ABSTRACT This study provides a composite record of 87 Sr/ 86 Sr, δ 18 O, and δ 13 C for three sections in the Tethyan Lower Cretaceous Maiolica formation, a pelagic limestone from the Umbria-Marche Apennines of Italy, carefully tied to a magnetostratigraphically and biostratigraphically calibrated time scale. Although the 87 Sr/ 86 Sr record accurately follows the trend of the global marine 87 Sr/ 86 Sr reference curve, individual Sr isotope ratio values are relatively high for their inferred stratigraphic position, with all 87 Sr/ 86 Sr ratios yielding a fairly uniform +0.00007 to +0.0001 discrepancy. This offset likely results from incorporation of excess 87 Sr through isotopic reequilibration with interstitial pore waters during progressive lithification of the calcareous ooze. Although the process occurs principally through dissolution-reprecipitation, buffering the contemporaneous seawater Sr isotopic signature, diffusive communication with the overlying water column and porous sediments will compete with the dissolution-precipitation process, homogenizing pore-fluid concentrations and isotope ratios throughout the sediment column. Because the secular trend in 87 Sr/ 86 Sr throughout the Maiolica time frame is one of constant increase before rebounding to lower 87 Sr/ 86 Sr ratios in the Barremian, the ratios of the Maiolica carbonates are systematically displaced from that of the seawater in which they were deposited toward more radiogenic (higher 87 Sr/ 86 Sr) values. In addition, the carbon and oxygen isotope record of the Maiolica limestone allows identification of the mid-Valanginian Weissert event, characterized by a positive excursion in the δ 13 C and the δ 18 O records. Furthermore, the Weissert event correlates with a positive spike (+0.0001) in 87 Sr/ 86 Sr. Both the Sr and O isotopic peak signals predate the maximum peak in the δ 13 C excursion. This is likely a diagenetic artifact and may support the hypothesis of diffusive communication during lithification of the calcareous ooze.

ABSTRACT The geochemical signatures of sparry calcite-sealing expansion breccias, calcite veins, and host clasts were analyzed for their strontium ( 87 Sr/ 86 Sr) and oxygen and carbon (δ 18 O, δ 13 C) stable isotopic signatures. The breccias occur within the Lower Cretaceous Maiolica Formation. Related but different breccias are found in a few places in the Upper Cretaceous to Eocene Scaglia Rossa Formation of the Umbria-Marche Apennines fold-and-thrust belt (Italy). We propose hydraulic fracturing by fluid overpressure as a possible mechanism for generation of the breccias in these formations. Our data are compatible with the hypothesis of a hydraulically fractured breccia formed by cyclic buildup and rapid decompression of CO 2 -rich fluids, with overpressures generated by entrapment of CO 2 by structural and stratigraphic seals. Strontium and oxygen isotope ratio data suggest that the CO 2 -rich fluids may have originated from carbonate metasomatism of the mantle, resulting from subduction of carbonate-rich lithologies constituting the downgoing slab. This is consistent with previous conceptual models inferring that in the central part of the Northern Apennines, which is characterized by thick continental crust, CO 2 -rich fluids derived from mantle metasomatism would become trapped in structural seals, creating high fluid overpressures.

Fifteen impactites from various intervals within the Eyreville cores of the Chesapeake Bay impact structure were sampled to measure siderophile element concentrations. The sampled intervals include basement-derived rocks with veins, polymict impact breccias and associated rocks, and crater-fill sediments. The platinum group element (PGE) concentrations obtained are generally low (e.g., iridium concentrations less than 0.1 ng/g) and are fractionated relative to chondrites. There is no clear distinction in concentration between the different impactite units. So far in the Chesapeake Bay material, only the impact melt rocks from the 823-m-deep Cape Charles test hole, drilled over the central uplift of the structure, have generated a bulk chondritic signature of 0.01–0.1 wt% meteoritic contribution based on a mixing model of 187 Os/ 188 Os isotopic ratios and Os concentrations. However, none of the samples studied shows PGE abundances that enable identification of the type of projectile responsible for the formation of the structure. Hence, it is at present not possible to link the Chesapeake Bay impact to the proposed ordinary chondrite falls by projectiles recorded for other late Eocene craters, namely the 100-km-diameter Popigai impact structure in Siberia and 7.5-km-diameter Wanapitei structure in Canada. The absence of a clear projectile signature hinders further discussions on the existence and the nature of the late Eocene shower event (asteroid versus comet).

Abstract By using a set of granitic whole rock samples, originating from the Regensburg Forest (Germany) and dated (approximately 350 Ma old) previously by means of thermal ionization mass spectrometry (TIMS), the capability of dynamic reaction cell (DRC) inductively coupled plasma mass spectrometry (ICPMS) for Rb-Sr age determination was demonstrated. With DRC-ICPMS, chemical separation of Sr from Rb during the sample pretreatment is no longer required as interference-free determination of the 87 Sr/ 86 Sr isotope ratio can be accomplished by monitoring the signals of the SrF + adduct ions, formed as a result of the selective reaction between the Sr + ions extracted from the ICP and the reaction gas CH 3 F. The mass discrimination was established to depend strongly on the matrix composition. This drawback could be overcome by using the United States Geological Survey reference material G-2 as an external standard. Results obtained by DRC-ICPMS (age and initial 87 Sr/ 86 Sr ratio) showed an excellent agreement with both (a) experimental values obtained by means of quadrupole-based and sector field ICPMS after isolation of Sr via cation exchange chromatography and (b) TIMS literature values. In addition, DRC-ICPMS offers a smaller combined uncertainty on the isotope ratio results as a result of (a) an improved internal isotope ratio precision (<0.1% RSD when also using Ne as an additional non-reactive collision gas) and (b) the fact that, in contrast to quadrupole-based and sector field ICPMS, no correction for the remaining overlap between the signals of 87 Sr + and 87 Rb + is required.