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Allan H. Treiman - One of the best experts on this subject based on the ideXlab platform.
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Geochemistry of intermediate olivine-phyric shergottite Northwest Africa 6234, with similarities to basaltic shergottite Northwest Africa 480 and olivine-phyric shergottite Northwest Africa 2990
Meteoritics & Planetary Science, 2012Co-Authors: Justin Filiberto, Emily J. Chin, James M.d. Day, Ian A. Franchi, Richard C. Greenwood, Juliane Gross, Sarah C. Penniston-dorland, Susanne P. Schwenzer, Allan H. TreimanAbstract:The newly found meteorite Northwest Africa 6234 (NWA 6234) is an olivine (ol)-phyric shergottite that is thought, based on texture and mineralogy, to be paired with Martian shergottite meteorites NWA 2990, 5960, and 6710. We report bulk-rock major- and trace-element abundances (including Li), abundances of highly siderophile elements, Re-Os isotope systematics, oxygen isotope ratios, and the lithium isotope ratio for NWA 6234. NWA 6234 is classified as a Martian shergottite, based on its oxygen isotope ratios, bulk composition, and bulk element abundance ratios, Fe ⁄ Mn, Al ⁄ Ti, and Na ⁄ Al. The Li concentration and d 7 Li value of NWA 6234 are similar to that of basaltic Shergottites Zagami and Shergotty. The rare earth element (REE) pattern for NWA 6234 shows a depletion in the light REE (La-Nd) compared with the heavy REE (Sm-Lu), but not as extreme as the known ''depleted'' Shergottites. Thus, NWA 6234 is suggested to belong to a new category of shergottite that is geochemically ''intermediate'' in incompatible elements. The only other basaltic or ol-phyric shergottite with a similar ''intermediate'' character is the basaltic shergottite NWA 480. Rhenium-osmium isotope systematics are consistent with this intermediate character, assuming a crystallization age of 180 Ma. We conclude that NWA 6234 represents an intermediate compositional group between enriched and depleted Shergottites and offers new insights into the nature of mantle differentiation and mixing among mantle reservoirs in Mars.
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Geochemistry of Intermediate Olivine-Phyric Shergottite Northwest Africa 6234
2012Co-Authors: Justin Filiberto, Emily J. Chin, James M.d. Day, Juliane Gross, Sarah C. Penniston-dorland, Susanne P. Schwenzer, Allan H. TreimanAbstract:The SNC meteorites are our only samples from Mars and are, therefore, the key source of information regarding its igneous history and geochemical evolution. Shergottites, which form the largest sub-group of the SNC meteorites, provide some of the most significant insights into the igneous history of Mars due in part to their petrological and geochemical diversity. Among petrological divisions of Shergottites the olivine (ol) phyric Shergottites are extremely useful in extending our knowledge of the interior of Mars, because they have many primitive characteristics. Here we present major- and trace-element geochemistry, Li isotope composition and abundance, and Re-Os isotope and highly-siderophile element abundance data for the ol-phyric shergottite NorthWest Africa 6234 (NWA 6234). Meteorite NWA 6234 was found in 2009 at an undisclosed location in Mali and purchased by an anonymous collector in February 2010. It was a 55.7 g partly fusion-crusted stone. It is thought to have experienced a moderate shock stage and limited weathering. For this study, we purchased a 3.3 gram slice of the meteorite from Marmet Meteorites, and confirmed that it matched the description of NWA 6234.
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Experimental petrology, crystallization history, and parental magma characteristics of olivine‐phyric shergottite NWA 1068: Implications for the petrogenesis of “enriched” olivine‐phyric Shergottites
Meteoritics & Planetary Science, 2010Co-Authors: Justin Filiberto, Juliane Gross, Donald S. Musselwhite, Katherine Burgess, Allan H. TreimanAbstract:Abstract– Northwest Africa (NWA) 1068 is one of the few olivine-phyric Shergottites (e.g., NWA 1068, Larkman Nunatak [LAR] 06319, and Roberts Massif [RBT] 04262) that is not depleted in light rare earth elements (LREE). Its REE pattern is similar to that of the basaltic shergottite Shergotty, suggesting a possible connection between the olivine-phyric and the basaltic Shergottites. To test this possible link, we have investigated the high-pressure near-liquidus phase equilibria for the NWA 1068 meteorite bulk composition. Our results show that the NWA 1068 bulk composition does not represent an unmodified mantle-derived melt; the olivine and pyroxene in our near-liquidus experiments are more magnesian than in the rock itself, which suggests that NWA 1068 contains cumulate minerals (extra olivine). We have then used these experimental results combined with the pyroxene compositions in NWA 1068 to constrain the possible high-pressure crystallization history of the parental magma. These results suggest that NWA 1068 had a complex polybaric history. Finally, we have calculated a model parental magma composition for the NWA 1068 meteorite. The calculated parental magma is an evolved basaltic composition which is too ferroan to be a primitive melt directly derived from the mantle. We suggest that it ponded and crystallized at approximately the base of the crust. This provided an opportunity for the magma to become contaminated by an “enriched” crustal component prior to crystallization. The results and modeling from these experiments are applicable not only to the NWA 1068 meteorite, but also to LAR 06319 and possibly any other enriched olivine-phyric shergottite.
