Glass Veins Research Articles

Diverse Invasive Melts in Cascadia Mantle Xenoliths: No Subduction Connection

Mantle xenoliths (spinel lherzolite, dunite, harzburgite, websterite, rare phlogopite-bearing orthopyroxenite) in basaltic lavas of the Simcoe volcanic field, southern Washington Cascades arc are invaded by diverse alkaliand silica-rich glasses. The following melt variants are identified: (1) volatile-rich melts fresh glass veins with vesiculation bubbles (up to -57% SiO2, ~19% A1203); (2) potassic melts glass pools or veins in contact with phlogopite (up to -62% SiO2, ~12% total alkalies); and (3) in situ melts blebs of glass in contact with exsolution lamellae of orthopyroxene and spinel in clinopyroxene (~52-55% SiO2, -6% total alkalies, ~10% CaO) (Fig. 1). Glass/quench olivine (F088-91) assemblages in veins (1) and pockets (2) indicate high equilibrium temperatures ~1000~ Especially pyroxene-rich varieties contain pure CO2 fluid inclusions with high densities (~1.07) entrapped at minimum depths of = 35 km (Ertan and Leeman, 1996, 1998). These melts differ significantly from the host lavas (Fig. 1) (Leeman et al., 1990) with phenocrysts [olivine (Fo65-80); plagioclase (An45-58)] and they must have been emplaced after the deformation of mantle, but before entrainment of the xenoliths. The invasive melts resulted from decompressional melting of hydrous phases, magmatic corrosion along mineral grain boundaries, and in situ melting. The rare xenolith occurrences from island arc settings potentially provide important information about the subduction processes. For example, melts and CO2-rich fluids from a subducting sediment-rich oceanic plate may cause the metasomatism of the overlying lithospheric mantle wedge (Peacock, 1993). Trace element analyses of selected glasses were made to evaluate possible relations between metasomatism and subduction processes. Ba, Rb, K,

Magmatic infiltration and melting in the lower crust and upper mantle beneath the Cima volcanic field, California

Xenoliths of lower crustal and upper mantle rocks from the Cima volcanic field (CVF) commonly contain glass pockets, veins, and planar trains of glass and/or fluid inclusions in primary minerals. Glass pockets occupy spaces formerly occupied by primary minerals of the host rocks, but there is a general lack of correspondence between the composition of the glass and that of the replaced primary minerals. The melting is considered to have been induced by infiltration of basaltic magma and differentiates of basaltic magma from complex conduits formed by hydraulic fracturing of the mantle and crustal rocks, and to have occurred during the episode of CVF magmatism between ∼7.5 Ma and present. Variable compositions of quenched melts resulted from mixing of introduced melts and products of melting of primary minerals, reaction with primary minerals, partial crystallization, and fractionation resulting from melt and volatile expulsion upon entrainment of the xenoliths. High silica melts ( >∼60% SiO2) may result by mixing introduced melts with siliceous melts produced by reaction of orthopyroxene. Other quenched melt compositions range from those comparable to the host basalts to those with intermediate Si compositions and elevated Al, alkalis, Ti, P, and S; groundmass compositions of CVF basalts are consistent with infiltration of fractionates of those basalts, but near-solidus melting may also contribute to formation of glass with intermediate silica contents with infiltration only of volatile constituents.

MAC88105—A regolith breccia from the lunar highlands: Mineralogical, petrological, and geochemical studies

