Incompatible Element Abundance Ratios Research Articles

Geochemistry and Petrogenesis of Pliocene Alkaline Volcanic Rocks of Dok Island, Korea

Dok island comprises Pliocene volcanic products such as a series of volcanoclastic rocks and lavas ranging in composition from alkali basalts, and trachyandesites to trachytes. Compositional variation of the basaltic rocks can be attributed to fractional crystallization of olivine, clinopyroxene, plagioclase, and magnetite. Chemical variations among the trachyandesites are caused by fractionation of clinopyroxene, plagioclase, and magnetite with minor amphibole, while trachytes are controlled mainly by feldspar fractionation. Incompatible element abundance ratios and chondrite normalized LREE/HREE ratios (e.g., (La/Yb)c: 24.8 to 32.8 for basalts, 15.6 to 31.2 for trachyandesites) suggest that the origins of the basalts and trachyandesites involve both different degrees of partial melting and subsequent fractional crystallization processes. Trace element ratios of the basalts from Dok island are characterized by high Ba/Nb, La/Nb, Ba/Th and Th/U and isotopic ratios (Tasumoto and Nakamura, 1991) that are similar to the EM 1 type of oceanic island basalts such as Gough and Tristan da Cunha basalts.

Petrology of Stewart Mountain basalt field in central Arizona, U.S.A.: A lithospheric source with small-scale trace element and isotopic heterogeneities

The Stewart Mountain basalt field in central Arizona is composed of three horizons of Miocene lavas over ~4 km 2 . The youngest lava is ~15.5 Ma. The field is in the southern Basin and Range at its transition to the Colorado Plateau. It is also at the northwestern margin of the ~8000 km 2 Goldfield-Superstition volcanic province (G-SVP), where basaltic lavas are ~20–19 Ma. Stewart Mountain basalts are alkalic, and most have from 6–8 weight percent (wt%) MgO, but more primitive and evolved lavas (10.7 and 4 wt% MgO, respectively) are also present. Most incompatible element abundances differ widely for basalts within the 6–8 wt% MgO range, and they distinguish the three horizons (e.g., ranges for P 2 O 5 are 0.5–1.4 wt%; Zr 125–250 ppm; La 40–80 ppm). One lava has quartz and plagioclase xenocrysts and even lower incompatible element abundances (e.g., P 2 O 5 0.25 wt%; La 25 ppm). All Stewart Mountain basalts, however, have Nb-Ta negative anomalies, consistent with a lithospheric mantle source that had subduction characteristics. Isotopic compositions differ across the three basalt horizons (e.g., ranges for 87 Sr/ 86 Sr are 0.7049–0.7061; 206 Pb/ 204 Pb 17.7–19.2; e Nd -3.5 to -6.2), where the xenocrystic lava has the lowest Sr and Pb isotopic ratios. Over its life, the Stewart Mountain field radiogenic isotope ratios decreased to reflect source heterogeneities, and its 206 Pb/ 204 Pb range is as wide as that formed by Oligocene–Miocene basalts collectively across the southern Basin and Range and transition zone. Incompatible-element abundances and ratios also reflect source heterogeneities, whereby the greatest differences are observed as abundances decreasing from middle to upper horizon basalts. Several abundance ratios, such as Zr/Nb, Th/Ta, Th/Nb, and Zr/Hf, record some of the source heterogeneities that are manifested over the short geologic time represented by the successive lava horizons. These temporal compositional changes likely reflect partial melts from a variably metasomatized lithospheric mantle. Compared to the compositions of the older, neighboring G-SVP basalts, Stewart Mountain lavas are generally evolved (MgO <8 wt%). The absence of mantle xenoliths in any Stewart Mountain lava and the xenocrystic lava both point to the compositional evolution having occurred in crustal reservoirs; however, based on the lowest isotopic ratios present in the xenocrystic lava, the upper crust was not a reservoir. Comparing Stewart Mountain basalt incompatible-element abundance ratios to those in the neighboring G-SVP shows enough difference to conclude that these two Miocene basalt localities had lithospheric sources with distinct trace element characteristics. The G-SVP source also had higher, distinguishing e Nd (-1 to -2). All characteristics combined, the Stewart Mountain field shows that lithospheric source heterogeneities can be manifested both temporally and spatially over only a small surface area. Stewart Mountain lithospheric source indicates that magmatism in central Arizona did not have asthenospheric sources by 15 Ma.

