Clive R. Neal

The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous-Paleogene Boundary

The Fall of the Dinosaurs According to the fossil record, the rule of dinosaurs came to an abrupt end ∼65 million years ago, when all nonavian dinosaurs and flying reptiles disappeared. Several possible mechanisms have been suggested for this mass extinction, including a large asteroid impact and major flood volcanism. Schulte et al. (p. 1214) review how the occurrence and global distribution of a global iridium-rich deposit and impact ejecta support the hypothesis that a single asteroid impact at Chicxulub, Mexico, triggered the extinction event. Such an impact would have instantly caused devastating shock waves, a large heat pulse, and tsunamis around the globe. Moreover, the release of high quantities of dust, debris, and gases would have resulted in a prolonged cooling of Earths surface, low light levels, and ocean acidification that would have decimated primary producers including phytoplankton and algae, as well as those species reliant upon them. The Cretaceous-Paleogene boundary ~65.5 million years ago marks one of the three largest mass extinctions in the past 500 million years. The extinction event coincided with a large asteroid impact at Chicxulub, Mexico, and occurred within the time of Deccan flood basalt volcanism in India. Here, we synthesize records of the global stratigraphy across this boundary to assess the proposed causes of the mass extinction. Notably, a single ejecta-rich deposit compositionally linked to the Chicxulub impact is globally distributed at the Cretaceous-Paleogene boundary. The temporal match between the ejecta layer and the onset of the extinctions and the agreement of ecological patterns in the fossil record with modeled environmental perturbations (for example, darkness and cooling) lead us to conclude that the Chicxulub impact triggered the mass extinction.

A chemical model for generating the sources of mare basalts - Combined equilibrium and fractional crystallization of the lunar magmasphere

Abstract It is generally considered that mare basalts were generated by the melting of a cumulate mantle formed in an early Moon-wide magma ocean or magmasphere. However, the nature and chemistry of this cumulate mantle and the logistics of its origin have remained elusive. Extensive studies of terrestrial layered mafic intrusions over the past sixty years have emphasized the imperfection of fractional crystallization and attendant crystal-crystal and crystal-liquid separation in a convecting magma chamber. Crystal-liquid and crystal-crystal separations were similarly inefficient during evolution of the lunar magma ocean (LMO), allowing for the trapping of interstitial liquid and entrainment of a small proportion of less-dense plagioclase into the denser mafic cumulate mush. Indeed, petrography of lunar highlands samples demonstrates this for anorthosites (with 1–10% olivine). The residual liquid after 80–90% crystallization was very evolved (in fact KREEPy) and, even in small proportions (1–5%), would have a noticeable effect on the trace-element chemistry of melts generated from these cumulates. This trapped residual liquid would elevate total REE abundances in the cumulate pile, while synchronously deepening the already negative Eu anomaly. Essentially, this trapped liquid will make the cumulate more fertile for melting to generate both KREEP basalt and mare basalt magmas. Plagioclase entrained in the mafic cumulate pile adds an essential Al component to the high-Ti basalt source and will moderate the requisite negative Eu anomaly in the cumulate. Early in the evolution of the lunar mantle, when the LMO still was largely liquid, it is likely that vigorous convection was an important factor in crystallization. Such convection would allow crystals to remain suspended and in equilibrium with the LMO liquid for relatively long periods of time. This extended period of equilibrium crystallization would then have been followed by fractional crystallization once plagioclase became a liquidus phase and began to float to form the lunar highlands crust. Previous authors have proposed a three-component model for the evolution of high-Ti mare basalt source regions. This model includes KREEP, early (olivine-rich, high Mg#) cumulates, and late (ilmeniterich, low Mg#) cumulates in various proportions. However, we propose a model for high-Ti basalt parent magmas which is in accord with studies of terrestrial layered intrusions. This model for the high-Ti source includes trapped instantaneous residual liquid (TIRL; 1–3%) and entrainment of a small (2–5%) proportion of plagioclase into the late-stage cumulate pile in order to account for both the observed Al compositions and trace-element characteristics of high-Ti mare basalts. Melting of this relatively shallow, ilmenite- and clinopyroxene-bearing, late-stage cumulate can generate high-Ti mare basalt magmas. Furthermore, we are in agreement with other workers that only through a process of nonmodal melting will the high Ti values for the parent magmas be realized. Large-scale convective overturn of the cumulate pile and mixing of KREEP with early- and late-stage cumulates is not required. However, localized overturn of the upper tenth of the cumulate pile is likely and, in fact, required to achieve an appropriate major-element balance for the high-Ti mare basalt source region.

