Bulk-rock Analyses Research Articles

Chemical Petrology of Eclogitic Lenses in Serpentinite, Gruppo di Voltri, Ligurian Alps

Eclogitic lenses of metabasaltic composition-some units as much as 4 km in length and more than 200 m in thickness-are surrounded by and interlayered with deformed, completely serpentinized Beigua peridotite of the Voltri Massif. Relict primary textures indicate that most of the mafic layers were originally gabbros, but some units may represent finer-grained diabasic dikes; a few apparently were converted to rodingites prior to the high-pressure metamorphism. The mafic rocks have been subjected to a complex recrystallization history which involved both an earlier stage (A) and a later stage (B) production of the eclogitic assemblage garnet + omphacite + rutile, separated in time by a period of severe cataclasis. Subsequent recrystallization led to the formation of (C) garnet + glaucophane rocks, then (D) barrositic amphibole-bearing assemblages, and finally, through a stage of prasinitization (E), to the typical greenschist assemblage of albite + chlorite + epidote + actinolite + sphene. Some outcrops preserve evidence of the entire paragenetic sequence; green -schistic assemblages characteristically occur along the contact of the metagabbros with serpentinite, and more eclogitic assemblages are preserved in the interior portions of the mafic lenses. Of 95 specimens studied in detail, XRF bulk rock analyses have been obtained for 83. Combined with petro-graphic data, it is clear that most of the mafic units are especially titanium- and iron-rich compared to common terrestrial rocks of metabasaltic composition. The greenschists and barroisitic amphibo-lites exhibit much greater chemical diversity than do the parental eclogites. This means that either a more heterogeneous leucogabbro protolith was present in addition to the ferrogabbroic layers, and that this more magnesian rock type was selectively converted to the more hydrous, prasinitic assemblages, or that intense metasomatism has affected the later stage, lower grade assemblages. The 37 analyzed eclogitic rocks are relatively homogeneous in terms of their major element chemistry (e.g., ranges between 0.01 and 0.19 wt%), and they compare favorably with some ferro-grabbroic oceanic tholeiites. The parental gabbros were much too iron-rich to have been generated in situ from the enclosing Beigua ultramafic body. Accordingly, it is hypothesized that, at the onset of rifting of the lithosphere, mafic magma-derived elsewhere and already at an advanced stage of differentiation-was injected as a series of layers into relatively primitive mantle material. Relics of an early high-pressure, blue-schist-type recrystallization are preserved in the spatially associated "calcescisti," including rocks of the Sestri-Voltaggio zone and the Savona-type continental basement, hence this entire terrane may have been involved in an early Alpine( ?) subduction event. In contrast, the tectonically higher, deformed, plagioclase-bearing Erro-Tobbio lherzolites and derived serpentinites of the northern Voltri Massif contain only feebly metamorphosed gabbros lacking a high-pressure paragenesis. Apparently this latter unit was brought into juxtaposition with the underlying blueschist + eclogite-bearing terrane after the inferred subduction event.

Densities of fertile and sterile garnet peridotites

Densities of a fertile peridotite (PHN 1611) and a depleted sterile peridotite (PHN 1569) have been calculated to be 3.39 and 3.30, respectively. These densities were calculated from the cell dimensions of the component minerals, the mineral analyses, and the bulk rock analyses. Studies of kimberlite nodules suggest that lighter, sterile peridotites overlie heavier, more fertile peridotites uniformly in the upper mantle beneath southern Africa. A temperature difference of 500°C reduces the density of fertile peridotite PHN 1569 to approximately 3.33, which is insufficient to cause it to float in sterile peridotite. However, if fertile peridotite PHN 1569 undergoes 25% partial melting and the garnet is dissolved, its zero pressure density would be reduced to approximately 3.2, which is less than that of depleted, sterile mantle.