Trace element abundances of Mauna Kea basalt from phase 2 of the Hawaii Scientific Drilling Project: Petrogenetic implications of correlations with major element content and isotopic ratios

The temporal geochemical variations defined by lavas erupted throughout the growth of a single volcano provide important information for understanding how the Hawaiian plume works. The Hawaii Scientific Drilling Project (HSDP) sampled the shield of Mauna Kea volcano to a depth of 3100 meters below sea level during Phase 2 of the HSDP. Incompatible element abundance ratios, such as La/Yb, Sm/Yb, Nb/Zr, and Ti/Zr, in conjunction with SiO2 abundance and radiogenic isotopic ratios, especially He and Pb, in the reference sample suites of the Mauna Kea portion of cores from Phases 1 and 2 of the HSDP define three distinct geochemical groups. The upper 550 m of Mauna Kea lavas in the Phase 2 core include the Postshield Group with eruption ages of ∼200 ka to &lt;370 ka. These lavas have relatively low SiO2 content, 3He/4He and 206Pb/204Pb, and they define a trend to relatively high La/Yb, Sm/Yb, and Nb/Zr. The eruption of these lavas coincides with migration of the Mauna Kea shield off the hot spot. As a result, extent of melting decreased, melt segregation occurred at greater depth, within the garnet stability field, and a geochemically distinct component associated with the periphery of the plume was sampled. Deeper in the Phase 2 core two other geochemical groups of lava are intercalated. One group has relatively low SiO2 abundance and high Nb/Zr Ti/Zr, 3He/4He and high 208Pb/204Pb at a given 206Pb/204Pb. These are distinctive geochemical characteristics of lavas erupted at Loihi seamount. Variations in incompatible element abundance ratios (e.g., Sm/Yb versus Nb/Zr and La/Yb versus Ti/Zr) define mixing trends between these low SiO2 lavas (Loihi‐type) and lavas belonging to a high SiO2 group that are the dominant lava type in the shield part of the core (Kea‐type). These two groups are presumed to reflect components intrinsic to the plume. Correlations of incompatible element abundance ratios, such as La/Nb, with radiogenic isotope ratios show that Hawaiian shields contain different proportions of geochemically distinctive components. The Koolau shield contains a recycled sedimentary component that is not present in the Mauna Kea shield. The anomalously high Ba/Th in Hawaiian lavas is inferred to be a source characteristic. Ba/Th is correlated with some radiogenic isotope ratios in Kilauea and Mauna Loa lavas, but there is no correlation in Mauna Kea lavas which range in Ba/Th by a factor of 2.6.

Trace of the Kerguelen mantle plume: Evidence from seamounts between the Kerguelen Archipelago and Heard Island, Indian Ocean

The gravity and bathymetric highs on the Kerguelen Plateau in the southern Indian Ocean between the Kerguelen Archipelago and Heard Island are seamounts formed of Miocene alkalic basalts similar to those found on the islands. Dredging during the Kerimis survey cruise recovered &gt;1 ton of mostly basaltic rocks. One of the dredges (D6) yielded a large volume of in situ alkalic picritic pillow basalts, the first picritic lavas recovered that are related to the Kerguelen plume. K‐Ar and 40Ar‐39Ar ages are between 18 and 21 Ma for all but one sample, and these ages are only slightly younger than the main phase of volcanism on the archipelago. The dredged lavas form three distinct groups based on chemical and isotopic compositions. Incompatible element abundance ratios overlap with compositional groups defined by lavas from both the Kerguelen Archipelago and Heard Island indicating that alkalic volcanism in this region of the Kerguelen Plateau has been spatially diverse. Olivine and picritic basalts have Sr and Nd isotopic characteristics similar to most of the lavas exposed on the archipelago and those proposed for the Kerguelen plume. However, compared to Kerguelen Archipelago lavas, the picritic basalts have relatively low 206Pb/204Pb which is a characteristic of Cretaceous basalt forming some parts of the Kerguelen Plateau. We propose that the apparent age trend of the lavas from 34 Ma in the Northern Kerguelen Plateau (ODP Leg 183, Site 1140) to 24–30 Ma on the Kerguelen Archipelago to 18–21 Ma on the dredged submarine volcanoes, and even possibly to recent volcanism on Heard and McDonald Islands, may correspond to the Tertiary hot spot track of the Kerguelen plume.

