Transition between tholeiitic an alkali basalts: Petrographical and geochemical evidence from Fangataufa, Pacific Ocean, and Kerguelen, Indian Ocean (original) (raw)
Related papers
Geochemistry of diverse basalt types from Loihi Seamount, Hawaii: petrogenetic implications
Earth and Planetary Science Letters, 1983
The wide variety of basalt types, tholeiitic to basanitd, dredged from Loihi Seamount have minor and trace element abundances that are characteristic of subaerial Hawaiian basalts, thereby confirming that Loihi Seamount is a manifestation of the Hawaiian "hot spot". Within the Loihi sample suite there are well-defined positive correlations among abundances of highly incompatible elements (P, K, Rb, Ba, Nb, light REE and Ta) and moderately incompatible elements (Sr, Ti, Zr and Hf) and between MgO, Ni and Cr. However, within the Loihi suite abundance ratios of geochemically similar elements (Zr/Hf, Nb/Ta and La/Ce) vary by factors of 1.2-1.5 and abundance ratios of highly incompatible elements such as P/Ce, P/Th, K/Rb, Ba/Th and La/Nb vary by factors of 1.2-2.5. These abundance ratios are not readily changed by different degrees of fractionation and melting. Therefore, we conclude that these samples are not genetically related by different degrees of melting of a compositionally homogeneous source.
Contributions to Mineralogy and Petrology, 2006
This experimental study examines the role of clinopyroxene fractionation on major element trends and alkalinity variations in mildly alkalic basalts from the Kerguelen Archipelago, Southeast Indian Ocean. Equilibrium crystallization experiments were carried out on a natural basalt (MgO=5 wt.%, alkalinity in-dex=0.10) over a range of pressures (0-1.43 GPa) and water contents (nominally dry to hydrous, 1.2 wt.% H 2 O) under relatively oxidizing conditions (Dlog FMQ=+1 to +2) at 0 GPa and relatively reducing conditions (Dlog FMQ=0 to -2) at all higher pressures. The hydrous experiments at 0.93 GPa closely reproduce most of the compositional variations in the 24-25 Ma mildly alkalic lavas from the archipelago, which supports a major role for high-Al clinopyroxene fractionation (5-9 wt.% Al 2 O 3 ) at pressures corresponding to the base of the Northern Kerguelen Plateau (15-20 km). However, clinopyroxene fractionation at depth fails to produce important changes in the alkalinity of the residual melts. The transition from tholeiitic to mildly alkalic basalts on the Kerguelen Archipelago thus reflects primarily changes in melting conditions (lower extents of partial melting at higher pressures), which is related to crustal and lithospheric thickening as distance from the Southeast Indian Ridge increased over time from 43 to 24 Ma.
Pliocene and Pleistocene alkalic flood basalts on the seafloor north of the Hawaiian islands
Earth and Planetary Science Letters, 1990
The North Arch volcanic field is located north of Oahu on the Hawaiian Arch, a 200-m high flexural arch formed by loading of the Hawaiian Islands. These flood basalt flows cover an area of about 25,000 km2; the nearly flat-lying sheet-like flows extend about 100 km both north and south from the axis of the flexural arch. Samples from 26 locations in the volcanic field range in composition from nephelinite to alkalic basalt. Ages estimated from stratigraphy, thickness of sediment on top of the flows, and thickness of palagonlte alteration rinds on the recovered lavas, range from about 0.75-0.9 Ma for the youngest lavas to somewhat older than 2.7 Ma for the oldest lavas. Most of the flow field consists of extensive sheetflows of dense basanite and alkalic basalt. Small hills consisting of pillow basalt and hyaloclastite of mainly nephelinite and alkalic basalt occur within the flow field but were not the source vents for the extensive flows. Many of the vent lavas are highly vesicular, apparently because of degassing of CO 2. The lavas are geochemically similar to the rejuvenated-stage lavas of the Koloa and Honolulu Volcanics and were generated by partial melting of sources similar to those of the Koloa Volcanics. Prior to eruption, these magmas may have accumulated at or near the base of the lithosphere in a structural trap created by upbowing of the lithosphere.
The Austral Islands, a volcanic chain in the South-Central Pacific Ocean (French Polynesia) are composed mainly of alkali basalts and basanites with subordinate amounts of olivine tholeiites and strongly undersaturated rocks (phonolite foidites and phonolite tephrites). The basaltic rocks have geochemical features typical of oceanic island suites. The distribution of incompatible trace elements indicate that the lavas were derived from a heterogeneous mantle source. The chondrite-normalized patterns of the incompatible elements in basaltic rocks of the Austral Islands are complementary to those of island arc tholeiites. As supported by isotope data, the observed trace element heterogeneities of the source are probably due to mixing of the upper mantle with subducted oceanic crust from which island arc tholeiitic magma was previously extracted.
