Hilary Downes - Academia.edu (original) (raw)

Papers by Hilary Downes

Journal of Volcanology and Geothermal Research, Aug 1, 2021

Introduction: Spinel symplectites have been reported in lunar dunites from the Taurus–Littrow val... more Introduction: Spinel symplectites have been reported in lunar dunites from the Taurus–Littrow valley [1-6]. Samples 72415 to 72418 were taken from a 10 cm size clast from boulder 3, station 2. Sample 72417 yielded an age of 4.55 ± 0.1 Ga [7]. Hand specimens are brecciated and composed of pale-green, translucent olivine grains up to 10 mm in size, set in a fine-grained matrix. We have investigated Apollo 17 samples 72415,4 and 72417,9003 by electron microprobe analysis (TS 72415,53) and micro-CT. Electron microprobe results: The sample shows a brecciated texture of angular to sub-angular fragments (Fig. 1) of olivine Fo86-89 ([1] in a matrix of smaller angular fragments, with rare diopside, anorthite, spinel and Fe-Ni metal. Olivine grains show shock features with undulose extinction and mosaicism. Fractures crossing olivine crystals are common.

Journal of the Geological Society, Jun 15, 2011

Introduction: Ureilites are ultramafic achondrites, predominantly composed of olivine and pyroxen... more Introduction: Ureilites are ultramafic achondrites, predominantly composed of olivine and pyroxenes with accessory carbon, metal and sulfide. The majority of ureilites are believed to represent the mantle of the ureilite parent body (UPB) [1]. Although ureilites have lost much of their original metal [2], the metal that remains retains a record of the formative processes. Almahata Sitta is predominantly composed of unbrecciated ureilites with a wide range of silicate compositions [3,4]. As a fall it presents a rare opportunity to examine fresh ureilite metal in-situ, and analyzing their highly siderophile element (HSE) ratios gives clues to their formation. Bulk siderophile element analyses of Almahata Sitta fall within the range observed in other ureilites [5]. We have examined the metals in seven ureilitic samples of Almahata Sitta (AS) and one associated chondrite fragment (AS#25). Methods: Electron-beam techniques: Imaging (using a LEO 1455VP SEM with Oxford Instruments INCA software) and major element composition analyses (using a WDS Cameca SX100 EMPA) were conducted at the NHM. EBSD analysis was conducted at the University of Glasgow using an EDAX-TSL system attached to a Quanta 200F fieldemission SEM that was operated in low vacuum mode. LA-ICP-MS: Analyses to obtain trace element compositions were performed at NASA JSC using a New Wave UP-193 SS laser ablation system attached to a Thermo Element2-XR ICP-MS. Laser power was 1-2 GW/cm 2 , with a repetition rate of 10 Hz. Spot sizes of 20 to 100 µm were used, depending on metal grain size. Some grains were too small to analyze. The Filomena and Hoba iron meteorites, NIST 610 glass, and NBS 1168 and NBS 1178 steels were used as standards. Ni values from EPMA were used for internal calibration for all samples except AS#15, #25 and #33 where Fe+Co+Ni were summed to 100 wt%. The following isotopes were analyzed:

Meteoritics & Planetary Science, Jan 22, 2015

Lunar and Planetary Science Conference, 2007

Geochemical Perspectives Letters, 2020

Ureilite meteorites are ultramafic rocks derived from parts of the depleted silicate mantle of th... more Ureilite meteorites are ultramafic rocks derived from parts of the depleted silicate mantle of their parent planetesimal. We used Monte Carlo modelling to explain the observed array of oxygen isotopes and major element chemistry shown by bulk ureilites, after restoration of their missing core and silicate melt components. Despite using a wide range of primitive nebular material, our modelling shows that only a combination of proxy material resembling Allende-type FeO-rich and MgO-rich chondrules, can account for the ureilite oxygen isotope trend and the reconstructed ureilite major element chemistry. Our model predicts formation of a radial gradient in major elements and oxygen isotopes within the planetesimal, with a more Mg-rich silicate interior and a more Fe-rich silicate exterior. Temperatures recorded by ureilites were not high enough to form a magma ocean but were sufficiently high to form a metallic core and silicate melts. The ureilite parent planetesimal was then disrupted by impact. Re-accretion of the outer layers of more Fe-rich silicate material, at the expense of the more MgO-rich material and the core, explains the observed distribution of bulk rock and mineral compositions.

