Grey Breccia Research Articles

Compositional analysis of Apollo 12 Granitic Breccia 12013: Insights into protoliths and formation

We report bulk rock composition and mineral chemistry of previously unstudied fragments and thin sections of lunar granitic breccia 12013. Instrumental Neutron Activation Analysis data on 25 fragments indicate that the 12013 breccia can be described as a three-component system comprising granitic, rare earth element (REE)-rich, and mafic components. The granitic component is low in FeO and REE but rich in K2O, Th, and associated incompatible elements. The REE-rich component has an elevated concentration of REE, moderate FeO, and low concentrations of those elements that are high in the granitic component, in other words, its composition is complementary to that of the granitic component. The mafic component is richest in FeO, and low in those elements that are concentrated in the granitic and REE-rich components. Petrographically, the REE-rich component is an impact melt breccia with a composition unique to Apollo 12 impact melt breccias. Trace-element concentrations in the REE-rich component indicate that its protolith is possibly monzogabbro, although its composition is significantly more magnesian than other known lunar monzogabbros. The mafic component is dominated by fine-grained pyroxene and plagioclase that appear to have recrystallized after impact. However, preserved lithic clasts with an igneous texture occur in the mafic component and are inferred to represent its protolith. The textures of the preserved mafic lithic fragments and lack of Mg-Fe zoning in pyroxene indicate that the protolith of the mafic component formed either as a crustal intrusion or at the bottom of a thick lava flow. The composition of the mafic component is unlike any previously studied mare basalt samples. Textures of this complex breccia suggest that the granitic breccia was first incorporated with the mafic component to form the “gray breccia”, and that the gray breccia was then incorporated with the REE-rich component while the components were still in a hot, plastic state. The complementary trace-element compositions, including the REE patterns, indicate that the petrogenesis of the granitic and REE-rich components are related. We infer that silicate-liquid immiscibility is likely not the formation mechanism for the 12013 components. Instead, bimodal volcanism owing to basaltic underplating may have led to the formation of the 12013 components.

Editorial

Editorial

Effectiveness of Preservative Treatments on Coloured Ruditic Building Stones

Abstract: The dark grey Breccia di Billiemi and the Ammonitico Rosso of Piana degli Albanesi, two coloured stones widely used in monuments both outdoor and indoor in Sicily, undergo significant decay as effect of weathering: colour change and development of surface microcracks which affect not only the chromatic aspect but also the state of aggregation. In a previous study (Fracture and Failure of Natural Building Stones, 2006, Springer, Dordrecht, The Netherlands: 329), such effects were successfully reproduced on laboratory samples by means of artificial accelerated weathering and the different response of veins and grains to the ageing agents was related to the microstructure of stones. In this study, the effect of accelerated washout and acid etching is evaluated both on untreated samples and after the application of four commercial preservative products. The development of decay phenomena is monitored by means of weight measurements and image analysis, coupled with optical microscopy, the last evidencing a selectivity in the effectiveness of treatments on the different textural components of the lithotypes. The effect of treatments on the stones is also investigated by contact angle measurements on different areas.

Bone breccias, bone dumps, and sedimentary sequences of the western Limeworks, Makapansgat, South Africa

The sediments in the western side of the Makapansgat Limeworks were either precipitated as speleothems, represented in the earlier massive deposits, or were deposited as coarsening clastic sediments, mainly representing later deposits. Between the earlier deposits and the main sedimentary phase, the stratigraphic sequence was inverted twice to a considerable height by the unusual deposition of subaqueous speleothem. Bone-bearing deposits, including the Main Quarry Bone Breccia and the well-known Grey Breccia belong, in time, to the lower part of the clastic deposits called the Red Silts. Australopith fossils have been found in the Grey Breccia dumped material and, in situ, from the dolomite clast breccia on the Main Quarry entrance buttress. Whatever the problems may be in provenancing some of the material from the Limeworks dumps, there is no doubt that the three rows of blocks on the southern side of the dumps belong to the Grey Breccia, and other rows contain red sediment sufficient for them to be safely associated with the Red Silts. There is no reason why this material should not be prepared with confidence as to its stratigraphic provenance. In any case, stratigraphic evidence, presented here, shows that there is little difference in time between the deposition of the bone-bearing breccias.

