We report the petrology and geochemistry of NWA 3368, a new non-cumulate, monomict eucrite breccia with a variety of clast sizes and a pink-tinted matrix. Analytical techniques include electron microprobe, INAA, and ICP-MS

Domanik, K.J.

Drake, M.J.

Franchi, I.A.

Gardner, K.G.

Goreva, J.S.

Hill, D.H.

Lauretta, D.S.

English

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsPetrology and geochemistry of the NWA 3368 EucriteConference or Workshop ItemHow to cite:Gardner, K.G.; Lauretta, D.S.; Hill, D.H.; Goreva, J.S.; Domanik, K.J.; Franchi, I.A. and Drake, M.J. (2006).Petrology and geochemistry of the NWA 3368 Eucrite. In: 37th Lunar and Planetary Science Conference, 13-17 Mar2006, 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/lpsc2006/pdf/2389.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.ukPETROLOGY AND GEOCHEMISTRY OF THE NWA 3368 EUCRITE.  K. G. Gardner1, D. S. Lauretta1, D. H. Hill1, J. S. Goreva1, K. J. Domanik1, I. A. Franchi2, M. J. Drake1 1Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, 2Planetary and Space Sciences Research Institute, The Open University, Milton Keynes, MK7 6AA, United Kingdom;  kgardner@lpl.arizona.edu.   Introduction:  A considerable amount of debate exists on the petrogenesis of eucrites, basaltic meteor-ites believed to have originated on the asteroid 4 Vesta [1].  One school of thought, developed by Stolper [2], suggests that most eucrites are primary partial melts of chondritic material, while another by Mason  [3] sug-gests that eucrites are residual liquids that resulted from fractional crystallization from a chondritic magma that had previously crystallized diogenites.  Many scientists have further speculated on the forma-tion event, supplementing ideas to the models by Ma-son and Stolper ([4], [5], etc.).  In order to distinguish between hypotheses, an understanding of both the ma-jor and trace element compositions of all eucrites is imperative.  In this study, we report the petrology and trace element geochemistry of NWA 3368.      Figure 1: Cut slab of NWA 3368; fresh, black fusion crust is seen at the edge (1 cm3 block and cm-ruler at top for scale)  Petrology Methods: Two thin sections and one 20.5 gram polished slab were studied using optical microscopy and the Lunar and Planetary Laboratory’s  Cameca SX-50 electron microprobe.  Element maps of major elements were prepared, and analyses were made of all phases.   Geochemistry Methods:  Six sub-samples includ-ing four clasts, a section of light-colored matrix, and a section of dark-colored matrix were separated from the polished slab using a rock splitter, and each chunk was divided in two: one for use in INAA and one for use in ICP-MS.  Two small chips of NWA 3368 were sent to Open University to determine oxygen isotope compo-sition.     Instrumental Neutron Activation Analysis:  A suite of major, minor and trace elements were analyzed by INAA at the University of Arizona Nuclear Reactor Laboratory and LPL’s Gamma Ray Analysis Facility.  60-sec. irradiations in the Rabbit Facility of the UA-TRIGA reactor were followed by a 3-hr. Lazy Susan irradiation.  Samples were counted in series grouped by decay.   Inductively Coupled Plasma Mass Spectrometry: A similar suite of major, minor and trace elements were analyzed using a Thermo Finnigan Element2 ICP-MS.  Each of the six NWA 3368 clasts and matrix pieces were dissolved using standard dissolution procedure.  A series of custom-made solutions of basaltic compo-sition and the Geological Society of Japan standard basalt JB-2 were used as standards.   