Marine hdrothermal alteration at a Kuroko ore deposit,Kosaka, Japan

Isotopic compositions were determined for quartz, sericite and bulk rock samples surrounding the Uwamuki no. 4 Kuroko ore body, Kosaka, Japan. ¹⁸O values of quartz from Siliceous Ore (S.O.), main body of Black Ore B.O.) and the upper layer of B.O. are fairly uniform, +8.7 to +10.5. Formation temperatures calculated from fractionation of ¹⁸O between sericite and quartz from B.O. and upper S.O. are 250° to 300° C. The ore-forming fluids had ¹⁸O values of +1 and D values of –10, from isotope compositions of quartz and sericite.Tertiary volcanic rocks surrounding the ore deposits at Kosaka have uniform ¹⁸O values, +8.1±1.0 (n=50), although their bulk chemical compositions are widely varied because of different degrees of alteration. White Rhyolite, which is an intensely altered rhyolite occurring in close association with the Kuroko ore bodies, has also uniform ¹⁸O values, +7.9±0.9 (n=19). Temperatures of alteration are estimated to be around 300° C from the oxygen isotope fractionation between quartz and sericite. Paleozoic basement rocks phyllite and chert, have high ¹⁸O values, +18 and +19. The Sasahata formation of unknown age, which lies between Tertiary and Paleozoic formations, has highly variable ¹⁸O, +8 to +16 (n=4). High ¹⁸O values of the basement rocks and the sharp difference in ¹⁸O at their boundary suggest that the hydrothermal system causing Kuroko mineralization was mainly confined within permeable Tertiary rocks. D values of altered Tertiary volcanic rocks are highly variable ranging from –34 to –64% (n=12). The variation of D does not correlate with change of chemical composition, ¹⁸O values, nor distance from the ore deposits. The relatively high D values of the altered rocks indicate that the major constituent of the hydrothermal fluid was sea water. However, another fluid having lower D must have also participated. The fluid could be evolved sea water modified by interaction with rocks and the admixture of magmatic fluid. The variation in D may suggest that sea water mixed dispersively with the fluid.     

Meteoric hydrothermal origin of calcites in “Green Tuff” formations,Miocene age,Japan

Oxygen and carbon isotope compositions were determined for calcites from the Green Tuff formations of Miocene age in Japan. Values of ¹⁸O from 24 calcites in altered rocks from 5 districts range from –2 to +16_SMOW, in most cases from 0 to +8_SMOW. The low ¹⁸O values rule out the possibility of their low-temperature origin or any significant contribution of magmatic fluid in the calcite precipitation. These values, coupled with their mineral assemblages, suggest that the calcites formed from meteoric hydrothermal solutions which caused propylitic alteration after the submarine strata became emergent.Values of ¹³C from the calcites show a wide variation from –17 to 0_PDB. Calcites from different districts have different ranges of ¹³C values, indicating that there was no homogeneous reservoir of carbon at the time the calcite formed, and that the carbon had local sources. Carbon isotopic compositions of calcite within ore deposits in the Green Tuff formations range from –19 to 0_PDB, similar to those of calcite in the altered rocks in the same district, suggesting that the carbon in ore calcites was likely supplied from the surrounding rocks through activity of meteoric hydrothermal solutions.     

Flow behavior of large-scale pyroclastic flows — Evidence obtained from petrofabric analysis

The grain orientations within the matrix of two large-scale welded, two small-scale nonwelded and two nonwelded low-aspect ratio pyroclastic flow deposits are measured to analyze flow behavior. Preferred grain alignments are especially apparent in the middle part of layer 2 of each deposit. Preferred grain alignments do not vary laterally within a 10 m interval. The grain alignments obtained are radial from the source caldera, especially in proximal to medial and plateau-forming facies of pyroclastic flow deposits. Grain alignments are controlled by valley-channel directions for the valley-ponded facies of pyroclastic flow deposits, especially at medial to distal locations. Such local topographic factors strongly affect the data for high-aspect ratio and smallscale deposits, and weakly affect the data for widespread low-aspect ratio pyroclastic flow deposits. This work suggests that grain alignment analysis should be used with care when attempting to determine the location of an unknown source.     

