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Papers by Rachel Rahib

Research paper thumbnail of Petrology and Geochemistry of the Enriched Poikilitic Shergottite Northwest Africa 10169: Insight into the Martian Interior

The martian meteorite Northwest Africa (NWA) 10169 is classified as a new member of the geochemic... more The martian meteorite Northwest Africa (NWA) 10169 is classified as a new member of the geochemically enriched poikilitic shergottites, based on mineral composition, Lu-Hf isotope systematics, and rare earth element (REE) composition. Akin to other poikilitic shergottites, it shows a similar bimodal texture to other enriched and intermediate poikilitic shergottites. In addition, olivine and pyroxene in the poikilitic zone have higher Mg#'s (Mg/Mg+Fe) than those in the interstitial areas, suggesting that the poikilitic texture represents early-stage crystallization, opposed to late-stage non-poikilitic crystallization. Calculated fO2 values are reduced (FMQ -2.3 ± 0.23) within the poikilitic texture, and more oxidized (FMQ -1.07 ± 0.14) within the interstitial areas likely representing auto-oxidation and degassing during magma crystallization. Melt inclusions within olivine crystals provide snapshots of magma composition throughout parent melt evolution. The calculated parental m...

Research paper thumbnail of Aubrite and Enstatite Chondrite Impact Melt Meteorites: Analogs to Mercury?

MERCURY? Z. E. Wilbur, A. Udry, F. M. McCubbin, K. E. Vander Kaaden, R. R. Rahib, and T. J. McCoy... more MERCURY? Z. E. Wilbur, A. Udry, F. M. McCubbin, K. E. Vander Kaaden, R. R. Rahib, and T. J. McCoy. 1 Department of Geoscience, University of Nevada, Las Vegas, Las Vegas, NV 89154, USA (wilbur@unlv.nevada.edu), Jacobs, NASA Johnson Space Center, Mail Code XI3, Houston, TX 77058, ARES, NASA Johnson Space Center, 2101 NASA Parkway, Houston, TX 77058, Natural Museum of History, Smithsonian Institution, Washington, D.C., USA.

Research paper thumbnail of Constraining Formation and Emplacement Processes of Poikilitic Shergottites Using Quantitative Textural Analyses

Research paper thumbnail of  Petrology of the enriched poikilitic shergottite Northwest Africa 10169: Insight into the martian interior

Geochimica et Cosmochimica Acta, 2019

Research paper thumbnail of Reclassification of four aubrites as enstatite chondrite impact melts: Potential geochemical analogs for Mercury

Meteoritics & Planetary Science, 2019

Research paper thumbnail of Potential Mercurian Analogues: Aubrite and Enstatite Chondrite Impact Melt Meteorites

Annual UNLV Geosymposium 2018, Apr 20, 2018

Research paper thumbnail of Aubrite and Impact Melt Enstatite Chondrite Meteorites as Potential Analogs to Mercury

The MESSENGER (MErcury Sur-face, Space ENvironment, GEochemistry and Ranging) orbiter measured th... more The MESSENGER (MErcury Sur-face, Space ENvironment, GEochemistry and Ranging) orbiter measured the Mercurian surface abundances of key rock-forming elements to help us better understand the planet's surface and bulk geochemistry. A major discovery is that the Mercurian surface and interior are characterized by an extremely low oxygen fugacity (ƒO2; Iron-Wustite (IW) -7.3 to IW-2.6. This is supported by low Fe and high S abundances on the surface. This low ƒO2 causes a different elemental partioning from what is observed on Earth. Using surface composition, it was shown that the Mercurian surface mainly consists of normative plagioclase, pyroxene, olivine, and exotic sulfides, such as niningerite ((Mg,Mn, Fe)S) and oldhamite (CaS).

Research paper thumbnail of Formation and Emplacement of Martian Poikilitic Shergottites

R. R. Rahib, A. Udry, G. H. Howarth, J. Gross, M. Paquet, L. M. Combs, D. L. Laczniak, and J. M. ... more R. R. Rahib, A. Udry, G. H. Howarth, J. Gross, M. Paquet, L. M. Combs, D. L. Laczniak, and J. M. D. Day, Department of Geosciences, University of Nevada Las Vegas, 4505 S. Maryland Pkwy, Las Vegas, NV 890154 (rahibr@unlv.nevada.edu; arya.udry@unlv.edu), Department of Geological Sciences, University of Cape Town, Rondebosch 7701, South Africa, Department of Earth and Planetary Sciences, Rutgers University, Piscataway NJ 08854, Scripps Institution of Oceanography, University of California San Diego, La Jolla CA 92093-0244, Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN 47907.