Petrogenesis of lavas from the AMAR Valley and Narrowgate region of the FAMOUS Valley, 36°-37°N on the Mid-Atlantic Ridge

Basaltic lavas from the AMAR Valley and the Narrowgate region of the FAMOUS Valley on the Mid-Atlantic Ridge (36° to 37° N) range in texture from aphyric to highly plagioclase phyric (>25% large plagioclase phenocrysts). Based on 87Sr/86Sr and 143Nd/144Nd ratios, most of these lavas can be subdivided into two distinct, isotopically homogeneous, groups: Group I has lower 87Sr/86Sr (0.70288±1) and higher 143Nd/144Nd (0.51312±1) ratios; Group II has higher 87Sr/86Sr (0.70296±1) and lower 143Nd/144Nd (0.51309±2) ratios. Most Group II lavas are aphyric, whereas Group I lavas are primarily plagioclase phyric. Lavas from both groups show a wide range in incompatible element abundance ratios (e.g., Zr/Nb =6–29; (La/Sm)n=0.6–1.7). Aphyric lavas have relatively constant Sc (40±1.5 ppm) abundances and CaO/Al2O3 ratios (0.80±0.02). Group I lavas are confined primarily to the AMAR rift valley floor whereas Group II lavas are found along the east and west marginal highs. We interpret the isotopic differences between the two groups as reflecting a temporal change in the upwelling mantle beneath this region of the Mid-Atlantic Ridge which is south of the Azore Islands. For each group, a petrogenetic model consistent with the geochemical data is multi-stage decompression melting of an initially enriched, homogeneous, mantle source region. If the early derived, incompatible-element enriched, melt increments are not always pooled with subsequent increments, the erupted magma batches may have the major element characteristics of melts derived by 10 to 20% melting, but with incompatible element abundance ratios reflecting the change from an enriched to depleted source during the incremental melting process. In this process an initially homogeneous source can generate primary magmas with the required range in incompatible element abundance ratios shown by each group. The nearly constant CaO/Al2O3 ratios and Sc contents of the aphyric lavas with decreasing Mg ? reflects subsequent polybaric fractionation of clinopyroxene, plagioclase and olivine over the pressure interval 8–6 kbar (24–18 km), followed by rapid transport to the surface and eruption. There is no geochemical evidence for a crustal magma chamber beneath this section of the Mid-Atlantic Ridge.

Role of the Kerguelen Plume in generating the eastern Indian Ocean seafloor

Mid‐ocean ridge basalts (MORB) in the Indian Ocean have Sr‐Nd‐Pb isotopic characteristics that distinguish them from seafloor basalts in the Atlantic and Pacific Oceans. These differences have important implications for mantle dynamics. We discuss the isotopic variation with eruption age of seafloor basalts recovered by deep sea drilling at 10 sites in the eastern Indian Ocean ranging in age from Eocene to Late Jurassic. Except for alkalic basalts recovered from near Christmas Island in the northeast Indian Ocean, the basement lavas are tholeiitic basalts that are characterized by a wide range in incompatible element abundance ratios, such as La/Yb and Zr/Nb. Most of the tholeiitic basalts from seven sites are geochemically similar to recent Indian Ocean MORB, but the alkalic basalts and tholeiitic lavas from two other sites have isotopic and incompatible element abundance ratios similar to lavas associated with the Kerguelen Plume. Two of these three sites, however, are not close to the track of this plume. The Dupal isotopic signature (relatively high 87Sr/86Sr and high 208Pb/204Pb at a given 206Pb/204Pb) is characteristic of lavas that have been attributed to the Kerguelen Plume, i.e., the Kerguelen Archipelago, Ninetyeast Ridge, and Kerguelen Plateau. Among eastern Indian Ocean seafloor basalts, a Dupal component is apparent in basement lavas from six of the seven drill sites in the eastern Indian Ocean that range in inferred age from ∼57 to 125 Ma. The oldest (∼155 Ma) seafloor lavas recovered from the Indian Ocean, derived from a spreading center in the Argo Abyssal Plain near northwest Australia, have high 143Nd/144Nd and low 87/86Sr similar to the most depleted recent Indian MORB. Because the oldest volcanism on the Kerguelen Plateau (∼118 Ma) is the first evidence of the activity of the Kerguelen Plume, this plume is inferred to be the source of Dupal isotopic characteristics in Indian Ocean MORBs. Some recent Indian Ocean MORB are also distinctive because many have relatively low 206Pb/204Pb (&lt;17.4). Some of the oldest (110 to 155 Ma) seafloor lavas in the eastern Indian Ocean also have relatively low 206Pb/204Pb ratios. This low 206Pb/204Pb signature predates volcanism associated with the Kerguelen Plume and may reflect a significant role for continental lithosphere as a long‐term source component for Indian Ocean MORB.