Seagrasses from the Nansei Islands, Southern Japanese Archipelago: species composition, distribution and biogeography

The Nansei Islands in the southern Japanese Archipelago have 15 taxa of seagrasses from seven genera within three families. Seagrasses in this region grow on coral sands or coral debris in shallow reefs and on sandy or muddy substrata in the shallow areas of bays and inlets. Certain Halophila species grow in deep water off some islands. Enhalus acoroides only reaches to Ishigaki I. with winter sea water temperature (WST) at 23 °C, while Okinawa I. (WST at 21.6 °C) is the northern biogeographic limit for Halophila decipiens, H. okinawensis, H. major and H. gaudichaudii. Amami‐oshima I. (WST at 20.7 °C) is the northern border for Thalassia hemprichii, H. minor, H. ovalis, Cymodocea serrulata, Cymodocea rotundata, Syringodium isoetifolium, Halodule uninervis and Halodule pinifolia. Halophila mikii the sole seagrass collected from Yakushima I. (WST at 19.3 °C), is of volcanic origin. The distribution of tropical seagrasses in the Nansei Islands is clearly associated with the warm Kuroshio Current, WST and habitat availability. Zostera japonica is the only temperate species occurring in the region. Meadows of Z. japonica, H. ovalis and Halodule pinifolia have disappeared from certain localities in the Archipelago, due probably to human activities and natural siltation.     

Seasonal and interannual variability of sea surface chlorophyll a concentration in the Japan/East Sea (JES)

Seasonal and interannual variability of chlorophyll a concentration in the Japan/East Sea (JES) was detected spatially by ocean color satellite remote sensing. Start timing of the spring bloom was different spatially. The spring bloom started at the subpolar front and southward of it in March, northward of subpolar front, along the Primorye coast and off Hokkaido in April and in the middle of the Japan Basin in May. The start of the spring bloom showed interannual variability that corresponded with the wind speed in the area. The spring bloom in 1998 and 2002 appeared about four weeks earlier than in 1997, 1999 and 2001, and it corresponded with weak winds that can lead to an early development of the thermocline. The bloom was late in 1999 and 2001 in the Japan Basin and along the Primorye coast, and in the southern area in 2000. It corresponded with stronger wind stress that delayed seasonal thermocline formation. The bloom along the Primorye coast appeared later in 1999, and it corresponded with stronger wind stress, and at the same time, it seemed to be related with the delay of melting of sea ice in Mamiya Strait. The fall bloom appeared from early October to early December, and it did not have a clear temporal transition. The area where chlorophyll a concentration exceeded 0.8 μg l⁻¹ was wider in the western area than in the eastern area every year. The magnitude of the fall bloom was different between years, but it did not show a correlation with average wind speed in fall. Those results indicated that the timing of the seasonal bloom in the JES is largely affected by the variability of global climate such as ENSO events.     

Spatial and Temporal Variability of Satellite Primary Production in the Japan Sea from 1998 to 2002

The spatial and seasonal variability of primary production in the Japan Sea from 1998 to 2002 was estimated using a satellite primary production model. A size-fractionated primary production model was validated by in situ primary production data measured in the Japan Sea. Estimated primary production and in situ primary production showed a good positive correlation. Estimated primary production showed spatial variability. Annual primary production levels were 170, 161, 191 and 222 gC m⁻²year⁻¹ at the Russian coast, in the middle of the Japan Basin, the southeastern area and the southwestern area, respectively. It was higher to the south around 40°N than to the north, and higher in the western area than in the eastern one. Peaks of primary production appeared twice, in spring and fall, in the southern area, while a single peak appeared in the northern area. Primary production along the Russian coast was higher than in other areas during summer. The spring bloom contributed 42% to the annual primary production in these four areas. Furthermore, estimated primary production showed an interannual variability that was largest in spring. Primary production in fall also showed interannual variability, especially in the middle of the Japan Basin and the southwestern area. This corresponded mainly to the size of the phytoplankton bloom in each year. Winter convection by wind and the depth of nutrient-rich, cold subsurface water underlying the Tsushima Current may contribute to the nutrient supply to upper layer and interannual variations of primary production in spring.     

