Element analyses and isotope composition of basalts from the West Valley segment, Juan de Fuca Ridge, northeast Pacific

The 50 km-long West Valley segment of the northern Juan de Fuca Ridge is a young, extension-dominated spreading centre, with volcanic activity concentrated in its southern half. A suite of basalts dredged from the West Valley floor, the adjacent Heck Seamount chain, and a small near-axis cone here named Southwest Seamount, includes a spectrum of geochemical compositions ranging from highly depleted normal (N-) MORB to enriched (E-) MORB. Heck Seamount lavas have chondrite-normalized La/Sm en -0.3, 87Sr/86Sr = 0.70235 - 0.70242, and 206Pb/204Pb = 18.22 - 18.44, requiring a source which is highly depleted in trace elements both at the time of melt generation and over geologic time. The E-MORB from Southwest Seamount have La/Sm en -1.8, 87Sr/86Sr = 0.70245 - 0.70260, and 206Pb/204Pb = 18.73 - 19.15, indicating a more enriched source. Basalts from the West Valley floor have chemical compositions intermediate between these two end-members. As a group, West Valley basalts from a two-component mixing array in element-element and element-isotope plots which is best explained by magma mixing. Evidence for crustal-level magma mixing in some basalts includes mineral-melt chemical and isotopic disequilibrium, but mixing of melts at depth (within the mantle) may also occur. The mantle beneath the northern Juan de Fuca Ridge is modelled as a plum-pudding, with "plums" of enriched, amphibole-bearing peridotite floating in a depleted matrix (DM). Low degrees of melting preferentially melt the "plums", initially removing only the amphibole component and producing alkaline to transitional E-MORB. Higher degrees of melting tap both the "plums" and the depleted matrix to yield N-MORB. The subtly different isotopic compositions of the E-MORBs compared to the N-MORBs require that any enriched component in the upper mantle was derived from a depleted source. If the enriched component crystallized from fluids with a DM source, the "plums" could evolve to their more evolved isotopic composition after a period of 1.5-2.0 Ga. Alternatively, the enriched component could have formed recently from fluids with a lessdepleted source than DM, such as subducted oceanic crust. A third possibility is that enriched material might be dispersed as "plums" throughout the upper mantle, transported from depth by mantle plumes.

Chemical and isotopic compositions of basaltic glasses and crusts from the Mid-Atlantic Ridge rift valley

Data presented in the paper suggest significant differences between thermodynamic conditions, under which magmatic complexes were formed in MAR at 29°-34°N and 12°-18°N. Melts occurring at 29°-34°N were derived by melting of a mantle source with homogeneous distribution of volatile components and arrived at the surface without significant fractionation, likely, due to their rapid ascent. The MAR segments between 12° and 18°N combine contrasting geodynamic environments of magmatism, which predetermined development of a large plume region with widespread mixing of melting products of geochemically distinct mantle sources. At the same time, this region is characterized by conditions favorable for origin of localized zones of anomalous plume magmatism. These sporadic magmatic sources were spatially restricted to MAR fragments with the Hess crust, whose compositional and mechanical properties were, perhaps, favorable for focusing and localization of plume magmatism. The plume source between 12° and 18°N beneath MAR may be geochemically heterogeneous.