Evolution of Mauna Kea volcano: Inferences from lava compositions recovered in the Hawaii Scientific Drilling Project

The lower 776 m of core recovered during the initial phase of the Hawaii Scientific Drilling Project (HSDP) contains lavas erupted from Mauna Kea volcano. Tholeiitic and alkalic basalts, including an Fe‐Ti rich flow, are intercalated in the upper 58 m of Mauna Kea lavas. Similar basaltic sections are subaerially exposed on the lower east flank of Mauna Kea. The Fe‐Ti rich lavas reflect large amounts of clinopyroxene, plagioclase, and olivine fractionation within the crust and upper mantle, but the range from tholeiitic to alkalic compositions reflects variable extents of melting of a garnet‐bearing source. Based on abundances of incompatible elements, the extent of melting for a basanitoid was a factor of 2 less than that for nearly coeval tholeiitic lavas. All flow units in the lower 718 m of the HSDP core are tholeiitic lavas. Their variability in major element compositions reflect variable accumulation of olivine. Incompatible element abundance ratios in these lavas reflect a complex temporal variation in extent of melting. Within the tholeiitic part of the core, lavas from 800 m to 950 m formed by the largest extent of melting, whereas tholeiitic lavas from the bottom of the core and from just below the tholeiitic to alkalic transition formed by lower degrees of melting. Inferred melt compositions at 16% MgO show that the ∼200 to 400 ka Mauna Kea lavas from the HSDP core and the &lt;250 ka subaerial exposures define an inverse correlation between SiO2 and FeO contents. Based on experimental studies, this correlation is caused by differing pressures of melt segregation. Furthermore, abundances of Nb and SiO2 are also inversely correlated in these calculated melts. In general, the younger lavas are relatively enriched in FeO and incompatible elements but are depleted in SiO2. These trends are interpreted to reflect an overall trend of increasing pressure of melt segregation and decreasing extent of melting with decreasing eruption age. There are, however, geochemical variations which indicate short‐term reversals in this long‐term trend. Previously, the geochemical trends accompanying the transition from tholeiitic to alkalic volcanism at Hawaiian volcanoes have been interpreted as reflecting the effects of increasing distance from the plume axis. The long‐term geochemical trends of tholeiitic lavas in the HSDP core also reflect migration of Mauna Kea away from the Hawaiian plume.

Temporal evolution of the kerguelen plume: Geochemical evidence from 38 to 82 ma lavas forming the Ninetyeast ridge

Abstract Basaltic basement has been recovered by deep-sea drilling at seven sites on the linear Ninetyeast Ridge in the eastern Indian Ocean. Studies of the recovered lavas show that this ridge formed from ~ 82 to 38 Ma as a series of subaerial volcanoes that were created by the northward migration of the Indian Plate over a fixed magma source in the mantle. The Sr, Nd and Pb isotopic ratios of lavas from the Ninetyeast Ridge range widely, but they largely overlap with those of lavas from the Kerguelen Archipelago, thereby confirming previous inferences that the Kerguelen plume was an important magma source for the Ninetyeast Ridge. Particularly important are the ~ 81 Ma Ninetyeast Ridge lavas from DSDP Site 216 which has an anomalous subsidence history (Coffin 1992). These lavas are FeTi-rich tholeiitic basalts with isotopic ratios that overlap with those of highly alkalic, Upper Miocene lavas in the Kerguelen Archipelago. The isotopic characteristics of the latter which erupted in an intraplate setting have been proposed to be the purest expression of the Kerguelen plume (Weis et al. 1993a,b). Despite the overlap in isotopic ratios, there are important compositional differences between lavas erupted on the Ninetyeast Ridge and in the Kerguelen Archipelago. The Ninetyeast Ridge lavas are dominantly tholeiitic basalts with incompatible element abundance ratios, such as La/Yb and Zr/Nb, which are intermediate between those of Indian Ocean MORB (mid-ocean ridge basalt) and the transitional to alkalic basalts erupted in the Kerguelen Archipelago. These compositional differences reflect a much larger extent of melting for the Ninetyeast Ridge lavas, and the proximity of the plume to a spreading ridge axis. This tectonic setting contrasts with that of the recent alkalic lavas in the Kerguelen Archipelago which formed beneath the thick lithosphere of the Kerguelen Plateau. From ~ 82 to 38 Ma there was no simple, systematic temporal variation of Sr, Nd and Pb isotopic ratios in Ninetyeast Ridge lavas. Therefore all of the isotopic variability cannot be explained by aging of a compositionally uniform plume. Although Class et al. (1993) propose that some of the isotopic variations reflect such aging, we infer that most of the isotopic heterogeneity in lavas from the Ninetyeast Ridge and Kerguelen Archipelago can be explained by mixing of the Kerguelen plume with a depleted MORB-like mantle component. However, with this interpretation some of the youngest, 42–44 Ma, lavas from the southern Ninetyeast Ridge which have206pb/204Pb ratios exceeding those in Indian Ocean MORB and Kerguelen Archipelago lavas require a component with higher206Pb/204Pb, such as that expressed in lavas from St. Paul Island.