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experimental petrology crystallization history and parental magma characteristics of olivine phyric shergottite nwa 1068 implications for the petrogenesis of enriched olivine phyric Shergottites
Meteoritics & Planetary Science, 2010Co-Authors: Justin Filiberto, Juliane Gross, Donald S. Musselwhite, Katherine Burgess, L E Loan, Allan H. TreimanAbstract:Abstract– Northwest Africa (NWA) 1068 is one of the few olivine-phyric Shergottites (e.g., NWA 1068, Larkman Nunatak [LAR] 06319, and Roberts Massif [RBT] 04262) that is not depleted in light rare earth elements (LREE). Its REE pattern is similar to that of the basaltic shergottite Shergotty, suggesting a possible connection between the olivine-phyric and the basaltic Shergottites. To test this possible link, we have investigated the high-pressure near-liquidus phase equilibria for the NWA 1068 meteorite bulk composition. Our results show that the NWA 1068 bulk composition does not represent an unmodified mantle-derived melt; the olivine and pyroxene in our near-liquidus experiments are more magnesian than in the rock itself, which suggests that NWA 1068 contains cumulate minerals (extra olivine). We have then used these experimental results combined with the pyroxene compositions in NWA 1068 to constrain the possible high-pressure crystallization history of the parental magma. These results suggest that NWA 1068 had a complex polybaric history. Finally, we have calculated a model parental magma composition for the NWA 1068 meteorite. The calculated parental magma is an evolved basaltic composition which is too ferroan to be a primitive melt directly derived from the mantle. We suggest that it ponded and crystallized at approximately the base of the crust. This provided an opportunity for the magma to become contaminated by an “enriched” crustal component prior to crystallization. The results and modeling from these experiments are applicable not only to the NWA 1068 meteorite, but also to LAR 06319 and possibly any other enriched olivine-phyric shergottite.
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chemical compositions of martian basalts Shergottites some inferences on b formation mantle metasomatism and differentiation in mars
Meteoritics & Planetary Science, 2003Co-Authors: Allan H. TreimanAbstract:Bulk chemical compositions of the shergottite basalts provide important constraints on magma genesis and mantle processes in Mars. Abundances of many major and trace elements in the Shergottites covary in 2 distinct groups: Group 1 (G1) includes mostly highly incompatible elements (e.g., La, Th), and Group 2 (G2) includes mostly moderately incompatible elements (e.g., Ti, Lu, Al, Hf). Covariations of G2 elements (not necessarily linear) are consistent with partitioning between basalt magma and orthopyroxene + olivine. This fractionation represents partial melting to form the Shergottites and their crystallization; the restite minerals cannot include aluminous phase(s), phosphate, ilmenite, zircon, or sulfides. Overall, abundances of G1 elements are decoupled from those of G2. In graphing abundances of a G1 element against those of a G2 element, G1/G2 abundance ratios do not appear to be random but are restricted to 4 values. Shergottites with a given G1/G2 value need not have the same crystallization age and need not fall on a single fractionation trajectory involving compatible elements (e.g., Ti versus Fe*). These observations imply that the G1/G2 families were established before basalt formation and suggest metasomatic enrichment of their source region (major carrier of G2 elements) by a component rich in G1 elements. Group 1 elements were efficiently separated from G2 elements very early in Mars' history. Such efficient fractionation is not consistent with simple petrogenesis; it requires multiple fractionations, "complex" petrogenetic processes, or minerals with unusual geochemistry. The behavior of phosphorus in this early fractionation event is inexplicable by normal petrogenetic processes and minerals. Several explanations are possible, including significant compatibility of P in majoritic garnet and the presence of P-bearing iron metal (or a phosphide phase) in the residual solid assemblage (carrier of G2 elements). If the latter, Mars' mantle is more oxidized now than during the ancient fractionation event.
L. E. Nyquist - One of the best experts on this subject based on the ideXlab platform.
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Identification of Martian Regolith Sulfur Components In Shergottites Using Sulfur K XANES and Fe/S Ratios.
2014Co-Authors: S. R. Sutton, M. N. Rao, D. K. Ross, L. E. NyquistAbstract:Based on isotopic anomalies in Kr and Sm, Sr-isotopes, S-isotopes, XANES results on S-speciation, Fe/S ratios in sulfide immiscible melts [5], and major element correlations with S determined in impact glasses in EET79001 Lith A & Lith B and Tissint, we have provided very strong evidence for the occurrence of a Martian regolith component in some impact melt glasses in Shergottites. Using REE measurements by LA-ICP-MS in shergottite impact glasses, Barrat and co-workers have recently reported conflicting conclusions about the occurrence of Martian regolith components: (a) Positive evidence was reported for a Tissint impact melt, but (b) Negative evidence for impact melt in EET79001 and another impact melt in Tissint. Here, we address some specific issues related to sulfur speciation and their relevance to identifying Martian regolith components in impact glasses in EET79001 and Tissint using sulfur K XANES and Fe/S ratios in sulfide immiscible melts. XANES and FE-SEM measurements in approx. 5 micron size individual sulfur blebs in EET79001 and Tissint glasses are carried out by us using sub-micron size beams, whereas Barrat and coworkers used approx. 90 micron size laser spots for LA- ICP-MS to determine REE abundances in bulk samples of the impact melt glasses. We contend that Martian regolith components in some shergottite impact glasses are present locally, and that studying impact melts in various Shergottites can give evidence both for and against regolith components because of sample heterogeneity.