Meteoritics & Planetary Science, 2019

Almahata Sitta (AhS), an anomalous polymict ureilite, is the first meteorite observed to originat... more Almahata Sitta (AhS), an anomalous polymict ureilite, is the first meteorite observed to originate from a spectrally classified asteroid (2008 TC3). However, correlating properties of the meteorite with those of the asteroid is not straightforward because the AhS stones are diverse types. Of those studied prior to this work, 70–80% are ureilites (achondrites) and 20–30% are various types of chondrites. Asteroid 2008 TC3 was a heterogeneous breccia that disintegrated in the atmosphere, with its clasts landing on Earth as individual stones and most of its mass lost. We describe AhS 91A and AhS 671, which are the first AhS stones to show contacts between ureilitic and chondritic materials and provide direct information about the structure and composition of asteroid 2008 TC3. AhS 91A and AhS 671 are friable breccias, consisting of a C1 lithology that encloses rounded to angular clasts (<10 μm to 3 mm) of olivine, pyroxenes, plagioclase, graphite, and metal‐sulfide, as well as chondr...

Meteoritics & Planetary Science, 2015

Lithos, 2020

We have investigated the origin of garnet in garnet-spinel peridotite mantle xenoliths from two w... more We have investigated the origin of garnet in garnet-spinel peridotite mantle xenoliths from two wellknown localities, Pali-Aike (Patagonia) and Vitim (Siberia). We used micro-CT scanning to examine the textures of garnet-spinel clusters and the relationship between garnet and spinel. CT scanning yields different mineral modes in terms of spinel and garnet than observed in thin-section, because distribution of these phases is inhomogeneous, and a larger volume is visible in a CT scan than in a thinsection. We conclude that garnet formed after spinel in both localities, but the xenoliths record a complex tectonic history, in which the original garnets in garnet peridotites were transformed to spinelpyroxene clusters in spinel peridotites, via lithospheric thinning. This stage was followed by deformation of the spinel peridotites, producing a lineation. A final stage of isobaric cooling caused garnet to re-grow around the spinel clusters. Aspect ratios of garnets differ from those of garnet-spinel clusters; the latter are more similar to those of spinel pyroxene clusters. Kelyphite is a secondary feature resulting from heating and rapid decompression during entrainment in the host alkali basaltic lavas and has no relationship with the formation of the garnet-spinel clusters.

Lithos, Apr 1, 2017

Spinel pyroxene-clusters, which are intergrowths of spinel, orthopyroxene and clinopyroxene in ma... more Spinel pyroxene-clusters, which are intergrowths of spinel, orthopyroxene and clinopyroxene in mantle xenoliths, have been investigated through the use of micro-CT (μ-CT) in this study. Samples have been studied from two different tectonic settings: (1) the northern Massif Central, France, an uplifted and rifted plateau on continental lithosphere and (2) Lanzarote in the Canary Islands, an intraplate volcanic island on old oceanic lithosphere. μ-CT analysis of samples from both locations has revealed a range of spinel textures from small < 2 mm microcrystals which can be either spatially concentrated or distributed more evenly throughout the rock with a lineation, to large 4–12 mm individual clusters with ellipsoidal complex vermicular textures in random orientation. Microprobe analyses of pyroxenes inside and outside the clusters show broadly similar compositions. Spinel–pyroxene clusters are the result of a transition of shallow lithospheric mantle from the garnet stability field to the spinel stability field. Both the northern Massif Central and Lanzarote are regions that have experienced significant lithospheric thinning. This process provides a mechanism where the sub-solidus reaction of olivine + garnet = orthopyroxene + clinopyroxene + spinel is satisfied by providing a pathway from garnet peridotite to spinel peridotite. We predict that such textures would only occur in the mantle beneath regions that show evidence of thinning of the lithospheric mantle. Metasomatic reactions are seen around spinel–pyroxene clusters in some Lanzarote xenoliths, so metasomatism post-dated cluster formation.