Feldspathic clasts in Yamato-86032: Remnants of the lunar crust with implications for its formation and impact history

Low concentrations of Th and Fe in the Yamato (Y)-86032 bulk meteorite support earlier suggestions that Y-86032 comes from a region of the moon far distant from the Procellarum KREEP Terrain (PKT), probably from the lunar farside. 39Ar– 40Ar, Rb–Sr, Sm–Nd, and Sm-isotopic studies characterize the chronology of Y-86032 and its precursors in the mega regolith. One of the rock types present in a light gray breccia lithology is an anorthosite characterized by plagioclase with An ∼93, i.e., more sodic than lunar FANs, but with very low 87Rb/ 86Sr and 87Sr/ 86Sr similar to those of FANs. (FAN stands for Ferroan Anorthosite). This “An93 anorthosite” has Nd-isotopic systematics similar to those of nearside norites. A FAN-like “An97 anorthosite” is present in a second light-colored feldspathic breccia clast and has a more negative ε Nd value consistent with residence in a LREE-enriched environment as would be provided by an early plagioclase flotation crust on the Lunar Magma Ocean (LMO). This result contrasts with generally positive values of ε Nd for Apollo 16 FANs suggesting the possibility of assymetric development of the LMO. Other possible explanations for the dichotomy in ε Nd values are advanced in the text. The Y-86032 protolith formed at least 4.43 ± 0.03 Ga ago as determined from a Sm–Nd isochron for mineral fragments from the breccia clast composed predominantly of An93 anorthosite and a second clast of more varied composition. We interpret the mineral fragments as being predominatly from a cogenetic rock suite. An 39Ar– 40Ar age of 4.36–4.41 ± 0.035 Ga for a third clast composed predominantly of An97 anorthosite supports an old age for the protolith. Initial 143Nd/ 144Nd in that clast was −0.64 ± 0.13 ε-units below 143Nd/ 144Nd in reservoirs having chondritic Sm/Nd ratios, consistent with prior fractionation of mafic cumulates from the LMO. A maximum in the 39Ar– 40Ar age spectrum of 4.23 ± 0.03 Ga for a second sample of the same feldspathic breccia clast probably reflects some diffusive 40Ar loss. Lack of solar wind and lunar atmosphere implanted Ar in the light gray breccia clast allows determination of an 39Ar/ 40Ar age of 4.10 ± 0.02 Ga, which is interpreted as the time of initial brecciation of this litholgy. After correction for implanted lunar atmosphere 40Ar, impact melt and dark regolith clasts give Ar ages of 3.8 ± 0.1 Ga implying melt formation and final breccia assembly ∼3.8 Ga ago. Some breccia lithologies were exposed to thermal neutron fluences of ∼2 × 10 15 n/cm 2, only about 1% of the fluence experienced by some other lunar highlands meteorites. Other lithologies experienced neutron fluences of ∼1 × 10 15 n/cm 2. Thus, Y-86032 spent most of the time following final brecciation deeply buried in the megaregolith. The neutron fluence data are consistent with cosmogenic 38Ar cos cosmic ray exposure ages of ∼10 Ma. Variations among differing lithologies in the amount of several regolith exposure indicators, including cosmogenic noble gas abundances, neutron capture induced variations in Sm isotopic abundances, and Ir contents, are consistent with a period of early (>∼3.8 Ga ago) lunar regolith exposure, subsequent deep burial at >∼5 m depth, and ejection from the moon ∼7–10 Ma ago.

Re-examination of the lower stratigraphy in the classic section, limeworks site, Makapansgat, South Africa

At the end of Member 2 times, remnant mammillary-form stalactites in the roof and wall contacts show that an irregular wedge-shaped space existed from the Classic Section out to the Cone area. It was into this space that hyenas and other denning animals were able to gain access from an as yet unfilled Cone area. During the mining operations of the 1920’s, Eitzman (1958) recollects entering an impressive bone breccia resembling a charnel house. We suggest that it is most probable that the Grey Breccia corresponds to his main breccia as only the area on top of Member 2, between the dolomite walls, roof and stalagmite boss has the required volume. It also lies at the end of an incline first dug by the miners, (still in existence), and at the end of which he was able to note some of the deposits. The bone breccias at the same level at the back of the Cone suggest that other dens were created at about this time.

