W. I. Ridley

Mineral chemistry of peridotite xenoliths from the Lashaine volcano, Tanzania

Eight garnet peridotite and ten garnet-free peridotite xenoliths from the Lashaine volcano in northern Tanzania, and one garnet peridotite from the Matsoku pipe in Lesotho, were studied petrographically and by electron-microprobe techniques. The primary assemblages are ol + opx + cpx + ga, ol + opx + cpx + chr, ol + opx + ga + chr, ol + opx + ga, ol + opx + chr, and ol + cpx + chr. With the exception of the ol + cpx + chr assemblage, which is more iron-rich, the composition of each of the primary mineral species is essentially the same for all the xenoliths and is independent of the abundances of the minerals. The primary minerals are Ni-rich very low Ca olivine (Fo92), low Ca, Al, Cr, Ti, Mn,enstatite (Wo1En93Fs7), low Al, Ti, Mn chrome diopside (WO44En52Fs4), chrome pyrope and Mg, Al-rich chromite. Comparisons with phase equilibria studies suggest that the primary assemblages are stable at approximately 1050°C and 50 kbar, equivalent to a depth of approximately 150 km. The primary xenolith assemblages have been locally altered by three types of secondary processes. (1) Garnet has reacted with the adjacent olivine to produce reaction rims of aluminous orthopyroxene plus aluminous clinopyroxene plus spinel. (2) Some chrome-diopsides show marginal or total alteration that may result from secondary melting. (3) Extremely localized partial melting has occurred at some grain boundaries and the interstitial melt quenched to various combinations of olivine, clinopyroxene, orthopyroxene, spinel, phlogopite and glass. Garnet-bearing and garnet-free assemblages differ in bulk composition but have formed under similar physical conditions. The xenoliths derive from a region of the upper mantle that is homogeneous in terms of mineral compositions but heterogeneous, at least on the scale of the Lashaine xenoliths, in mineral proportions and bulk composition. Phlogopite in one garnet peridotite appears to be primary and several peridotites contain regions that have formed by localized melting of K-rich areas. Either the peridotites are not refractory residues from partial melting in the mantle or K-rich material has been added to them after partial melting but within the mantle, before incorporation into the ankaramite host.

The Igwisi Hills extrusive 'kimberlites'

Abstract The Igwisi Hills are a group of volcanic hills in Tanzania where a unique series of eruptions has produced volcanic rocks that have many of the features of kimberlites. The rocks are “igneous conglomerates” that are characterized by the presence of abundant ellipsoids of olivine in a fine grained matrix. The matrix contains major amounts of carbonate, serpentine, and complexly zoned MgAlCr spinels, minor perovskite, apatite and limonite. Large magnesian olivines are highly rounded and some have partially recrystallized to more Fe-rich and Ni-poor subhedral to euhedral grains. Several olivine ellipsoids are partly or wholly rimmed by a black coating consisting of perovskite, and MgAl spinel. Mineral inclusions within the olivines include chrome pyrope, similar in composition to garnets in kimberlites, low Al enstatite, low Al magnesian chrome diopside, MgAl chromite, and a highly magnesian phlogopite ( Mg Mg + Fe ∼.94 ). All of these are apparently primary phases in equilibrium with olivine. The total assemblage is similar in many respects to that found in garnet peridotite xenoliths such as those from the Lashaine volcano in northern Tanzania. The Igwisi irruptives apparently contain material derived from phlogopite-bearing, garnet peridotites with a primary mineral assemblage (assuming that all these phases coexist at depth) indicative of equilibrium at upper mantle temperatures and pressures. This primary assemblage was disrupted and brought rapidly to the surface in a gas-charged, carbonate-rich fluid. Rapid upward transport, extrusion, and rapid cooling have preserved inclusions of upper mantle ultramafics in a carbonate-rich matrix and have tended to prevent reaction between inclusions and matrix that might otherwise have yielded a more typical kimberlite.

Apollo 15 green glasses.

Abstract Apollo 15 breccia 15427 and soils 15101, 15261 and 15301 contain abundant spheres and fragments of a green glass that is remarkably constant in composition. The glass is rich in Fe and Mg, and low in Ti, unlike any known lunar basalt, and may be derived from material of pyroxenitic composition in the Apennine Front.

Highly aluminous glasses in lunar soils and the nature of the lunar highlands.

Abstract Approximately 25 per cent of the glasses in two Apollo 14 soil samples and in the soils at two levels in the Luna 16 core have compositions equivalent to anorthositic gabbro. Reassessment of the non-mare glass components in the Apollo 11 and 12 soils shows that glasses with the composition of anorthositic gabbro are common to both; gabbroic anorthosite glasses are less common, and anorthositic glasses, rare. Anorthositic gabbro glasses have the same major element composition at all four sites, and resemble the Surveyor 7 analysis from a ‘highland’ site. Thus, strong presumptive evidence exists that material with this specific composition is abundant in the lunar highlands.

Luna 20 soil - Abundance and composition of phases in the 45-125 micron fraction.

Abstract Glass compositions in the Luna 20 soil indicate a minor contribution of mare rocks and a major contribution of highly feldspathic highland material. Glasses with the composition of Highland basalt (anorthositic gabbro or norite) predominate in a range of highly aluminous glasses. The analyses of minerals in the soil show that the highland rocks have a unique assemblage of minerals that can readily be distinguished from the mineral assemblages of either mare or KREEP basalts. The soils are characterized by abundant anorthitic (An 92–99 ), low-Fe plagioclase. Highly magnesian orthopyroxenes, pigeonites and augites are the most prominent pyroxenes. Unlike mare basalt pyroxenes, clinopyroxenes with intermediate Ca values are not abundant, but extreme iron enrichment towards pyroxferroite does occur. Olivines are more abundant than at other sites and are Mg-rich, low in Ca and Cr. Spinels with compositions approaching MgAl 2 O 4 predominate over pleonastes and chromites. Ilmenite and metal are present but not abundant. These data establish the unique nature of the minerals in the highland soils. The mineral compositions are consistent with derivation from a suite of highly feldspathic rocks in which Highland basalt compositions predominate. Some of the mineral data, particularly from the pyroxenes, are suggestive of surface or near-surface processes, rather than plutonic crystallization.

