Geochemical and geochronological constraints on the origin and emplacement of the Shimo-ondori diorites in Shikoku,Southwest Japan

In this study, the LA-ICP-MS zircon U–Pb dating of the Shimo-ondori diorites in the Shimanto accretionary complex of SW Japan provides ~130 Ma, representing the timing of their crystallization ages. Combined with the geological occurrence, that age clearly indicates that the diorites occur as blocks, not as intrusive rocks as suggested by previous studies. Moreover, the ages of the Shimo-ondori diorites are suggesstive that they could be influential for the estimate of the early-Cretaceous tectonic evolution for the eastern Asian margin. Their whole-rock chemical compositions show high MgO, Ni and Cr contents, and low total FeO/MgO ratios, indicating that they were crystallized from high magnesian andesite (HMA) magmas. Moreover, their TiO₂ and REE compositions suggest that they were formed by the same processes as the sanukites. And, the zircon Hf isotopic ratios (ε_Hf [~130 Ma] = +9.9 − +17.5), which is close to or slightly lower than that of the ~130 Ma depleted mantle, suggest that the wedge-mantle materials were predominantly involved in the formation of the dioritic magmas. Their geochronological and geochemical similarities of the Shimo-ondori diorites with the early Cretaceous adakites and HMAs in the eastern Asian margin suggest that they might have been formed possibly by the same slab rollback of the Izanagi plate at the early Cretaceous. After the crystallization of the Shimo-ondori diorites, they were delivered and deposited as blocks in a trench site with the surrounding sedimentary rocks of the Shimanto accretionary complex.     

Dawson tephra,a widespread 29-ka marker bed,in a marine core from Patton Seamount off the Alaska Peninsula and its potential marine–terrestrial correlation

A tephra layer with normal grading in the sub-bottom depth interval 119–122 cm in marine core SO202-27-6 was collected on Patton Seamount in the northeast North Pacific Ocean. Based on the geochemistry of volcanic glass shards determined by a wavelength dispersive electron probe micro-analyser and an X-ray fluorescence analyser, this layer is correlated to the Dawson tephra, a widespread late Pleistocene time marker tephra in Alaska and the Yukon. The age of the Dawson tephra in the core is 29.03 ± 0.178 ka (1 sigma) based on a published age model. The Dawson tephra is revealed to have been deposited in the transition from marine isotope stage 3 to 2, i.e. the last stage of Heinrich Stadial 3 derived from the ice-rafted debris signal. According to the correlation between Greenland (NGRIP ice core) and this core, the Dawson tephra occupies the record immediately before inter stadial 4 in the δ¹⁸O stratigraphy of NGRIP. The Dawson tephra on Patton Seamount includes lithic fragments, which suggests that it was deposited not only by fall-out but also in part via another mechanism, such as icebergs from the Cordilleran ice sheet or seasonal sea ice.     

Calcite U–Pb age of the Cretaceous vertebrate‐bearing Bayn Shire Formation in the Eastern Gobi Desert of Mongolia: Usefulness of caliche for age determination

To verify the usefulness of calcite U–Pb measurement for vertebrate‐bearing strata in the Eastern Gobi, Mongolia, we performed laser ablation‐inductively coupled plasma‐mass spectrometry calcite U–Pb and trace element analyses of three caliche (calcrete) of the Bayn Shire Formation. The trace element analysis demonstrates high concentration of U in the calcites. Two meaningful calcite U–Pb ages were obtained; 95.9 ± 6.0 and 89.6 ± 4.0 Ma, which are consistent with published ages from the Bayn Shire Formation. Our results demonstrate that the calcite U–Pb method can be powerful tool for age determination of vertebrate‐bearing strata in the Gobi that do not contain index fossils or beds, but do contain caliches. This would make it possible for a comparison of biostratigraphy between the Gobi and other areas yielding abundant vertebrate fossils in Asia, North America and Europe, based on chronological data.     