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Compositions of Magmatic and Impact Melt Sulfides in Tissint and EETA79001: Precursors of Immiscible Sulfide Melt Blebs in Shergottite Impact Melts
2013Co-Authors: D. K. Ross, L. E. Nyquist, M. N. Rao, C. Agee, S. SuttonAbstract:Immiscible sulfide melt spherules are locally very abundant in shergottite impact melts. These melts can also contain samples of Martian atmospheric gases [1], and cosmogenic nuclides [2] that are present in impact melt, but not in the host shergottite, indicating some components in the melt resided at the Martian surface. These observations show that some regolith components are, at least locally, present in the impact melts. This view also suggests that one source of the over-abundant sulfur in these impact melts could be sulfates that are major constituents of Martian regolith, and that the sulfates were reduced during shock heating to sulfide. An alternative view is that sulfide spherules in impact melts are produced solely by melting the crystalline sulfide minerals (dominantly pyrrhotite, Fe(1-x)S) that are present in Shergottites [3]. In this abstract we report new analyses of the compositions of sulfide immiscible melt spherules and pyrrhotite in the Shergottites Tissint, and EETA79001,507, and we use these data to investigate the possible origins of the immiscible sulfide melt spherules. In particular, we use the metal/S ratios determined in these blebs as potential diagnostic criteria for tracking the source material from which the numerous sulfide blebs were generated by shock in these melts.
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Sm–Nd and Rb–Sr studies of lherzolitic shergottite Yamato 984028
Polar Science, 2011Co-Authors: C. Y. Shih, L. E. Nyquist, Young Reese, Keiji MisawaAbstract:Abstract The distribution pattern of the trace elements Rb, Sr, Nd and Sm for Yamato 984028 (Y984028) is consistent with its classification as a lherzolitic shergottite. The Sm–Nd mineral isochron of this lherzolitic shergottite defines its age to be 170 ± 10 Ma for an initial ɛ Nd = +11.6 ± 0.2. The corresponding Rb–Sr mineral isochron yields an identical age of 170 ± 9 Ma and an initial 87 Sr/ 86 Sr = 0.710389 ± 0.000029. The concordant Sm–Nd and Rb–Sr isochron ages suggest that Y984028 crystallized 170 ± 7 Ma ago contemporaneously with five other lherzolitic Shergottites and ten enriched basaltic and olivine-phyric Shergottites. The age, Sr- and Nd- isotopic signatures further suggest that Y984028 and Y-793605, and also probably Y000097 could come from a single magmatic body. Using a two-stage evolution model, the time-averaged 87 Rb/ 86 Sr-ratio for the mantle source of the parent magma of Y984028 is ∼0.182, within the range of 0.178–0.182 that has been reported for other lherzolitic Shergottites. The corresponding time-averaged 147 Sm/ 144 Nd-ratio for the source mantle of its parent magma is super-chondritic at ∼0.217, implying its source was a depleted mafic part of the Martian mantle similar to that of diabasic shergottite Northwest Africa (NWA) 1460. Rb, Sr, Sm and Nd distributions in Y984028 are likely produced by pyroxene and olivine accumulation, probably from a NWA 1460-like parental melt, in an intrusive magma body.
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Rb-Sr Isotopic Studies Of Antarctic Lherzolitic Shergottite Yamato 984028
2009Co-Authors: C. Y. Shih, L. E. Nyquist, Y. Reese, K. MisawaAbstract:Yamato 984028 is a Martian meteorite found in the Yamato Mountains of Antarctica. It is classified as a lherzolitic shergottite and petrographically resembles several other lherzolitic Shergottites, i.e. ALHA 77005, LEW 88516, Y-793605 and Y-000027/47/97 [e.g. 2-5]. These meteorites have similarly young crystallization ages (152-185 Ma) as enriched basaltic Shergottites (157-203 Ma), but have very different ejection ages (approximately 4 Ma vs. approximately 2.5 Ma), thus they came from different martian target crater areas. Lherzolitic Shergottites have mg-values approximately 0.70 and represent the most mafic olivine-pyroxene cumulates. Their parental magmas were melts derived probably from the primitive Martian mantle. Here we present Rb-Sr isotopic data for Y-984028 and compare these data with those obtained from other lherzolitic and olivine-phyric basaltic Shergottites to better understand the isotopic characteristics of their primitive mantle source regions. Corresponding Sm-Nd analyses for Y-984028 are in progress.
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Rb-Sr and Sm-Nd Studies of Olivine-Phyric Shergottites RBT 04262 and LAR 06319: Isotopic Evidence for Relationship to Enriched Basaltic Shergottites
2009Co-Authors: C. Y. Shih, L. E. Nyquist, Y. ReeseAbstract:RBT 04262 and LAR 06319 are two Martian meteorites recently discovered in Antarctica. Both contain abundant olivines, and were classified as olivine-phyric Shergottites. A detailed petrographic study of RBT 04262 suggested it should be reclassified as a lherzolitic shergottite. However, the moderately LREE-depleted REE distribution pattern indicated that it is closely related to enriched basaltic Shergottites like Shergotty, Zagami, Los Angeles, etc. In earlier studies of a similarly olivinephyric shergottite NWA 1068 which contains 21% modal olivine, it was shown that it probably was produced from an enriched basaltic shergottite magma by olivine accumulation . As for LAR 06319, recent petrographic studies suggested that it is different from either lherzolitic Shergottites or the highly LREE-depleted olivine-phyric Shergottites. We performed Rb-Sr and Sm-Nd isotopic analyses on RBT 04262 and LAR 06319 to determine their crystallization ages and Sr and Nd isotopic signatures, and to better understand the petrogenetic relationships between them and other basaltic, lherzolitic and depleted olivine-phyric Shergottites.
Justin Filiberto - One of the best experts on this subject based on the ideXlab platform.
