Research background and current research interests
I have over fifteen years of research experience undertaking high-precision oxygen isotope analysis of an extensive range of terrestrial and extraterrestrial materials using the laser-assisted fluorination technique (Open University). This work is complimented by three years of detailed mineralogical studies of meteorites and related extraterrestrial materials (Natural History Museum, London). In addition, I have undertaken Mg isotope studies of refractory inclusions (Edinburgh and Cambridge Universities), as well as Pb, Sr and O isotope analysis of terrestrial igneous rocks (East Kilbride).
I also have extensive geological fieldwork experience, gained both as part of my academic studies (B.Sc., M.Sc. and Ph.D.) and through employment experience (Noranda exploration, Canada; Ontario Geological Survey, Canada).
My current research is focused on understanding the origin and early evolution of the Solar System, including the formation of the Earth-Moon system. This work involves detailed studies of extraterrestrial materials (meteorites, micrometeorites, lunar samples and impactites) using a wide range of techniques, including: isotope-ratio mass spectrometry, optical microscopy, quantitative electron microbeam analysis, FIB-SEM, ICP-MS. These current research interests can be divided into six themes, as set out below.
Theme 1 – Solar system formation processes
Understanding the underlying mechanisms involved in producing the observed oxygen isotope variation in extraterrestrial materials has been a fundamental part of cosmochemical research since the initial discovery the slope 1 anomaly in Allende CAIs by Clayton and co-workers (Clayton et al., 1973). Confirmation by the NASA Genesis mission that the Earth and bulk Solar System have fundamentally different oxygen isotope compositions has reinforced the importance of attempts to understand how this primordial variation was formed (McKeegan et al., 2011). While self-shielding of CO, either in the early solar nebula (Clayton, 2002; Lyons and Young, 2005), or the precursor molecular cloud (Yurimoto and Kurimoto, 2004), is the currently favoured mechanism, alternative models have also been proposed (Dominguez, 2010; Krot et al., 2010).
Fig. 1 Oxygen isotope composition of acaplucoites, lodranites, winonaites and CR chondrites in relation to the Y&R and CCAM lines. The most primitive CR chondrites, including QUE 99177, plot close to the Young & Russell line (Young and Russell, 1998) and so provide important evidence supporting its primordial significance Acapulcoite-lodranite and winonaite data: Greenwood et al. 2012; CR chondrite data: Schrader et al., 2011. (Key: TFL: Terrestrial Fractionation Line; Y&R: slope 1 line (Young and Russell, 1998); CCAM: Carbonaceous Chondrite Anhydrous Mineral line (Clayton et al., 1977; Clayton and Mayeda, 1999).
The research I have been undertaking at the Open University has involved high-precision oxygen isotope measurements of a diverse range of chondrites and primitive achondrites, as well as separated components from chondrites (chondrules, dark inclusions, matrix etc.), with the overall aim of determining the slope of the primordial oxygen isotope distribution (Greenwood et al., 2012, 2016a,b; Schrader et al., 2011, 2014) (Fig. 1, 2). Results to date are broadly consistent with previous studies (Young and Russel, 1998), indicating that primordial variation was of exactly slope 1, and hence does not conform with results of some self-shielding experiments (Chakraborty et al., 2008).
Fig. 2 Oxygen isotopic composition of Allende chondrules and Dark Inclusions from Greenwood et al. 2016b. PCM = Primitive Chondrule Minerals line (Ushikubo et al., 2012; Tenner et al., 2015).
Primordial oxygen isotope variation most likely reflects conditions inherited from the precursor molecular cloud prior to solar system formation (Yurimoto and Kurimoto, 2004). However, a perplexing aspect of this problem is that invariably the most pristine CAIs do not display slope-1 behaviour, but instead have almost uniform 16O-enriched compositions (Bodenan et al., 2014). This is not the case for chondrules, which formed over a more protracted time interval (Connelly et al., 2012). The most pristine chondrules in CR chondrites define a distinct array with a slope close to 1, known as the PCM line (Ushikubo et al., 2012; Tenner et al., 2015). I am actively working on this problem with colleagues in the US, Mike Zolensky (NASA) and Paul Buchanan. A more complete understanding of the relationship between the various slope ~1 oxygen isotope reference lines (CCAM, PCM, Y&R) necessarily involves unravelling the nature and extent of the secondary alteration processes that took place on early formed asteroids (Fu et al., 2017) (see Theme 2 below).
Theme 2 – Hydrothermal alteration of primitive chondrites
Fig. 3 The bright spots seen on Ceres by the NASA Dawn spacecraft may be the result of hydrothermal activity. Red in the right-hand image indicates a high abundance of carbonates, which may have formed due to the presence of liquid water on Ceres in the recent past (De Sanctis et al., 2016).