Journal of Volcanology and Geothermal Research, 1993
Basalts from the Taupo Volcanic Zone (TVZ), New Zealand, the Kermadec Island Arc (KA) and its back-arc basin, the Havre Trough show systematic variations in trace-element and isotope geochemistry which are attributed to differences in tectonic setting and source heterogeneity along a more or less continuous plate boundary. Basalts from the Kermadec Arc are characterised by low abundances of high field strength elements (HFSE) such as Ti, Zr, Nb, Ta and Hfand have high ratios of Ti/Zr and low ratios of Ti/Sc and Ti/V relative to typical MORB. Basalts from TVZ also show low abundances of the HFS elements relative to MORB but show lower Ti/Zr, higher Ti/V and Ti/Sc ratios and generally higher Zr abundances than KA most basalts. The Havre Trough basalt is mildly alkaline (< 1% nor.mative nepheline) like many back-arc basin basahs from the Pacific rim, contrasting with the hypersthene normative TVZ and KA rocks. It has higher Zr than most TVZ basahs and all KA basalts. Ratios such as Ti/V, Ti/Sc and Ti/Zr are within the range of TVZ and MORB basalls but distinct from KA basalts. The depleted (relative to MORB) HFSE characteristics of the KA and TVZ basalts are complemented by high abundunces of large ion lithophile elements (LIL), such as Ba, Rb and K. when compared to MORB. yielding the distinctive ElL-enriched pattern of subduction related rocks on a normalised multi-element plot. In contrast, the Havre Trough basalt is MORB-tike. Chondrite-normalised Rare Earth Element (REE) patterns for the TVZ basalts show a field overlapping with that defined by the southern KA (Rumble Sea Mounts), with light REE enriched patterns (Cc/Yb~=-1.8-3) and fiat heavy REE (Tb-Lu). Basalts from the northern KA are typically light REE depleted (Ce/Yb.~=0.5) or slightly enriched (Ce/Yb, = 1.5). The REE pattern of the Havre Trough basalt is distinctive from both the KA and TVZ fields, being richer in the heavy REE, yet similar to many basalts frnm back-arc basins. Sr and Nd isotopes show considerable variation from a depleted MORB-Iike ratio in the Havre Trough basalt (TVZ-1% ~TSr/86Sr=0.702556, ~Nd= +9.3) to radiogenic ratio,,; in the Tarawcra basalt (TVZ-4, SVSr/S6Sr=0.7052, ~N~= + 2.2} fiom TVZ suggestive of crustal contamination. Between these extreme values the data from the KA and TVZ define distinctive arrays trending towards the evolved andesites, rhyolites and supracrustal basement rocks of central New Zealand. This curvilinear Nd-Sr array of the TVZ data is best explained as a result of crustal contamination. At the unradiogenic-Sr end of the array, the Sr and Nd isotopic ratios of primitive TVZ basalts overlap with those from southern KA suggesting that primitive TVZ basalts were derived from sources that are isotopically similar to those from which the southern K\ basalts derived. Basalls from northern KA (Herald, Raoul and Macauley Islands) are appreciably less radiogenic than those from the central part (Curtis and L'Esperance) and the southern (Rumble Seamounts) part of the arc. Using non-slab-derived HFSE ratios and abundances as a guide to relative source depletion, it is inferred that the K-\ source varies from strongly depleted in the north to less depleted in the TVZ in the south. The KA source has generally experienced more extensive extraction of mafic magmas and hence is impoverished in incompatible HFSE. This leads to high Ti/Zr and low (absolute) Zr, HE Ta and Nb abundances. The Havre Trough source is relatively fertile with regard
The concentrations of incompatible elements in basaltic rocks from the Cook–Austral and Pitcairn–Gambier island chains of French Polynesia were determined to refine geochemical models for the origin of EM I-type ocean island basalts. It has been proposed previously that the addition of pelagic sediment to a HIMU source could yield mantle with EMI-like characteristics. The very high abundances of many large ion lithophile elements (e.g. Pb) in pelagic sediments would control incompatible element ratios in the proposed EMI source. Calculations indicate that partial melts of this HIMU–pelagic sediment mix would have incompatible element ratios that do not overlap with those of EMI basalts. An investigation of the effect of subduction on a pelagic sediment composition shows that this is a critical process: the sediment undergoes significant loss of large ion lithophile elements during sub-arc devolatilization and metamorphism. Incompatible element ratios in partial melts of a model EMI source which includes this residual metasediment component are shown to be consistent with the range of EMI-type lavas from French Polynesia. In addition, the EMI source may include a smaller ratio of altered oceanic crust to depleted mantle peridotite than is present in a HIMU source.