Raman spectroscopy and heat capacity measurement of calcium ferrite type MgAl2O4 and CaAl2O4

Raman spectroscopy of calcium ferrite type MgAl₂O₄ and CaAl₂O₄ and heat capacity measurement of CaAl₂O₄ calcium ferrite were performed. The heat-capacity of CaAl₂O₄ calcium ferrite measured by a differential scanning calorimeter (DSC) was represented as C_P(T)=190.6–1.116 × 10⁷T^–2 + 1.491 × 10⁹T^–3 above 250 K (T in K). The obtained Raman spectra were applied to lattice dynamics calculation of heat capacity using the Kieffer model. The calculated heat capacity for CaAl₂O₄ calcium ferrite showed good agreement with that by the DSC measurement. A Kieffer model calculation for MgAl₂O₄ calcium ferrite similar to that for CaAl₂O₄ calcium ferrite was made to estimate the heat capacity of the former. The heat capacity of MgAl₂O₄ calcium ferrite was represented as C_P(T)=223.4–1352T ^–0.5 – 4.181 × 10⁶T ^–2 + 4.300 × 10⁸T ^–3 above 250 K. The calculation also gave approximated vibrational entropies at 298 K of calcium ferrite type MgAl₂O₄ and CaAl₂O₄ as 97.6 and 114.9 J mol^–1 K^–1, respectively.     

Sanidine—olivine—diopside Association in a Metasomatized Garnet Lherzolite Xenolith from Southeastern China

A garnet lherzolite xenolith hosted in olivine nephelinite from Xilong,Zhejiang Province ,southeastern China, shows clear evidence of modal metasomatism involving a special sanidine-olivine-diopside(SOD) association which was produced by replacement of primary orthopyroxene.The fluid responsible for the measomatism was a silica-undersaturated vapour,rich in K,Ba,Sr and Ti.It is suggested that the SOD assemblage is the product of metasomatism of a depleted lherzolite precursor,and formed in the upper mantle prior to entrainment and eruption.     

Rare gases in a phlogopite nodule and a phlogopite-bearing peridotite in South African kimberlites

Rare gas isotopes in a phlogopite nodule and a phlogopite-bearing peridotite nodule in South African kimberlites were studied to examine the state of rare gases in the deep interior of the kimberlite region.Within the experimental error of 1 ～ 2%, rare gas isotopic compositions are atmospheric except for radiogenic⁴He and⁴⁰Ar. No excess¹²⁹Xe was observed.In phlogopite, Ne is more depleted, whereas the heavier rare gases are more enriched than the atmospheric rare gases relative to³⁶Ar.Together with other data these results suggest that the state of rare gases in the upper mantle of the South African kimberlite region might have been changed from the typical primitive mantle by a process such as mixing of crustal materials.     

Content and behavior of fluorine in Japanese quaternary volcanic rocks and petrogenetic application

Fluorine contents in about 160 representative Quaternary volcanic rocks and 15 hornblende and biotite phenocrysts in a calc-alkali series in Japan have been determined by a selective ion-electrode method. Tholeiites have the lowest contents and the narrowest range (58–145 ppm), while alkali basalts have the highest contentws and the widest range (301–666 ppm), high-alumina basalts have intermediate values (188–292 ppm). F contents in basalts clearly increase from east to west across the Japanese Islands, as do alkalies, P₂O₅ REE, U, Th and H₂O.The volcanic rocks studied are divided into two groups on the basis of F: (1) witt, increasing % SiO₂ or advancing fractionation, F contents show either progressive enrichment; or (2) with increasing fractionation, F contents show rather constant values. The former is produced by fractionation of anhydrous phases from basalt to mafic andesite magmas; the tholeiite series of Nasu volcanic zone (outer zone), northeastern, Japan is a typical example. The latter group is derived through separation of amphibole-bearing phases from basaltic magmas at various depths from upper mantle (about 30 km) to upper crust; the alkali series in southwestern Japan and the calc-alkali series of Chokai volcanic zone (inner zone), northeastern Japan, are examples.     