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Crystallization History of Gabbroic Shergottite NWA 6963 as Revealed by Pyroxene Zoning
2017Co-Authors: Andrea L. Meado, Susanne P. Schwenzer, Samantha J. Hammond, Justin FilibertoAbstract:NorthWest Africa (NWA) 6963 is an intriguing new coarse grained Martian meteorite that further extends our sample collection both compositionally and texturally. It was originally documented as a basaltic shergottite because the pyroxene compositions and modal abundances are similar to Shergotty. However, recent work reclassified NWA 6963 as an intrusive, gabbroic Martian meteorite because of the large oriented pyroxene crystals. This investigation will focus on NWA 6963 pyroxene zoning profiles and interpreting igneous processes related to its crystallization history. Compositional zoning of pyroxene crystals in basaltic Shergottites have previously been interpreted for petrology and degrees of undercooling. In addition, cooling rates of Martian magmas have been determined experimentally to constrain volcanic processes based on both pyroxene and olivine zoning. These studies interpret crystallization histories of basaltic Shergottites and provide valuble insight of volcanic flows on Mars. However, igneous conditions below the Martian surface can now be constrained with instrusive shergottite NWA 6963. The crystallization history of a gabbroic shergottite is expected to differ from basaltic Martian meteorites due to the nature of instrusive and extrusive igneous conditions. Determining rates of cooling from pyroxene zoning will further constrain the petrogenetic history of NWA 6963. This will provide new information on the nature of instrusive igneous processes within the shallow Martian subsurface.
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Geochemistry of intermediate olivine-phyric shergottite Northwest Africa 6234, with similarities to basaltic shergottite Northwest Africa 480 and olivine-phyric shergottite Northwest Africa 2990
Meteoritics & Planetary Science, 2012Co-Authors: Justin Filiberto, Emily J. Chin, James M.d. Day, Ian A. Franchi, Richard C. Greenwood, Juliane Gross, Sarah C. Penniston-dorland, Susanne P. Schwenzer, Allan H. TreimanAbstract:The newly found meteorite Northwest Africa 6234 (NWA 6234) is an olivine (ol)-phyric shergottite that is thought, based on texture and mineralogy, to be paired with Martian shergottite meteorites NWA 2990, 5960, and 6710. We report bulk-rock major- and trace-element abundances (including Li), abundances of highly siderophile elements, Re-Os isotope systematics, oxygen isotope ratios, and the lithium isotope ratio for NWA 6234. NWA 6234 is classified as a Martian shergottite, based on its oxygen isotope ratios, bulk composition, and bulk element abundance ratios, Fe ⁄ Mn, Al ⁄ Ti, and Na ⁄ Al. The Li concentration and d 7 Li value of NWA 6234 are similar to that of basaltic Shergottites Zagami and Shergotty. The rare earth element (REE) pattern for NWA 6234 shows a depletion in the light REE (La-Nd) compared with the heavy REE (Sm-Lu), but not as extreme as the known ''depleted'' Shergottites. Thus, NWA 6234 is suggested to belong to a new category of shergottite that is geochemically ''intermediate'' in incompatible elements. The only other basaltic or ol-phyric shergottite with a similar ''intermediate'' character is the basaltic shergottite NWA 480. Rhenium-osmium isotope systematics are consistent with this intermediate character, assuming a crystallization age of 180 Ma. We conclude that NWA 6234 represents an intermediate compositional group between enriched and depleted Shergottites and offers new insights into the nature of mantle differentiation and mixing among mantle reservoirs in Mars.
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Geochemistry of Intermediate Olivine-Phyric Shergottite Northwest Africa 6234
2012Co-Authors: Justin Filiberto, Emily J. Chin, James M.d. Day, Juliane Gross, Sarah C. Penniston-dorland, Susanne P. Schwenzer, Allan H. TreimanAbstract:The SNC meteorites are our only samples from Mars and are, therefore, the key source of information regarding its igneous history and geochemical evolution. Shergottites, which form the largest sub-group of the SNC meteorites, provide some of the most significant insights into the igneous history of Mars due in part to their petrological and geochemical diversity. Among petrological divisions of Shergottites the olivine (ol) phyric Shergottites are extremely useful in extending our knowledge of the interior of Mars, because they have many primitive characteristics. Here we present major- and trace-element geochemistry, Li isotope composition and abundance, and Re-Os isotope and highly-siderophile element abundance data for the ol-phyric shergottite NorthWest Africa 6234 (NWA 6234). Meteorite NWA 6234 was found in 2009 at an undisclosed location in Mali and purchased by an anonymous collector in February 2010. It was a 55.7 g partly fusion-crusted stone. It is thought to have experienced a moderate shock stage and limited weathering. For this study, we purchased a 3.3 gram slice of the meteorite from Marmet Meteorites, and confirmed that it matched the description of NWA 6234.
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Experimental petrology, crystallization history, and parental magma characteristics of olivine‐phyric shergottite NWA 1068: Implications for the petrogenesis of “enriched” olivine‐phyric Shergottites
Meteoritics & Planetary Science, 2010Co-Authors: Justin Filiberto, Juliane Gross, Donald S. Musselwhite, Katherine Burgess, Allan H. TreimanAbstract:Abstract– Northwest Africa (NWA) 1068 is one of the few olivine-phyric Shergottites (e.g., NWA 1068, Larkman Nunatak [LAR] 06319, and Roberts Massif [RBT] 04262) that is not depleted in light rare earth elements (LREE). Its REE pattern is similar to that of the basaltic shergottite Shergotty, suggesting a possible connection between the olivine-phyric and the basaltic Shergottites. To test this possible link, we have investigated the high-pressure near-liquidus phase equilibria for the NWA 1068 meteorite bulk composition. Our results show that the NWA 1068 bulk composition does not represent an unmodified mantle-derived melt; the olivine and pyroxene in our near-liquidus experiments are more magnesian than in the rock itself, which suggests that NWA 1068 contains cumulate minerals (extra olivine). We have then used these experimental results combined with the pyroxene compositions in NWA 1068 to constrain the possible high-pressure crystallization history of the parental magma. These results suggest that NWA 1068 had a complex polybaric history. Finally, we have calculated a model parental magma composition for the NWA 1068 meteorite. The calculated parental magma is an evolved basaltic composition which is too ferroan to be a primitive melt directly derived from the mantle. We suggest that it ponded and crystallized at approximately the base of the crust. This provided an opportunity for the magma to become contaminated by an “enriched” crustal component prior to crystallization. The results and modeling from these experiments are applicable not only to the NWA 1068 meteorite, but also to LAR 06319 and possibly any other enriched olivine-phyric shergottite.