苏鲁超高压变质带胡家林超镁铁质岩成因及构造意义

胡家林超镁铁质杂岩体产于苏鲁超高压变质带中部,纯橄岩和(石榴)单斜辉石岩呈不连续透镜体产于蛇纹石化橄榄岩中。纯橄岩遭受了部分蛇纹石化(烧失量=6.6%～13.2%),全岩富集强相容元素(Ni、Cr、Co)和Ir族PGE(IPGE;Ir、Os、Ru)及高IPGE/PPGE值,亏损Al、Ti、V,具高Mg~#橄榄石(Fo=91.7～92.4)和高Cr~#(0.68～0.76)尖晶石。纯橄岩高耐熔地球化学及矿物化学特征和古老的大陆岩石圈地幔相一致,表明其原岩来源于弧前地幔,代表了华北克拉通古老的大陆岩石圈地幔残留。(石榴)单斜辉石岩全岩呈相对低含量的强相容元素(Cr、Ni、Co)和IPGE,高含量的Al、Ti、V和流体迁移元素(Sr、Pb和Ba),球粒陨石标准化REE配分图呈明显"上凸"型,具低Mg~#橄榄石(Fo=76.6～76.8)和低Al_2O_3(2.76%)和高SiO_2(54.56%～56.87%)的单斜辉石。全岩组成和矿物化学表明其原岩为俯冲带内超镁铁质火成堆晶岩,最初岩浆由地幔岩高程度部分熔融的熔体和俯冲带中富H_2O流体和/或熔体构成。(石榴)单斜辉石岩原岩曾被地幔流带入扬子大陆俯冲板片和上覆地幔楔之间的俯冲通道,经历了超高压变质作用和生成大量石榴石。(石榴)单斜辉石岩在折返过程中,与大陆岩石圈地幔楔剥离的蛇纹石化橄榄岩及纯橄岩相结合,形成超镁铁质杂岩体,整体被低密度的俯冲板片(主要由花岗质片麻岩和变质沉积岩组成)裹挟,折返至地壳浅部。     

Subduction of mantle wedge peridotites: Evidence from the Higashi-akaishi ultramafic body in the Sanbagawa metamorphic belt

The Higashi-akaishi garnet-bearing ultramafic body in the Sanbagawa metamorphic belt, Southwest Japan, represents a rare example of oceanic-type ultrahigh-pressure metamorphism. The body of 2 km × 5 km is composed mostly of anhydrous dunite with volumetrically minor lenses of clinopyroxene-rich rocks. Dunite samples contain high Ir-type platinum group elements (PGE) and Cr in bulk rocks, high Mg and Ni in olivine, and high Cr in spinel. On the other hand, clinopyroxene-rich rocks contain low concentrations of Ir-type PGE and Cr, high concentrations of fluid-mobile elements in bulk rocks, and low Ni and Mg in olivine. Clinopyroxene is diopsidic with low Al₂O₃. The compositions of bulk rocks and mineral chemistry of spinel, olivine, and clinopyroxene suggest that the olivine-dominated rocks are residual mantle peridotites after high degrees of influx partial melting, and that the clinopyroxene-rich rocks are cumulates of subduction-related melts. Thus, the Higashi-akaishi ultramafic body originated from the interior of the mantle wedge, most likely the forearc upper mantle. It was then incorporated into the Sanbagawa subduction channel by a mantle flow, and underwent high pressure metamorphism to a depth greater than 100 km. Such a strong active flow in the mantle wedge is likely facilitated by the lack of serpentinites along the interface between the slab and the overlying mantle, as it was too hot for serpentine. These unusually hot conditions and strong active mantle flow may reflect conditions in the earliest stage of development of subduction, and may have been maintained by massive upwelling and subsequent eastward flow of asthenospheric mantle in the northeastern Asian continent in Cretaceous time when the Sanbagawa belt began to form.     

吉林东部汪清新生代玄武岩中地幔包体的矿物化学：对该地区岩石圈地幔形成过程的启示

在对吉林汪清新生代玄武岩中橄榄岩包体进行全岩主、微量元素分析的基础上,重点分析了矿物的主、微量元素含量,讨论了华北克拉通东北缘与兴蒙造山带结合部位陆下岩石圈地幔所经历的部分熔融作用和地幔交代作用。橄榄石的Fo、单斜辉石以及尖晶石的Mg#和Cr#均表明该地区陆下岩石圈地幔相对饱满。对地幔橄榄岩透辉石微量元素模拟计算证实,汪清陆下岩石圈地幔经历了1%~3%的部分熔融作用,并遭受后期硅酸盐熔体交代作用。这些地幔特征与中国东部其他地区新生代主体岩石圈相似。研究结果证明,吉林汪清中、新生代以来岩石圈性质的转变是在汇聚板块构造体制下通过“拆沉作用”实现的。