A number of important groups of carbonaceous chondrites (CI, CM, CR) show extensive evidence for pervasive, early hydrothermal alteration (Krot et al., 2006). In collaboration with colleagues based in Copenhagen, Paris, London, Arizona and Washington, I have been undertaking oxygen isotope analysis of various carbonaceous chondrite groups with the aim of understanding the origin, extent and setting of aqueous alteration on early-formed asteroids (Greenwood et al., 2004, 2015a, 2016b,c; Haack et al., 2012; Hewins et al., 2014; Schrader et al., 2011, 2014; Howard et al., 2016; Alexander et al., 2017). This work has provided further evidence indicating that certain CR chondrites, including the highly primitive sample QUE 99177 (Fig. 1), have largely escaped parent body alteration (Abreu and Brearley, 2010; Schrader et al., 2011, 2014). In contrast, CM chondrites have generally been more extensively altered (Hewins et al., 2014). These findings are consistent with recent NanoSIMS studies, indicating that CR chondrites retain primitive isotopic signatures, whereas CMs do not (Hashizume et al., 2011). In addition, the work undertaken on the important Danish CM2 fall Maribo is consistent with the possibility that at least the earliest stages of hydrothermal alteration may have commenced in the solar nebula prior to formation of the final parent asteroid (Haack et al., 2012).
Fig. 4 Oxygen isotope composition of CM2 chondrites. The Paris meteorite plots at the 16O enriched end of the CM2 array, consistent with mineralogical evidence that it experienced relatively limited parent body hydrothermal alteration when compared to other CM2 samples (Hewins et al., 2014) . The Danish CM2 fall Maribo is more altered than Paris, but still significantly less so than almost all other CMs (Haack et al., 2012). Data for CV-CO-CKs (Greenwood and Franchi, 2004, Greenwood et al., 2010), Murchison (unpublished Open University), other CM2s (Clayton and Mayeda, 1999).
The extent to which hydrothermal circulation took place on early-formed primitive asteroids remains controversial, with some workers favouring large-scale flow (Young et al., 1999), while others suggest that fluids were essentially stagnant (Bland et al., 2009). In collaboration with colleagues at MIT, UCLA and ASU, I was invited to look at this problem in further detail for a chapter in the recently published Planetesimals book (Cambridge University Press) (Fu et al, 2016). Our conclusions strongly favoured large-scale fluid flow on early-formed asteroids. However, it was also clear from our analysis that this is an area requiring significant further research.
Theme 3 – Metamorphism, melting and differentiation of early-formed asteroids
Fig. 5 Asteroid 4 Vesta, as imaged by the NASA Dawn spacecraft. Vesta is widely considered to be a remnant intact protoplanet from the earliest epoch of Solar System formation (Russell et al., 2012). It is also the source of the HED meteorites (McSween et al., 2013) and possibly also the mesosiderites (Greenwood et al., 2015b, 2016a).
It is now well established that melting and differentiation of planetesimals took place within 1-2 Myr of solar system formation (Kruijer et al., 2014). These planetesimals then rapidly accreted to produce Moon-to-Mars-size planetary embryos, which in turned collided to form the large terrestrial planets on timescales of 10-100 Myr (Chambers, 2004). As a consequence, investigating the processes involved in the origin and evolution of the earliest formed planetesimals is important to a complete understanding of planet formation and in particular that of the Earth.
Fig. 5 Oxygen isotope composition of angrites, HEDs and main-group pallasites showing that all three groups have distinct compositions and hence were derived from separate parent bodies. (Data from Greeenwood et al., 2005, 2006, 2014, 2015b, 2016a)
A major theme of my research has been the study of differentiated meteorites (angrites, main-group pallasites, mesosiderites, HEDs (howardite, eucrite, diogenite group)). Using high-precision laser-assisted fluorination, I have been able to demonstrate that angrites, main-group pallasites and HEDs have distinct asteroidal sources (Greenwood et al., 2005, 2006, 2014, 2015b, 2016a) (Fig. 5). In contrast, the HEDs and mesosiderites have nearly identical oxygen isotope compositions and hence may be derived from the same parent body, believed to be the asteroid 4 Vesta (Greenwood et al., 2006, 2015b, 2016a) (Fig. 6).
Fig. 6 Oxygen isotopic composition of olivine-rich clasts in mesosiderites compared to main-group pallasites, HEDs (references as Fig. 5) and data for other mesosiderite samples (Greenwood et al., 2006). ±2σ variation on the HED group mean value (grey shaded box) is almost fully enclosed by the slightly wider±2σ zone for the mesosiderites (green shaded box). The thick continuous line represents the mesosiderite fractionation line and the dashed line represents the HED fractionation line. Lam = Lamont, VM = Vaca Muerta, Pad = Mount Padbury. TFL = terrestrial fractionation line.