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experimental petrology crystallization history and parental magma characteristics of olivine phyric shergottite nwa 1068 implications for the petrogenesis of enriched olivine phyric Shergottites
Meteoritics & Planetary Science, 2010Co-Authors: Justin Filiberto, Juliane Gross, Donald S. Musselwhite, Katherine Burgess, L E Loan, Allan H. TreimanAbstract:Abstract– Northwest Africa (NWA) 1068 is one of the few olivine-phyric Shergottites (e.g., NWA 1068, Larkman Nunatak [LAR] 06319, and Roberts Massif [RBT] 04262) that is not depleted in light rare earth elements (LREE). Its REE pattern is similar to that of the basaltic shergottite Shergotty, suggesting a possible connection between the olivine-phyric and the basaltic Shergottites. To test this possible link, we have investigated the high-pressure near-liquidus phase equilibria for the NWA 1068 meteorite bulk composition. Our results show that the NWA 1068 bulk composition does not represent an unmodified mantle-derived melt; the olivine and pyroxene in our near-liquidus experiments are more magnesian than in the rock itself, which suggests that NWA 1068 contains cumulate minerals (extra olivine). We have then used these experimental results combined with the pyroxene compositions in NWA 1068 to constrain the possible high-pressure crystallization history of the parental magma. These results suggest that NWA 1068 had a complex polybaric history. Finally, we have calculated a model parental magma composition for the NWA 1068 meteorite. The calculated parental magma is an evolved basaltic composition which is too ferroan to be a primitive melt directly derived from the mantle. We suggest that it ponded and crystallized at approximately the base of the crust. This provided an opportunity for the magma to become contaminated by an “enriched” crustal component prior to crystallization. The results and modeling from these experiments are applicable not only to the NWA 1068 meteorite, but also to LAR 06319 and possibly any other enriched olivine-phyric shergottite.
C. Y. Shih - One of the best experts on this subject based on the ideXlab platform.
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sm nd and rb sr isotopic systematics of a heavily shocked martian meteorite tissint and petrogenesis of depleted Shergottites
LPI, 2014Co-Authors: C. Y. Shih, Laurence E. Nyquist, Jisun Park, Carl B AgeeAbstract:Tissint is a very fresh Martian meteorite that fell near the town of Tissint in Morocco on July 18, 2011. It contains abundant olivine megacrysts (~23%) in a fine-grained matrix of pyroxene (~55%), maskelynitized plagioclase (~15%), opaques (~4%) and melt pockets (~3%) and is petrographically similar to lithologies A and C of picritic shergottite EETA 79001 [1,2]. The presence of 2 types of shock-induced glasses and all 7 high-pressure mineral phases that were ever found in melt pockets of Martian meteorites suggests it underwent an intensive shock metamorphism of ~25 GPa and ~2000 C localized in melt pockets [2]. Mineral textures suggest that olivines, pyroxenes and plagioclases probably did not experience such hightemperature. Earlier determinations of its age yielded 596+/-23 Ma [3] and 616+/-67 Ma [4], respectively, for the Sm-Nd system and 583+/-86 Ma for the Lu-Hf system [4], in agreement with the 575+/-18 Ma age of the oldest olivine-phyric depleted shergottite Dho 019 [5]. However, the exposure ages of Tissint (~1 Ma [1, 6, 7]) and Dho 019 (~20 Ma [8]) are very different requiring two separate ejection events. These previously determined Sm-Nd and Lu-Hf ages are older than the Ar-Ar maskelynite plateau age of 524+/-15 Ma [9], reversing the pattern usually observed for Martian meteorites. In order to clarify these age issues and place models for Tissint's petrogenesis on a firm basis, we present new Rb-Sr and Sm- Nd isotopic results for Tissint, and discuss (a) the shock effects on them and the Ar-Ar chronometer, (b) correlation of the determined ages with those of other depleted Shergottites, and (c) the petrogenesis of depleted Shergottites. Since the meteorite is a recent fall, terrestrial contamination is expected to be minimal, but, the strong shock metamorphism might be expected to compromise the equilibrium of the isotopic systems.