The new large lunar meteorite MAC88105 is a dense breccia, with lithic and mineral clasts and fragments set into a welded matrix. It is a regolith breccia which shows some recrystallization and evidence for a late shock event during which anorthositic glass veins were formed. Shock effects (most probably due to the impact ejection from the moon) are present throughout the sample and require a shock pressure of about 25–30 GPa, in agreement with observations made on other lunar meteorites. Some components of MAC88105 have been subjected to a shock pressure of about 40–45 GPa as evident from melt pockets in a clast. The population of lithic clasts in MAC88105 is similar to other lunar highland breccias. Vitric breccias are more abundant than granulitic breccias and plutonic rock fragments. The presence of devitrified glass (spheres and shards) supports a regolith origin. Most common are metameltbreccias consisting of abundant anorthitic plagioclase clasts and a dense, fine-grained matrix. Some fine-grained hornfelsic to granulitic metabreccias are also present. Lithic clast compositions are predominantly anorthositic noritic (or noritic anorthositic), and anorthositic troctolitic. Spinel-bearing rocks are present, but do not belong to the spinel-troctolite group. A spinel-bearing clast in MAC88105 consists of anorthite + pigeonite + spinel and indicates a different heritage, possibly similar to spinel cataclasites described from Apollo 17. The pyroxenes in MAC88105 have somewhat unusual compositions; orthopyroxenes and augites appear to be rare. Most lithologies (breccias and igneous rocks) contain solely pigeonite. We have found one igneous mafic rock, a gabbro, which has a low mg-number and is possibly of mare origin. A metal grain of a composition similar to metal in H chondrites supports the interpretation of MAC88105 as a regolith breccia. The bulk composition of MAC88105 is similar to the other lunar highland meteorites. The REE contents are slightly higher than for the other anorthositic meteorites, with a smaller positive Eu anomaly. This is in agreement with a possible KREEP contribution, possibly introduced from glasses. MACS 8105 is a mixture of rocks with a predominant contribution from the anorthosite suite and small admixtures from other rock types (mare basalt, Mg suite), which becomes obvious in plots of molar mg or Sm content vs. the Ti Sm ratio. The siderophile element abundances in MAC88105 are similar to other highland meteorites and show the characteristic Co excess and a Au Ir ratio similar to the other anorthositic meteorites and lower than the hyperchondritic ratio in Apollo 16 rocks, thus support the notion that the lunar meteorites are a more representative sample of the lunar highlands than the Apollo samples. The chemistry and mineralogy of MAC88105 is different from that of the other lunar meteorites and suggests a different source, which is supported by cosmic-ray and noble gas data. At this time it seems likely that about four individual impact events have been responsible for delivering the seven highland meteorites. MAC88105 and other lunar meteorites are important as they are probably random samples from the lunar surface and may thus be more representative than the Apollo and Luna rocks which come from only a small area of the moon.

Impact melting of the Cachari eucrite 3.0 Gy ago

Both the host phase and glass veins of the Cachari eucrite have been analyzed by microprobe and neutron activation analysis for their chemical compositions and by mass spectrometry for their 39Ar- 40Ar gas retention ages. Cachari is chemically similar to other non-cumulate eucrites. The vesicular glass veins vary from pure glass, to devitrified glass, to areas that are substantially crystalline. The glassy areas have nearly the same concentrations of major and trace elements as the unmelted portions of Cachari, but some differences, probably due to preferential dissolution, occur along melt contacts. The glass formed by shock melting of Cachari host or of rock identical to it. 39Ar- 40Ar data for the host and glass suggest distinctly different ages of 3.04 ±.07 Gy and 3.47 ±.04 Gy, respectively. The time of glass formation, which may also be the time of brecciation, is most likely given by the 3.0 Gy age of the host. The higher age for the glass is interpreted to represent incomplete Ar degassing during the 3.0 Gy event due to the greater resistance to Ar diffusion shown by the glass compared to the host. Event ages significantly younger than 4.5 Gy have now been determined for several eucrites and howardites and suggest a long dynamic regolith history for the parent body.

Structure and Petrology of a Cumulus Norite Boulder Sampled by Apollo 17 in Taurus-Littrow Valley, the Moon

A glass-coated half-meter-size boulder was sampled by the Apollo 17 crew at station 8 near the foot of the Sculptured Hills. The rock proved to be a coarse-grained (0.5-cm) plagioclase-orthopyroxene cumulate, and the samples are the only true norites returned from the lunar surface. Photographs of the boulder showed it to contain at least nine structural surfaces and four glass veins. Orientation and inspection of three of the returned samples resulted in the identification of six surfaces and one vein. One of the structural surfaces visible in the boulder was identified as primary cumulus planar lamination, which was folded through an angle of at least 35° between two oriented samples, whereas fracture sets representing the other surfaces were coincident. The boulder is believed to be a sample of the deeper highlands or submare lunar crust, derived from a depth of 8 to 30 km and somewhat shock-metamorphosed during at least two excavation events. The chemical composition of the norites, when determined, should be of special interest in view of the large amount of literature concerning glass, cataclasite, hornfels, and “basalt” of noritic composition returned by other Apollo missions. However, the cumulus texture of the boulder precludes its being representative of any magmatic liquid composition, suggests that the lunar crust is heterogeneously layered, and that plagioclase sank, not floated, in magmatic liquids that formed the lunar crust.