Ofioliti, 1999

Mt. Vulture volcano is located at the eastern margin of the Apennines, S. Italy and belongs to th... more Mt. Vulture volcano is located at the eastern margin of the Apennines, S. Italy and belongs to the Intra- mountain Ultra-alkaline Magmatic Province (Stoppa & Lavecchia , 1993). It is composed of Recent tephrites, basanites, phonolites and melilitites (De Fino et.al, 1982). Mantle xenoliths are found in the youngest unit of the volcano, a sequence of melilititic and carbonatitic tuffs. They are very fresh, mainly lherzolites and wehrlites. A few harzburgites and orthopyroxene (En89, Wo 2.0), clinopyroxene (Mg# 90-92, Al2O3 = 3-6 wt%, Cr2O3% ~1%) and one composite xenolith have also been found. The main mineral phases are olivine (Fo 90- 92), Cr-spinel ( MgO = 12-20 wt%, Cr2O3 = 30-40 wt%). Glasses are frequently found and are thought to represent in situ partial melting. Mica (phlogopite) is found in a few xenoliths. Carbonate is also present occasionally. The xenoliths have been derived from a depth between 55-75 km, corresponding to the spinel stability field in the upper mantle (J...

The massive ~530 ka Cumbre Nueva collapse on the Canary Island of La Palma separates the pre-coll... more The massive ~530 ka Cumbre Nueva collapse on the Canary Island of La Palma separates the pre-collapse Cumbre Nueva rift sequence from the post-collapse Bejenado edifice. Such collapses are characteristic of the shield stage of oceanic island growth, and these Canarian volcanoes form an ideal case study to determine how the composition of lavas produced within a magma supply system varies in response to the structural evolution of the island. The Cumbre Nueva rift formed as the result of focused activity along a central rift zone, with the flanks of the edifice becoming progressively more unstable as growth continued. Conversely, the Bejenado volcano erupted entirely within the collapse embayment in a brief, intense period of activity after which volcanism permanently ceased. All flows are composed of alkaline trend lavas, including basanites, tephrites, tephriphonolites and porphyritic ankaramites, with lavas from both edifices showing a tendency towards increased evolution over tim...

International Journal of Earth Sciences, 2017

Introduction: Numerous fragments of opal (SiO 2 .nH 2 O) have previously been reported in 5 inter... more Introduction: Numerous fragments of opal (SiO 2 .nH 2 O) have previously been reported in 5 internal chips of polymict ureilite EET83309 [1]. The largest fragments are >300µm in the longest dimension and show clear banding. The bands are terminated by the adjacent olivine clasts, indicating that the opal clasts were not formed in situ. Opal is also found in contact with ureilitic olivine, forming a single clast with a terrestrial weathering rim around both minerals. It also occurs as thin (10µm) rims completely surrounding suessite (Fe 3 Si) grains, and can also contain inclusions of schreibersite. It shows no relationship whatsoever with terrestrial weathering rims. These petrographic observations indicate that the opal is (a) extraterrestrial in origin, and (b) native to the ureilite parent body. Suessite and schreibersite are considered to be the products of reduction during shock metamorphism. Therefore formation of the opal occurred after the formation of suessite but before the formation of the regolith represented by polymict ureilites. Opal composition: Opal is hydrated silica containing 10-30 wt% H 2 O. XRD analysis suggests that the opal in EET83309 is of the amorphous opal-A variety, although it appears to have undergone some recrystallization to a SiO 2 phase. The largest opal clast has a composition of approximately 65.5 wt.% SiO 2 , 6 wt.% FeO and <1wt.% MgO. In contrast, terrestrial opals only contain up to 1.0 wt% FeO and 0.06 wt. % MgO [2]. The origin of the water or ice which reacted with Si-rich minerals to form the opal is unknown but must be extra-terrestrial in origin, possibly due to impact of an icy body such as a comet on the surface of the ureilite parent body. The identification of hydrated silica (opal) by the Mars Exploration Rover (MER) Spirit in the Gusev Crater, Mars [3] and presence of opal in ureilite EET83309 has implications for the presence of water and its delivery by cometary bodies during the early formation of the Inner Solar System.