Results: Petrology:  NWA 3368 has a variety of dark and light angular clasts that range in size from several millimeters to a couple of centimeters in length. They range in texture from coarse- to fine-grained.  The fine-grained matrix has both a light and dark pink hue, and the two colored matrix regions are separated by a slight foliation in the matrix texture (marked in Fig 1 as a red dashed line).  Only a minor amount of weathering is present.       Electron microprobe analyses yielded pyroxene compositions of Wo5En36Fs59 for low-Ca pyroxene and Wo43En30Fs27 for high-Ca pyroxene (Fig 2).  All Fe/Mn ratios lie between 28 and 32, typical for eucrites [6].   Fs0 10 20 30 40 50 60 70 80 90 100WoEn5060708090100 Figure 2: Range of pyroxene compositions and their associated exsolution lamellae (atomic %)   Plagioclase in NWA 3368 has a composition of An90Ab10Or0.4.  Ilmenite and troilite grains are abun-dant, along with chromite containing 5 to 27% TiO2 and ~5% Al2O3.  Other phases include iron metal and silica.  Figure 3 shows a BSE electron microprobe im-Lunar and Planetary Science XXXVII (2006) 2389.pdfage of a high-Ti content chromite grain with ilmenite exsolution lamellae.   Two main texture types exist in NWA 3368.  A BSE image of the fine-grained texture is shown in Fig-ure 4, and a BSE image of the coarse-grained texture is shown in Figure 5.  The fine-grained texture is com-posed of abundant plagioclase and lamellae-free py-roxene, while the coarse-grained texture contains abundant pyroxenes with varying degrees of exsolu-tion lamellae.          Figure 3: Chromite with ilmenite exsolution lamellae   Figure 4: Example of fine-grained texture   Figure 5: Coarse-grained texture and exsolution ob-served in some clasts     Oxygen isotope data are consistent with all other HED meteorites (Table 1).    δ17‰ δ18‰ ∆17‰ 1.714 3.753 -0.237 Table 1: Oxygen isotope data  INAA:  INAA data reveal a flat REE pattern with a slightly negative Eu anomaly.  Figure 6 represents REE data for both matrix regions and several represen-tative clasts.     110100La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb LuREEConcentration/CI chondritesLight matrix Dark matrixClast 1 Rectangular ClastClast 6Figure 6: REE data measured by INAA   ICP-MS:  At the date of submission of this abstract, ICP-MS data was not finalized for publication.  Please see conference presentation or further publications for these results.     Conclusions: NWA 3368 is a non-cumulate, monomict eucrite breccia related to known normal eucrites.   The two separate lithologies, particularly the pyroxene exsolution, probably represent two separate thermal events that may be either metamorphic or pri-mary igneous in origin.  The REE abundances and patterns, as well as other trace element abundances, are typical of normal or main group eucrites.  At this point in the study, the trace element data do not reveal the nature of the pink coloration.     References:  [1] Consolmagno G. J. and Drake M. J. (1977) Geochim. Cosmochim. Acta, 41, 1271-1282. [2] Stolper E. M. (1977) Geochim. Cosmochim. Acta, 41, 587-611. [3] Mason B. (1962) Meteorites.  274 pp.  [4] Ikeda Y. and Takeda H. (1985) LPSC XV; JGR, 90 (suppl), C649-C663. [5] Righter K. and Drake M. J. (1997) M&PS, 32, 929-944. [6]  Mittlefehldt, D. W. et al. (1998) Planetary Materials, 36, 4-103—4-130.  Acknowledgements:  This work was supported by NASA grant NAG 12795 to M. J. Drake.  Special thanks go to William Boynton for the use of his Gamma Ray Laboratory to count the irradiated sam-ples from INAA.   20μm 20μm 200μm Lunar and Planetary Science XXXVII (2006) 2389.pdf