Platinum-group, major, and trace element abundances in ODP Leg 183 basalts

Seventeen basalts from Ocean Drilling Program (ODP) Leg 183 to the Kerguelen Plateau (KP) were analyzed for the platinum-group elements (PGEs: Ir, Ru, Rh, Pt, and Pd), and 15 were analyzed for trace elements. Relative concentrations of the PGEs ranged from ~0.1 (Ir, Ru) to ~5 (Pt) times primitive mantle. These relatively high PGE abundances and fractionated patterns are not accounted for by the presence of sulfide minerals; there are only trace sulfides present in thin-section. Sulfur saturation models applied to the KP basalts suggest that the parental magmas may have never reached sulfide saturation, despite large degrees of partial melting (~30%) and fractional crystallization (~45%). First order approximations of the fractionation required to produce the KP basalts from an ~30% partial melt of a spinel peridotite were determined using the PELE program. The model was adapted to better fit the physical and chemical observations from the KP basalts, and requires an initial crystal fractionation stage of at least 30% olivine plus Cr-spinel (49:1), followed by magma replenishment and fractional crystallization (RFC) that included clinopyroxene, plagioclase, and titanomagnetite (15:9:1). The low Pd values ([Pd/Pt]_pm < 1.7) for these samples are not predicted by currently available Kd values. These Pd values are lowest in samples with relatively higher degrees of alteration as indicated by petrographic observations. Positive anomalies are a function of the behavior of the PGEs; they can be reproduced by Cr-spinel, and titanomagnetite crystallization, followed by titanomagnetite resorption during the final stages of crystallization. Our modeling shows that it is difficult to reproduce the PGE abundances by either depleted upper or even primitive mantle sources. Crustal contamination, while indicated at certain sites by the isotopic compositions of the basalts, appears to have had a minimal affect on the PGEs. The PGE abundances measured in the Kerguelen Plateau basalts are best modeled by melting a primitive mantle source to which was added up to 1% of outer core material, followed by fractional crystallization of the melt produced. This reproduces both the abundances and patterns of the PGEs in the Kerguelen Plateau basalts. An alternative model for outer core PGE abundances requires only 0.3% of outer core material to be mixed into the primitive mantle source. While our results are clearly model dependent, they indicate that an outer core component may be present in the Kerguelen plume source.

Major element chemistr yof fallout tephra from ODP Hole 125-782A

New and published analyses of major element oxides (SiO2, TiO2, Al2O3, FeO*, MnO, MgO, CaO, K2O, Na2O and P2O5) from the central Izu Bonin and Mariana arcs (IBM) were compiled in order to investigate the evolution of the IBM in terms of major elements since arc inception at ~49 million years ago. The database comprises ?3500 volcanic glasses of distal tephra fallout and ?500 lava samples, ranging from the Quaternary to mid-Eocene in age. The data were corrected to 4 wt% MgO in order to display the highly resolved temporal trends. These trends show that the IBM major elements have always been "arc-like" and clearly distinct from N-MORB. Significant temporal variations of some major element oxides are apparent. The largest variations are displayed by K4.0. The data support a model wherein the K2O variability is caused by the addition of slab component with strongly differing K2O contents to a fairly depleted subarc mantle; variable extents of melting, or mantle heterogeneity, appear to play a negligible role. The other major element oxides are controlled by the composition and processes of the subarc mantle wedge. The transition from the boninitic and tholeiitic magmatism of the Eocene and Oligocene to the exclusively tholeiitic magmatism of the Neogene IBM is proposed to reflect a change in the composition of the subarc mantle wedge. The early boninitic magmas originate from an ultra-depleted subarc mantle, that is residual to either the melting of E-MORB mantle, or of subcontinental lithospheric mantle. During the Eocene and Oligocene, this residual mantle is gradually replaced by Indian MORB mantle advected from the backarc regions. The Indian MORB mantle is more radiogenic in Nd isotope ratios but also more fertile with respect to major and trace elements. Therefore the Neogene tholeiites have higher Al2O3 and TiO2 contents and lower mg# numbers at given SiO2 content. After the subarc mantle replacement was complete in the late Oligocene or early Miocene, the Neogene IBM entered a "steady state" that is characterized by the continuous advection of Indian MORB mantle from the reararc, which is fluxed by fluids and melt components from slab. The thickness of the IBM crust must have grown with time, but any effects of crustal thickening on the major element chemistry of the IBM magmas appear to be minor relative to the compositional changes that are related to source composition. Therefore next to the processes of melting, the composition of the mantle sources must play a major role in creating substantiative heterogeneities in the major element chemistry of the arc crust.

