Makoto Arima

Proterozoic Events in the Eastern Ghats Granulite Belt, India: Evidence From Rb‐Sr, Sm‐Nd Systematics, and Shrimp Dating

Metamorphic and protolith ages of five rock types (mafic granulite, orthopyroxene granulite, leptynite, sillimanite granite, and metapelite) from Rayagada, in the north‐central part of the Eastern Ghats Granulite Belt (EGGB), India, were determined from Rb‐Sr and Sm‐Nd whole rock and mineral isochrons in combination with SHRIMP U‐Pb zircon data. Most of the whole rock isochron ages in both Sm‐Nd and Rb‐Sr systems point to either ∼1450 or ∼1000, Ma, and the mineral isochron ages are ∼1000, ∼800, and ∼550 Ma. SHRIMP U‐Pb zircon ages of ∼940 Ma were obtained from metapelite, which are in close agreement with the Sm‐Nd and Rb‐Sr isochron ages. From all these data, four age clusters (∼1450, ∼1000, ∼800, and ∼550 Ma) have been noted. The 1450 Ma ages are interpreted to represent igneous protolith formation of mafic granulite and leptynite. The 1000 Ma age cluster is regarded as the intrusion ages of sillimanite granite, and charnockite, and associated granulite facies metamorphism. Two other age clusters (800 and 550 Ma) are regarded as metamorphic heating events. Earlier reports from the EGGB show two major age‐groupings, one around 1450 Ma, characterized by alkaline magmatism and anorthositic intrusions, and the other at 1000 Ma, considered to be the major metamorphic and tectonothermal event. The present data are broadly similar with those reported from parts of East Antarctica with respect to the 1000 Ma and 550 Ma events and reconfirm that EGGB has been an integral part of eastern Gondwana.

India-Antarctica-Australia-Laurentia connection in the Paleoproterozoic–Mesoproterozoic revisited: Evidence from new zircon U-Pb and monazite chemical age data from the Eastern Ghats Belt, India

We present zircon and monazite U-Pb data from ultrahigh-temperature (UHT) metamorphosed orthogneisses and paragneisses collected from key areas of the Eastern Ghats Belt, India. The results show contrasting tectonothermal histories in different isotopic domains of the Eastern Ghats Belt that were identified by previous workers. Of particular importance is the discovery of a ca. 1760 Ma event (concordia age) in the southern domain 1A, which is interpreted to be the age of an early UHT metamorphism event. This was followed by a second granulite-facies metamorphism event and partial melting at ca. 1600 Ma. This domain was presumably cratonized with India at around 1600 Ma. The record of the ca. 1760–1600 Ma events in domain 1A of the Eastern Ghats Belt allows us to speculate on modeling the Paleoproterozoic–Mesoproterozoic transcontinental correlation. The accretionary orogenic processes in the supercontinent Columbia encompassed Australia, Antarctica, Laurentia, and parts of India. The central part of Eastern Ghats Belt (isotopic domain 2), on the other hand, contains zircons showing inherited ages of ca. 1880–1700 Ma, with a concordant age group of ca. 1760 Ma. Moderately to strongly discordant ages in the time span of ca. 1600–1100 Ma in domain 2 are interpreted to be mixing ages as a result of strong overprint of a ca. 1030–900 Ma tectonothermal event(s) that affected this domain. An early UHT metamorphism event in this domain is inferred to have occurred at ca. 1030–990 Ma (chemical dating of included monazite grains). Zircon records the most pervasive tectonothermal event in this domain at ca. 980–900 Ma, which is correlative with the Rayner orogeny in East Antarctica as a part of the formation of Rodinia.

Petrological model of the northern Izu–Bonin–Mariana arc crust: constraints from high-pressure measurements of elastic wave velocities of the Tanzawa plutonic rocks, central Japan

Abstract Ultrasonic compressional wave velocities (Vp) and shear wave velocities (Vs) were measured with varying pressure up to 1.0 GPa in a temperature range from 25 to 400 °C for a suite of tonalitic–gabbroic rocks of the Miocene Tanzawa plutonic complex, central Japan, which has been interpreted as uplifted and exposed deep crust of the northern Izu–Bonin–Mariana (IBM) arc. The Vp values of the tonalitic–gabbroic rocks increase rapidly at low pressures from 0.1 to 0.4 GPa, and then become nearly constant at higher pressures above 0.4 GPa. The Vp values at 1.0 GPa and 25 °C are 6.3–6.6 km/s for tonalites (56.4–71.1 wt.% SiO2), 6.8 km/s for a quartz gabbro (53.8 wt.% SiO2), and 7.1–7.3 km/s for a hornblende gabbro (43.2–47.7 wt.% SiO2). Combining the present data with the P wave velocity profile of the northern IBM arc, we infer that 6-km-thick tonalitic crust exists at mid-crustal depth (6.1–6.3 km/s Vp) overlying 2-km-thick hornblende gabbroic crust (6.8 km/s Vp). Our model shows large differences in acoustic impedance between the tonalite and hornblende gabbro layers, being consistent with the strong reflector observed at 12-km-depth in the IBM arc. The measured Vp of Tanzawa hornblende-bearing gabbroic rocks (7.1–7.3 km/s) is significantly lower than that Vp modeled for the lowermost crustal layer of the northern IBM arc (7.3–7.7 km/s at 15–22 km depth). We propose that the IBM arc consists of a thick tonalitic middle crust and a mafic lower crust.