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Sm–Nd and Rb–Sr studies of lherzolitic shergottite Yamato 984028
Polar Science, 2011Co-Authors: C. Y. Shih, L. E. Nyquist, Young Reese, Keiji MisawaAbstract:Abstract The distribution pattern of the trace elements Rb, Sr, Nd and Sm for Yamato 984028 (Y984028) is consistent with its classification as a lherzolitic shergottite. The Sm–Nd mineral isochron of this lherzolitic shergottite defines its age to be 170 ± 10 Ma for an initial ɛ Nd = +11.6 ± 0.2. The corresponding Rb–Sr mineral isochron yields an identical age of 170 ± 9 Ma and an initial 87 Sr/ 86 Sr = 0.710389 ± 0.000029. The concordant Sm–Nd and Rb–Sr isochron ages suggest that Y984028 crystallized 170 ± 7 Ma ago contemporaneously with five other lherzolitic Shergottites and ten enriched basaltic and olivine-phyric Shergottites. The age, Sr- and Nd- isotopic signatures further suggest that Y984028 and Y-793605, and also probably Y000097 could come from a single magmatic body. Using a two-stage evolution model, the time-averaged 87 Rb/ 86 Sr-ratio for the mantle source of the parent magma of Y984028 is ∼0.182, within the range of 0.178–0.182 that has been reported for other lherzolitic Shergottites. The corresponding time-averaged 147 Sm/ 144 Nd-ratio for the source mantle of its parent magma is super-chondritic at ∼0.217, implying its source was a depleted mafic part of the Martian mantle similar to that of diabasic shergottite Northwest Africa (NWA) 1460. Rb, Sr, Sm and Nd distributions in Y984028 are likely produced by pyroxene and olivine accumulation, probably from a NWA 1460-like parental melt, in an intrusive magma body.
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Rb-Sr Isotopic Studies Of Antarctic Lherzolitic Shergottite Yamato 984028
2009Co-Authors: C. Y. Shih, L. E. Nyquist, Y. Reese, K. MisawaAbstract:Yamato 984028 is a Martian meteorite found in the Yamato Mountains of Antarctica. It is classified as a lherzolitic shergottite and petrographically resembles several other lherzolitic Shergottites, i.e. ALHA 77005, LEW 88516, Y-793605 and Y-000027/47/97 [e.g. 2-5]. These meteorites have similarly young crystallization ages (152-185 Ma) as enriched basaltic Shergottites (157-203 Ma), but have very different ejection ages (approximately 4 Ma vs. approximately 2.5 Ma), thus they came from different martian target crater areas. Lherzolitic Shergottites have mg-values approximately 0.70 and represent the most mafic olivine-pyroxene cumulates. Their parental magmas were melts derived probably from the primitive Martian mantle. Here we present Rb-Sr isotopic data for Y-984028 and compare these data with those obtained from other lherzolitic and olivine-phyric basaltic Shergottites to better understand the isotopic characteristics of their primitive mantle source regions. Corresponding Sm-Nd analyses for Y-984028 are in progress.
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Rb-Sr and Sm-Nd Studies of Olivine-Phyric Shergottites RBT 04262 and LAR 06319: Isotopic Evidence for Relationship to Enriched Basaltic Shergottites
2009Co-Authors: C. Y. Shih, L. E. Nyquist, Y. ReeseAbstract:RBT 04262 and LAR 06319 are two Martian meteorites recently discovered in Antarctica. Both contain abundant olivines, and were classified as olivine-phyric Shergottites. A detailed petrographic study of RBT 04262 suggested it should be reclassified as a lherzolitic shergottite. However, the moderately LREE-depleted REE distribution pattern indicated that it is closely related to enriched basaltic Shergottites like Shergotty, Zagami, Los Angeles, etc. In earlier studies of a similarly olivinephyric shergottite NWA 1068 which contains 21% modal olivine, it was shown that it probably was produced from an enriched basaltic shergottite magma by olivine accumulation . As for LAR 06319, recent petrographic studies suggested that it is different from either lherzolitic Shergottites or the highly LREE-depleted olivine-phyric Shergottites. We performed Rb-Sr and Sm-Nd isotopic analyses on RBT 04262 and LAR 06319 to determine their crystallization ages and Sr and Nd isotopic signatures, and to better understand the petrogenetic relationships between them and other basaltic, lherzolitic and depleted olivine-phyric Shergottites.
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Concordant Rb–Sr, Sm–Nd, and Ar–Ar ages for Northwest Africa 1460: A 346 Ma old basaltic shergottite related to “lherzolitic” Shergottites
Geochimica et Cosmochimica Acta, 2009Co-Authors: L. E. Nyquist, C. Y. Shih, Young Reese, Donald D. Bogard, J. Park, Anthony J. IrvingAbstract:Abstract Multiple lines of evidence show that the Rb–Sr, Sm–Nd, and Ar–Ar isotopic systems individually give robust crystallization ages for basaltic (or diabasic) shergottite Northwest Africa (NWA) 1460. In contrast to other Shergottites, NWA 1460 exhibits minimal evidence of excess 40Ar, thus allowing an unambiguous determination of its Ar–Ar age. The concordant Rb–Sr, Sm–Nd, and Ar–Ar results for NWA 1460 define its crystallization age to be 346 ± 17 Ma (2σ). In combination with petrographic and trace element data for this specimen and paired meteorite NWA 480, these results strongly refute the suggestion by others that the Shergottites are ∼4.1 Ga old. Current crystallization and cosmic-ray exposure (CRE) age data permit identification of a maximum of nine ejection events for Martian meteorites (numbering more than 50 unpaired specimens as of 2008) and plausibly as few as five such events. Although recent high resolution imaging of the Martian surface has identified limited areas of sparsely cratered terrains, the meteorite data suggest that either these areas are representative of larger areas from which the meteorites might come, or that the cratering chronology needs recalibration. Time-averaged 87Rb/86Sr = 0.16 for the mantle source of the parent magma of NWA 1460/480 over the ∼4.56 Ga age of the planet is consistent with previously estimated values for bulk silicate Mars in the range 0.13–0.16, and similar to values of ∼0.18 for the “lherzolitic” Shergottites. Initial eNd for NWA 1460/480 at 350 ± 16 Ma ago was +10.6 ± 0.5, which implies a time-averaged 147Sm/144Nd of 0.217 in the Martian mantle prior to mafic melt extraction, similar to values of 0.211–0.216 for the “lherzolitic” Shergottites. These time-averaged values do not imply a simple two-stage mantle/melt evolution, but must result from multiple episodes of melt extractions from the source regions. Much higher “late-stage” eNd values for the depleted Shergottites imply similar processes carried to a greater degree. Thus, NWA 1460/480, the “lherzolitic” Shergottites and perhaps EET 79001 give the best (albeit imperfect) estimate of the Sr- and Nd-isotopic characteristics of bulk silicate Mars.
