Geochemistry of basalts from the Caribbean Large Igneous Province

Oceanic flood basalts are poorly understood, short-term expressions of highly increased heat flux and mass flow within the convecting mantle. The uniqueness of the Caribbean Large Igneous Province (CLIP, 92-74 Ma) with respect to other Cretaceous oceanic plateaus is its extensive sub-aerial exposures, providing an excellent basis to investigate the temporal and compositional relationships within a starting plume head. We present major element, trace element and initial Sr-Nd-Pb isotope composition of 40 extrusive rocks from the Caribbean Plateau, including onland sections in Costa Rica, Colombia and Curaçao as well as DSDP Sites in the Central Caribbean. Even though the lavas were erupted over an area of ~3*10**6 km**2, the majority have strikingly uniform incompatible element patterns (La/Yb=0.96+/-0.16, n=64 out of 79 samples, 2sigma) and initial Nd-Pb isotopic compositions (e.g. 143Nd/144Ndin=0.51291+/-3, epsilon-Nd i=7.3+/-0.6, 206Pb/204Pbin=18.86+/-0.12, n=54 out of 66, 2sigma). Lavas with endmember compositions have only been sampled at the DSDP Sites, Gorgona Island (Colombia) and the 65-60 Ma accreted Quepos and Osa igneous complexes (Costa Rica) of the subsequent hotspot track. Despite the relatively uniform composition of most lavas, linear correlations exist between isotope ratios and between isotope and highly incompatible trace element ratios. The Sr-Nd-Pb isotope and trace element signatures of the chemically enriched lavas are compatible with derivation from recycled oceanic crust, while the depleted lavas are derived from a highly residual source. This source could represent either oceanic lithospheric mantle left after ocean crust formation or gabbros with interlayered ultramafic cumulates of the lower oceanic crust. High 3He/4He in olivines of enriched picrites at Quepos are ~12 times higher than the atmospheric ratio suggesting that the enriched component may have once resided in the lower mantle. Evaluation of the Sm-Nd and U-Pb isotope systematics on isochron diagrams suggests that the age of separation of enriched and depleted components from the depleted MORB source mantle could have been <=500 Ma before CLIP formation and interpreted to reflect the recycling time of the CLIP source. Mantle plume heads may provide a mechanism for transporting large volumes of possibly young recycled oceanic lithosphere residing in the lower mantle back into the shallow MORB source mantle.

Chemical, and Sr and Li isotopic composition of pore fluids and sediments from ODP Leg 170 and Leg 205 sites

Pore fluid and sediment chemical and isotopic data were obtained for samples from Ocean Drilling Program (ODP) Leg 205 Sites 1253, 1254, and 1255 in the Costa Rica subduction zone. The chemical and isotopic data reported here were generated in our shore-based laboratories to complement shipboard inorganic geochemical data. Li isotopic analyses were carried out by L.-H. Chan at Louisiana State University (USA). The data reported herein include fluoride, bromide, rubidium, cesium, and barium concentrations; Li and Sr isotopic compositions in pore fluids; and Rb, Cs, and Ba concentrations in representative bulk sediments. The data also include new pore fluid fluoride and bromide concentrations from corresponding ODP Leg 170 Sites 1039, 1040, and 1043. O.M. Saether's Site 1039 and 1040 fluoride concentration data are shown for comparison. Basal sediment fluoride concentrations and Li and Sr isotope ratios at both Sites 1253 and 1039 show reversals that approach modern seawater values. Br/Cl ratios are, however, conservative throughout the sediment section at Sites 1039 and 1253. The observed sharp F and Br concentration maxima, Rb and K concentration minima, the most radiogenic 87Sr/86Sr ratios, and highest 7Li values along the décollement and fracture zone (Sites 1040, 1043, 1254, and 1255) strengthen the evidence obtained during Leg 170 that a deeply sourced fluid, originating from fluid-rock reactions at ~150°C and corresponding to between 10 and 15 km depth, is transporting solutes to the ocean.