Geochemistry of basalts from ODP Leg 49, North Atlantic Ocean

The geochemistry of basalts recovered from seven sites in the North Atlantic is described with particular reference to minor elements. Three sites (407, 408, and 409) along the same mantle flow line, transverse to the Reykjanes Ridge at about 63°N, provide information on the composition of basalts erupted over a 34-m.y. interval between 2.3 and 36 m.y. ago. At Site 410, at 45°N, penetration into 10 m.y.-old crust west of the ridge axis permits comparisons with young basalts dredged from the median valley at 45°N. Three sites in the FAMOUS area at about 36°N provided material from very young (1 m.y.) basaltic crust (Site 411), and material to test the geochemical coherence of basalts of different ages (1.5 and 3.5 m.y.) on either side of a fracture zone (Sites 412 and 413). These sites complement earlier data from dredged and drilled sites (Leg 37) in the FAMOUS area. At Site 407, four geochemically distinct basalt units occur, with different normative and rare-earth element (REE) characteristics, and there is a clear correlation with magnetic stratigraphy. Yet there is a remarkable consistency in incompatible element ratios between these units, indicating derivation from an essentially similar mantle source. The basalts from the younger sites, 408 and 409, show a similar range of normative and REE variation, but incompatible element ratios are identical to those at Site 407, indicating that basalts at all three sites were produced from a mantle source which was geochemically relatively uniform. Rare-earth differences between the basalts can be interpreted in terms of variations in the degree and depth of partial melting causing HREE (+Y) retention in the source, although there may be some inter-site differences with respect to REE. A similar picture is presented at 45°N. Apparently a range of tholeiitic, transitional, and alkalic basalts were being erupted 10 m.y. ago, which have almost identical geochemical characteristics to those recently erupted in the median valley at 45°N. Incompatible element ratios are markedly different from those recorded at the Reykjanes Ridge. Basalts recovered from the FAMOUS sites are geochemically similar to previous samples recovered from the FAMOUS area, and their incompatible element ratios are similar, but not identical, to those at 45°N. However, total trace element levels are consistently lower than in 45°N basalts, which might imply smaller degrees of partial melting and/or greater depths of magma generation at 45°N, or higher trace element levels in the mantle source at 45°N. Few of the basalts recovered on Leg 49 have the geochemical characteristics of typical "MORB" (e.g., Nazca Plate, Leg 34). The data strongly support models invoking geochemical inhomogeneity in the source regions of basalts produced at the Mid-Atlantic Ridge. However, the data also introduce an additional time factor into such models and demonstrate the uniformity of the mantle source at a particular ridge sector (over periods in excess of 30 m.y.), while emphasizing the marked differences along the ridge. Mixing models invoking "depleted" and "enriched" mantle sources would seem to be inadequate to account for the observed variations.

Annotated record of the detailed examination of Mn deposits from DEEPSONDE Expedition stations near the East Pacific Rise

Basalts from the base of a small seamount on ~1.5-m.y.-old crust west of the East Pacific Rise (EPR) at 9°N are intermediate in chemical and isotopic composition between light-rare-earth-element-depleted tholeiite (normal midocean ridge basalt (MORB)) and alkali basalt. Like oceanic alkali basalt, these rocks contain significantly more Ba, K, P, Sr, Ti, U, and Zr than normal MORB. Since the absolute abundances of these elements are still well below alkali basalt levels, the label transitional is adopted for these basalts. A series of fractionated MORB also occurs in this area, northwest of the Siqueiros Fracture Zone - Transform Fault. The normal tholeiites are either olivine-plagioclase or plagioclase-clinopyroxene phyric, while the transitional basalts are spinel-olivine phyric. Fractional crystallization quantitatively accounts for the chemical variability of the tholeiitic series but not for the transitional basalts. The tholeiitic series probably evolved in a crustal magma chamber ~4 km below the crest of the East Pacific Rise. 143Nd/144Nd and other chemical data suggest that the large-ion-lithophile-enriched transitional basalts may represent a hybrid of normal MORB and Siqueiros area alkali basalt. Incompatible element plots of K, P, and U indicate possible derivation of the transitional basalts by magma mixing. Magma mixing of unfractionated normal MORB and Siqueiros alkali basalt has been quantified. Derivation of the transitional basalts from a 1:1 mixture is supported by all available chemical data, including Cr, Cu, Nd, Ni, Sm, Sr, U, and V. This magma mixing apparently occurred at ?<~30 km depth within a few tens of kilometers from the EPR axis. These Siqueiros area EPR transitional basalts are compared with Mid-Atlantic Ridge (MAR) transitional basalts from the Iceland and Azores areas. The Siqueiros area basalts reflect a profound chemical and isotopic heterogeneity in the upper mantle, similar to that found along the MAR. Unlike the MAR, the EPR shows no evidence of plumelike bulges and associated large-scale outpourings of nonnormal MORB resulting from these mantle heterogeneities. Siqueiros alkali basalt and MORB, as well as transitional basalt and MORB, were recovered from single dredge hauls. Such close spatial and temporal proximity of the inferred mantle sources places severe constraints on geometric and physicochemical upper mantle models.