Research paper thumbnail of Highly siderophile elements in shergottite sulfides and the sulfur content of the martian mantle

Geochimica et Cosmochimica Acta, 2021

Abstract Shergottite meteorites are ultramafic to mafic igneous rocks derived from partial meltin... more Abstract Shergottite meteorites are ultramafic to mafic igneous rocks derived from partial melting of distinct regions of the martian mantle. As such, they trace magmatic processes, including fractional crystallization and mixing processes in Mars. New chalcophile (Cu, Se, Zn, Pb), siderophile (Ni, Co, W), and highly siderophile element (HSE: Au, Re, Pd, Rh, Pt, Ru, Ir, Os) abundance data are reported for sulfide assemblages in a suite of thirteen incompatible trace-element depleted, intermediate and enriched shergottites, along with new whole-rock HSE abundance and 187Os/188Os data for seven shergottites. Sulfide grains in depleted and intermediate shergottites typically have the highest absolute abundances of HSE, with broadly flat CI-chondrite normalized patterns. Enriched shergottite sulfide grains typically have highly variable Au, elevated Pd and Rh and are relatively depleted in Zn, Ir and Os. The new HSE whole-rock data for enriched (Northwest Africa [NWA] 7397, NWA 7755, NWA 11043), and intermediate shergottites (NWA 10961, NWA 11065, NWA 12241, and NWA 12536) are generally consistent with existing 187Os/188Os and HSE abundance data for these geochemical groupings. Enriched shergottites with >1 ppb bulk rock Os have measured 187Os/188Os ranging between 0.1296 and 0.1471, with variable Pd and Pt contents. Intermediate shergottites with >1 ppb bulk rock Os have chondrite-relative proportions of the HSE at ∼0.01 to 0.001 × CI chondrites and 187Os/188Os from 0.1284 and 0.1295. Sulfides are the major host of the HSE, and they control the behavior of the HSE during petrogenetic processes in shergottite magmas, enabling the determination of HSE compatibility for martian magmatism in the order: Os > Ir ≥ Ru ≥≥ Rh ≥ Pd ≥ Re ≥ Pt ≥ Au. Fractionation models of removal of an olivine-dominated cumulate recreate HSE patterns for the whole-rock shergottites. Enriched shergottites are best reproduced by 25 to 30% of fractionation from a degassed parent melt (250 ± 50 ppm of S), whereas depleted and intermediate shergottites can be explained by slightly lower fractionation (10 to 15%) from higher S content parent melts (350 ± 100 ppm of S). Sulfur contents in the melt that are ∼50% higher than these estimates yield earlier S-saturation during fractional crystallization, leading to an abrupt decrease of the more compatible HSE (Ru, Ir, Os), which is not observed. These results indicate that the martian mantle and partial melts derived from it, are probably not anomalously enriched in S, and instead are similar to slightly higher than those of the terrestrial mantle and its partial melts.

Research paper thumbnail of Mantle source to near-surface emplacement of enriched and intermediate poikilitic shergottites in Mars

Geochimica et Cosmochimica Acta, 2019

Abstract Poikilitic shergottites make up >20% of the current martian meteorite collection, wit... more Abstract Poikilitic shergottites make up >20% of the current martian meteorite collection, with a total of 27 samples. These meteorites are intrusive gabbroic to lherzolitic rocks and represent igneous materials recording important processes in the martian crust. To further constrain petrogenetic relationships amongst enriched and intermediate poikilitic shergottites, we studied a comprehensive suite of poikilitic shergottites — including four newly recovered samples (Northwest Africa [NWA] 11065, NWA 11043, NWA 10961, NWA 10618) — using bulk rock major- and trace-element compositions, mineral major-element compositions, oxygen fugacity (ƒO2) values, crystallization temperatures, phosphorus maps of olivine grains, and quantitative textural analyses. The characteristic bimodal textures (poikilitic and non-poikilitic textures) of poikilitic shergottites record evolving magmatic conditions at different stages of crystallization. Higher temperatures and more reducing conditions during early-stage crystallization are recorded in the poikilitic textures, while lower temperature and more oxidizing conditions are recorded in the non-poikilitic textures during late-stage crystallization. Oxygen fugacity estimates relative to the quartz-fayalite-magnetite (QFM) buffer for early-stage olivine-pyroxene-spinel assemblages of enriched and intermediate poikilitic shergottites suggest decoupling of ƒO2 and the degree of light rare earth element (LREE)-enrichment (i.e., [La/Yb]CI). An increase in ƒO2 exceeding 1 log unit from poikilitic to non-poikilitic textures implies degassing, with possible auto-oxidation, and/or crustal contamination. Quantitative textural analyses support the emplacement of both enriched and intermediate poikilitic shergottites as various shallow intrusive bodies, as well as a potentially widespread emplacement mechanism responsible for a major lithology of the martian crust. In addition, early assemblages (i.e., pyroxene oikocrysts) of all the poikilitic shergottites likely formed close to the crust-mantle boundary, implying a possible widespread presence of magma staging chambers at these depths. Fractional crystallization and magma storage in these chambers could have possibly resulted in all of the different enriched and intermediate shergottites that have been analyzed from Mars.