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the origin of alteration orangettes in dhofar 019 implications for the age and aqueous history of the Shergottites
Meteoritics & Planetary Science, 2017Co-Authors: L J Hallis, L Kemppinen, M R Lee, Lawrence A. TaylorAbstract:The Shergottites are the largest group of Martian meteorites, and the only group that has not been found to contain definitive evidence of Martian aqueous alteration. Given recent reports of current liquid water at the surface of Mars, this study aimed to investigate in detail the possibility of Martian phyllosilicate within shergottite Dhofar 019. Optical and scanning electron microscopy, followed by transmission electron microscopy, confirmed the presence of alteration orangettes, with a layered structure consisting of poorly ordered Mg-phyllosilicate and calcite. These investigations identified maskelynite dissolution, followed by Mg-phyllosilicate and calcite deposition within the dissolution pits, as the method of orangette production. The presence of celestine within the orangette layers, the absence of shock dislocation features within calcite, and the Mg-rich nature of the phyllosilicate, all indicate a terrestrial origin for these features on Dhofar 019.
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evidence for the exsolution of cl rich fluids in martian magmas apatite petrogenesis in the enriched lherzolitic shergottite northwest africa 7755
Geochimica et Cosmochimica Acta, 2015Co-Authors: Geoffrey H Howarth, John F Pernetfisher, Robert J Bodnar, Lawrence A. TaylorAbstract:Abstract Martian meteorite Northwest Africa 7755 is a new example of an enriched, lherzolitic shergottite, containing some of the coarsest-grained apatite yet identified in shergottite meteorites. Their size has permitted detailed observations of volatile distributions within single grains. We have demonstrated that some apatites have been invaded by shock melts, which act to devolatilize parts of grains, resulting in significant Cl-enrichment in the adjacent regions. The extent of chemical heterogeneity within single grains must be carefully considered in other Shergottites, so that the effects of secondary modification of apatites are well-constrained, prior to interpreting the volatile contents and primary magmatic processes. Apatite grains unaffected by shock melts are OH–F enriched and Cl-poor (∼F 50 Cl 14 OH 36 ), relative to interstitial apatites reported in other Shergottites. The volatile compositions are similar to interstitial apatites reported in terrestrial mafic intrusions. Such apatites in terrestrial intrusions are argued to have formed after significant Cl-loss due to the exsolution and migration of Cl-rich brines. Calculated relative F 2 , Cl 2 , and H 2 O fugacities for NWA 7755 apatites show a trend of degassing rather than fractionation, noted in previous studies. Indeed, we interpret the volatile contents of apatites analyzed in the cumulate shergottite NWA 7755 to represent snapshots of the evolving late-stage residual liquid during exsolution of a Cl-rich brine. This fluid phase has subsequently been lost from an open magma system, migrating upward through the cumulate sequence enriching residual liquids in Cl. Alternatively, it formed a hydrothermal system in the martian crust surrounding the intrusion. Furthermore, by comparison with terrestrial examples, we suggest that the late-stage evolution of volatile-bearing phases in NWA 7755 is similar to that of comparable terrestrial mafic rocks. Primary cumulus apatites are F-rich, whereas interstitial apatites may be either Cl-rich, forming as a result of modification through the interaction with Cl-rich brines, or OH–F-rich forming from a magma having exsolved or degassed a Cl-rich fluid phase.
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Two-stage polybaric formation of the new enriched, pyroxene-oikocrystic, lherzolitic shergottite, NWA 7397
Meteoritics & Planetary Science, 2014Co-Authors: Geoffrey H Howarth, J. Brian Balta, Robert J Bodnar, John F. Pernet-fisher, Peter H. Barry, Lawrence A. TaylorAbstract:Northwest Africa (NWA) 7397 is a newly discovered, enriched, lherzolitic shergottite, the third described example of this group. This meteorite consists of two distinct textural lithologies (1) poikilitic—comprised of zoned pyroxene oikocrysts, with chadacrysts of chromite and olivine, and (2) nonpoikilitic—comprised of olivine, low-Ca and high-Ca pyroxene, maskelynite, and minor abundances of merrillite, spinel, ilmenite, and pyrrhotite. The constant Ti/Al ratios of pyroxene oikocrysts suggests initial crystallization of the poikilitic lithology at depth (equivalent to pressures of approximately 10 kbar), followed by crystallization of the nonpoikilitic lithology at shallower levels. Oxygen fugacity conditions become more oxidizing during crystallization ranging from fO2 conditions of approximately QFM-2 to QFM-0.7. Magma calculated to be in equilibrium with the major rock-forming minerals is LREE-enriched relative to depleted or intermediate Shergottites and has flat overall profiles. Therefore, we suggest that the parental magma for NWA 7397 had sampled an enriched, oxidized, Martian geochemical source, similar to that of other enriched basaltic and olivine-phyric Shergottites. We present a polybaric formation model for the lherzolitic shergottite NWA 7397, to account for the petrologic constraints. Three successive stages in the development of NWA 7397 are discussed (1) formation of a REE-enriched parental magma from a distinct Martian mantle reservoir; (2) magma ponding and development of a staging chamber concomitant with initial crystallization of the poikilitic lithology; and (3) magma ascent to the near surface, with entrainment of cumulates from the staging chamber and subsequent crystallization of the nonpoikilitic lithology en route to the surface.