Some Aleutian Andesites: Their Nature and Source

Selected Late Tertiary and Quaternary lavas from the Aleutian localities of Adak, Great Sitkin, Cold Bay, and Amak have been studied. Bulk rock analyses as well as electron probe analyses of the major phases are presented. The lavas, basaltic andesites, and andesites are characterized by high $$SiO_{2}, Al_{2}O_{3}, K_{2}O, Sr$$, and Ba and low MgO, Ni, Cr, Co, and Yb (relative to ocean ridge tholeiites). Anorthite-rich plagioclase, magnetite, and large clinopyroxenes typify the phenocryst assemblage, while, in general, orthopyroxene occurs in those lavas possessing more than about 51.5 wt % $$SiO_{2}$$ and olivine replaces it in the less silicic lavas. No alkali feldspar, ilmenite, or hydrous phases occur in these lavas. A xenocryst of mantle-like olivine was analyzed in one lava. The limited phenocryst zoning and small phenocryst-groundmass compositional contrast suggests crystallization within perhaps 10 km of the earth's surface. The suggested sequence of crystallization is plagioclase, olivine and/or orthopyroxene, magnetite, and clinopyroxene, which is consistent with an oxygen fugacity similar to that denned by the Ni-NiO buffer. The plagioclase phenocrysts are relatively unzoned or slightly reversed in zoning, suggesting that crystallization took place under relatively high temperature (~1,200°C), low pressure, and essentially anhydrous conditions. Since addition of water to a melt greatly reduces the thermal stability of plagioclase and promotes the appearance of olivine (replacing orthopyroxene and becoming the liquidus phase) in even the more silica-rich lavas, the magma from which these lavas were produced probably contained less than 2.0 wt % water. This implies that hydrous melting of mantle peridotite is precluded as a source for these lavas. A more likely source is the subducted oceanic crust. The distinctly low $$TiO_{2}$$ contents typical of andesitic lavas may reflect the stability of rutile in the subducted eclogite assemblage under partial melting conditions. The well-known island arc relationship between $$K_{2}O$$ and depth to the Benioff zone may arise from the buffering of this component by sanidine in the eclogite assemblage.

Chloritoid-bearing metapelites associated with glaucophane rocks in western Crete, Greece

In western Crete, Greece, a widespread occurrence of chloritoid-bearing metapelites with the main mineral assemblage chloritoid-phengitic white mica-Fe-rich chlorite-quartz was recorded to form the country rock of glaucophane-bearing metabasalts. Six bulk rock analyses of the metapelites conform to the compositional restrictions evaluated by Hoschek (1967) for the formation of chloritoid. Three microprobe analyses revealed chloritoid compositions low in Mg and Mn, and, consequently, high in Fe. The metamorphic grade documented in the metapelites is obviously related to a subsequent prograde metamorphism by which, in the adjacent meta-basalts, epidote is formed at the expense of lawsonite. No relict of a high-P, low-T assemblage, in part well preserved in the meta-basalts, was recognized in the chloritoid schists. The significance of the metamorphic history is briefly discussed.

Silicate melt inclusions and glasses in lunar soil fragments from the Luna 16 core sample

Abstract More than 2000 fragments were studied microscopically, and electron microprobe analyses were made of 39 selected areas, from a few square mm of polished surface, through 75- to 425-μm fragments of lunar soil from two samples of the Luna 16 core. The silicate melt inclusions and glasses differ in important details from those observed earlier in the Apollo samples. Melt inclusions in olivine contain epitaxially oriented daughter crystals, but also show a similar epitaxy around the outside of the crystals not observed in previous lunar samples. Melt inclusions in ilmenite suggest trapping at successive stages in a differentiation sequence. There is abundant evidence for late-stage silicate liquid immiscibility, with melt compositions similar but not identical to those from Apollo 11 and 12. A comparison of the alkali ratio of any given bulk rock analysis with that of its late-stage, high-silica melt shows gross differences for different rocks. This is pertinent to understanding late-stage differentiation processes. Glass fragments and spherules exhibit a wide range of crystallization textures, reflecting their wide range of compositions and cooling histories. No significant differences were found between the two portions of core examined (Zones A and D).

Subsolidus Reduction of Lunar Spinels

IT is now generally accepted that lunar basalts crystallized at extremely low oxygen fugacities. This contention is supported by the ubiquitous occurrence of metallic iron, metallic Ni–Fe alloys and the absence of Fe+3 in bulk rock analyses. Electrolytic cell measurements on the Apollo 12 basalts1 indicate crystallization oxygen fugacities (f02) at 1,200° C between 10−10.8 and 10−12.8 atm. The evidence presented in this communication shows that some basalts must have exceeded the lower limit by several orders of magnitude in fO2 and is based primarily on the decomposition of Cr-Al-ulvospinel; it is further supported in one case by the decomposition of olivine. Optical and analytical data on the opaque phase mineralogy of ulvospinel, chromite and ilmenite are presented in this communication and thermodynamic calculations relating to the decomposition of Fe+2-bearing spinels are considered. These data suggest that some rocks have undergone intense non-equilibrium subsolidus reduction. The reduction phenomenon is widespread and is considered to have developed either during initial deuteric cooling or as a result of a post-crystallization reduction event.