Spinel In Peridotites Research Articles

Mineralogy and PGE geochemistry of chromitites and peridotites of the sapat complex in the indus suture zone, northern Pakistan: implications for magmatic processes in the supra-subduction zone

ABSTRACT The Sapat Complex in Northern Pakistan contains remnants of the northern Neo-Tethys Ocean, presently exposed along the Indus Suture Zone. The mantle peridotites of the Sapat Complex include harzburgites, dunites, and subordinate lherzolites. Harzburgites are the dominant peridotite variety over dunites. The dunites are hosted by harzburgites and occur exclusively as ‘envelopes’ surrounding chromitite pods. The podiform chromitites show disseminated, banded, and massive textures. Chromitites exhibit variable Cr# [Cr/(Cr + Al)] and Mg# [Mg/(Mg + Fe2+)], which range from 0.76 to 0.77, and from 0.64 to 0.66, respectively, while TiO2 contents are <0.2 wt.%. These features perhaps reflect crystallization of the chromian spinel from a boninitic magma. Similarly, chromian spinel in peridotites manifest a wide range of Cr# and Mg#, from 0.49 to 0.83 and 0.41 to 0.57, respectively, and are characterized by very low TiO2 values, averaging at 0.1 wt.%. Chromian spinel of chromitites and peridotites of the Sapat Complex have also very low Fe3+# (<0.01), which indicate their crystallization under low oxygen fugacities. The platinum group elements (PGE) distributions show high (Os + Ir + Ru)/(Rh + Pt + Pd), very low Pd/Ir values, and are defined by a prominently fractionated chondritic normalized PGE pattern, hence, this deposit is a typical example of an ophiolitic chromitite. The studied peridotites are highly depleted in PGE compared to chondritic values. The PdN/IrN values, averaging to 1.5 in dunites are unfractionated, while PGE spidergrams of harzburgites and lherzolites depict minor positive slopes, a minor positive Ru anomaly, and have average PdN/IrN values of 2.3 and 2.4, respectively. Furthermore, the harzburgites, dunites, and lherzolites display generally flat chondritic and primitive mantle normalized PGE patterns, and therefore, are nearly identical to highly depleted mantle peridotites. The mineralogical and PGE geochemical imprints of Sapat Complex chromitites and peridotites establish a strong affinity to supra-subduction zone ophiolites. Moreover, calculated parental melts of the chromitites and various geochemical discrimination diagrams elucidate that the chromitites were derived from boninitic magma produced by melting of depleted mantle peridotites in an oceanic arc, characterized by low oxygen fugacity, similar to a supra-subduction zone tectonic setting. This research highlights the use of mineralogy and geochemical compositions of chromitites and peridotites to reveal deep magmatic processes in a supra-subduction zone environment.

Mineralogy and geochemistry of the peridotites and high-Cr podiform chromitites from the Tangbale Ophiolite Complex, West Junggar (NW China): Implications for the origin and tectonic environment of formation

The Tangbale Ophiolite Complex is located in southwestern part of the West Junggar, NW China, which is a major component of the Central Asian Orogenic Belt (CAOB). The Tangbale Ophiolite Complex consists of voluminous dunites, harzburgites and high-Cr podiform chromitites. Harzburgite contains chromian spinel of low Cr# (100 × Cr/(Cr + Al) = 38–64) but high Mg# (100 × Mg/(Mg + Fe2+) = 55–61), whereas dunite contains spinel of higher Cr# (56–82) and lower Mg# (41–55). Numerical modeling of heavy rare earth element (HREE) contents in harzburgites are consistent with derivation from ~15% to 20% partial melting of a fertile mantle source, and dunites with low concentrations of HREE are suggested to be generated by higher degrees (~20%–25%) melting of the same source. The peridotites have REE signatures with enrichments in LREE, indicating the involvement of metasomatism and interaction by subduction-related melts in their formation. The chromite in podiform chromitite displays the highest Cr# (80–84) and Mg# (65–76), as well as moderate TiO2 contents, suggesting crystallization from hydrous, high-Si and high-Mg melts in supra-subduction zones, which is consistent with the calculated compositions of parental melts for spinels in chromitites. The genesis of high-Cr chromitites is likely to be triggered by the metasomatism and interaction between high-Mg boninitic-like melts and mantle peridotites. The addition of SiO2 and chrome in boninitic-like melts via interaction with peridotites trigger high-Cr chromite to precipitate and separate from parental magma, and finally concentrate into chromitites with boninitic-like signature at SSZ zone. The calculated parental melts for spinels in peridotites show hybrid compositions between MORB (mid-ocean ridge basalt) and arc melts, suggesting a two-stage formation in an evolving from MOR (mid-ocean ridge) to SSZ (supra-subduction zone) setting. The peridotites in the Tangbale body were most likely formed at a MOR setting and trapped above an intra-oceanic subduction zone as part of the mantle wedge, where the upper peridotites were metasomatized and partially re-melted under fluid-saturated and high temperature conditions to generate boninitic melts. The interaction between upward-migrating boninitic melts and/or rising asthenospheric mantle-derived melts with the peridotites beneath the Moho triggered the generation of high-Cr podiform chromitites and associated dunite envelopes.