Exsolved garnet-bearing pyroxene megacrysts from some South African kimberlites

Clinopyroxene and orthopyroxene megacrysts containing garnet lamellae up to 1.2 mm thick as an exsolved phase are found rarely in kimberlites from Frank Smith and Bellsbank. Chemically the clinopyroxenes are characteristically subcalcic, being within the range of 100 Ca/Ca + Mg + Fe = 27 to 36, and the orthopyroxenes are characterized by high Al₂O₃ and Cr₂O₃. Immediately after crystallization during very slow cooling, clinopyroxene and orthopyroxene exsolve wide-spaced orthopyroxene and clinopyroxene phases parallel to (100) of the host phases, respectively, then both host and exsolved phases exsolve garnet lamellae. Topotactic relations between pyroxenes and garnet are determined by X-ray for the first time. Partitioning of major and minor elements among the coexisting clinopyroxene, orthopyroxene and garnet in pyroxene megacrysts is the same as that of the granular-type garnet peridotite xenoliths in Lesotho and South African kimberlies. Mineralogy and chemistry indicate that subcalcic clinopyroxene and orthopyroxene megacrysts contain respectively about 10 and 3 mole % of the garnet molecule in solid solution.     

Fluorine geochemistry of basaltic rocks from continental and oceanic regions and petrogenetic application

Fluorine contents in about 300 samples of various types of basalts and related rocks from continental (southwestern U.S.A.; Zaire; Deccan and South Africa) and oceanic regions (Hawaii and Mid-Atlantic Ridge between 23° N and 40° N) were determined by a selective ion-electrode method.Of all of the major components in these basaltic rocks, F shows good correlation only with K₂O. It increases regularly from tholeiite to perpotassic basalt on continents, and from tholeiite to nephelinite on Hawaii. In the F-K₂O diagram all the basaltic rocks from continents and Hawaii plot between the origin of the coordinate axes and the field of phlogopite in peridotite xenoliths in South African kimberlites. Accordingly, the major proportions of F, K₂O and also H₂O in these basaltic magmas are derived from phlogopite at the source regions in the upper mantle. On the other hand, F in abyssal tholeiites is relatively higher than that of the other tholeiites at equal K₂O content, and it is suggested that most of F, K₂O and H₂O are derived from pargasites.When it is assumed that the upper mantle phlogopite contains about 10% K₂O, 0.4% (0.3–0.5%) F and 4% H₂O, H₂O content in basaltic magmas from continental including island arc and oceanic island regions can be qualitatively estimated based on their proportions of K₂OFH₂O. Similarly, H₂O content in abyssal basaltic rocks is also estimated on the basis of FH₂O in pargasites (Table 2).A suite of Deccan tholeiites shows remarkable F enrichment with increasing K₂O due to separation of anhydrous and K-free minerals during fractionation. F in tholeiitic and alkali basalt magmas in Hawaii also increases regularly with K₂O during progressive fractionation until the later stages, where rhyodacite and trachyte exhibit a relative decrease owing to the effective subtraction of F-bearing amphibole and apatite in addition to anhydrous minerals.     

Pyroxenes from lherzolite inclusions of Itinome-gata,Japan

The lherzolites have recrystallized to plagioclase lherzolites consisting of olvine, pyroxenes, chromian spinel, plagioclase and pargasite at a depth of 20 to 25 km in the uppermost part of the mantle. It is believed that the garnet lherzolites and spinel lherzolites were originally derived from depths of 50–75 km and 30–50 km respectively. The clinopyroxenes contained about 10 mol. % of jadeite and Tschermak's molecules, respectively and the orthopyroxenes also included about 5–10% of Tschermak's component. Transported upward, the garnet was transformed through pyroxene-spinel symplectite to olivine, plagioclase and spinel aggregates, and most of the jadeite amd some Tschermak's components in the pyroxenes formed secondary pyroxenes and pargasite, and finally plagioclase under isochemical conditions.