Research paper thumbnail of Petrogenesis of Enriched and Intermediate Poikilitic Shergottites: From Magmatic Source to Emplacement

Geological Society of America Abstracts with Programs, 2018

Research paper thumbnail of Textural Analysis of Enriched Poikilitic Shergottites

Geological Society of America Abstracts with Programs, 2016

Research paper thumbnail of Formation and Emplacement Processes of Martian Poikilitic Shergottite Meteorites

Introduction: Shergottites, the most prolific group of martian meteorites, have been subdivided b... more Introduction: Shergottites, the most prolific group of martian meteorites, have been subdivided based on their mineralogies and textures into three types: Olivinephyric, basaltic, and poikilitic [1-3]. A further geochemical classification has been made for the shergottites on the basis of their light rare earth element (LREE) concentrations into: Enriched, intermediate, and depleted. The focus of this study are the poikilitic shergottites, which only include enriched and intermediate rocks to date (e.g., [3-9]). The poikilitic shergottites now make up >20% of the entire martian meteorite suite, with a total of 27 samples to date. Although the poikilitic shergottites may represent a dominant and important lithology on Mars as gabbroic intrusive rocks [10], the petrologic relationships between the enriched and intermediate poikilitic shergottites, as well as with other shergottites are not completely understood (e.g., [3-12]). The poikilitic shergottites display a unique and charac...

Research paper thumbnail of Petrology and Geochemistry of the Enriched Poikilitic Shergottite Northwest Africa 10169: Insight into the Martian Interior

The martian meteorite Northwest Africa (NWA) 10169 is classified as a new member of the geochemic... more The martian meteorite Northwest Africa (NWA) 10169 is classified as a new member of the geochemically enriched poikilitic shergottites, based on mineral composition, Lu-Hf isotope systematics, and rare earth element (REE) composition. Akin to other poikilitic shergottites, it shows a similar bimodal texture to other enriched and intermediate poikilitic shergottites. In addition, olivine and pyroxene in the poikilitic zone have higher Mg#'s (Mg/Mg+Fe) than those in the interstitial areas, suggesting that the poikilitic texture represents early-stage crystallization, opposed to late-stage non-poikilitic crystallization. Calculated fO2 values are reduced (FMQ -2.3 ± 0.23) within the poikilitic texture, and more oxidized (FMQ -1.07 ± 0.14) within the interstitial areas likely representing auto-oxidation and degassing during magma crystallization. Melt inclusions within olivine crystals provide snapshots of magma composition throughout parent melt evolution. The calculated parental m...

Research paper thumbnail of Aubrite and Enstatite Chondrite Impact Melt Meteorites: Analogs to Mercury?

MERCURY? Z. E. Wilbur, A. Udry, F. M. McCubbin, K. E. Vander Kaaden, R. R. Rahib, and T. J. McCoy... more MERCURY? Z. E. Wilbur, A. Udry, F. M. McCubbin, K. E. Vander Kaaden, R. R. Rahib, and T. J. McCoy. 1 Department of Geoscience, University of Nevada, Las Vegas, Las Vegas, NV 89154, USA (wilbur@unlv.nevada.edu), Jacobs, NASA Johnson Space Center, Mail Code XI3, Houston, TX 77058, ARES, NASA Johnson Space Center, 2101 NASA Parkway, Houston, TX 77058, Natural Museum of History, Smithsonian Institution, Washington, D.C., USA.