Petrological study of spinel peridotites of Nidar ophiolite, Ladakh Himalaya, India

Petrological study of the ultramafic rocks from the Nidar Ophiolite Complex (NOC) of the Indus Suture Zone is carried out. The study of Cr-spinels along with olivine and pyroxenes emphasizes the genesis and tectonic setting of the ultramafites. Olivine from the harzburgite is Mg-rich, with the molar ratio Mg# [Mg/(Mg + Fe2+)] varying between 0.91 and 0.94 and olivine in dunite between 0.92 and 0.94. Clinopyroxene from the harzburgite is TiO2 and Na2O-poor diopside (Wo47–50En47–50Fs2–4). Spinel in harzburgite shows wide Cr#, molar ratio varied between 0.26 and 0.72, and significantly higher in dunites with Cr# ranges from 0.69–0.85. Cr# of the peridotite spinel follow a depletion trend. Calculated equilibrium conditions of the samples are 800–900°C temperature, 32 and 40 kbar pressure, oxygen fugacity −0.09 to 0.55 log units above the FMQ buffer. Residual nature of the harzburgites and the presence of high and low Cr# spinels may be due to the genetic artifact of the different ultramafic units.

Spinel, olivine, and pyroxene chemistry of the Eoarchaean Manfred Complex (Yilgarn Craton, Western Australia), with implications for the tectonic setting of Archaean layered mafic intrusions and the stabilisation of continental nuclei

Anorthosite complexes are distinctive components of Archaean cratons and their petrogenesis has implications for ancient mantle evolution, early Earth geodynamics, and growth of continental crust. However, the origin and tectonic setting of Archaean anorthosites and their complementary mafic and ultramafic cumulates is uncertain due to metamorphism and deformation. Here, we report spinel, olivine, and pyroxene chemical compositions from rare, well preserved ultramafic cumulates of the Eoarchaean Manfred Complex in the Yilgarn Craton of Western Australia, which contains the oldest anorthosites on Earth. Relict magmatic spinel in peridotite and pyroxenite enclaves define a distinctive high Al, low Ti-Fe3+ layered mafic intrusion trend that establishes a petrogenetic link to the ca. 3730 Ma anorthosites. Olivine chemistry (Fo85), Ni concentrations, FeO/MgO ratio trends for coexisting olivine and liquid, and spinel Mg# identify the parental magma as picritic to tholeiitic. Unique spinel Cr#, Fe#, and Ti contents are attributed to derivation of the parental magmas from a spinel peridotite source, in contrast to younger Archaean anorthosite complexes, reflecting an evolution of Archaean mantle. Magmatic spinel Al3+, Fe3+, and Cr3+ systematics are consistent with the emplacement and differentiation of the ca. 3730 Ma Manfred Complex parental magma within a thickened oceanic plateau. This implies a similar, intraplate tectonic environment for formation of the surrounding, coeval tonalite-trondhjemite-granodiorite gneisses in the Narryer Terrane. We therefore infer that early Archaean oceanic plateaus may be important tectonic settings for producing and preserving ancient continental crust, as well as the variety of supracrustal rocks commonly observed in gneiss complexes of Archaean cratons.

Spinel chemistry from Madawara Mafic-Ultramafic Complex, Lalitpur District, Uttar Pradesh, India

Spinel in peridotite of the Pipariya area in Madawara Mafic and Ultramafic Complex (MMUMC) occur as euhedral to subhedral inclusion within the silicates. They exhibit regular to irregular shape and is altered to ferritchromite to magnetite along the cracks and boundaries. Cr-spinel is characterised by high values of Cr# (79.7-98.4) and low Mg# and Al# values (i.e., 20.41-9.66 and 0.49- 17.71, respectively) and are identified as Fe-chromite. Chemical discontinuity/zones between the core and rim can be observed in the analysed grain. The textural and chemical features of investigated suggest low- to sub greenschist facies metamorphism consistent with the estimated metamorphic equilibrium temperature of 500°C-550°C.&#x0D; Mineral chemistry of these spinel suggests that spinel in peridotite of Pipariya in the Madawara Mafic and Ultramafic Complex depicts that these Cr-spinel may have been derived from boninitic related magmas at arc to suprasubduction zone tectonic setting at low degree of partial melting.