Calbuco Volcano and minor eruptive centers distributed along the Liqui�e-Ofqui Fault Zone, Chile (41�?42� S): contrasting origin of andesitic and basaltic magma in the Southern Volcanic Zone of the Andes

Calbuco volcano is a Late Pleistocene-Holocene composite stratovolcano located at 41°20′ S, in the southern region of the Southern Volcanic Zone of the Andes (SSVZ; 37°–46° S). In contrast to basalt and basaltic andesite, which are the dominant lava types on the volcanic front from 37° to 42° S, Calbuco lavas are porphyritic andesites which contain a wide variety of crustal xenoliths. They have SiO2 contents in the 55–60% range, and have comparatively low K2O, Rb, Ba, Th and LREF abundances relative to other SSVZ centers. Incompatible element abundance ratios are similar to those of most SSVZ volcanics, but 87Sr/86Sr and 143Nd/144Nd are respectively higher and lower than those of adjacent volcanic centers. Basalts from nearby Osorno stratovolcano, 25 km to the northeast, are similar to other basaltic SSVZ volcanoes. However, basalts from several minor eruptive centers (MEC), located east of Calbuco and Osorno volcano along the Liquine-Ofqui fault zone (LOFZ), are enriched in Ba, Nb, Th and LREE, and have higher La/Yb and lower Ba/La, K/La and Rb/La. 87Sr/86Sr and 143Nd/144Nd in MEC basalts are respectively lower and higher than those of Osorno and Calbuco lavas. We suggest that MEC basalts were produced by lower extents of mantle melting than basalts from Osorno and other SSVZ stratovolcanoes, probably as a result of lower water content in the source of MEC basalts. Calbuco andesites formed from basaltic parents similar to Osorno basalts, by moderate pressure crystallization of a hornblende-bearing assemblage accompanied by crustal assimilation. Hornblende stability in the Calbuco andesites was promoted by the assimilation of hydrous metasedimentary crustal rocks, which are also an appropriate endmember for isotopic trends, together with magma storage at mid-crustal depths. The unique characteristics of Calbuco volcano, i.e. the stability of hornblende at andesitic SiO2 contents, low 143Nd/144Nd and high 87Sr/86Sr, and abundant crustal xenoliths, provide evidence for crustal assimilation that is not apparent at more northerly volcanoes in the SSVZ.

Geochemical characteristics of Koolau Volcano: Implications of intershield geochemical differences among Hawaiian volcanoes

The voluminous shields of Hawaiian volcanoes are dominantly composed of tholeiitic basalts, but there are important intershield geochemical differences. The subaerial lavas forming the ~2–3 Ma Koolau shield have several extreme characteristics: relatively high abundances of SiO2, low abundances of total iron and CaO, and high ratios of LaNb and SrNb. In addition, they range to near bulk-earth strontium, neodymium, and lead isotopic ratios. Although postmagmatic alteration has significantly affected the compositions of some Koolau lavas (decreases in SiO2, K2O, and rubidium contents, increases in total iron and in unusual cases, increases in yttrium and REE abundances), the geochemical charac teristics of unaltered Koolau lavas reflect a distinctive primary magma composition. Within a stratigraphic sequence of lavas, Koolau lavas vary significantly in incompatible element abundance and isotopic ratios, but these variations are not systematic with eruption age, and they are smaller than the differences between Hawaiian shields. Intershield differences in some incompatible element abundance ratios, LaNb and SrNb, are correlated with intershield differences in isotopic ratios, thereby indicating that each shield formed from a compositionally distinct source. However, other intershield compositional differences are not correlated with differences in radiogenic isotope ratios. Some of these compositional differences probably reflect variations in the melting process; e.g., inverse correlations between SiO2 and total iron contents may reflect differences in the pressure of melt segregation, differences in abundances of incompatible elements may reflect variations in mean degree of melting, and variations in ratios like SmNd may reflect the presence of residual garnet. Each shield appears to reflect a unique combination of source components and variables, such as extent of melting and pressure of melt segregation. Consequently, the intershield geochemical differences have important implications for plume structure. Either a relatively large plume has a spatially systematic distribution of geochemical heterogeneities which are sampled by the overlying shields, or each shield is derived from a small radius (<20 km) conduit composed of geochemically distinct diapirs or solitary waves.