An unexpected result of high-precision oxygen isotope studies has been the identification of anomalous basaltic achondrites, believed to be samples from differentiated asteroids unrelated to Vesta (HED parent body) (Wiechert et al., 2004; Greenwood et al., 205, 2016a; Scott et al., 2009; Bland et al. 2012; Barrett et al., 2017). In collaboration with colleagues in Durham and Oxford, we were able to demonstrate that in a similar manner to the Earth and Mars, the highly siderophile element budget of these early-formed planetesimals was controlled by late accretion processes (Dale et al., 2012). An important, and still poorly understood problem, is the extent to which collisional processes modified the bulk chemistry of these bodies and how in turn this might have influenced the final composition of the terrestrial planets (Greenwood et al., 2005). It is clear from our work on olivine-rich clasts in mesosiderites that the destruction and loss of asteroidal mantle materials may have been an important process in modifying the composition of the terrestrial planets (Greenwood et al., 2015b, 2016a).
Fig. 7 Oxygen isotope composition of anomalous basaltic achondrites shown in relation to the HED analyses of Greenwood et al. (2016a). Central yellow zone: ±2σ precision for eucrite and diogenite falls only (n = 26). Grey zone: ±3σ precision for eucrite and diogenite falls only (n = 26). References and data for the anomalous basaltic achondrites given in Greenwood et al. (2016a) Data for angrites from Greenwood et al. (2005). Abbreviations, EFL: eucrite fractionation line, AFL: angrite fractionation line, TFL: terrestrial fractionation line, BR-F: Bunburra Rockhole fine-grained lithology, BR-M: Bunburra Rockhole medium-grained lithology, BR-C Bunburra Rockhole coarse-grained lithology (BR analyses from Bland et al. 2009).
Based on similar Al/Si and Mg/Si ratios to those measured by the NASA MESSENGER spacecraft, it was suggested by Irving et al. (2013) that the ungrouped achondrite NWA 7325 could be a sample from Mercury. This was clearly a proposal that needed to be investigated further. I was involved in two studies that examined in detail the likely origin of NWA 7325 (Barrat et al., 2015; Weber et al., 2016).
Fig. 8 False colour image of Mercury taken by the NASA MESSENGER spacecraft. The possibility that we might have samples from Mercury in our meteorite collections has been proposed on a number of occasions. Based on the MESSENGER spacecraft measurements, NWA 7325 appeared to be a reasonable possibility.
The studies of Barrat et al. (2015) and Weber et al. (2016) demonstrate conclusively, on the basis of spectroscopy, mineralogy, major and trace element geochemistry and isotopic systematics, that NWA 7325 is not a sample from Mercury. However, despite this somewhat disappointing result, it turns out to be a very unusual meteorite after all. Barrat et al. (2015) suggest that the parental melt to NWA 7325 formed by total melting of preexisting crustal material that may have been as old as 4566 Myr. NWA 7325 therefore provides evidence for the existence of the oldest crust on a differentiated asteroid yet studied.
Theme 4 – Linking meteorites to asteroids
The planetary or asteroidal sources of most meteorite groups are generally difficult to define (notable exceptions being the SNCs, HEDs and lunar meteorites). Based on an integrated study of the mineralogy and geochemistry of the CK and CV3 carbonaceous chondrite groups, it has been possible to demonstrate that both may be derived from a single parent body, which may represented by the Eos asteroid family (Greenwood et al., 2010). Similarities between the isotopic, mineralogical and geochemical characteristics of the mesosiderites and HEDs suggest that both may be derived from asteroid 4 Vesta (Greenwood et al., 2006; 2015b, 2016a). A detailed mineralogical and high-precision oxygen isotope study of the IIE irons and H chondrites, indicates that both may have a common source (McDermott et al., 2016), which remote sensing studies suggests is the large main belt asteroid 6 Hebe (Gaffey and Gilbert, 1988).
Fig. 9 Possible parent body structures for the CV and CK chondrite groups based on their oxygen isotopic and mineralogical compositions (Greenwood et al., 2010). (A) The CK and each of the three CV3 subgroups are from distinct parent asteroids. (B) The CV3 subgroups are samples from a single asteroid and the CK chondrites from another. (C) The CV and CK chondrites are samples from a single, large, heterogeneous asteroid. Our study (Greenwood et al., 2010) favoured this third scenario and suggested that the CV-CK parent body may have been the source of the Eos asteroid family.