Processes of primary and re-equilibration mineralization affecting chromitite ore geochemistry within the Vourinos ultramafic sequence, Vourinos ophiolite (West Macedonia, Greece)

The Vourinos ophiolite is a complete ophiolite sequence covering an area of ∼450 km2, that hosts the largest chromite mining district in Europe. Chrome ore deposits range in size from several millions of tonnes of Xerolivado-Skoumtsa mine to some thousand tonnes of the smallest exploited ores. Widespread mining activity closed in the 1990s, leaving behind good exposures of chromitite bodies within the closed mines. Samples from 10 of these mines, 4 located in the Northern part of the massif and 6 located in its Southern part, were collected in order to study the spatial variability of chromite and olivine mineral chemistry in these ores at different scales, and to infer the roles of primary magmatic and secondary post-magmatic re-equilibration processes. Chromites from chromitites show higher XCr (0.75–0.87) than scattered spinels in peridotites and serpentinites (0.45–0.70) and largely variable XMg (0.40–0.78). Chromite compositions of North and South Vourinos ores largely overlap, while they show differences at the scale of single deposits, with a remarkable higher XMg at Xerolivado-Skoumtsa. High variability internal to each deposit is due to different re-equilibration conditions at variable olivine/spinel volume ratios. Olivine and chromite XMg values are positively correlated to chromite modal content, while XCr of chromite is not significantly affected by re-equilibration. Calculated re-equilibration temperatures are higher for the larger Xerolivado-Skoumtsa deposit (777–837 °C) than for the other smaller ones (715–746 °C). The inferred magmatic temperature is 1243 °C for Xerolivado-Skoumtsa and 982 °C for the other chromite deposits. Calculated melt compositions are very consistent for the 10 studied mines that all show a boninitic affinity. The whole set of data shows that Vourinos chromitites were formed in an island arc environment from a heterogeneous mantle source that underwent different degrees of melting. Subsolidus re-equilibration occurred during cooling at temperatures as low as 700 °C, partially obliterating primary magmatic features.

Major and Trace Element Chemistry of Chromian Spinels in Peridotites from Yesilova Ophiolite (SW Turkey): Petrogenetic Interpretation

Major and Trace Element Chemistry of Chromian Spinels in Peridotites from Yesilova Ophiolite (SW Turkey): Petrogenetic Interpretation

Geochemistry of platinum-group elements and mineral composition in chromitites and associated rocks from the Abdasht ultramafic complex, Kerman, Southeastern Iran

The Abdasht complex is a major ultramafic complex in south-east Iran (Esfandagheh area). It is composed mainly of dunite, harzburgite, podiform chromitites, and subordinate lherzolite and wehrlite. The podiform chromitites display massive, disseminated, banded and nodular textures. Chromian spinels in massive chromitites exhibit a uniform and restricted composition and are characterized by Cr# [=Cr/(Cr+Al)] ranging from 0.76 to 0.77, Mg# [=Mg/(Mg+Fe2+)] from 0.63 to 0.65 and TiO2<0.2wt.%. These values may reflect crystallization of the chromian spinels from boninitic magmas. Chromian spinels in peridotites exhibit a wide range of Cr# from 0.48 to 0.86, Mg# from 0.26 to 0.56 and very low TiO2 contents (averaging 0.07wt.%). The Fe3+# is very low, (<0.08wt.%) in the chromian spinel of chromitites and peridotites of the Abdasht complex which reflects crystallization under low oxygen fugacities.The distribution of platinum group elements (PGE) in Abdasht chromitites displays a high (Os+Ir+Ru)/(Rh+Pt+Pd) ratio with strongly fractionated chondrite-normalized PGE patterns typical of ophiolitic chromitites. Moreover, the Pd/Ir value, which is an indicator of PGE fractionation, is very low (<0.1) in the chromitites.The harzburgite, dunite and lherzolite samples are highly depleted in PGE contents relative to chondrites. The PdN/IrN ratios in dunites are unfractionated, averaging 0.72, whereas the harzburgites and lherzolites show slightly positive slopes PGE spidergrams, together with a small positive Ru anomaly, and their PdN/IrN ratio averages 2.4 and 2.3 respectively. Moreover, the PGE chondrite and primitive mantle normalized patterns of harzburgite, dunite and lherzolite are relatively flat which are comparable to the highly depleted mantle peridotites.The mineral chemistry data and PGE geochemistry indicate that the Abdasht chromitites and peridotites were generated from a melt with boninitic affinity under low oxygen fugacity in a supra-subduction zone setting. The composition of calculated parental melts of the Abdasht chromitites is consistent with the differentiation of arc-related magmas.