Kerguelen Archipelago revisited: geochemical and isotopic study of the Southeast Province lavas

In order to constrain an isotopically extreme mantle component involved in the genesis of oceanic basalts, we studied some of the youngest lavas of the Kerguelen Archipelago. According to the model developed by Gautier et al. [1], these young lavas were derived from the Kerguelen mantle plume, with little, if any, depleted MORB component. The alkaline lavas in the Southeast Province of the Kerguelen Archipelago correspond to two main volcanic phases: (a) a 20–22 Ma old lower Miocene series with basalts and trachytes belonging to a mildly alkaline series comparable to the mildly alkaline series of Gautier et al. [1] and (b) a 6.6–10.2 Ma old [2] upper Miocene series with basanites, tephri-phonolites and phonolites, forming a highly alkaline series. The lower Miocene lavas haveε Nd, 87Sr/ 86Sr and Pb isotopic ratios overlapping with those of Kerguelen transitional and mildly alkaline series, i.e.ε Nd from +2.2 to −0.6 and 87Sr/ 86Sr between 0.70493 and 0.70522. The upper Miocene lavas have higher 87Sr/ 86Sr ratios (> 0.7054) and lowerε Nd ( ≪ 0), i.e. comparable to the highly alkaline series of Gautier et al. [1]. In contrast, compared to other lavas from the Kerguelen Archipelago, the upper Miocene lavas have distinctly less radiogenic 206Pb/ 204Pb ratios. They plot to the left of the Indian OIB field in Pb-Pb diagrams, i.e. in a direction opposite to that expected for contamination with sediment or young continental crust. In both series, the evolved lavas and associated basalts/basanites overlap in isotopic ratios, although incompatible element abundance ratios in the evolved lavas were modified by fractionation of amphibole, Fe-Ti oxides, apatite and sphene. We propose that the isotopic characteristics of the upper Miocene lavas correspond to the pure Kerguelen plume signature, which is: 87Sr/ 86Sr= 0.7054−0.7058,ε Nd = −0.2 to −2.9, 206Pb/ 204Pb= 18.06−18.27, 207Pb/ 204Pb= 15.54−15.58 and 208Pb/ 204Pb= 38.68−39.16. The Kerguelen plume therefore has an intermediate isotopic composition and corresponds neither to the EM I nor the EM II component [3]. A continuum between EM I and EM II is expected for mantle contaminated by recycled continental crust or continent-derived material.

Geochemical characteristics of basaltic glasses from the amar and famous axial valleys, Mid-Atlantic Ridge (36°–37°N): Petrogenetic implications

The axial valley of the Mid-Atlantic Ridge from 36° to 37°N was intensively sampled by submersible during the famous and amar projects. Our research focussed on the compositional and isotopic characteristics of basaltic glasses from the amar valley and the narrowgate region of the famous valley. These basaltic glasses are characterized by: (1) major element abundance trends that are consistent with control by multiphase fractionation (olivine, plagioclase and clinopyroxene) and magma mixing, (2) near isotopic homogeneity δ 18O= 5.2to6.4 , 87Sr/ 86Sr= 0.70288to0.70299 and 206Pb/ 204Pb= 18.57to18.84 , and (3) a wide range of incompatible element abundance ratios; e.g., within the amar valley chondrite-normalized La/Sm ranges from 0.7 to 1.5 and La/Yb from 0.6 to 1.6. These ratios increase with decreasing MgO content. Because of the limited variations in isotopic ratios of Sr, Nd and Pb, it is plausible that these compositional variations reflect post-melting magmatic processes. However, it is not possible to explain the correlated variation in MgO content and incompatible element abundance ratios, such as La/Sm and Zr/Nb, by fractional crystallization or more complex processes such as boundary layer fractionation. These geochemical trends can be explained by mixing of parental magmas that formed by very different extents of melting. In particular, the factor of three variation in Ce content in samples with ∼ 2.1% Na 2O and 8% MgO requires a component derived by < 1% melting. If the large variations in abundance ratios of incompatible elements reflect the melting process, a large, long-lived magma chamber was not present during eruption of these amar lavas. The geological characteristics of the amar valley and the compositions of amar lavas are consistent with episodic volcanism; i.e., periods of magma eruption were followed by extensive periods of tectonism with little or no magmatism.