The evidence from oxygen isotopes, in conjunction with that from other techniques, indicates that we have samples of approximately 110 asteroidal parent bodies (~60 irons, ~35 achondrites and stony-irons, and ~15 chondrites) in our worlwide meteorite collections (Greenwood et al., 2016a). However, compared to the likely size of the original protoplanetary asteroid population this value is extremely low and the samples that have been studied to date appear to be derived from extensively deformed bodies (Greenwood et al., 2016a).
Theme 5 – The nature of the Terrestrial Fractionation Line (TFL)
The terrestrial fractionation line (TFL) is generally assumed by geochemists to be a unique reference line with a single well-constrained slope in triple oxygen isotope space. However, as demonstrated by our joint study with the Carnegie Institution, this is not in fact the case (Rumble et al., 2007). We were able to show that metamorphic garnets and hydrothermal quartz samples plot on distinct lines, each with a well-constrained slope (0.5262±0.0008 and 0.5240±0.0010 respectively). From a theoretical perspective, it was already well established that the slope of the TFL should vary depending on the nature of the fractionation process involved (Young et al., 2002). However, our study was the first to provide clear evidence of this in terrestrial silicate samples.
Fig. 10 Slope values (λ) obtained in the study of Rumble et al. (2007) compared to literature values. For references see Rumble et al. (2007). Values marked by vertical tick. Length of horizontal bar gives 95% confidence interval. Vertical dashed line marks λ =0.529, the equilibrium value for water vapor-liquid fractionation (Barkan and Luz 2005). (1) GL Quartz = Geophysical Lab measurements for quartz and chalk flint, OU = Open University, see table 3; (2) Landais et al. 2006; (3) Spicuzza et al. 2007; (4) Barkan and Luz 2005; (5) Miller et al. 1999; Miller 2002; (6) Li and Meijer 1998; (7) Jabeen and Kusakabe 1997; (8) Robert et al. 1992.
The nature of the TFL has come into further sharp focus following the recent study of Pack and Herwartz (2014). Their conclusion that at high precision there is no single TFL is in agreement with our earlier findings. However, the methodology they employed is controversial, as we pointed out in a recent published comment on their paper (Miller et al., 2015).
Theme 6 – Impact history of the Earth and other Solar-System bodies
Fig. 11 Artist’s impression of how the Earth-Moon system formed according to the conventional model involving a collision between the proto-Earth and a Mars-sized impactor (Canup and Asphaug, 2001). This model has been increasingly questioned (Canup, 2012; Cuk and Stewart, 2012) in the light of the results from high-precision oxygen isotope studies of terrestrial and lunar samples, which show no detectable difference between the Earth and Moon (Wiechert et al., 2001; Spicuzza et al., 2007; Hallis et al., 2010). A recent study, indicating a small, but detectable, oxygen isotope difference between the Earth and Moon (Herwartz et al., 2014) is the subject of ongoing debate (Young et al., 2016).
It is widely held that pristine meteoritic material does not survive large-scale terrestrial impact events. However, the results of a number of studies I have undertaken demonstrate that this is not the case. Oxygen isotope analysis of meteoritic material recovered from the site of the late Pliocene Eltanin oceanic impact structure (Gersonde et al., 1997), has confirmed that the impactor was a mesosiderite projectile (Greenwood et al., 2006).
Fig. 12 Eltanin impact (2.15 Myr) event off the coast of South America produced a major Tsunami (Gersonde et al., 1997). Oxygen isotope analysis of material recovered from drilling the Eltanin structure confirmed that it was mesosiderite-related (Greenwood et al., 2006).
Oxygen isotope analysis of projectile material from the 145 Myr old Morokweng crater in South Africa has confirmed it to be an LL chondrite (Maier et al., 2008) The disruption of the L-group ordinary chondrite parent body has been dated by Ar-Ar methods at approximately 500 Myr ago (Bogard, 1995). Oxygen isotope analysis of Ordovician fossil meteorites from Sweden has confirmed that they are L-group in composition and so linked to the disruption of their parent body in the main belt (Greenwood et al., 2007). This study demonstrates that the flux of meteoritic material arriving on Earth can show significant changes in intensity and composition over time. A geochemical investigation of the howardite breccia JaH 556 demonstrates that it is possible to produce target rock materials with highly variable oxygen isotope compositions by impact processes (Janots et al., 2012).
Howardite breccia JaH 556 is highly siderophile element enriched and contains relict chondrules derived from H chondrite impactor material. JaH 556 has an anomlaous bulk oxygen isotope composition, but contains isotopically normal HED clasts (Janots et al., 2011).
JaH 556 contains isotopically normal HED clasts (CO-1, 2, 3) and yet has an anomalous bulk composition as a result of impact mixing involving H chondrile projectile material (Janots et al. 2011).
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