Trace element geochemistry at DSDP Leg 82 Holes

Forty-three samples from DSDP Holes 556-559 and 561-564 were analyzed for rare earth elements (REE), Sc, Cr, Co, Hf, Ta, and Th by instrumental neutron activation analysis. The recovered basalts range from those depleted in light REE (LREE) to those enriched in LREE. The two types of basalts occur together in Holes 558 and 561. The depleted basalts have remarkably constant La/Yb, La/Sm, and La/Ti ratios and apparently derive from a large, homogeneous, mantle source underneath a segment (1200 km long) of the Mid-Atlantic Ridge. The almost twofold variation in the concentrations of incompatible trace elements in the depleted basalts is primarily due to different degrees of batch partial melting. The variation of highly to moderately incompatible elements in the Leg 82 enriched basalts can be successfully explained in terms of source mixing between depleted mantle sources and alkaline or nephelinitic magmas similar to Azores Islands magmas. However, the correlation of LREE enrichment with distance from the Azores Triple Junction is tenuous at best, and the enriched alkaline component is probably not directly related to the Azores volcanism.

Major and trace elements and Nd and Sr isotope geochemistry of basalts at DSDP Leg 74 Holes

Basement intersected in Holes 525A, 528, and 527 on the Walvis Ridge consists of submarine basalt flows and pillows with minor intercalated sediments. These holes are situated on the crest and mid- and lower NW flank of a NNW-SSE-trending ridge block which would have closely paralleled the paleo mid-ocean ridge. The basalts were erupted approximately 70 Ma, a date consistent with formation at the paleo mid-ocean ridge. The basalt types vary from aphyric quartz tholeiites on the Ridge crest to highly Plagioclase phyric olivine tholeiites on the flank. These show systematic differences in incompatible trace element and isotopic composition, and many element and isotope ratio pairs form systematic trends with the Ridge crest basalts at one end and the highly phyric Ridge flank basalts at the other. The low 143Nd/144Nd (0.51238) and high 87Sr/86Sr (0.70512) ratios of the Ridge crest basalts suggest derivation from an old Nd/Sm and Rb/Sr enriched mantle source. This isotopic signature is similar to that of alkaline basalts on Tristan da Cunha but offset by somewhat lower 143Nd/144Nd values. The isotopic ratio trends may be extrapolated beyond the Ridge flank basalts (which have 143Nd/144Nd of 0.51270 and 87Sr/86Sr of 0.70417) in the direction of typical MORB compositions. These isotopic correlations are equally consistent with mixing of depleted and enriched end-member melts or partial melting of an inhomogeneous, variably enriched mantle source. However, observed Zr-Ba-Nb-Y interelement relationships are inconsistent with any simple two-component model of magma mixing or partial melting. They also preclude extensive involvement of depleted (N-type) MORB material or its mantle sources in the petrogenesis of Walvis Ridge basalts.