Petrology and geochemistry of the NWA 3368 Eucrite

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsPetrology and geochemistry of the NWA 3368 EucriteConference or Workshop ItemHow to cite:Gardner, K.G.; Lauretta, D.S.; Hill, D.H.; Goreva, J.S.; Domanik, K.J.; Franchi, I.A. and Drake, M.J. (2006).Petrology and geochemistry of the NWA 3368 Eucrite. In: 69th Annual Meteoritical Society Meeting, 6-11 Aug 2006,Zurich, Switzeralnd.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/metsoc2006/pdf/5152.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.ukPETROLOGY AND GEOCHEMISTRY OF THE NWA 3368 EUCRITE K. G. Gardner1, D. S. Lauretta1, D. H. Hill1, J. S. Goreva1, K. J. Domanik1, I. A. Franchi2, and M. J. Drake1. 1Lunar and Planetary Laboratory, University of Arizona. 2Planetary and Space Sci-ences Research Institute, Open University.  kgard-ner@lpl.arizona.edu  Introduction: A considerable amount of debate exists on the petrogenesis of eucrites, basaltic meteorites believed to have originated on 4 Vesta [1].  Many scientists have speculated on the formation event, supplementing ideas to Mason’s fractional crystallization model and Stolper’s partial melting model ([2-5]). Thus, an understanding of major and trace element compositions of all eucrites is imperative.  Here, we report the petrology and trace element geochemistry of NWA 3368.    Methods: Two thin sections and one 20.5 gram polished slab were studied using a Cameca SX-50 electron microprobe.  Six sub-samples including four clasts, a section of light-colored ma-trix, and a section of dark-colored matrix were extracted from the polished slab, and each piece was analyzed by both INAA and ICP-MS using the UA Nuclear Reactor Laboratory-LPL Gamma Ray Analysis Facility and the LPL ICP-MS facility, respectively [6]. A series of custom-made solutions of basaltic composition and the Geological Society of Japan standard basalt JB-2 were used as standards. Two small chips of NWA 3368 were sent to Open University to determine oxygen isotope composition. Results: NWA 3368 has a variety of dark and light angular clasts that range in size from several millimeters to a couple of centimeters in length. They range in texture from coarse- to fine-grained. The fine-grained texture is composed of abundant pla-gioclase and lamellae-free pyroxene, while the coarse-grained texture contains abundant pyroxenes with varying degrees of ex-solution lamellae. Electron microprobe analyses yielded pyrox-ene compositions of Wo5En36Fs59 for low-Ca pyroxene and Wo43En30Fs27 for high-Ca pyroxene.  Plagioclase has a composi-tion of An90Ab10Or0.4. Ilmenite and troilite grains are abundant, along with chromite containing 5 to 27% TiO2 and ~5% Al2O3.  Other phases include iron metal and silica. All Fe/Mn ratios lie between 28 and 32, typical for eucrites. Trace element data re-veal a flat REE pattern with a slightly negative Eu anomaly.  Sm vs. Sc, La, and Eu plots all reveal a pattern normal to eucrites.  Similarly, O-isotope data are consistent with other HEDs [7].  Conclusions: NWA 3368 is a non-cumulate, monomict eucrite breccia related to known eucrites. The two separate lithologies, particularly the pyroxene exsolution, probably repre-sent two separate thermal events that may be either metamorphic or primary igneous in origin.  The REE abundances and patterns, as well as other trace element abundances, are typical of normal or main group eucrites.   References: [1] Consolmagno G. J. and Drake M. J. (1977) Geochim. Cosmochim. Acta, 41, 1271-1282. [2] Stolper E. M. (1977) Geochim. Cosmochim. Acta, 41, 587-611. [3] Mason B. (1962) Meteorites.  274 pp.  [4] Ikeda Y. and Takeda H. (1985) LPSC XV; JGR, 90 (suppl), C649-C663. [5] Righter K. and Drake M. J. (1997) M&PS, 32, 929-944. [6] Gardner K. G. et al. (2006) Lunar and Planetary Science XXXVII, #2389. [7] Mittle-fehldt, D. W. et al. (1998) Planetary Materials, 36, 4-103-130.  Acknowledgements: This work was supported by NASA grant NAG 12795 to M. J. Drake. Special thanks go to William Boynton for use of the LPL Gamma Ray Analysis Facility.   69th Annual Meteoritical Society Meeting (2006) 5152.pdf

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Petrology and geochemistry of the NWA 3368 Eucrite

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