Lithos, 2000

... We thank Nina Levkovich for help with conventional zircon U–Pb chemistry, and Jessica Vestin ... more ... We thank Nina Levkovich for help with conventional zircon U–Pb chemistry, and Jessica Vestin and Martin Whitehouse for help with SIMS analyses and fruitful discussions. ... Geochronology of the early Proterozoic Pechenga–Varzuga structure of the Kola peninsula. ...

Lithos, 2019

In a continuing study of the textures of mantle peridotites, we have analysed thin-sections of ad... more In a continuing study of the textures of mantle peridotites, we have analysed thin-sections of additional samples including spinel and garnet peridotite xenoliths from a range of locations, using a grain-section skeleton outline method. Peridotites from ultramafic massifs have also been analysed using the same methodology. The results for all these samples lie on the same linear trend of grain-section area vs standard deviation as seen in our previous study. This confirms the utility of the quantitative method, providing observer-independent objective numerical descriptions of textures in peridotite rocks. Two spinel peridotite xenoliths have been disaggregated using an Electric Discharge Disaggregation technique and sieved them to produce a grain size distribution. SEM imaging has been used to show that the 3D shapes of grains of the constituent minerals have concave features. CT-scanning of separated grains and rock cores has also demonstrated the concave features of the constituent minerals and their consequent interlocking structures. A 'perimeter-area' relation technique has been used to show the 2-dimensional grain-section skeleton outlines clearly display fractal characteristics, authenticating our method by reference to established Euclidian and fractal patterns. The fractal nature of textures of mantle peridotite rocks is further supported by an alternative method for fractal assessment (Box Counting). *Manuscript Click here to download Manuscript: TXMPRR Textures of mantle peridotite rocks revisited.docx Click here to view linked References

Journal of Volcanology and Geothermal Research, Aug 1, 2021

Introduction: Spinel symplectites have been reported in lunar dunites from the Taurus–Littrow val... more Introduction: Spinel symplectites have been reported in lunar dunites from the Taurus–Littrow valley [1-6]. Samples 72415 to 72418 were taken from a 10 cm size clast from boulder 3, station 2. Sample 72417 yielded an age of 4.55 ± 0.1 Ga [7]. Hand specimens are brecciated and composed of pale-green, translucent olivine grains up to 10 mm in size, set in a fine-grained matrix. We have investigated Apollo 17 samples 72415,4 and 72417,9003 by electron microprobe analysis (TS 72415,53) and micro-CT. Electron microprobe results: The sample shows a brecciated texture of angular to sub-angular fragments (Fig. 1) of olivine Fo86-89 ([1] in a matrix of smaller angular fragments, with rare diopside, anorthite, spinel and Fe-Ni metal. Olivine grains show shock features with undulose extinction and mosaicism. Fractures crossing olivine crystals are common.