Research paper thumbnail of Constraining Formation and Emplacement Processes of Poikilitic Shergottites Using Quantitative Textural Analyses

Research paper thumbnail of  Petrology of the enriched poikilitic shergottite Northwest Africa 10169: Insight into the martian interior

Geochimica et Cosmochimica Acta, 2019

Research paper thumbnail of Reclassification of four aubrites as enstatite chondrite impact melts: Potential geochemical analogs for Mercury

Meteoritics & Planetary Science, 2019

Research paper thumbnail of Potential Mercurian Analogues: Aubrite and Enstatite Chondrite Impact Melt Meteorites

Annual UNLV Geosymposium 2018, Apr 20, 2018

Research paper thumbnail of Aubrite and Impact Melt Enstatite Chondrite Meteorites as Potential Analogs to Mercury

The MESSENGER (MErcury Sur-face, Space ENvironment, GEochemistry and Ranging) orbiter measured th... more The MESSENGER (MErcury Sur-face, Space ENvironment, GEochemistry and Ranging) orbiter measured the Mercurian surface abundances of key rock-forming elements to help us better understand the planet's surface and bulk geochemistry. A major discovery is that the Mercurian surface and interior are characterized by an extremely low oxygen fugacity (ƒO2; Iron-Wustite (IW) -7.3 to IW-2.6. This is supported by low Fe and high S abundances on the surface. This low ƒO2 causes a different elemental partioning from what is observed on Earth. Using surface composition, it was shown that the Mercurian surface mainly consists of normative plagioclase, pyroxene, olivine, and exotic sulfides, such as niningerite ((Mg,Mn, Fe)S) and oldhamite (CaS).

Research paper thumbnail of Formation and Emplacement of Martian Poikilitic Shergottites

R. R. Rahib, A. Udry, G. H. Howarth, J. Gross, M. Paquet, L. M. Combs, D. L. Laczniak, and J. M. ... more R. R. Rahib, A. Udry, G. H. Howarth, J. Gross, M. Paquet, L. M. Combs, D. L. Laczniak, and J. M. D. Day, Department of Geosciences, University of Nevada Las Vegas, 4505 S. Maryland Pkwy, Las Vegas, NV 890154 (rahibr@unlv.nevada.edu; arya.udry@unlv.edu), Department of Geological Sciences, University of Cape Town, Rondebosch 7701, South Africa, Department of Earth and Planetary Sciences, Rutgers University, Piscataway NJ 08854, Scripps Institution of Oceanography, University of California San Diego, La Jolla CA 92093-0244, Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN 47907.

Research paper thumbnail of Highly siderophile elements in shergottite sulfides and the sulfur content of the martian mantle

Geochimica et Cosmochimica Acta, 2021

Abstract Shergottite meteorites are ultramafic to mafic igneous rocks derived from partial meltin... more Abstract Shergottite meteorites are ultramafic to mafic igneous rocks derived from partial melting of distinct regions of the martian mantle. As such, they trace magmatic processes, including fractional crystallization and mixing processes in Mars. New chalcophile (Cu, Se, Zn, Pb), siderophile (Ni, Co, W), and highly siderophile element (HSE: Au, Re, Pd, Rh, Pt, Ru, Ir, Os) abundance data are reported for sulfide assemblages in a suite of thirteen incompatible trace-element depleted, intermediate and enriched shergottites, along with new whole-rock HSE abundance and 187Os/188Os data for seven shergottites. Sulfide grains in depleted and intermediate shergottites typically have the highest absolute abundances of HSE, with broadly flat CI-chondrite normalized patterns. Enriched shergottite sulfide grains typically have highly variable Au, elevated Pd and Rh and are relatively depleted in Zn, Ir and Os. The new HSE whole-rock data for enriched (Northwest Africa [NWA] 7397, NWA 7755, NWA 11043), and intermediate shergottites (NWA 10961, NWA 11065, NWA 12241, and NWA 12536) are generally consistent with existing 187Os/188Os and HSE abundance data for these geochemical groupings. Enriched shergottites with >1 ppb bulk rock Os have measured 187Os/188Os ranging between 0.1296 and 0.1471, with variable Pd and Pt contents. Intermediate shergottites with >1 ppb bulk rock Os have chondrite-relative proportions of the HSE at ∼0.01 to 0.001 × CI chondrites and 187Os/188Os from 0.1284 and 0.1295. Sulfides are the major host of the HSE, and they control the behavior of the HSE during petrogenetic processes in shergottite magmas, enabling the determination of HSE compatibility for martian magmatism in the order: Os > Ir ≥ Ru ≥≥ Rh ≥ Pd ≥ Re ≥ Pt ≥ Au. Fractionation models of removal of an olivine-dominated cumulate recreate HSE patterns for the whole-rock shergottites. Enriched shergottites are best reproduced by 25 to 30% of fractionation from a degassed parent melt (250 ± 50 ppm of S), whereas depleted and intermediate shergottites can be explained by slightly lower fractionation (10 to 15%) from higher S content parent melts (350 ± 100 ppm of S). Sulfur contents in the melt that are ∼50% higher than these estimates yield earlier S-saturation during fractional crystallization, leading to an abrupt decrease of the more compatible HSE (Ru, Ir, Os), which is not observed. These results indicate that the martian mantle and partial melts derived from it, are probably not anomalously enriched in S, and instead are similar to slightly higher than those of the terrestrial mantle and its partial melts.