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Evolution of the martian mantle inferred from the 187Re–187Os isotope and highly siderophile element abundance systematics of shergottite meteorites
Geochimica et Cosmochimica Acta, 2012Co-Authors: Alan D. Brandon, James M.d. Day, Igor S. Puchtel, Richard J. Walker, Anthony J. Irving, Lawrence A. TaylorAbstract:Abstract Shergottite meteorites are a suite of mafic to ultramafic igneous rocks whose parental magmas probably derived from the martian mantle. In this study, a suite of 23 Shergottites, spanning their known range in bulk compositions, Rb–Sr, Sm–Nd, and Lu–Hf isotopes, were measured for 187Re–187Os isotopic systematics and highly siderophile element abundances (HSE: including Os, Ir, Ru, Pt, Pd, Re). The chief objective was to gain new insight on the chemical evolution of the martian mantle by unraveling the long-term HSE budget of its derivative melts. Possible effects upon HSEs related to crustal contamination, as well as terrestrial and/or martian surface alteration are also examined. Some of the Shergottites are hot arid-desert finds. Their respective acetic acid leachates and residues show that both Re and Os display open-system behavior during sample residence at or near the martian and/or terrestrial surfaces. In some meteorites, the alteration effects can be circumvented by analysis of the leached residues. For those Shergottites believed to record robust Re–Os isotopic systematics, calculated initial 187Os/188Os are well correlated with the initial 143Nd/144Nd. Shergottites from mantle sources with long-term melt-depleted characteristics (initial e143Nd of +36 to +40) have chondritic initial γ187Os ranging from −0.5 to +2.5. Shergottites with intermediate initial e143Nd of +8 to +17 have a range in initial γ187Os of −0.6 to +2.3, which overlaps the range for depleted Shergottites. Shergottites from long-term enriched sources, with initial e143Nd of ∼−7, are characterized by suprachondritic γ187Os values of +5 to +15. The initial γ187Os variations for the Shergottites do not show a correlation with indices of magmatic differentiation, such as MgO, or any systematic differences between hot arid-desert finds, Antarctic finds, or observed falls. The strong correlation between the initial e143Nd and γ187Os in Shergottites from approximately +40 and 0 to −7 and +15, respectively, is assessed in models for mixing depleted mantle-derived melts with ancient crust (modeled to be similar to evolved shergottite in composition), and with assimilation-fractional crystallization. These models show that the correlation is unlikely to result from participation of martian crust. More likely, this correlation relates to contributions from depleted and enriched reservoirs formed in a martian magma ocean at ca. 4.5 Ga. These models indicate that the shergottite endmember sources were generated by mixing between residual melts and cumulates that formed at variable stages during solidification of a magma ocean. The expanded database for the HSE abundances in Shergottites suggests that their martian mantle sources have similar HSE abundances to the terrestrial mantle, consistent with prior studies. The relatively high HSE abundances in both planetary mantles likely cannot be accounted for by high pressure–temperature metal–silicate partitioning at the bases of magma oceans, as has been suggested for Earth. If the HSE were instead supplied by late accretion, this event must have occurred prior to the crystallization of the last martian magma ocean.
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Martian meteorite Dhofar 019: A new shergottite
Meteoritics & Planetary Science, 2002Co-Authors: Lawrence A. Taylor, Clive R. Neal, H. Y. Mcsween, Charles K. Shearer, Mikhail A. Nazarov, Joshua T.s. Cahill, Marina A. Ivanova, L. D. Barsukova, Rachel C. F. Lentz, Robert N. ClaytonAbstract:Dhofar 019 is a new martian meteorite found in the desert of Oman. In texture, mineralogy, and major and trace element chemistry, this meteorite is classified as a basaltic shergottite. Olivine megacrysts are set within a groundmass composed of finer grained olivine, pyroxene (pigeonite and augite), and maskelynite. Minor phases are chromite-ulvospinel, ilmenite, silica, K-rich feldspar, merrillite, chlorapatite, and pyrrhotite. Secondary phases of terrestrial origin include calcite, gypsum, celestite, Fe hydroxides, and smectite. Dhofar 019 is most similar to the Elephant Moraine (EETA) 79001 lithology A and Dar al Gani (DaG) 476/489 Shergottites. The main features that distinguish Dhofar 019 from other Shergottites are lack of orthopyroxene; lower Ni contents of olivine; the heaviest oxygen-isotopic bulk composition; and larger compositional ranges for olivine, maskelynite, and spinel, as well as a wide range for pyroxenes. The large compositional ranges of the minerals are indicative of relatively rapid crystallization. Modeling of olivine chemical zonations yield minimum cooling rates of 0.5-0.8 °C/h. Spinel chemistry suggests that crystallization took place under one of the most reduced conditions for martian meteorites, at an f O2 3 log units below the quartz-fayalite-magnetite (QFM) buffer. The olivine megacrysts are heterogeneously distributed in the rock. Crystal size distribution analysis suggests that they constitute a population formed under steady-state conditions of nucleation and growth, although a few grains may be cumulates. The parent melt is thought to have been derived from partial melting of a light rare earth element- and platinum group element-depleted mantle source. Shergottites, EETA79001 lithology A, DaG 476/489, and Dhofar 019, although of different ages, comprise a particular type of martian rocks. Such rocks could have formed from chemically similar source(s) and parent melt(s), with their bulk compositions affected by olivine accumulation.