Origin and evolution of a submarine large igneous province: the Kerguelen Plateau and Broken Ridge, southern Indian Ocean

Oceanic plateaus form by mantle processes distinct from those forming oceanic crust at divergent plate boundaries. Eleven drillsites into igneous basement of Kerguelen Plateau and Broken Ridge, including seven from the recent Ocean Drilling Program Leg 183 (1998–99) and four from Legs 119 and 120 (1987–88), show that the dominant rocks are basalts with geochemical characteristics distinct from those of mid-ocean ridge basalts. Moreover, the physical characteristics of the lava flows and the presence of wood fragments, charcoal, pollen, spores and seeds in the shallow water sediments overlying the igneous basement show that the growth rate of the plateau was sufficient to form subaerial landmasses. Most of the southern Kerguelen Plateau formed at ~110 Ma, but the uppermost submarine lavas in the northern Kerguelen Plateau erupted during Cenozoic time. These results are consistent with derivation of the plateau by partial melting of the Kerguelen plume. Leg 183 provided two new major observations about the final growth stages of the Kerguelen Plateau. 1: At several locations, volcanism ended with explosive eruptions of volatile-rich, felsic magmas; although the total volume of felsic volcanic rocks is poorly constrained, the explosive nature of the eruptions may have resulted in globally significant effects on climate and atmospheric chemistry during the late-stage, subaerial growth of the Kerguelen Plateau. 2: At one drillsite, clasts of garnet–biotite gneiss, a continental rock, occur in a fluvial conglomerate intercalated within basaltic flows. Previously, geochemical and geophysical evidence has been used to infer continental lithospheric components within this large igneous province. A continental geochemical signature in an oceanic setting may represent deeply recycled crust incorporated into the Kerguelen plume or continental fragments dispersed during initial formation of the Indian Ocean during breakup of Gondwana. The clasts of garnet–biotite gneiss are the first unequivocal evidence of continental crust in this oceanic plateau. We propose that during initial breakup between India and Antarctica, the spreading center jumped northwards transferring slivers of the continental Indian plate to oceanic portions of the Antarctic plate.

Signatures of the highly siderophile elements in the SNC meteorites and Mars: a review and petrologic synthesis

We have evaluated the highly siderophile element (HSE) signatures of the martian (SNC) meteorites using new and literature data. These Ir and Os concentrations correlate with the Mg# [molar Mg/(Mg+Fe)], Cr and Ni, suggesting that olivine or chromite acts as a host for compatible siderophiles. Our analysis agrees with others who have suggested that the martian mantle has chondritic relative abundances of siderophiles. We also agree that, unlike the Sr and Nd isotopic systems, there is no evidence from Os isotopes for crustal assimilation. Comparisons of the siderophile element ratios of ALH 84001 to younger SNCs give no indication of a change in the martian siderophile element pattern over time. D 2002 Published by Elsevier Science B.V.