Melt stagnation in peridotites from the Godzilla Megamullion Oceanic Core Complex, Parece Vela Basin, Philippine Sea

The Godzilla Megamullion, located in the Parece Vela Backarc Basin of the Izu–Bonin–Mariana (IBM) system, is the largest known example of an Oceanic Core Complex (OCC). Peridotites recovered from the megamullion are divided petrographically into fertile (e.g. lherzolites), depleted (e.g. harzburgites), and plagioclase-bearing groups (Ohara et al., 2003a). A total of 151 thin sections were studied from the Kairei KR03-01, Hakuho Maru KH07-02, and Yokosuka YK09-05 cruises. Melt stagnation is studied via the incidence of plagioclase-bearing peridotites and the major element chemistry of Cr-spinels in the plag-bearing samples. A distinct trend in melt stagnation is evident along the length of the megamullion representing a secular evolution in the entrapment of melts rising through the lithosphere. The distal (furthest from the termination of spreading), depleted portion of the mullion represents a robust mantle section that was still producing abundant melt and can be compared to typical oceanic spreading with its relatively “normal” percentage of plagioclase peridotites and average spinel Cr# of 0.35. The medial, fertile portion of the mullion represents a steep falloff in melt productivity represented by fertile spinel compositions (i.e. Cr# <0.25) and the presence of plagioclase-free lherzolites. The proximal (closest to termination of spreading), heavily plagioclase impregnated portion (with spinel Cr#s covering nearly the entire range of abyssal peridotite spinel compositions) of the mullion then represents a period of increasing stagnation of melt into a lithosphere that was undergoing progressive thickening. We infer that the processes of mantle evolution through melt stagnation and impregnation, as evidenced by the systematic variations in plag–peridotites along Godzilla Megamullion, represent a possibly common way in which the mantle reacts to OCC formation. In this case, Godzilla Megamullion may represent an extreme endmember in OCC formation.

Petrogenesis of Tertiary Mafic Alkaline Magmas in the Hocheifel, Germany

Primitive nephelinites and basanites from the Tertiary Hocheifel area of Germany (part of the Central European Volcanic Province; CEVP) have high Mg-number (>0� 64), high Cr and Ni contents and strong light rare earth element enrichment but systematic depletion in Rb, K and Ba relative to trace elements of similar compatibility in anhydrous mantle. Alkali basalts and more differentiated magmatic rocks have lower Mg-number and lower abundances of Ni and Cr, and have undergone fractionation of mainly olivine, clinopyroxene, Fe–Ti oxide, amphibole and plagioclase. Some nephelinites and basanites approach the Sr–Nd–Pb isotope compositions inferred for the EAR (European Asthenospheric Reservoir) component. The Nd–Sr–Pb isotope composition of the differentiated rocks indicates that assimilation of lower crustal material has modified the composition of the primary mantle-derived magmas. Rare earth element melting models can explain the petrogenesis of the most primitive mafic magmatic rocks in terms of mixing of melt fractions from an amphibole-bearing garnet peridotite source with melt fractions from an amphibole-bearing spinel peridotite source, both sources containing residual amphibole. It is inferred that amphibole was precipitated in the asthenospheric mantle beneath the Hocheifel, close to the garnet peridotite–spinel peridotite boundary, by metasomatic fluids or melts from a rising mantle diapir or plume. Melt generation with amphibole present suggests relatively low mantle potential temperatures (<1200 � C); thus the mantle plume is not thermally anomalous. A comparison of recently published Ar/Ar ages for Hocheifel basanites with the geochemical and isotopic composition of samples from this study collected at the same sample sites indicates that eruption of earlier lavas with an EM signature was followed by the eruption of later lavas derived from a source with EAR or HIMU characteristics, suggesting a contribution from the advancing plume. Thus, the Hocheifel area represents an analogue for magmatism during continental rift initiation, during which interaction of a mantle plume with the overlying lithosphere may have led to the generation of partial melts from both the lower lithosphere and the asthenosphere.