Intershield geochemical differences among Hawaiian volcanoes: implications for source compositions, melting process and magma ascent paths

As Hawaiian volcanoes develop, their lavas systematically change in composition and isotopic ratios of Sr, Nd and Pb. These trends provide important constraints for understanding plume-related volcanism as a volcano migrates away from the hotspot. There are also geochemical differences between Hawaiian shields. In particular, lavas from adjacent shields such as Kilauea and Mauna Loa on Hawaii and Koolau and Waianae on Oahu have significant differences in abundances of some major and incompatible elements and isotopic ratios of Sr, Nd and Pb. Some incompatible element abundance ratios, such as Zr/Nb and Sr/Nb, are correlated with intershield differences in Sr and Nd isotope ratios, but these isotopic ratios are not correlated with intershield differences in major element composition, or even parent/daughter abundance ratios such as Rb/Sr and Sm/Nd. Moreover, at Kilauea and Mauna Loa the intershield differences have apparently persisted for a relatively long time, perhaps 100 ka. These intershield geochemical differences provide important constraints on plume volcanism. Specifically, (i) each volcano must have distinct magma ascent paths from the region of melt segregation; (ii) the 25-50 km distance between adjacent, but geochemically distinct, shields requires that the sources vary on a similar scale, and that the melt production region is similarly restricted. The absence of correlations between lava compositions and radiogenic isotope ratios provides evidence for significant differences in melting process such as each shield forming by a different mean extent of melting with melt segregation at different mean pressures. Two types of models are consistent with the intershield geochemical differences: (i) a relatively large radius, ca . 40 km, plume conduit with a systematic spatial distribution of geochemical heterogeneities; or (ii) a small radius, less than 20 km, plume conduit composed of geochemically distinct diapirs. Because relatively small radius diapirs of limited vertical extent are too small to create the large Hawaiian shields, a possible alternative is a continuous conduit containing solitary waves which transport geochemically distinct packets of material.

The tholeiite to alkalic basalt transition at Haleakala Volcano, Maui, Hawaii

Previous studies of alkalic lavas erupted during the waning growth stages (<0.9 Ma to present) of Haleakala volcano identified systematic temporal changes in isotopic and incompatible element abundance ratios. These geochemical trends reflect a mantle mixing process with a systematic change in the proportions of mixing components. We studied lavas from a 250-m-thick stratigraphic sequence in Honomanu Gulch that includes the oldest (∼1.1 Ma) subaerial basalts exposed at Haleakaka. The lower 200 m of section is intercalated tholeiitic and alkalic basalt with similar isotopic (Sr, Nd, Pb) and incompatible element abundance ratios (e.g., Nb/La, La/Ce, La/Sr, Hf/Sm, Ti/Eu). These lava compositions are consistent with derivation of alkalic and tholeiitic basalt by partial melting of a compositionally homogeneous, clinopyroxene-rich, garnet lherzolite source. The intercalated tholeiitic and alkalic Honomanu lavas may reflect a process which tapped melts generated in different portions of a rising plume, and we infer that the tholeiitic lavas reflect a melting range of ∼10% to 15%, while the intercalated alkalic lavas reflect a range of ∼6.5% to 8% melting. However, within the uppermost 50 m of section. 87Sr/86Sr decreases from 0.70371 to 0.70328 as eruption age decreased from ∼0.97 Ma to 0.78 Ma. We infer that as lava compositions changed from intercalated tholeiitic and alkalic lavas to only alkalic lavas at ∼0.93 Ma, the mixing proportions of source components changed with a MORB-related mantle component becoming increasingly important as eruption age decreased.

Geochemistry of Hannuoba basalts, eastern China: Constraints on the origin of continental alkalic and tholeiitic basalt

The Cenozoic Hannuoba basaltic province, 200 km northwest of Beijing, China, is composed of intercalated tholeiitic, transitional and alkalic basalt. Although the alkalic basalts contain abundant upper-mantle xenoliths, they range in MgO content from 5.7% to 10.6% MgO. Most of the major- and trace-element abundance variations in the alkalic basalts can be explained by segregation of a clinopyroxene-rich assemblage containing significant amounts of garnet and minor Fe-Tioxides. Incompatible-element abundances in these alkalic basalts are remarkably similar to those of alkalic basalts from oceanic islands. Obviously, there is a worldwide source for alkalic basalts that can be tapped in oceanic and continental settings. Compared with Hannuoba alkalic basalts of similar MgO content, Hannuoba tholeiitic lavas have lower incompatible-element abundances and different incompatible-element abundance ratios (e.g., Ba/La, La/Sm, Sm/Nd, Zr/Nb and Zr/Hf). In contrast to arc basalts and many continental flood basalts, Hannuoba tholeiitic lavas are not anomalously depleted in Nb and Ta. Although radiogenic isotope data (Sr, Nd and Pb) show that the alkalic and tholeiitic lavas are isotopically distinct, variations in incompatible-element abundances and abundance ratios in all lava types define coherent trends suggestive of a mantle mixing process.