Journal of the Geological Society, Jun 15, 2011

Introduction: Ureilites are ultramafic achondrites, predominantly composed of olivine and pyroxen... more Introduction: Ureilites are ultramafic achondrites, predominantly composed of olivine and pyroxenes with accessory carbon, metal and sulfide. The majority of ureilites are believed to represent the mantle of the ureilite parent body (UPB) [1]. Although ureilites have lost much of their original metal [2], the metal that remains retains a record of the formative processes. Almahata Sitta is predominantly composed of unbrecciated ureilites with a wide range of silicate compositions [3,4]. As a fall it presents a rare opportunity to examine fresh ureilite metal in-situ, and analyzing their highly siderophile element (HSE) ratios gives clues to their formation. Bulk siderophile element analyses of Almahata Sitta fall within the range observed in other ureilites [5]. We have examined the metals in seven ureilitic samples of Almahata Sitta (AS) and one associated chondrite fragment (AS#25). Methods: Electron-beam techniques: Imaging (using a LEO 1455VP SEM with Oxford Instruments INCA software) and major element composition analyses (using a WDS Cameca SX100 EMPA) were conducted at the NHM. EBSD analysis was conducted at the University of Glasgow using an EDAX-TSL system attached to a Quanta 200F fieldemission SEM that was operated in low vacuum mode. LA-ICP-MS: Analyses to obtain trace element compositions were performed at NASA JSC using a New Wave UP-193 SS laser ablation system attached to a Thermo Element2-XR ICP-MS. Laser power was 1-2 GW/cm 2 , with a repetition rate of 10 Hz. Spot sizes of 20 to 100 µm were used, depending on metal grain size. Some grains were too small to analyze. The Filomena and Hoba iron meteorites, NIST 610 glass, and NBS 1168 and NBS 1178 steels were used as standards. Ni values from EPMA were used for internal calibration for all samples except AS#15, #25 and #33 where Fe+Co+Ni were summed to 100 wt%. The following isotopes were analyzed:

Meteoritics & Planetary Science, Jan 22, 2015

Lunar and Planetary Science Conference, 2007

Geochemical Perspectives Letters, 2020

Ureilite meteorites are ultramafic rocks derived from parts of the depleted silicate mantle of th... more Ureilite meteorites are ultramafic rocks derived from parts of the depleted silicate mantle of their parent planetesimal. We used Monte Carlo modelling to explain the observed array of oxygen isotopes and major element chemistry shown by bulk ureilites, after restoration of their missing core and silicate melt components. Despite using a wide range of primitive nebular material, our modelling shows that only a combination of proxy material resembling Allende-type FeO-rich and MgO-rich chondrules, can account for the ureilite oxygen isotope trend and the reconstructed ureilite major element chemistry. Our model predicts formation of a radial gradient in major elements and oxygen isotopes within the planetesimal, with a more Mg-rich silicate interior and a more Fe-rich silicate exterior. Temperatures recorded by ureilites were not high enough to form a magma ocean but were sufficiently high to form a metallic core and silicate melts. The ureilite parent planetesimal was then disrupted by impact. Re-accretion of the outer layers of more Fe-rich silicate material, at the expense of the more MgO-rich material and the core, explains the observed distribution of bulk rock and mineral compositions.

Meteoritics & Planetary Science, 2019

Almahata Sitta (AhS), an anomalous polymict ureilite, is the first meteorite observed to originat... more Almahata Sitta (AhS), an anomalous polymict ureilite, is the first meteorite observed to originate from a spectrally classified asteroid (2008 TC3). However, correlating properties of the meteorite with those of the asteroid is not straightforward because the AhS stones are diverse types. Of those studied prior to this work, 70–80% are ureilites (achondrites) and 20–30% are various types of chondrites. Asteroid 2008 TC3 was a heterogeneous breccia that disintegrated in the atmosphere, with its clasts landing on Earth as individual stones and most of its mass lost. We describe AhS 91A and AhS 671, which are the first AhS stones to show contacts between ureilitic and chondritic materials and provide direct information about the structure and composition of asteroid 2008 TC3. AhS 91A and AhS 671 are friable breccias, consisting of a C1 lithology that encloses rounded to angular clasts (<10 μm to 3 mm) of olivine, pyroxenes, plagioclase, graphite, and metal‐sulfide, as well as chondr...