Mid-crustal Metasomatic Reaction Veins ina Spinel Peridotite

In Central Dronning Maud Land, East Antarctica, rare metresized lenses of spinel peridotite are enclosed in high-grade metamorphic rocks. The rocks experienced a medium-P granulite-facies metamorphism at 575Ma and a low-P amphibolite-facies overprint at 530Ma. The latter is probably related to extensive granitoid magmatism between 530 and 500Ma, which produced large volumes (about half of the outcrops today) of granitic to syenitic rocks as well as abundant K-feldspar±quartz pegmatites. One of the spinel peridotite lenses in the Schirmacher Oasis of Central Dronning Maud Land is crosscut by several small (up to 10 cm wide) veins with a characteristic zoned sequence of mineral assemblages, which was formed by reaction of a hydrous, SiO2-saturated fluid or pegmatitic melt with the peridotite. The zoned sequence consists of the following mineral assemblages (from the centre of the vein towards the outer margin): zone 0, plagioclase ‡ quartz; zone 1, green biotite intergrown with zircon ‡ clinoamphibole; zone 2, cummingtonite ‡ dark brown biotite intergrown with rutile ‡ clinoamphibole; zone 3, cummingtonite ‡ light brown biotite ‡ spinel; zone 4, olivine ‡ orthopyroxene ‡ spinel clinopyroxene (unaltered peridotite). This sequence was investigated with respect to its conditions of formation, modal mineralogy, mineral chemistry, fluid inclusions, and oxygen and hydrogen isotope compositions of selected minerals. Based on the stability of cummingtonite and on equilibrium calculations in the MgO±SiO2±H2O system and on quartz±biotite oxygen isotope thermometry, the reaction vein formed at 650 C, which is in accord with typical pegmatite crystallization temperatures. The pegmatite of zone 0 is interpreted to have formed in an open fissure whereas, on textural grounds, zone 3 replaces former peridotite. On the basis of mass balance constraints, the boundary between zones 1 and 2 is interpreted to approximately represent the former boundary between peridotite and the open fissure before reaction. Oxygen isotope systematics show that the infiltrating fluid had an isotopic composition of 9±101⁄2 SMOW. All minerals of the reaction vein with the exception of the inherited spinel and olivine in the adjacent peridotite are in equilibrium with such a fluid. Spinel in the peridotite is depleted in O compared with coexisting olivine, which suggests isotopic disequilibrium. Spinel in zone 3 has a distinctly different isotopic composition compared with that in the peridotite, apparently approaching but not reaching equilibrium. The combination of mineral chemistry and mass balance constraints of the modal mineralogy constrains the volume change during metasomatism and the direction of elemental diffusion. It is indicated that Mg, Cr and Ni always diffused towards the vein, whereas Si, Al, K, Na, H2O and possibly Fe diffused into the peridotite.

Mantle melting beneath Gakkel Ridge (Arctic Ocean): abyssal peridotite spinel compositions

The ultraslow spreading Gakkel Ridge represents one of the most extreme spreading environments on the Earth. Full spreading rates there of 0.6–1.3 cm/year and Na 8.0 in basalts of 3.3 imply an extremely low degree of mantle partial melting. For this reason, the complementary degree of melting registered by abyssal peridotite melting residues is highly interesting. In a single sample of serpentinized peridotite from Gakkel Ridge, we found spinels which, though locally altered, have otherwise unzoned and thus primary compositions in the cores of the grains. These reflect a somewhat higher degree of melting of the uppermost oceanic mantle than indicated by basalt compositions. Cr/(Cr+Al) ratios of these grains lie between 0.23 and 0.24, which is significantly higher than spinels from peridotites collected along the faster spreading Mid-Atlantic and Southwest Indian Ridges. Crustal thickness at Gakkel Ridge can be calculated from the peridotite spinel compositions, and is thicker than the crustal thickness of less than 4 km estimated from gravity data, or predicted from global correlations between spreading rate and seismically determined crustal thickness. The reason for this unexpected result may be local heterogeneity due to enhanced melt focussing at an ultraslow spreading ridge.

