Mamoru Hayashi

Plate Tectonics at 3.8–3.7 Ga: Field Evidence from the Isua Accretionary Complex, Southern West Greenland

A 1:5000 scale mapping was performed in the Isukasia area of the ca. 3.8‐Ga Isua supracrustal belt, southern West Greenland. The mapped area is divided into three units bounded by low‐angle thrusts: the Northern, Middle, and Southern Units. The Southern Unit, the best exposed, is composed of 14 subunits (horses) with similar lithostratigraphy, bound by layer‐parallel thrusts. Duplex structures are widespread in the Isua belt and vary in scale from a few meters to kilometers. Duplexing proceeded from south to north and is well documented in the relationship between link‐ and roof‐thrusts. The reconstructed lithostratigraphy of each horse reveals a simple pattern, in ascending order, of greenstone with low‐K tholeiitic composition with or without pillow lava structures, chert/banded iron‐formation, and turbidites. The cherts and underlying low‐K tholeiites do not contain continent‐ or arc‐derived material. The lithostratigraphy is quite similar to Phanerozoic “oceanic plate stratigraphy,” except for the abundance of mafic material in the turbidites. The evidence of duplex structures and oceanic plate stratigraphy indicates that the Isua supracrustal belt is the oldest accretionary complex in the world. The dominantly mafic turbidite composition suggests that the accretionary complex was formed in an intraoceanic environment comparable to the present‐day western Pacific Ocean. The duplex polarity suggests that an older accretionary complex should occur to the south of the Isua complex. Moreover, the presence of seawater (documented by a thick, pillow, lava unit at the bottom of oceanic plate stratigraphy) indicates that the surface temperature was less than ca. 100°C in the Early Archean. The oceanic geotherm for the Early Archean lithosphere as a function of age was calculated based on a model of transient half‐space cooling at given parameters of surface and mantle temperatures of 100° and 1450°C, respectively, suggesting that the Archean oceanic lithosphere was rigid. These conclusions—rigidity and lateral plate movement—support the idea that the modern style of plate tectonics was in operation only 0.7–0.8 G.yr. after the formation of the Earth.

Archean Regional Metamorphism of the Isua Supracrustal Belt, Southern West Greenland: Implications for a Driving Force for Archean Plate Tectonics

The Isua supracrustal belt (∼3.8 Ga) constitutes the oldest accretionary complex in the world. Petrochemical and geothermobarometric studies of more than 1500 rock samples of the Isua belt have enabled us to estimate the extent of regional metamorphism, the petrotectonic environment, and the subduction-zone geothermal gradient in the Archean. The following line of evidence indicates progressive, prograde metamorphism from greenschist (Zone A) through albite-epidote-amphibolite (Zone B) to amphibolite facies (Zones C and D) in the northeastern part of the Isua supracrustal belt: (1) the systematic change of mineral paragenesis in metabasites and metapelites; (2) progressive change of the composition of major metamorphic minerals, including plagioclase, amphibole, chlorite, epidote, and garnet; (3) normal zoning of amphibole and garnet; and (4) the absence of relict minerals of high-grade amphibolitic metamorphism even in the lowest metamorphic zone. Metabasites of the Isua belt vary extremely in Mg#, causing a complex mineral paragenesis throughout the area. For example, a high FeO content of metabasites expands the stability field of hornblende to both lower and higher grades. The compositional and mineralogical characteristics above also indicate that the Isua supracrustal belt underwent a single regional metamorphic event, involving minor contact metamorphism and mylonitization; however, weak ocean-floor metamorphism and low-grade regional metamorphism during accretion cannot be ruled out. Metamorphic pressures and temperatures are estimated to be 5–7 kbar from garnet-hornblende-plagioclase-quartz geobarometry and 380–550°C from garnet-biotite geothermometry in Zones B to D. These P-T estimates indicate an intermediate P/T ratio metamorphic facies series. Geological investigations and chronological constraints of the Isua metamorphic belt indicate that the regional metamorphism was related to the subduction of Archean lithosphere, and records a geothermal gradient for the Archean subduction zone that is much higher than geotherms for Phanerozoic subduction zones. The high geothermal gradient may have resulted from the young age of subducted lithosphere and high potential temperature of the mantle. The Archean high geothermal gradient led to melting of thick oceanic crust in a thin, descending oceanic plate, creating many huge granitic (tonalite-trondhjemite-granodiorite [TTG]) batholiths. Slab melting changed the oceanic crust (density = 3.07) to denser garnet-bearing assemblages (density = 3.55), implying that TTG melt extraction provided a potential driving force for Archean plate tectonics.

Ultrasonic-Assisted Incremental Microforming of Thin Shell Pyramids of Metallic Foil

Single point incremental forming is used for rapid prototyping of sheet metal parts. This forming technology was applied to the fabrication of thin shell micropyramids of aluminum, stainless steel, and titanium foils. A single point tool used had a tip radius of 0.1 mm or 0.01 mm. An ultrasonic spindle with axial vibration was implemented for improving the shape accuracy of micropyramids formed on 5–12 micrometers-thick aluminum, stainless steel, and titanium foils. The formability was also investigated by comparing the forming limits of micropyramids of aluminum foil formed with and without ultrasonic vibration. The shapes of pyramids incrementally formed were truncated pyramids, twisted pyramids, stepwise pyramids, and star pyramids about 1 mm in size. A much smaller truncated pyramid was formed only for titanium foil for qualitative investigation of the size reduction on forming accuracy. It was found that the ultrasonic vibration improved the shape accuracy of the formed pyramids. In addition, laser heating increased the forming limit of aluminum foil and it is more effective when both the ultrasonic vibration and laser heating are applied.

High Speed Machining of Difficult-to-Machine Materials under Different Lubrication Conditions

This paper describes the applicability of air jet assisted (AJA) machining to stainless steel and titanium alloy at high cutting speeds in terms of tool wear and tool life. A specially designed tool holder with an air nozzle very close to the tool tip was prepared for turning stainless steel. From the experimental results, it was found that the application of flood coolant from the side of the end flank face leads to better result in tool life in AJA machining of stainless steel than that from the side of the side flank face. The assistance of air jet can improve the tool life of the M35 CVD coated insert in machining of the stainless steel by 36 to 100% under the optimal conditions in comparison with wet machining. It was also found that the air jet assistance extended the tool life of the S10 PVD coated insert by 48% in turning titanium alloy. The tool life extension of the coated insert in AJA machining titanium alloy is much longer than that of an uncoated carbide insert.

High performance cutting using micro-textured tools and low pressure jet coolant

Tool inserts with different kinds of microtexture on the flank face were fabricated by laser irradiation for promoting the heat transfer from the tool face to the coolant. In addition to the micro-...