Shugo Ohi

3 – Diamond and Other Possible Ultradeep Evidence Discovered in the Orogenic Spinel-Garnet Peridotite from the Moldanubian Zone of the Bohemian Massif, Czech Republic

Publisher Summary In the Plesovice peridotite, several lines of evidence suggesting an ultradeep condition are found. All of these facts suggest that the Plesovice peridotite once resided in the diamond-stability field, which could not be identified by the previous geothermobarometric studies. This suggests that diamond was stable in the rock before Stage I (high-temperature spinel6garnet peridotite stage), which is the oldest generation of mineral assemblage previously recognized in the Plesovice peridotite. Furthermore, the presence of pyroxene exsolution lamellae in the earlier generation of spinel (Stage I) implies the preexisting high-pressure stage before Stage I. If graphite in MS inclusions were originally crystallized as diamond, parental fluids responsible for MS inclusion was most likely entrapped in the host mineral before Stage I, and they should have crystallized daughter minerals during the cooling/exhumation trajectory from the diamond-stability field, through Stage I, to the chlorite peridotite stage. For MS inclusions within Spinel-I, there is a good possibility original fluid/melt was entrapped in the diamond-stability field, if host Spinel-I was originally its high-pressure polymorph, Ca-ferrite, or Ca-titanite. On the other hand, one could not find the typical pyroxene exsolution texture in garnet. This probably suggests that most garnet was secondary formed at the expense of Spinel-I at relatively low-pressure condition. Therefore, if graphite in garnet was originally diamond, it is most likely that the diamond was graphitized before the entrapment within garnet crystal.

An isosymmetric phase transition of orthopyroxene found by high-temperature X-ray diffraction

Abstract High-temperature synchrotron X-ray powder diffraction experiments for the composition of (Ca0.06Mg1.94)Si2O6 have been carried out in the present study to clarify whether orthopyroxene has a transition between low- and high-temperature phases. Our results show that discontinuous changes of unit-cell dimensions and volume occur at 1170 °C during both heating and cooling processes and that the space group of Pbca does not change during this reversible phase transition. These facts indicate a first-order and isosymmetric phase transition. This high-temperature phase is thermodynamically distinct from the low-temperature phase, i.e., orthoenstatite in the Mg-rich portion of Mg2Si2O6- CaMgSi2O6 phase diagram, although they have the same space group.

Stability field of the high-temperature orthorhombic phase in the enstatite-diopside system

Abstract This research investigated the phase transition between low- and high-temperature orthopyroxenes with composition (Ca0.06Mg1.94)Si2O6 using differential scanning calorimetry experiments and in situ high-temperature X-ray diffraction. The transition enthalpy, temperature, volume change, and slope were estimated to be 6.2 kJ/mol, 1170 °C, 10.25 Å3/unit cell, and 0.0056 GPa/°C, respectively. The phase boundary between low- and high-temperature orthopyroxene was defined as P (GPa) = 0.0056T (°C) - 6.55. This relationship shows that the invariant point for four-phase equilibria (protoenstatite + high-temperature orthopyroxene + pigeonite + diopside) is approximately 1240-1280 °C and 0.1-0.2 GPa, rather than the equivalent system involving low-temperature orthopyroxene as described in previous studies. We developed phase diagrams for Mg2Si2O6 and the Mg2Si2O6-CaMgSi2O6 system taking into account the results of previous synthetic experiments and the phase boundary that we determined between low- and high-temperature orthopyroxene. The developed phase diagrams for Mg2Si2O6 showed that high-temperature orthoenstatite is more stable than protoenstatite at pressure above ~0.8 GPa, and that the boundary between high-temperature orthoenstatite and protoenstatite has a gentle negative slope. As pressure is increased from 1 atm to about 0.2 GPa, the lower temperature limit of stability of high-temperature orthopyroxene decreases from ~1370 to ~1200 °C. Above 0.9 GPa, the stability field of protoenstatite disappears and high-temperature Ca-free orthopyroxene is stable. On the basis of these results, it is suggested that further high-resolution analyses of the thermodynamics of the enstatite-diopside system at high temperatures and high pressures are required.

Phase transitions between high- and low-temperature orthopyroxene in the Mg2Si2O6-Fe2Si2O6 system

Abstract We observed isosymmetric phase transitions of orthopyroxene in the Mg2Si2O6-Fe2Si2O6 system during high-temperature in situ X-ray powder diffraction experiments with a multiple-detector system and a high-temperature strip heater chamber in an atmosphere of Ar plus 1% H2. The transition temperatures we determined for natural orthopyroxenes were 1113–1147, 1120–1139, and around 1200 °C for Fs10, Fs14, and Fs37, respectively, and those for synthetic orthopyroxenes were 1048–1075, 961–1048, and 1037–1148 °C for Fs20, Fs30, and Fs46, respectively. Our experiments showed that the transition from low- to high-temperature orthopyroxene in the Mg2Si2O6-Fe2Si2O6 system occurred at about 1000–1200 °C. We concluded that the stability field of low-temperature orthopyroxene was below 1000 °C and that of high-temperature orthopyroxene was above 1200 °C.

Mineralogical composition of sediment determines the preference for smooth particles by caddisfly larvae during case construction

1. The mineralogical/petrological composition of the substratum influences aquatic organisms in several ways. However, the actual mechanisms are often unclear. Some caddisfly larvae actively concentrate smooth quartz particles in their portable cases thus producing a smooth inner surface of the case wall.

Clinopyroxene exsolution in wollastonite from Namaqualand granulite, South Africa

Abstract Chemical and crystallographic properties of clinopyroxene exsolution in wollastonite are described from metamorphosed calc-silicate granulite, Namaqualand, South Africa. The wollastonite is Ca1.96Fe0.01Al0.01Si2.01-O6 belonging to space group P21/a (2M polytype) and the clinopyroxene is Ca0.99Mg0.75-0.80Fe0.17-0.21Na0.02Al0.03Si1.99-2.00O6 belonging to C2/c. An electron backscattered diffraction investigation suggests that the clinopyroxene lamellae elongated along [11̅1] lie on (120) and (100) of the wollastonite-2M, and [11̅0] of both lamellae are parallel to [001] of the wollastonite-2M. The formation of the exsolution probably results from the relatively high peak metamorphic temperature (800-860ºC) of the Namaqualand granulite and its slow cooling rate.