Evolution of alkalic lavas at Haleakala Volcano, east Maui, Hawaii

The postshield and posterosional stages of Haleakala Volcano contain intercalated alkalic basalt and evolved alkalic lavas. Isotopic and incompatible element abundance ratios in the Haleakala postshield basalts changed systematically with time, providing evidence for significant temporal changes in the mantle components contributing to the magmatic sources. Specifically, a depleted, i.e. low87Sr/86Sr and high143Nd/144Nd, mantle component is more abundant in younger lavas. However, as magma-production rates decreased during the postshield and posterosional stages, basaltic melts in magma reservoirs cooled and fractionated, leading to evolved residual melts such as hawaiite. Because primary basalt compositions changed with time, the evolved Haleakala lavas formed from a range of parental compositions. However, basalts and evolved lavas of similar age and isotopic ratios (Sr and Nd) have major and trace element contents that are consistent with a crystal-fractionation model. Although alkalic basalt and hawaiite are the dominant lavas of the postshield stages of both Haleakala and Mauna Kea volcanoes, there are important differences between their lavas. For example, compositional differences between the hawaiite suites at Haleakala and Mauna Kea indicate that, on average, the evolved lavas at Haleakala formed at lower pressures. Also, at Haleakala basalts are intercalated with hawaiites, whereas at Mauna Kea basalts and hawaiites are separated by a sharp boundary. These differences probably reflect a higher magma supply rate to the Haleakala volcano.

Basalt basement from the Kerguelen Plateau and the trail of a Dupal plume

The first samples of volcanic basement recovered from the Kerguelen Plateau are Lower Cretaceous transitional tholeiites. Isotope and incompatible element abundance ratios for these rocks are similar to ocean island basalts from the southern hemisphere Dupal anomaly region, and geochemical, geological and geophysical data are consistent with volcanic activity associated with a mantle plume. A reconstruction of plate motions suggests that the Kerguelen Plateau formed above a mantle plume in the interval 118-95 Ma, during the opening of the Indian Ocean between India and Australia-Antarctica. This plume was the source of other plateaus and ridges of the eastern Indian Ocean and possibly the Bunbury Basalt of southwestern Australia, and is now beneath Heard Island.

Trace element and isotopic geochemistry of lavas from Haleakala Volcano, east Maui, Hawaii: Implications for the origin of Hawaiian basalts

Haleakala volcano on East Maui, Hawaii, consists of a tholeiitic basalt shield which grades into a younger alkalic series that was followed by a posterosional alkalic series. Tholeiitic, transitional, and alkalic basalts range widely in Sr and Nd isotopic ratios (from mid‐ocean ridge basalt to bulk earth ratios) and incompatible element (P, K, Rb, Sr, Zr, Nb, Ba, REE, Hf, Ta, and Th) abundances, but isotopic ratios and incompatible element abundance ratios (e.g., Ba/La, Nb/La, La/Ce, La/Sm) vary systematically with age. The youngest series (posterosional alkalic lavas) has the highest Rb/Sr, Ba/La, Nb/La, La/Ce, and 143Nd/144Nd ratios and the lowest 87sr/86sr ratios, whereas the oldest series (dominantly tholeiitic basalts) has the lowest Rb/Sr, Ba/La, Nb/La, La/Ce, and 143Nd/144Nd ratios and the highest 87sr/86sr ratios. The most striking features of the trace element and isotopic data are the inverse correlations between isotopic ratios and parent/daughter abundance ratios in the Sr and Nd systems. Although some of the geochemical variations can be explained by shallow level fractional crystallization (e.g., alkali basalt to mugearite [Chen et al., 1984, and manuscript in preparation, 1985]), the temporal geochemical trends require a major role for mixing. We propose a model in which melts from a diaper interact with incipient melts of its wall rocks, presumed to be oceanic lithosphere. Because of motion between the lithosphere and mantle hot spot the relative contribution of melts from the diapir (mantle plume) material to the lavas decreases with time; consequently, with decreasing age the basalts become more enriched in incompatible trace elements and acquire Sr and Nd isotopic ratios which overlap with mid‐ocean ridge basalts. This model quantitatively explains the isotopic ratios and incompatible trace element abundances in representative samples from the three Haleakala volcanic series. On the basis of the degrees of melting inferred for the mixing components we conclude that the lower lithosphere and much of the asthenosphere beneath Hawaiian volcanoes are involved in creating these volcanoes.