Meteoritics & Planetary Science, 2015

Lithos, 2020

We have investigated the origin of garnet in garnet-spinel peridotite mantle xenoliths from two w... more We have investigated the origin of garnet in garnet-spinel peridotite mantle xenoliths from two wellknown localities, Pali-Aike (Patagonia) and Vitim (Siberia). We used micro-CT scanning to examine the textures of garnet-spinel clusters and the relationship between garnet and spinel. CT scanning yields different mineral modes in terms of spinel and garnet than observed in thin-section, because distribution of these phases is inhomogeneous, and a larger volume is visible in a CT scan than in a thinsection. We conclude that garnet formed after spinel in both localities, but the xenoliths record a complex tectonic history, in which the original garnets in garnet peridotites were transformed to spinelpyroxene clusters in spinel peridotites, via lithospheric thinning. This stage was followed by deformation of the spinel peridotites, producing a lineation. A final stage of isobaric cooling caused garnet to re-grow around the spinel clusters. Aspect ratios of garnets differ from those of garnet-spinel clusters; the latter are more similar to those of spinel pyroxene clusters. Kelyphite is a secondary feature resulting from heating and rapid decompression during entrainment in the host alkali basaltic lavas and has no relationship with the formation of the garnet-spinel clusters.

Lithos, Apr 1, 2017

Spinel pyroxene-clusters, which are intergrowths of spinel, orthopyroxene and clinopyroxene in ma... more Spinel pyroxene-clusters, which are intergrowths of spinel, orthopyroxene and clinopyroxene in mantle xenoliths, have been investigated through the use of micro-CT (μ-CT) in this study. Samples have been studied from two different tectonic settings: (1) the northern Massif Central, France, an uplifted and rifted plateau on continental lithosphere and (2) Lanzarote in the Canary Islands, an intraplate volcanic island on old oceanic lithosphere. μ-CT analysis of samples from both locations has revealed a range of spinel textures from small < 2 mm microcrystals which can be either spatially concentrated or distributed more evenly throughout the rock with a lineation, to large 4–12 mm individual clusters with ellipsoidal complex vermicular textures in random orientation. Microprobe analyses of pyroxenes inside and outside the clusters show broadly similar compositions. Spinel–pyroxene clusters are the result of a transition of shallow lithospheric mantle from the garnet stability field to the spinel stability field. Both the northern Massif Central and Lanzarote are regions that have experienced significant lithospheric thinning. This process provides a mechanism where the sub-solidus reaction of olivine + garnet = orthopyroxene + clinopyroxene + spinel is satisfied by providing a pathway from garnet peridotite to spinel peridotite. We predict that such textures would only occur in the mantle beneath regions that show evidence of thinning of the lithospheric mantle. Metasomatic reactions are seen around spinel–pyroxene clusters in some Lanzarote xenoliths, so metasomatism post-dated cluster formation.

Ofioliti, 1999

Mt. Vulture volcano is located at the eastern margin of the Apennines, S. Italy and belongs to th... more Mt. Vulture volcano is located at the eastern margin of the Apennines, S. Italy and belongs to the Intra- mountain Ultra-alkaline Magmatic Province (Stoppa & Lavecchia , 1993). It is composed of Recent tephrites, basanites, phonolites and melilitites (De Fino et.al, 1982). Mantle xenoliths are found in the youngest unit of the volcano, a sequence of melilititic and carbonatitic tuffs. They are very fresh, mainly lherzolites and wehrlites. A few harzburgites and orthopyroxene (En89, Wo 2.0), clinopyroxene (Mg# 90-92, Al2O3 = 3-6 wt%, Cr2O3% ~1%) and one composite xenolith have also been found. The main mineral phases are olivine (Fo 90- 92), Cr-spinel ( MgO = 12-20 wt%, Cr2O3 = 30-40 wt%). Glasses are frequently found and are thought to represent in situ partial melting. Mica (phlogopite) is found in a few xenoliths. Carbonate is also present occasionally. The xenoliths have been derived from a depth between 55-75 km, corresponding to the spinel stability field in the upper mantle (J...

The massive ~530 ka Cumbre Nueva collapse on the Canary Island of La Palma separates the pre-coll... more The massive ~530 ka Cumbre Nueva collapse on the Canary Island of La Palma separates the pre-collapse Cumbre Nueva rift sequence from the post-collapse Bejenado edifice. Such collapses are characteristic of the shield stage of oceanic island growth, and these Canarian volcanoes form an ideal case study to determine how the composition of lavas produced within a magma supply system varies in response to the structural evolution of the island. The Cumbre Nueva rift formed as the result of focused activity along a central rift zone, with the flanks of the edifice becoming progressively more unstable as growth continued. Conversely, the Bejenado volcano erupted entirely within the collapse embayment in a brief, intense period of activity after which volcanism permanently ceased. All flows are composed of alkaline trend lavas, including basanites, tephrites, tephriphonolites and porphyritic ankaramites, with lavas from both edifices showing a tendency towards increased evolution over tim...