Geochemistry on the Kekesai composite intrusion

The late Carboniferous to Permian is a critical period for final amalgamation of the Central Asian Orogenic Belt (CAOB), which is characterized by voluminous igneous rocks, particularly granitoids. The Kekesai composite granitoid porphyry intrusion, situated in the Chinese western Tianshan (southwest part of CAOB) includes two intrusive phases, a monzogranite phase, intruded by a granodiorite phase. LA-ICPMS U-Pb zircon analyses suggest that the monzogranitic rocks formed at 305.5±1.1 Ma, with a wide age range of inherited zircons (358-488 Ma and 1208-1391 Ma), whereas the granodioritic rocks formed at 288.7±1.5 Ma. The monzogranitic and granodioritic phases have similar geochemical features and Sr-Nd-Hf isotopic compositions. They exhibit high and variable SiO2 (66-71 wt.%) and MgO (0.41-2.14 wt.%) contents with some arc-like geochemical characteristics (e.g., enrichment of large ion lithophile elements and negative anomalies of Nb, Ta and Ti) and relatively high initial 87Sr/86Sr ratios (ISr=0.7055-0.7059), low positive eNd(t) (+0.84 to +1.03) as well as a large variation in Hf isotopic compositions with eHf(t) between +3.43 to +14.8, implying both of them were derived from similar source materials. These geochemical characteristics suggest that they might be mainly derived from the partial melting of arc-derived Mesoproterozoic mafic lower crust with involvement of a mantle-derived component in variable proportions by mantle-derived magma underplating. The presence of late-Ordovician to earliest early Carboniferous inherited zircons and the Hf isotopic compositions in the monzogranitic sample, similar to that of the widespread juvenile arc rocks, indicates some crust contamination during magma emplacement. Our new data, combined with previous studies, imply that extensive post-collisional magmatism due to underplating of mantle-derived magma, could plausibly be explained by slab break-off regime.

Geochemistry of North Atlantic Igneous Province basalts

New Sr-Nd-Pb-Hf data require the existence of at least four mantle components in the genesis of basalts from the the North Atlantic Igneous Province (NAIP): (1) one (or more likely a small range of) enriched component(s) within the Iceland plume, (2) a depleted component within the Iceland plume (distinct from the shallow N-MORB source), (3) a depleted sheath surrounding the plume and (4) shallow N-MORB source mantle. These components have been available since the major phase of igneous activity associated with plume head impact during Paleogene times. In Hf-Nd isotope space, samples from Iceland, DSDP Leg 49 (Sites 407, 408 and 409), ODP Legs 152 and 163 (southeast Greenland margin), the Reykjanes Ridge, Kolbeinsey Ridge and DSDP Leg 38 (Site 348) define fields that are oblique to the main ocean island basalt array and extend toward a component with higher 176Hf/177Hf than the N-MORB source available prior to arrival of the plume, as indicated by the compositions of Cretaceous basalts from Goban Spur (~95 Ma). Aside from Goban Spur, only basalts from Hatton Bank on the oceanward side of the Rockall Plateau (DSDP Leg 81) lie consistently within the field of N-MORB, which indicates that the compositional influence of the plume did not reach this far south and east ~55 Ma ago. Thus, Hf-Nd isotope systematics are consistent with previous studies which indicate that shallow MORB-source mantle does not represent the depleted component within the Iceland plume (Thirlwall, J. Geol. Soc. London 152 (1995) 991-996; Hards et al., J. Geol. Soc. London 152 (1995) 1003-1009; Fitton et al., 1997 doi:10.1016/S0012-821X(97)00170-2). They also indicate that the depleted component is a long-lived and intrinsic feature of the Iceland plume, generated during an ancient melting event in which a mineral (such as garnet) with a high Lu/Hf was a residual phase. Collectively, these data suggest a model for the Iceland plume in which a heterogeneous core, derived from the lower mantle, consists of 'enriched' streaks or blobs dispersed in a more depleted matrix. A distinguishing feature of both the enriched and depleted components is high Nb/Y for a given Zr/Y (i.e. positive DeltaNb), but the enriched component has higher Sr and Pb isotope ratios, combined with lower epsilon-Nd and epsilon-Hf. This heterogeneous core is surrounded by a sheath of depleted material, similar to the depleted component of the Iceland plume in its epsilon-Nd and epsilon-Hf, but with lower 87Sr/86Sr, 208Pb/204Pb and negative DeltaNb; this material was probably entrained from near the 670 km discontinuity when the plume stalled at the boundary between the upper and lower mantle. The plume sheath displaced more normal MORB asthenosphere (distinguished by its lower epsilon-Hf for a given epsilon-Nd or Zr/Nb ratio), which existed in the North Atlantic prior to plume impact. Preliminary data on MORBs from near the Azores plume suggest that much of the North Atlantic may be 'polluted' not only by enriched plume material but also by depleted material similar to the Iceland plume sheath. If this hypothesis is correct, it may provide a general explanation for some of the compositional diversity and variations in inferred depth of melting (Klein and Langmuir, 1987 doi:10.1029/JB092iB08p08089) along the MAR in the North Atlantic.