Petrogenesis of mare basalts - A record of lunar volcanism

Returned rock and soil samples from our nearest planetary neighbor have provided the basis for much of our understanding of the origin and evolution of the Moon. Of particular importance are the mare basalts, which have revealed considerable information about lunar volcanism and the nature of the mantle, as well as post-magma-generation processes. This paper is a critical review of the petrogenetic models for the generation of mare basalts formulated over the last twenty years. We have used all available mare basalt analyses to define a six-fold classification scheme using TiO2 contents as the primary division (i.e., ∗ 1 wt% = very low-Ti or VLT; 1–6 wt% = low-Ti; > 6 wt% = high-Ti). A secondary division is made using Al2O3 contents (i.e., ∗ 11 wt% = low-Al; > 11 wt% = high-Al), and a tertiary division is defined using K contents (i.e., ∗ 2000 ppm = low-K; > 2000 ppm = high-K). Such divisions and subdivisions yield a classification containing twelve categories, of which six are accounted for by the existing Apollo and Luna collections. Therefore, we present our discussions in the form of six mare basalt rock types: 1. (1) high-Ti/low-Al/low-K (referred to as “high-Ti/low-K”). 2. (2) high-Ti/low-Al/high-K (referred to as “high-Ti/high-K”). 3. (3) low-Ti/low-Al/low-K (referred to as “low-Ti”). 4. (4) low-Ti/high-Al/low-K (referred to as “high-alumina”). 5. (5) low-Ti /high-Al/ high-K (referred to as “VHK”). 6. (6) VLT/low-Al/low-K basalts (referred to as “VLT”). A variety of post-magma-generation processes have been invoked, such as fractional crystallization, either alone or combined with wallrock assimilation, to explain the compositional ranges of the various mare basalt suites. In order to evaluate these proposed petrogenetic processes, this review is by rock type and is non-site specific, but for each rock type, reference to particular lunar sample return missions is brought forth. This permits a comparison of similarities and differences of broadly similar rock types correlated with geography on the Moon, which, in turn, allows a more thorough petrogenetic evaluation. High-Ti mare basalts (i.e., high-Ti/low-Al/low-K) are found at Apollo 11 and Apollo 17 sites; however, the A-11 basalts contain lower TiO2 abundances, a considerably larger range in trace-element contents, and the only occurrence of high-Ti/high-K mare basalts. Fractional crystallization and source heterogeneity within each site are the keys to understanding the petrogenesis of the high-Ti basalts. Low-Ti basalts (including both low-Al/K and high-Al/K varieties) are found at Apollo 12, 14, and 15, and Luna 16 sites. The low-Ti basalts exhibit a wide range of major- and trace-element compositions and require source heterogeneity, fractional crystallization, and some assimilation. The high-alumina mare basalts (i.e., low-Ti/high-Al/low-K) are found at Apollo 14 and Luna 16 sites and exhibit a wide range of major-and trace-element compositions. However, in these examples, source heterogeneity is not a major factor. Indeed, fractional crystallization coupled with KREEP assimilation, particularly for the Apollo 14 variants, can explain the compositional ranges of these high-alumina basalts. The VHK mare basalts (i.e., low-Ti/high-Al/high-K) have been sampled only at the Apollo 14 locale and are products of a parental highalumina magma assimilating lunar granite. Very low-Ti (VLT) mare basalts (i.e., VLT/low-Al/low-K) are found at Apollo 17 and Luna 24 sites. Fractional crystallization has had a major influence upon the range in VLT compositions, but Luna 24 VLT basalts have been derived from a source slightly different in composition from that for Apollo 17 VLT varieties. For example, the Luna 24 VLT basalts generally exhibit positive Eu anomalies, a unique property for mare basalts, which almost always have negative Eu anomalies. The concept of a lunar magma ocean (LMO) is generally accepted, and source modelling of all basalts invokes a “mafic LMO cumulate source.” This is the only unifying model for mare basalt petrogenesis, but the semantics and logistics of it are and will be debated for many years. For example, major convective overturn of the LMO appears plausible, but whether this occurred on a local- or planet-wide scale to produce source heterogeneity remains to be determined.

Eclogites with Oceanic Crustal and Mantle Signatures from the Bellsbank Kimberlite, South Africa, Part I: Mineralogy, Petrography, and Whole Rock Chemistry

Three groups of eclogite xenoliths have been identified from the DeBruyn and Martin Mine of the Bellsbank kimberlite, South Africa. These eclogites are divided into the three groups on the basis of petrography, clinopyroxene and garnet mineral chemistry, and rare earth element (REE) contents of the whole rocks, clinopyroxenes, and garnets. Abundances of the REE and garnet-clinopyroxene Kds are consistent with a petrogenesis by fractional crystallization for Group A eclogites, but not Groups and We suggest, on the basis of bulk-rock major-element and REE analyses and reconstructed REE patterns, that eclogites from Groups and are the metamorphosed products of ancient subducted oceanic crust. Group eclogites have major element chemistry similar to Archean basalts, and Group eclogites have high

Structure and deformation of north and central Malaita, Solomon Islands: tectonic implications for the Ontong Java Plateau-Solomon arc collision, and for the fate of oceanic plateaus

Al_{2}O_{3}

Beyond EM-1: Lavas from Afanasy-Nikitin Rise and the Crozet Archipelago, Indian Ocean

abundances. Metasomatism has affected all groups of eclogites, producing phlogopite, feldspar, amphibole, and a breakdown of primary clinopyroxene. On the basis of large ion lithophile (LIL) elements, this metasomatism can be traced to the host kimberlite. Our study demonstrates that petrogenesis by fractional crystallization cannot account for all eclogite xenolith compositions found in kimberlite.