrestrial standards a number of major peaks remain poorly matched. Further work is aimed at better matching terrestrial standards to matrix phases and addressing the effects of any post-formation, parent body aqueous alteration (e.g. gentle metamorphism), which may have affected the water (OH) release profile from hydrated minerals.         Figure 1 Peak EGA temperatures for Murchison and Mg, Fe-phyllosilicates 2468101214010203040500 200 400 600 800 1000Ion Current *10-10 (Murchison)Ion Current *10-10 (Phyllosilicates)Temperature (oC)LizarditeChrysotileAntigorite10341753969416659687748CronstedtiteMurchison Figure 2 Peak EGA temperatures for chamosite and Orgueil 01020304050600510152025300 200 400 600 800 1000Ion Current *10-10 (Orguiel)Ion Current *10-10 (Saponite/Chamosite)Temperature (oC)OrguielChamositeSaponite122229 553450720836714125 702540  References: [1] Clayton R. N., and Mayeda T.K. (1999) GCA, 63, 2089–2104. [2] Baker et al (2002) MAPS, 37, 977-985. [3] Zolensky M. E. and McSween H.Y., JR. (1988) In Meteorites and the Early Solar System., 114-143. [4] Bland et al. (2004) MAPS 39, 3-16. [5] Wicks F.J. and Ramik R.A. (1990). In Thermal Analysis in Clay Science. 160-189. [6] Tomeoka K and Buseck P.R. (1988) GCA,52, 1627-1640.  [7] Brearly A. J. and Jones R.H. (1998) In Planetary Materials, Vol.43. [8] Lauer et al (2000) LPSC XXX1 Abstract #1990 Acknowledgments: We thank Dr Gordon Cressey and Mike Rumsey of the Natural History Museum, London, for suppling min-eral samples for this work. Lunar and Planetary Science XXXVI (2005) 1925.pdf

Evolved gas analysis of hydrated phases in Murchison and Orgueil

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Evolved gas analysis of hydrated phases in Murchison and Orgueil

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