Geochemistry of Lesser Antilles lavas

We present new U-series disequilibrium and radiogenic isotope data for 7 mafic lavas from the Lesser Antilles arc. These are combined with published data in an internally consistent model that quantitatively estimates the amount of sediment and fluid added to the source of the Lesser Antilles arc system. Some lavas form an array consistent with bulk sediment addition (0.2-2%) whereas others appear to require addition of 0.4-2% sediment melt, particularly in the south of the arc. Evidence for both bulk sediment and sediment melt addition can be found within both the northern and central sections of the arc suggesting a thermal structure whereby the upper portions of the subducted sediment pile lie close to their solidus beneath much of the arc. Addition of up to 5% fluid derived from altered oceanic crust to these sediment enriched mantle wedge source regions can simulate the majority of the lavas on a plot of 207Pb/204Pb versus Ce/Pb. By taking into account the range in calculated wedge compositions and allowing for some mobility of Th in the fluid, the same model can also account for much of the observed range of U-Th-Ra disequilibria, especially if the eclogitic residue contains trace amounts of rutile. The implication of this more complex model is that the time scales for fluid addition and differentiation could be significantly shorter than those estimated in some previous studies.

U-series ages from calcitic cold-water corals from the Amundsen Sea

Radiocarbon and uranium-thorium dating results are presented from a genus of calcitic Antarctic cold-water octocorals (family Coralliidae), which were collected from the Marie Byrd Seamounts in the Amundsen Sea (Pacific sector of the Southern Ocean) and which to date have not been investigated geochemically. The geochronological results are set in context with solution and laser ablation-based element/Ca ratios (Li, B, Mg, Mn, Sr, Ba, U, Th). Octocoral radiocarbon ages on living corals are in excellent agreement with modern ambient deep-water D14C, while multiple samples of individual fossil coral specimens yielded reproducible radiocarbon ages. Provided that local radiocarbon reservoir ages can be derived for a given time, fossil Amundsen Sea octocorals should be reliably dateable by means of radiocarbon. In contrast to the encouraging radiocarbon findings, the uranium-series data are more difficult to interpret. The uranium concentration of these calcitic octocorals is an order of magnitude lower than in the aragonitic hexacorals that are conventionally used for geochronological investigations. While modern and Late Holocene octocorals yield initial d234U in good agreement with modern seawater, our results reveal preferential inward diffusion of dissolved alpha-recoiled 234U and its impact on fossil coral d234U. Besides alpha-recoil related 234U diffusion, high-resolution sampling of two fossil octocorals further demonstrates that diagenetic uranium mobility has offset apparent coral U-series ages. Combined with the preferential alpha-recoil 234U diffusion, this process has prevented fossil octocorals from preserving a closed system U-series calendar age for longer than a few thousand years. Moreover, several corals investigated contain significant initial thorium, which cannot be adequately corrected for because of an apparently variable initial 232Th/230Th. Our results demonstrate that calcitic cold-water corals are unsuitable for reliable U-series dating. Mg/Ca ratios within single octocoral specimens are internally strikingly homogeneous, and appear promising in terms of their response to ambient temperature. Magnesium/lithium ratios are significantly higher than usually observed in other deep marine calcifiers and for many of our studied corals are remarkably close to seawater compositions. Although this family of octocorals is unsuitable for glacial deep-water D14C reconstructions, our findings highlight some important differences between hexacoral (aragonitic) and octocoral (calcitic) biomineralisation. Calcitic octocorals could still be useful for trace element and some isotopic studies, such as reconstruction of ambient deep water neodymium isotope composition or pH, via boron isotopic measurements.