Major element composition of Luna 20 glasses

Abstract Ten percent of the 50–150 micron size fraction of Luna 20 soil is glass. A random suite of 270 of these glasses has been analyzed by electron microprobe techniques. The major glass type forms a strong cluster around a mean value corresponding to Highland basalt (anorthositic gabbro) with 70% normative feldspar. Minor glass groups have the compositions of mare basalts and of low-K Fra Mauro type basalts. The glass data indicate that Highland basalt is the major rock type in the highlands north of Mare Fecunditatis.

Major element chemistry of glasses in Apollo 14 soil 14156

Abstract Glasses in a soil sample (14156) from the middle layer of the trench at the Fra Mauro landing site show a wide range of compositions clustered around certain preferred compositions. Ninety per cent of the glasses are of two major types—Fra Mauro basalt (63 per cent) with high K and 17 weight per cent Al 2 O 3 and Highland basalt or anorthositic gabbro (27 per cent) with low K and 25–26 weight per cent Al 2 O 3 . The glass population is almost identical with that of the comprehensive soil 14259 ( Apollo Soil Survey , 1971).

16 – THE IGWISI HILLS EXTRUSIVE “KIMBERLITES”

The Igwisi Hills are a group of volcanic hills in Tanzania where a unique series of eruptions has produced volcanic rocks that have many of the features of kimberlites. The rocks are “igneous conglomerates” that are characterized by the presence of abundant ellipsoids of olivine in a fine grained matrix. The matrix contains major amounts of carbonate, serpentine, and complexly zoned Mg–Al–Cr spinels, minor perovskite, apatite and limonite. Large magnesian olivines are highly rounded and some have partially recrystallized to more Fe-rich and Ni-poor subhedral to euhedral grains. Several olivine ellipsoids are partly or wholly rimmed by a black coating consisting of perovskite, and Mg–Al spinel. Mineral inclusions within the olivines include chrome pyrope, similar in composition to garnets in kimberlites, low Al enstatite, low Al magnesian chrome diopside, Mg–Al chromite, and a highly magnesian phlogopite (Mg/Mg + Fe ~ .94). All of these are apparently primary phases in equilibrium with olivine. The total assemblage is similar in many respects to that found in garnet peridotite xenoliths such as those from the Lashaine volcano in northern Tanzania. The Igwisi irruptives apparently contain material derived from phlogopite-bearing, garnet peridotites with a primary mineral assemblage (assuming that all these phases coexist at depth) indicative of equilibrium at upper mantle temperatures and pressures. This primary assemblage was disrupted and brought rapidly to the surface in a gas-charged, carbonate-rich fluid. Rapid upward transport, extrusion, and rapid cooling have preserved inclusions of upper mantle ultramafics in a carbonate-rich matrix and have tended to prevent reaction between inclusions and matrix that might otherwise have yielded a more typical kimberlite.

36 – MINERAL CHEMISTRY OF PERIDOTITE XENOLITHS FROM THE LASHAINE VOLCANO, TANZANIA

Eight garnet peridotite and ten garnet-free peridotite xenoliths from the Lashaine volcano in northern Tanzania, and one garnet peridotite from the Matsoku pipe in Lesotho, were studied petrographically and by electron-microprobe techniques. The primary assemblages are ol + opx + cpx + ga, ol + opx + cpx + chr, ol + opx + ga + chr, ol + opx + ga, ol + opx + chr, and ol + cpx + chr. With the exception of the ol + cpx + chr assemblage, which is more iron-rich, the composition of each of the primary mineral species is essentially the same for all the xenoliths and is independent of the abundances of the minerals. The primary minerals are Ni-rich very low Ca olivine (Fo92), low Ca, Al, Cr, Ti, Mn enstatite (Wo1En93Fs7), low Al, Ti, Mn chrome diopside (WO44En52Fs4), chrome pyrope and Mg, Al-rich chromite. Comparisons with phase equilibria studies suggest that the primary assemblages are stable at approximately 1050°C and 50 kbar, equivalent to a depth of approximately 150 km. The primary xenolith assemblages have been locally altered by three types of secondary processes. (1) Garnet has reacted with the adjacent olivine to produce reaction rims of aluminous orthopyroxene plus aluminous clinopyroxene plus spinel. (2) Some chrome-diopsides show marginal or total alteration that may result from secondary melting. (3) Extremely localized partial melting has occurred at some grain boundaries and the interstitial melt quenched to various combinations of olivine, clinopyroxene, orthopyroxene, spinel, phlogopite and glass. Garnet-bearing and garnet-free assemblages differ in bulk composition but have formed under similar physical conditions. The xenoliths derive from a region of the upper mantle that is homogeneous in terms of mineral compositions but heterogeneous, at least on the scale of the Lashaine xenoliths, in mineral proportions and bulk composition. Phlogopite in one garnet peridotite appears to be primary and several peridotites contain regions that have formed by localized melting of K-rich areas. Either the peridotites are not refractory residues from partial melting in the mantle or K-rich material has been added to them after partial melting but within the mantle, before incorporation into the ankaramite host.