Geochemistry of Central American Volcanic Arc basement samples

The Central American Volcanic Arc (CAVA) has been the subject of intensive research over the past few years, leading to a variety of distinct models for the origin of CAVA lavas with various source components. We present a new model for the NW Central American Volcanic Arc based on a comprehensive new geochemical data set (major and trace element and Sr-Nd-Pb-Hf-O isotope ratios) of mafic volcanic front (VF), behind the volcanic front (BVF) and back-arc (BA) lava and tephra samples from NW Nicaragua, Honduras, El Salvador and Guatemala. Additionally we present data on subducting Cocos Plate sediments (from DSDP Leg 67 Sites 495 and 499) and igneous oceanic crust (from DSDP Leg 67 Site 495), and Guatemalan (Chortis Block) granitic and metamorphic continental basement. We observe systematic variations in trace element and isotopic compositions both along and across the arc. The data require at least three different endmembers for the volcanism in NW Central America. (1) The NW Nicaragua VF lavas require an endmember with very high Ba/(La, Th) and U/Th, relatively radiogenic Sr, Nd and Hf but unradiogenic Pb and low d18O, reflecting a largely serpentinite-derived fluid/hydrous melt flux from the subducting slab into a depleted N-MORB type of mantle wedge. (2) The Guatemala VF and BVF mafic lavas require an enriched endmember with low Ba/(La, Th), U/Th, high d18O and radiogenic Sr and Pb but unradiogenic Nd and Hf isotope ratios. Correlations of Hf with both Nd and Pb isotopic compositions are not consistent with this endmember being subducted sediments. Granitic samples from the Chiquimula Plutonic Complex in Guatemala have the appropriate isotopic composition to serve as this endmember, but the large amounts of assimilation required to explain the isotope data are not consistent with the basaltic compositions of the volcanic rocks. In addition, mixing regressions on Nd vs. Hf and the Sr and O isotope plots do not go through the data. Therefore, we propose that this endmember could represent pyroxenites in the lithosphere (mantle and possibly lower crust), derived from parental magmas for the plutonic rocks. (3) The Honduras and Caribbean BA lavas define an isotopically depleted endmember (with unradiogenic Sr but radiogenic Nd, Hf and Pb isotope ratios), having OIB-like major and trace element compositions (e.g. low Ba/(La, Th) and U/Th, high La/Yb). This endmember is possibly derived from melting of young, recycled oceanic crust in the asthenosphere upwelling in the back-arc. Mixing between these three endmember types of magmas can explain the observed systematic geochemical variations along and across the NW Central American Arc.