Research paper thumbnail of Mantle source to near-surface emplacement of enriched and intermediate poikilitic shergottites in Mars

Geochimica et Cosmochimica Acta, 2019

Abstract Poikilitic shergottites make up >20% of the current martian meteorite collection, wit... more Abstract Poikilitic shergottites make up >20% of the current martian meteorite collection, with a total of 27 samples. These meteorites are intrusive gabbroic to lherzolitic rocks and represent igneous materials recording important processes in the martian crust. To further constrain petrogenetic relationships amongst enriched and intermediate poikilitic shergottites, we studied a comprehensive suite of poikilitic shergottites — including four newly recovered samples (Northwest Africa [NWA] 11065, NWA 11043, NWA 10961, NWA 10618) — using bulk rock major- and trace-element compositions, mineral major-element compositions, oxygen fugacity (ƒO2) values, crystallization temperatures, phosphorus maps of olivine grains, and quantitative textural analyses. The characteristic bimodal textures (poikilitic and non-poikilitic textures) of poikilitic shergottites record evolving magmatic conditions at different stages of crystallization. Higher temperatures and more reducing conditions during early-stage crystallization are recorded in the poikilitic textures, while lower temperature and more oxidizing conditions are recorded in the non-poikilitic textures during late-stage crystallization. Oxygen fugacity estimates relative to the quartz-fayalite-magnetite (QFM) buffer for early-stage olivine-pyroxene-spinel assemblages of enriched and intermediate poikilitic shergottites suggest decoupling of ƒO2 and the degree of light rare earth element (LREE)-enrichment (i.e., [La/Yb]CI). An increase in ƒO2 exceeding 1 log unit from poikilitic to non-poikilitic textures implies degassing, with possible auto-oxidation, and/or crustal contamination. Quantitative textural analyses support the emplacement of both enriched and intermediate poikilitic shergottites as various shallow intrusive bodies, as well as a potentially widespread emplacement mechanism responsible for a major lithology of the martian crust. In addition, early assemblages (i.e., pyroxene oikocrysts) of all the poikilitic shergottites likely formed close to the crust-mantle boundary, implying a possible widespread presence of magma staging chambers at these depths. Fractional crystallization and magma storage in these chambers could have possibly resulted in all of the different enriched and intermediate shergottites that have been analyzed from Mars.

Research paper thumbnail of Petrogenesis of Enriched and Intermediate Poikilitic Shergottites: From Magmatic Source to Emplacement

Geological Society of America Abstracts with Programs, 2018

Research paper thumbnail of Textural Analysis of Enriched Poikilitic Shergottites

Geological Society of America Abstracts with Programs, 2016

Research paper thumbnail of Formation and Emplacement Processes of Martian Poikilitic Shergottite Meteorites

Introduction: Shergottites, the most prolific group of martian meteorites, have been subdivided b... more Introduction: Shergottites, the most prolific group of martian meteorites, have been subdivided based on their mineralogies and textures into three types: Olivinephyric, basaltic, and poikilitic [1-3]. A further geochemical classification has been made for the shergottites on the basis of their light rare earth element (LREE) concentrations into: Enriched, intermediate, and depleted. The focus of this study are the poikilitic shergottites, which only include enriched and intermediate rocks to date (e.g., [3-9]). The poikilitic shergottites now make up >20% of the entire martian meteorite suite, with a total of 27 samples to date. Although the poikilitic shergottites may represent a dominant and important lithology on Mars as gabbroic intrusive rocks [10], the petrologic relationships between the enriched and intermediate poikilitic shergottites, as well as with other shergottites are not completely understood (e.g., [3-12]). The poikilitic shergottites display a unique and charac...