Zircon U-Pb and Hf-O analysis of porphyries from the Duolong porphyry Cu-Au deposit in the Bangongco copper belt, central Tibet

The Duolong porphyry Cu-Au deposit (5.4 Mt at 0.72% Cu, 41 t at 0.23 g/t Au), which is related to the granodiorite porphyry and the quartz-diorite porphyry from the Bangongco copper belt in central Tibet, formed in a continental arc setting. Here, we present the zircon U-Pb ages, geochemical whole-rock, Sr-Nd whole-rock and zircon in-situ Hf-O isotopic data for the Duolong porphyries. Secondary ion mass spectrometry (SIMS) zircon U-Pb analyses for six samples yielded consistent ages of ~118 Ma, indicating a Cretaceous formation age. The Duolong porphyries (SiO2 of 58.81-68.81 wt.%, K2O of 2.90-5.17 wt.%) belong to the high-K calc-alkaline series. They show light rare earth element (LREE)-enriched distribution patterns with (La/Yb)N = 6.1-11.7, enrichment in large ion lithophile elements (e.g., Cs, Rb, and Ba) and depletion of high field strength elements (e.g., Nb), with negative Ti anomalies. All zircons from the Duolong porphyries share relatively similar Hf-O isotopic compositions (d18O=5.88-7.27 per mil; eHf(t)=3.6-7.3), indicating that they crystallized from a series of cogenetic melts with various degrees of fractional crystallization. This, along with the general absence of older inherited zircons, rules out significant crustal contamination during zircon growth. The zircons are mostly enriched in d18O relative to mantle values, indicating the involvement of an 18O-enriched crustal source in the generation of the Duolong porphyries. Together with the presence of syn-mineralization basaltic andesite, the mixing between silicic melts derived from the lower crust and evolved H2O-rich mafic melts derived from the metsomatizied mantle wedge, followed by subsequent fractional crystallization (FC) and minor crustal contamination in the shallow crust, could well explain the petrogenesis of the Duolong porphyries. Significantly, the hybrid melts possibly inherited the arc magma characteristics of abundant F, Cl, Cu, and Au elements and high oxidation state, which contributed to the formation of the Duolong porphyry Cu-Au deposit.

A deep-sea coral record of the North Atlantic

Our record of Younger Dryas intermediate-depth seawater D14C from North Atlantic deep-sea corals supports a link between abrupt climate change and intermediate ocean variability. Our data show that northern source intermediate water (~1700 m) was partially replaced by 14C-depleted southern source water at the onset of the event, consistent with a reduction in the rate of North Atlantic Deep Water formation. This transition requires the existence of large, mobile gradients of D14C in the ocean during the Younger Dryas. The D14C water column profile from Keigwin (2004) provides direct evidence for the presence of one such gradient at the beginning of the Younger Dryas (~12.9 ka), with a 100 per mil offset between shallow (<~2400 m) and deep water. Our early Younger Dryas data are consistent with this profile and also show a D14C inversion, with 35 per mil more enriched water at ~2400 m than at ~1700 m. This feature is probably the result of mixing between relatively well 14C ventilated northern source water and more poorly 14C ventilated southern source intermediate water, which is slightly shallower. Over the rest of the Younger Dryas our intermediate water/deepwater coral D14C data gradually increase, while the atmosphere D14C drops. For a very brief interval at ~12.0 ka and at the end of the Younger Dryas (11.5 ka), intermediate water D14C (~1200 m) approached atmospheric D14C. These enriched D14C results suggest an enhanced initial D14C content of the water and demonstrate the presence of large lateral D14C gradients in the intermediate/deep ocean in addition to the sharp vertical shift at ~2500 m. The transient D14C enrichment at ~12.0 ka occurred in the middle of the Younger Dryas and demonstrates that there is at least one time when the intermediate/deep ocean underwent dramatic change but with much smaller effects in other paleoclimatic records.