Down-core sedimentary Sr-Nd isotope records from two Cape Basin sediment cores in the South Atlantic: GeoB3603-2 (Meteor cruise M34/1) and MD02-2594 (Marion Dufresne cruise MD128)

We report down-core sedimentary Nd isotope (epsilon Nd) records from two South Atlantic sediment cores, MD02-2594 and GeoB3603-2, located on the western South African continental margin. The core sites are positioned downstream of the present-day flow path of North Atlantic Deep Water (NADW) and close to the Southern Ocean, which makes them suitable for reconstructing past variability in NADW circulation over the last glacial cycle. The Fe-Mn leachates epsilon Nd records show a coherent decreasing trend from glacial radiogenic values towards less radiogenic values during the Holocene. This trend is confirmed by epsilon Nd in fish debris and mixed planktonic foraminifera, albeit with an offset during the Holocene to lower values relative to the leachates, matching the present-day composition of NADW in the Cape Basin. We interpret the epsilon Nd changes as reflecting the glacial shoaling of Southern Ocean waters to shallower depths combined with the admixing of southward flowing Northern Component Water (NCW). A compilation of Atlantic epsilon Nd records reveals increasing radiogenic isotope signatures towards the south and with increasing depth. This signal is most prominent during the Last Glacial Maximum (LGM) and of similar amplitude across the Atlantic basin, suggesting continuous deep water production in the North Atlantic and export to the South Atlantic and the Southern Ocean. The amplitude of the epsilon Nd change from the LGM to Holocene is largest in the southernmost cores, implying a greater sensitivity to the deglacial strengthening of NADW at these sites. This signal impacted most prominently the South Atlantic deep and bottom water layers that were particularly deprived of NCW during the LGM. The epsilon Nd variations correlate with changes in 231Pa/230Th ratios and benthic d13C across the deglacial transition. Together with the contrasting 231Pa/230Th: epsilon Nd pattern of the North and South Atlantic, this indicates a progressive reorganization of the AMOC to full strength during the Holocene.

Lavas from the Elbrus volcano, Greater Caucasus

Comprehensive geochronological and isotope-geochemical studies showed that the Late Quaternary Elbrus Volcano (Greater Caucasus) experienced long (approximately 200 ka) discrete evolution with protracted periods of igneous quiescence (approximately 50 ka) between large-scale eruptions. Volcanic activity of Elbrus is subdivided into three phases: Middle Neopleistocene (225-170 ka), Late Neopleistocene (110-70 ka), and Late Neopleistocene - Holocene (earlier than 35 ka). Petrogeochemical and isotope (Sr-Nd-Pb) signatures of Elbrus lavas point to their mantle-crustal origin. It was shown that hybrid parental magmas of the volcano formed due to mixing and/or contamination of deep-seated mantle melts by Paleozoic upper crustal material of the Greater Caucasus. Mantle reservoir that participated in genesis of Elbrus lavas as well as most other Neogene-Quaternary magmatic rocks of Caucasus was represented by the lower mantle "Caucasus" source. Primary melts generated by this source in composition corresponded to K-Na subalkali basalts with the following isotopic characteristics: 87Sr/86Sr = 0.7041+/-0.0001, e-Nd = +4.1+/-0.2, 147Sm/144Nd = 0.105-0.114, 206Pb/204Pb = 18.72, 207Pb/204Pb = 15.62, and 208Pb/204Pb = 38.78. Temporal evolution of isotope characteristics for lavas of the Elbrus Volcano is well described by a Sr-Nd mixing hyperbole between "Caucasus" source and estimated average composition of the Paleozoic upper crust of the Greater Caucasus. It was shown that, with time, proportions of mantle material in parental magmas of Elbrus gently increased: from ~60% at the Middle-Neopleistocene phase of activity to ~80% at the Late Neopleistocene - Holocene phase, which indicates an increase of activity of a deep-seated source at decreasing input of crustal melts or contamination with time. Unraveled evolution of the volcano with discrete eruption events, lacking signs of cessation of the Late Neopleistocene - Holocene phase, increasing contribution of the deep-seated mantle source in genesis of Elbrus lavas with time as deduced from isotope-geochemical data, as well as numerous geophysical and geological evidence indicate that Elbrus is a potentially active volcano and its eruptions may be resumed. Possible scenarios were proposed for evolution of the volcano, if its eruptive activity continued.