The Canadian Mineralogist, 2007
We report the compositions of phenocrysts, xenocrysts, and reaction coronas from basaltic lavas sampled from the ~700m-thick Mt. Marion Dufresne section, located in the southern part of the Plateau Central region of the Kerguelen Archipelago, southern Indian Ocean. Compositional variations are used to constrain environments of phenocryst crystallization, temporal changes in magma activity, and magma-mixing relationships. The basal 300 m of the section consists of predominantly aphyric, mildly alkalic basaltic lavas that are increasingly intercalated with plagioclase-phyric to plagioclase-ultraphyric (up to 60 vol.% phenocrysts ranging from An 63 to An 86 ) basalt flows with increasing stratigraphic height. Growth of the plagioclase phenocrysts occurred in relatively shallow (<5-6 km) crystal-rich magma reservoirs, and mixed populations of phenocrysts were collected during periods of renewed magmatic activity and erupted within evolved magmas. Above an elevation of 400 m, there is a thick succession of tholeiitic olivine-phyric (up to 20 vol.%) high-MgO basalts that is interpreted to represent an interval of increased supply of magma and eruptive flux. Olivine -whole-rock Fe-Mg relations indicate that olivine phenocrysts with ~Fo 80-86 are in equilibrium with parental magma compositions of 8-10 wt.% MgO, which represents the maximum MgO content of the melt erupted as lavas on the Kerguelen Archipelago. Three quartz-bearing basaltic andesites that occur within this upper high-MgO succession contain olivine, plagioclase, and quartz phenocrysts. These phenocrysts exhibit extreme disequilibrium textures, including rounded, resorbed quartz surrounded by fine-grained pyroxene coronas, and rounded, reversely zoned, and sievetextured plagioclase, which are consistent with incorporation of a quartz-bearing magma into a high-MgO basaltic magma. The presence of evolved quartz-saturated magmas requires sporadic decreases in magmatic activity to allow for locally extensive fractionation. The phenocryst textures and compositions of basaltic lavas from the Marion Dufresne section documented in this study demonstrate the important role of changing magma-flux conditions in magma-conduit systems beneath the Kerguelen Archipelago at ~25 Ma.
Journal of Volcanology and Geothermal Research, 1990
Volcanism in the Taupo Volcanic Zone (TVZ) and the Kermadec arc-Havre Trough (KAHT) is related to westward subduction of the Pacific Plate beneath the Indo-Australian Plate. The tectonic setting of the TVZ is continental whereas in KAHT it is oceanic and in these two settings the relative volumes of basalt differ markedly. In TVZ, basalts form a minor proportion (< 1%) of a dominant rhyolite (97%)-andesite association while in KAHT, basalts and basaltic andesites are the major rock types. Neither the convergence rate between the Pacific and Indo-Australian Plates nor the extension rates in the back-arc region or the dip of the Pacific Plate Wadati-Benioff zone differ appreciably between the oceanic and continental segments. The distance between the volcanic front and the axis of the back-arc basin decreases from the Kermadec arc to TVZ and the distance between trench and volcanic front increases from around 200 km in the Kermadec arc to 280 km in TVZ. These factors may prove significant in determining the extent to which arc and backarc volcanism in subduction settings are coupled. All basalts from the Kermadec arc are porphyritic (up to 60% phenocrysts) with assemblages generally dominated by plagioclase but with olivine, clinopyroxene and orthopyroxene. A single dredge sample from the Havre Trough back arc contains olivine and plagioclase microphenocrysts in glassy pillow rind and is mildly alkaline (< 1% normative nepheline) contrasting with the tholeiitic nature of the other basalts. Basalts from the TVZ contain phenocryst assemblages of olivine + plagioclase + clinopyroxene; orthopyroxene phenocrysts occur only in the most evolved basalts and basaltic andesites from both TVZ and the Kermadec Arc. Sparsely porphyritic primitive compositions (Mg/(Mg+Fe2*)>70) are high in A120 a (>16.5%), and project in the olivine volume of the basalt tetrahedron. They contain olivine (FOsT) phenocrysts and plagioclase (> An60) microphenocrysts. These magmas have ratios of CaO/A1203, A1203/TiO2 and CaO/TiO 2 in the range of MORB and MORB picrites and can evolve to the low-pressure MORB cotectic by
The Marquesas, Society and Austral-Cook Islands, three volcanic chains in the central Pacific Ocean {French Polynesia), are composed mainly of alkali basalts, basanites and tholeiites, which have geochemical characteristics typical of ocean island basalts. The lavas from the Marquesas and Society Islands display generally chondritic ratios of highly incompatible trace elements and have higher 87Sr/86Sr than the basalts from the Austral-Cook Islands which have many trace-element ratios similar to those of mid-ocean ridge basalts. This grouping probably reflects differences in the composition of an ancient subducted and recycled lithosphere incorporated into the mantle source of the Polynesian basalts. Compared to Marquesas and Society Islands basalts, the mantle source of the Austral-Cook Islands basalts contains refractory oceanic lithosphere from which a larger amount of basaltic melt was extracted during subduction.