Nature and origin of cone-forming volcanic breccias in the Te Herenga Formation, Ruapehu, New Zealand

Volcanic breccias form large parts of composite volcanoes and are commonly viewed as containing pyroclastic fragments emplaced by pyroclastic processes or redistributed as laharic deposits. Field study of cone-forming breccias of the andesitic middle Pleistocene Te Herenga Formation on Ruapehu volcano, New Zealand, was complemented by paleomagnetic laboratory investigation permitting estimation of emplacement temperatures of constituent breccia clasts. The observations and data collected suggest that most breccias are autoclastic deposits. Five breccia types and subordinate, coherent lava-flow cores constitute nine, unconformity-bounded constructional units. Two types of breccia are gradational with lava-flow cores. Red breccias gradational with irregularly shaped lava-flow cores were emplaced at temperatures in excess of 580 °C and are interpreted as aa flow breccias. Clasts in gray breccia gradational with tabular lava-flow cores, and in some places forming down-slope-dipping avalanche bedding beneath flows, were emplaced at varying temperatures between 200 and 550 °C and are interpreted as forming part of block lava flows. Three textural types of breccia are found in less intimate association with lava-flow cores. Matrix-poor, well-sorted breccia can be traced upslope to lava-flow cores encased in autoclastic breccia. Unsorted boulder breccia comprises constructional units lacking significant exposed lava-flow cores. Clasts in both of these breccia types have paleomagnetic properties generally similar to those of the gray breccias gradational with lava-flow cores; they indicate reorientation after acquisition of some, or all, magnetization and ultimate emplacement over a range of temperatures between 100 and 550 °C. These breccias are interpreted as autoclastic breccias associated with block lava flows. Matrix-poor, well-sorted breccia formed by disintegration of lava flows on steep slopes and unsorted boulder breccia is interpreted to represent channel-floor and levee breccias for block lava flows that continued down slope. Less common, matrix-rich, stratified tuff breccias consisting of angular blocks, minor scoria, and a conspicuously well-sorted ash matrix were generally emplaced at ambient temperature, although some deposits contain clasts possibly emplaced at temperatures as high as 525 °C. These breccias are interpreted as debris-flow and sheetwash deposits with a dominant pyroclastic matrix and containing clasts likely of mixed autoclastic and pyroclastic origin. Pyroclastic deposits have limited preservation potential on the steep, proximal slopes of composite volcanoes. Likewise, these steep slopes are more likely sites of erosion and transport by channeled or unconfined runoff rather than depositional sites for reworked volcaniclastic debris. Autoclastic breccias need not be intimately associated with coherent lava flows in single outcrops, and fine matrix can be of autoclastic rather than pyroclastic origin. In these cases, and likely many other cases, the alternation of coherent lava flows and fragmental deposits defining composite volcanoes is better described as interlayered lava-flow cores and cogenetic autoclastic breccias, rather than as interlayered lava flows and pyroclastic beds. Reworked deposits are probably insignificant components of most proximal cone-forming sequences.

Some Fossil Chelonian Fragments from Makapansgat

THE fossil chelonian material loaned to me by Prof. R. Dart, of the University of the Witwatersrand Medical School, for examination consists of 36 fragments from the early Middle Pleistocene grey breccia of Makapansgat Limeworks in the Transvaal. This fossil material was compared with complete and fragmented skeletal material of Testudo pardalis babcocki, Kinyxs belliana belliana and Pelusios sinuatus, all of which occur in the area today. The fossil fragments were all from large animals and none was referable to the genera Chersina, Psammopbates, Homopus or Pelomedusa, which were also available for comparison.

An ‘Australopithecine’ Scoop from Herefordshire

SHORTLY before Christmas last year, I was demonstrating a series of curious scoop-like cannon-bones recovered from the Makapansgat grey breccia during the past year to a group of visitors to show what looked like digging tools when one of the party, Mr. J. W. F. Hampton, said laughingly : “or apple-corers”.

The first australopithecine fragment from the Makapansgat pebble culture stratum.

DURING October 1954, Mr. C. K. Brain1 discovered 129 trimmed, utilized or damaged stones in situ in the 18-ft. red, gravel-bearing, sandy layer 25 ft. above the australopithecine-carrying grey breccia at the Makapansgat Limeworks site. Of these, seventeen were claimed by Prof. C. van Riet Lowe to be undoubted Developed Kafuan artefacts, fifteen being of deeply leached dolomite, one of vein quartz and one of dark grey quartzite.