Diamond nucleation and growth by reduction of carbonate melts under high-pressure and high-temperature conditions

We report for the first time experimental evidence for the nucleation and growth of diamonds from carbonatitic melts by reduction in reactions with silicon metal or silicon carbide. Experiments were carried out in the CaMg(CO 3 ) 2 -Si and CaMg(CO 3 ) 2 -SiC systems at 7.7 GPa and temperatures of 1500-1800 °C. No graphite was added to the run powder as a carbon source; the carbonate-bearing melts supply the carbon for diamond formation. Diamond grows spontaneously from the carbonatitic melt by reducing reactions: CaMg(CO 3 ) 2 + 2Si = CaMgSi 2 O 6 + 2C in the CaMg(CO 3 ) 2 -Si system, and CaMg(CO 3 ) 2 + 2SiC = CaMgSi 2 O 6 + 4C in the CaMg(CO 3 ) 2 -SiC system. Our results provide strong experimental support for the view that some natural diamonds crystallized from carbonatitic melts by metasomatic reducing reactions with mantle solid phases.

Crystallization of diamond from a silicate melt of kimberlite composition in high-pressure and high-temperature experiments

In high-pressure and high-temperature experiments (1800- 2200 °C and 7.0-7.7 GPa), diamond crystallized and grew in a volatile-rich silicate melt of kimberlite composition. This diamond has well- developed {111} faces, and its morphologic characteristics resemble those of natural diamond but differ from those of synthetic diamond grown from metallic solvent-catalysts. The kimberlite melt has a strong solvent-catalytic effect on diamond formation, supporting the view that some natural diamonds crystallized from volatile-rich melts in the upper mantle.

Experimental study on diamond dissolution in kimberlitic and lamproitic melts at 1300–1420 °C and 1 GPa with controlled oxygen partial pressure

Abstract To evaluate the dissolution processes of diamond crystals in kimberlitic and lamproitic magmas, a series of diamond dissolution experiments were carried out in the graphite stability field at 1300-1420 °C and 1 GPa under the WI, MW, and HM buffers. Dissolution agents used include an aphanitic kimberlite from Wesselton Mine, South Africa, and a lamproite from Mount North, West Kimberley, Australia. With increasing run duration, diamond morphology changed from a sharp octahedron through combinations of octahedron and tetrahexahedroidal forms to spherical tetrahexahedroid having rounded faces. Negatively oriented trigons were formed on the octahedral {111} face. As the degree of diamond dissolution increased, the trigons changed from smaller shallow types to larger deep types. The dissolution rate in the kimberlitic solvent at 1300 °C was 0.12 mm/h under the HM buffer, 0.0034 mm/h under the MW buffer, and 0.0017 mm/h under the WI buffer, whereas that at 1420 °C was 0.014 mm/h under the WI buffer. In the lamproitic solvent, the dissolution rate was 0.0024 mm/h at 1420 °C under the WI buffer. The data indicate that diamond dissolves in silicate melts as carbonate ions through an oxidizing reaction. The degree of dissolution strongly depends on temperature, oxidation state, and the compositional dependence of CO2 solubility in the melts.

Mesoarchaean-Palaeoproterozoic stratigraphic record of the Singhbhum crustal province, eastern India: a synthesis

Abstract The Mesoarchaean–Palaeoproterozoic stratigraphic record of the Singhbhum crustal province, eastern India, implies sedimentation and volcanism in a changing tectonic scenario, and thus assumes immense geological significance. Although efforts have been made by many researchers in the past several decades to summarize various geological aspects of the Singhbhum crustal province, a critical synthesis of various stratigraphic issues was long overdue. The present contribution is an updated critical synthesis of the Mesoarchaean–Palaeoproterozoic stratigraphic record of the Singhbhum crustal province. We have pointed out the problematic stratigraphic issues of the Singhbhum crustal province that deserve careful scrutiny in order to gain better insights into the mode of stratigraphic sequence building.