Table A1 LA-ICP-MS zircon U-Pb data of Laoniushan granitoids

The NWW-striking Qinling Orogen formed in the Triassic by collision between the North China and Yangtze Cratons. Triassic granitoid intrusions, mostly middle- to high-K, calc-alkaline in composition, are widespread in this orogen, but contemporaneous intrusions are rare in the southern margin of the North China Craton, an area commonly considered as the hinterland belt of the orogen. In this paper, we report zircon U-Pb ages, elemental geochemistry, and Sr-Nd-Hf isotope data for the Laoniushan granitoid complex that was emplaced in the southern margin of the North China Craton. Zircon U-Pb dating shows that the complex was emplaced in the late Triassic (228±1 to 215±4 Ma), indicating that it is part of the post-collisional magmatism in the Qinling Orogen. The complex consists of, from early to late, biotite monzogranite, quartz diorite, quartz monzonite, and hornblende monzonite, which have a wide compositional range, e.g., SiO2=55.9-70.6 wt%, K2O+Na2O=6.6-10.2 wt%, and Mg# of 24 to 54. Rocks of the biotite monzogranite have high Al2O3(15.5-17.4 wt%), Sr(396-1398 ppm) and Ba(1284-3993 ppm) contents and La/Yb(mostly 14-30) and Sr/Y(mostly 40-97) ratios, but low Yb(mostly 1.3-1.6 ppm) and Y(mostly14-19 ppm) contents, features typical of adakite. The quartz monzonite, hornblende monzonite and quartz diorite have a shoshonitic affinity, with K2O up to 5.58 wt% and K2O/Na2O ratios averaging 1.4. The rocks are characterized by strong LREE/HREE fractionation in chondrite-normalized REE pattern, without obvious Eu anomalies, and show enrichment in large ion lithophile elements but depletion in high field strength elements (Nb, Ta, Ti). The biotite monzogranite (228 Ma) has initial 87Sr/86Sr ratios of 0.7061 to 0.7067, eNd(t) values of -9.2 to -12.6, and ?Hf(t) values of -9.0 to -15.1; whereas the shoshonitic granitoids (mainly 217-215 Ma) have similar initial 87Sr/86Sr ratios (0.7065 to 0.7075) but more radiogenic eNd(t) (-12.4 to -17.0) and eHf(t) (-14.1 to -17.0). The Sr-Nd-Hf isotope data indicate that the rocks were likely generated by partial melting of an ancient lower continental crust with heterogeneous compositions, as partly confirmed by the widespread presence of the early Paleoproterozoic inherited zircons. Mafic microgranular enclaves (MMEs), characterized by fine-grained igneous textures and an abundance of acicular apatites, are common in the Laoniushan complex. Compared with the host rocks, they have lower SiO2 (48.6-53.7 wt.%) and higher Mg# (51-56), Cr (122-393 ppm), and Ni (24-79 ppm), but equivalent Sr-Nd isotope compositions, indicating that the MMEs likely originated from an ancient enriched lithospheric mantle. The abundance of MMEs in the granitoid intrusions suggests that magma mixing plays an important role in the generation of the Laoniushan complex. Collectively, it is suggested that the Laoniushan complex was a product of post-collisional magmatism related to lithospheric extension following slab break-off. Formation of the adakitic and shoshonitic intrusions in the Laoniushan complex indicates that the Qinling Orogen had evolved into a post-collisional setting by about 230-210 Ma.

Chemical composition of ocean phosphorites and hosted sediments

The book provides an overview of recent data on processes of dispersion and concentration of phosphorus in marine sedimentation. Distribution, chemical and mineral compositions, structure and age of phosphorites occurring on the floor are described. Phosphorites are one of potential mineral resources of the World Ocean. A scheme of multistage-oceanic phosphorite formation is motivated. Modern and pre-Quaternary phosphorite formations in the ocean are paralleled.

Ages and stable isotope ratios from speleothem calcite from Cold Water Cave, Iowa

High-precision uranium-thorium mass spectrometric chronology and 18O-13C isotopic analysis of speleothem calcite from Cold Water Cave in northeast Iowa have been used to chart mid-Holocene climate change. Significant shifts in d18O and d13C isotopic values coincide with well-documented Holocene vegetation changes. Temperature estimates based on 18O/16O ratios suggest that the climate warmed rapidly by about 3°C at 5900 years before present and then cooled by 4°C at 3600 years before present. Initiation of a gradual increase in ?d13C at 5900 years before present suggests that turnover of the forest soil biomass was slow and that equilibrium with prairie vegetation was not attained by 3600 years before present.