International Journal of Earth Sciences, 2017

Introduction: Numerous fragments of opal (SiO 2 .nH 2 O) have previously been reported in 5 inter... more Introduction: Numerous fragments of opal (SiO 2 .nH 2 O) have previously been reported in 5 internal chips of polymict ureilite EET83309 [1]. The largest fragments are >300µm in the longest dimension and show clear banding. The bands are terminated by the adjacent olivine clasts, indicating that the opal clasts were not formed in situ. Opal is also found in contact with ureilitic olivine, forming a single clast with a terrestrial weathering rim around both minerals. It also occurs as thin (10µm) rims completely surrounding suessite (Fe 3 Si) grains, and can also contain inclusions of schreibersite. It shows no relationship whatsoever with terrestrial weathering rims. These petrographic observations indicate that the opal is (a) extraterrestrial in origin, and (b) native to the ureilite parent body. Suessite and schreibersite are considered to be the products of reduction during shock metamorphism. Therefore formation of the opal occurred after the formation of suessite but before the formation of the regolith represented by polymict ureilites. Opal composition: Opal is hydrated silica containing 10-30 wt% H 2 O. XRD analysis suggests that the opal in EET83309 is of the amorphous opal-A variety, although it appears to have undergone some recrystallization to a SiO 2 phase. The largest opal clast has a composition of approximately 65.5 wt.% SiO 2 , 6 wt.% FeO and <1wt.% MgO. In contrast, terrestrial opals only contain up to 1.0 wt% FeO and 0.06 wt. % MgO [2]. The origin of the water or ice which reacted with Si-rich minerals to form the opal is unknown but must be extra-terrestrial in origin, possibly due to impact of an icy body such as a comet on the surface of the ureilite parent body. The identification of hydrated silica (opal) by the Mars Exploration Rover (MER) Spirit in the Gusev Crater, Mars [3] and presence of opal in ureilite EET83309 has implications for the presence of water and its delivery by cometary bodies during the early formation of the Inner Solar System.

Lithos, 2000

... We thank Nina Levkovich for help with conventional zircon U–Pb chemistry, and Jessica Vestin ... more ... We thank Nina Levkovich for help with conventional zircon U–Pb chemistry, and Jessica Vestin and Martin Whitehouse for help with SIMS analyses and fruitful discussions. ... Geochronology of the early Proterozoic Pechenga–Varzuga structure of the Kola peninsula. ...

Lithos, 2019

In a continuing study of the textures of mantle peridotites, we have analysed thin-sections of ad... more In a continuing study of the textures of mantle peridotites, we have analysed thin-sections of additional samples including spinel and garnet peridotite xenoliths from a range of locations, using a grain-section skeleton outline method. Peridotites from ultramafic massifs have also been analysed using the same methodology. The results for all these samples lie on the same linear trend of grain-section area vs standard deviation as seen in our previous study. This confirms the utility of the quantitative method, providing observer-independent objective numerical descriptions of textures in peridotite rocks. Two spinel peridotite xenoliths have been disaggregated using an Electric Discharge Disaggregation technique and sieved them to produce a grain size distribution. SEM imaging has been used to show that the 3D shapes of grains of the constituent minerals have concave features. CT-scanning of separated grains and rock cores has also demonstrated the concave features of the constituent minerals and their consequent interlocking structures. A 'perimeter-area' relation technique has been used to show the 2-dimensional grain-section skeleton outlines clearly display fractal characteristics, authenticating our method by reference to established Euclidian and fractal patterns. The fractal nature of textures of mantle peridotite rocks is further supported by an alternative method for fractal assessment (Box Counting). *Manuscript Click here to download Manuscript: TXMPRR Textures of mantle peridotite rocks revisited.docx Click here to view linked References