Thermal History of UHT Metamorphism in the Napier Complex, East Antarctica: Insights from Zircon, Monazite, and Garnet Ages

High‐grade gneisses from Mt. Riiser‐Larsen, East Antarctica, have been dated by whole‐rock‐mineral Sm‐Nd and SHRIMP zircon and monazite U‐Pb to help define the thermal history of ultrahigh temperature (UHT) metamorphism in the Napier Complex. Both the monazite and youngest zircon yield a range of apparent ages (∼2.51–2.47 Ga), consistent with crystallization during an extended period of metamorphism. Some zircon also preserves an isotopic record of earlier events, placing an upper limit of a few million years on the duration of peak metamorphic conditions. The similarity of the monazite and zircon U‐Pb ages implies rapid initial postpeak cooling to below the blocking temperature of these minerals (∼900°C). Consistently lower Sm‐Nd whole‐rock‐mineral isochron ages (∼2.38 Ga) indicate that cooling slowed before the temperature reached ∼650°C. The history of the UHT metamorphism is interpreted to be (1) protracted high‐temperature (≥800°C) conditions ∼2.51–2.47 Ga, (2) peak conditions (up to 1100°C) for at most a few million years, (3) rapid cooling (10°–60°C /m.yr.) immediately after peak metamorphism, and (4) very slow cooling (≤4°C/m.yr.) at midcrustal levels (∼30‐km depth) to a steady state geotherm by 2.38 Ga.

Dunite, Glimmerite and Spinellite in Achankovil Shear Zone, South India: Implications for Highly Potassic CO2-rich Melt Influx Along an Intra-continental Shear Zone

In the Kakkaponnu area within the Achankovil Shear Zone (ACSZ), southern India, an undeformed ultramafic body occurs within intensely deformed granulite facies metamorphic rocks of Pan-African age. The Kakkaponnu ultramafic body is composed of spinel-dunite, phlogopite-dunite, glimmerite, graphite-spinel-glimmerite, and phlogopite-graphite-spinellite. The spinel-dunite is a fine- to medium-grained rock composed mainly of olivine and aluminous spinel and is characterized by relatively high MgO (50.39–50.90 wt.%), (Mg/ (Mg+Fe) = 0.95), Al2O3 (7.8–8.98 wt.%), and low Ni (10–14 ppm). The phlogopite-dunite comprises serpentinized olivine, phlogopite and subordinate amounts of dolomite and is high in MgO (36.5 wt.%), Mg# [(Mg/(Mg+Fe) = 0.97], and K2O (%%5.5 wt.%). Olivine in the spinel-dunite is marked by unusually high MgO (Mg# = 0.96) and extremely low NiO (<0.14 wt.%). Spinels in all rock variants are highly aluminous with low Cr# [Cr/(Al+Cr)] ratio (<0.01). Magnesian ilmenite [Mg# = 59], rutile, zirconolite and baddeleyite are main accessory phases. No significant compositional variation is noted between large grains and small inclusions for all minerals. Abundant graphite, magnesite, melt and ubiquitous CO2 fluid inclusions are identified in the olivine and spinel grains. The data imply that the Kakkaponnu ultramafic body was formed by progressive crystallization of highly potassic CO2-rich melts injected into lower crustal levels. K-Ar ages of 470.5±9.3 and 464.5±9.2 Ma are obtained for phlogopite separates from glimmerite and phlogopite-dunite respectively. These ages are comparable to the phlogopite K-Ar ages reported from lithospheric shear zones in southern Madagascar, which was once conjugated to the Southern Peninsular India prior to the Gondwana breakup. This implies widespread highly potassic CO2-rich fluid/melt influx along shear zones in this part of East Gondwana continent.

Diamond dissolution rates in kimberlitic melts at 1300–1500 °C in the graphite stability field

A series of diamond dissolution experiments were conducted in the graphite stability field under the metal iron-wustite (IW) oxygen buffer to evaluate the rate of diamond dissolution reactions into two sets of kimberlitic solvents having different CO 2 and H 2 O contents. The solvents used were a natural kimberlite from Wesselton Mine, South Africa and a synthetic H 2 O-free and CO 2 -rich kimberlite. Experiments with the Wesselton kimberlite were carried out at 1400 and 1500 °C under 1.0 GPa and those with the synthetic kimberlite were conducted at 1500 °C under 2.5 GPa. In both series of experiments, the diamond crystal form changed from that of a sharp octahedron through a combination of octahedron and hexoctahedroidal forms to a spherical trioctahedroidal with increasing run duration. The natural and synthetic kimberlitic solvents produced contrasting etch characteristics on the diamond crystal surfaces. Negatively-oriented trigons were formed on the octahedral {111} faces in the runs with the natural kimberlite agent; while diamond surfaces were covered by both negatively and positively oriented smaller trigons and larger hexagonal etch pits in the runs with the synthetic kimberlite solvent. The degree of dissolution was highly sensitive to temperature and carbonate solubility of the solvent. The diamond dissolution rate (radius loss of the diamond crystal) was 0.012 mm/h at 1400 °C and 0.066 mm/h at 1500 °C in the Wesselton kimberlite solvent, and 0.024 mm/h at 1500 °C in the synthetic kimberlite solvent. The data give an activation energy of ~342 kJ/mol for the diamond dissolution reaction in the natural kimberlite solvent at 1.0 GPa under the IW buffer.