Factors Controlling Chemistry of Magmatic Spinel: an Empirical Study of Associated Olivine, Cr-spinel and Melt Inclusions from Primitive Rocks

Compositions of ~2500 spinel-olivine pairs and 400 melt inclusion-spinel pairs have been analysed from 36 igneous suites from oceanic, arc and intraplate tectonic settings. Our data confirm that Cr-spinel mg-number is largely controlled by melt composition, but also influenced by octahedral site substitutions, and rate of cooling. Lavas quenched in submarine environments tend to have higher mg-number at a given cr-number than slowly cooled subaerial lavas and peridotites. Unlike mg-number, Cr-spinel Al2O3 and TiO2 contents show good correlations with melt composition, with only limited post-entrapment modifications. Out data suggest that increased activity of Al2O3 decreases the partitioning of TiO2 into spinels. The Al2O3 content of Cr-spinel is a useful guide to the degree of partial melting of mantle peridotites; however, this same relationship is obscured in volcanic rocks. Al2O3 contents of volcanic Cr-spinels are mostly determined by melt composition rather than mantle source composition. The data also suggest that most spinels from residual mantle peridotites can be readily differentiated from those hosted in volcanic rocks. Mantle peridotite spinel tend to have lower TiO2 and higher Fe2+/Fe3+ ratios than spinel from volcanic rocks. The spinel compositions in our database can be subdivided on the basis of tectonic setting and mode of occurrence using an Al2O3 vs TiO2 diagram. A total of seven fields can be distinguished with varying degrees of overlap. This diagram can then be used to determine the tectonic setting of spinel from altered mafic igneous rocks such as serpentinites or meta-basalts, or detrital spinel in sandstones.

High-Al podiform chromitites in dunite-harzburgite complexes of the Sangun zone, central Chugoku district, Southwest Japan

There are many ultramafic complexes in the Sangun zone of the central Chugoku district, Southwest Japan, some of which have chromitite bodies. Lithological variations of these complexes were determined for primary mineral assemblages in order to understand the origin of podiform chromitites. They mainly consist of harzburgite, dunite and chromitite, and are intruded by some gabbroic dikes. Harzburgite is always dominant over dunite. Chromitite is closely associaied with dunite: a chromitite body is enclosed by a dunite envelope and large chromitite bodies are exclusively found in relatively dunite-dominant complexes. Cr # (=Cr/(Cr + Al) atomic ratio) of chromian spinet varies only slightly, from 0.4 to 0.6 for all ultramafic rocks from all ultramafic complexes, with only one exception, Ochiai-Hokubo complex, (Arai et al. 1988) of the Sangun zone, central Chugoku district, Southwest Japan. Spinels in peridotites from Wakamatsu mine of the Tari-Misaka complex, the largest chromitite body of this district, are slightly high in Cr # (around 0.55) and in Fe 3+ (Cr + Al + Fe 3+) atomic ratio. The podiform chromitite of this district is of interaction origin between melt and harzburgite. Themagma mixing between a secondary Si-rich melt formed by the interaction and a primitive magma in the upper mantle is essential to the chromitite precipitation. Mechanism of spinet precipitation is the same as that given by Irvine (1975, 1977). The magma interacted with the harzburgite in the Sangun zone could be MORB or back-arc basin basalt. The podiform chromitites of this district have relatively Al-rich chromian spinet, indicating involvement of both an exotic Al-rich melt and of relatively fertile harzburgite.

Evaluation of olivine-spinel geothermometry as an indicator of thermal history for peridotites

Olivine and spinel in peridotites from the Miyamori ultramafic complex and the Ichinogemata crater of Northeast Japan show a systematic variation in the Mg/ (Mg+Fe) ratio which is correlated mainly with the grain size of spinel. This correlation can be explained by a diffusion model assuming a semi-infinite composite sphere under cooling or heating conditions. In order to obtain absolute temperatures of thermal events, the olivine-spinel geothermometer is applied to pairs of spinel core and olivine core (average composition). The calculated temperatures range over two hundred degrees and have a systematic relationship with the grain size of spinel. In the Miyamori complex, the calculated temperatures decrease monotonically with decrease in grain size of spinel, whereas in the Ichinomegata lherzolite nodule those of spinel smaller than 0.2 mm increase as the grain size decreases and those of spinel larger than 0.2 mm remain constant regardless of further increase in grain size. These observations, in the light of the diffusion model, suggest that the Miyamori complex may have cooled from higher than 800° C to lower than 600° C and that the lherzolite nodule from the Ichinomegata crater may have been in equilibrium at 900° C before it was heated above 1,100° C for less than a few days. These two examples indicate that olivine-spinel pairs of peridotites do not always indicate an appropriate equilibration temperature. We cannot interpret the supposed equilibration temperatures until the existence of isothermal stages in the thermal history of peridotites is established by carefully checking the chemical heterogeneity.