2 research outputs found
Physical, Chemical, and Petrological Characteristics of Chondritic Materials and Their Relationships to Small Solar System Bodies
Chondrite materials with varying abundances of volatile-bearing phases are expected at the destinations for the asteroid sample-return missions Hayabusa2 and OSIRIS-REx. The targets of the missions are 162173 (1999 JU3) Ryugu and 101955 (1999 RQ36) Bennu. Spectroscopic analyses of these asteroids suggest that their surface materials are related to types 1 and 2 carbonaceous chondrites. Some studies suggest that the parent bodies of these chondrites may have also experienced some thermal and/or shock metamorphism. The physical properties of boulders at asteroid surfaces and fine particles in asteroid regoliths are consequences of the diverse processes that fragmented them, mobilized them, and redeposited them in unique accumulations. Sample-return missions are likely to encounter a broad range of carbonaceous chondrite (CC)-like materials, to which aqueous alteration, thermal, and shock metamorphism imparted changes affecting their sub-micron- to meter-scale physical properties. Consequently, implementation of scale-dependent analytical techniques to the study of the chemical, physical, and geotechnical characteristics of these CC-like materials is fundamental to safe mission operations, sample selection, and return. However, most of the available knowledge for informing and formulating expectations about regolith processes, products, and properties on carbonaceous small bodies comes from missions that studied anhydrous (e.g., Itokawa studied by Hayabusa) and/or much larger asteroids (e.g., Vesta studied by Dawn). No previous mission is likely directly relevant to small ice-free carbonaceous NEOs 162173 Ryugu or 101955 Bennu, although the Rosetta Spaceraft performed a flyby of the large asteroid Lutetia which has variously been classified as M and C type (Ptzold et al., 2011). Carbonaceous chondrites carry the best record of the history, distribution, and activity of water in the early solar system. Ordinary and Enstatite chondrites carry only partial records, but these are still critical to understanding the full story. We will describe the records of water-rock interactions on asteroids, as recorded in these meteorites, with particular emphasis on the timing, nature, settings, and fluid compositions. An integral part of this story is the rare, but fortunate, preservation of actual early solar system water as aqueous fluid inclusions
A petrological study of peridotite and pyroxenite xenoliths from the West Bismarck Arc and the Tabar-Lihir-Tanga-Feni Arc, Papua New Guinea
Some of the most refractory peridotite samples described in the
literature comprise clasts up to 15 cm in size, hosted in
satellite cones of Ritter Volcano in the West Bismarck Arc, Papua
New Guinea. Host lavas are MgO-rich (13.9-16.6 wt%), mostly
non-accumulative picritic tholeiites, representing the most
primitive magma types in the region. The lava can be divided into
two distinct geochemical groups: a low-Ti series (TiO2 0.25-0.3
wt%) and a high-TiO2 series (TiO2 0.4-0.45 wt%).
This thesis documents the chemical composition and
mineralogy of the picritic hosts and peridotite suite of Ritter,
and compares the latter with a peridotite suite from the Tubaf
Seamount in the Tabar-Lihir-Tanga-Feni Arc of Papua New Guinea.
The Ritter and Tubaf peridotite suites have experienced minimal
alteration through serpentinisation or chloritisation. Petrologic
study reveals however, that they have experienced various degrees
of melt depletion, host magma infiltration, metasomatism,
dissolution/re-precipitation and replacement. The sample suite
can be divided into three broad groups: residues from partial
melting, re-equilibrated samples and a third category comprising
samples from both the ‘residual’ and ‘re-equilibrated’
categories that have been ‘contaminated’ by secondary melt
infiltration processes.
Olivine-spinel exchange geothermometric calculations give
temperatures of ~670 to 1140 oC for Ritter, and 755 to 840 oC for
Tubaf, consistent with entrainment in host lavas from the sub-arc
lithosphere. However, the bulk compositions, crystalline phase
major element compositions coupled with trace element geochemical
characteristics of these suites reflects a complex petrogenetic
history, likely established in regions of magma generation in a
supra-subduction zone, mantle wedge setting. Olivine is highly
forsteritic (Fo# 86.8-95.7 for Ritter, and Fo# 87-91 for Tubaf),
spinel is extremely Cr-rich (Cr# 40.4-89.3 for Ritter, and Cr#
45.0-69.1 for Tubaf), CaO in olivine, and Al2O3 in orthopyroxene
are consistently very low (<0.05 wt% and <2 wt% respectively),
and primary clinopyroxene is absent. The trace element abundance
patterns of primary orthopyroxene and secondary clinopyroxene
display depletions relative to rare earth elements in high field
strength elements, consistent with equilibration with arc-type
magmas.
Olivine-spinel oxygen barometry shows a range from reduced to
oxidised conditions relative to the fayalite-magnetite-quartz
buffer for both Ritter (-1.43 to +1.84 log10 units fO2) and Tubaf
(-1.26 to +0.86). Evidence from Zn/Fe, V/Sc and Mn/Fe systematics
suggests that independent of tectono-magmatic setting, the source
of arc magmas, evidenced by these peridotites, may be
indistinguishable in terms of oxidation state to that of mid
ocean ridge basalts. This study gives a rare insight into the
nature of the sub-arc mantle and the generation of arc magmas