Showing papers on "Incompatible element published in 2003"

Relationships between mafic and peralkaline silicic magmatism in continental rift settings: A petrological, geochemical and isotopic study of the Gedemsa volcano, Central Ethiopian rift

Abstract: Petrological and geochemical data are reported for basalts and silicic peralkaline rocks from the Quaternary Gedemsa volcano, northern Ethiopian rift, with the aim of discussing the petrogenesis of peralkaline magmas and the significance of the Daly Gap occurring at local and regional scales. Incompatible element vs incompatible element diagrams display smooth positive trends; the isotope ratios of the silicic rocks (Sr/Sr ˆ 0 70406---0 70719; Nd/Nd ˆ 0 51274---0 51279) encompass those of the mafic rocks. These data suggest a genetic link between rhyolites and basalts, but are not definitive in establishing whether silicic rocks are related to basalts through fractional crystallization or partial melting. Geochemical modelling of incompatible vs compatible elements excludes the possibility that peralkaline rhyolites are generated by melting of basaltic rocks, and indicates a derivation by fractional crystallization plus moderate assimilation of wall rocks (AFC) starting from trachytes; the latter have exceedingly low contents of compatible elements, which precludes a derivation by basalt melting. Continuous AFC from basalt to rhyolite, with small rates of crustal assimilation, best explains the geochemical data. This process generated a zoned magma chamber whose silicic upper part acted as a density filter for mafic magmas and was preferentially tapped; mafic magmas, ponding at the bottom, were erupted only during post-caldera stages, intensively mingled with silicic melts. The large number of caldera depressions found in the northern Ethiopian rift and their coincidence with zones of positive gravity anomalies suggest the occurrence of numerous magma chambers where evolutionary processes generated silicic peralkaline melts starting from mafic parental magmas. This suggests that the petrological and volcanological model proposed for Gedemsa may have regional significance, thus furnishing an explanation for the large-volume peralkaline ignimbrites in the Ethiopian rift.

Origin of ocean island basalts: A new perspective from petrology, geochemistry, and mineral physics considerations

Abstract: Consideration of petrology, geochemistry, and mineral physics suggests that ancient subducted oceanic crusts cannot be the source materials supplying ocean island basalts (OIB). Melting of oceanic crusts cannot produce high-magnesian OIB lavas. Ancient oceanic crusts (>1 Ga) are isotopically too depleted to meet the required values of most OIB. Subducted oceanic crusts that have passed through subduction zone dehydration are likely to be depleted in water-soluble incompatible elements (e.g., Ba, Rb, Cs, U, K, Sr, Pb) relative to water-insoluble incompatible elements (e.g., Nb, Ta, Zr, Hf, Ti). Melting of residual crusts with such trace element composition cannot produce OIB. Oceanic crusts, if subducted into the lower mantle, will be >2% denser than the ambient mantle at shallow lower mantle depths. This negative buoyancy will impede return of the subducted oceanic crusts into the upper mantle. If subducted oceanic crusts melt at the base of the mantle, the resultant melts are even denser than the ambient peridotitic mantle, perhaps by as much as similar to15%. Neither in the solid state nor in melt form can bulk oceanic crusts subducted into the lower mantle return to upper mantle source regions of oceanic basalts. Deep portions of recycled oceanic lithosphere are important geochemical reservoirs hosting volatiles and incompatible elements as a result of metasomatism taking place at the interface between the low-velocity zone and the cooling and thickening oceanic lithosphere. These metasomatized and recycled deep portions of oceanic lithosphere are the most likely candidates for OIB sources in terms of petrology, geochemistry and mineral physics.

Petrogenesis of Group I Kimberlites from Kimberley, South Africa: Evidence from Bulk-rock Geochemistry

Abstract: Fresh samples of hypabyssal kimberlite from the five major kimberlite pipes in the Kimberley area of South Africa have been analysed for their bulk-rock major and trace element geochemistry. The geochemical data allow identification of the influence of crustal contamination in certain samples, best illustrated in terms of elevated SiO2 ,A l 2 O 3, Pb and heavy rare earth element (HREE) contents. Samples devoid of such crustal contamination show coherent major and fluid-immobile trace element variations, whereas fluid-mobile trace elements are scattered. Kimberlites rich in macrocrysts are shown to reflect substantial (up to 35%) entrainment of mantle peridotite, with Ni---SiO2 and Sc---SiO2 variations defining mixing trajectories towards garnet lherzolite. The likely primary magma(s) parental to the Kimberley kimberlites is suggested to have a composition of 26---27 wt % MgO, 26---27 wt % SiO2, 2 2w t % Al2O3 and Mg number 086. Subtle differences in chondritenormalized REE abundance patterns can be explained by small variations in the degree of partial melting within the range 04---15%, leaving residual garnet. The data are satisfied by melting a source enriched relative to chondrites by a factor of 10 in light REE (LREE), with chondritic or lower HREE abundances. Extended normalized trace element diagrams exhibit significant negative K, Rb, Sr and Ti anomalies that are interpreted to be primary magma characteristics, despite evidence for secondary mobility of K, Sr and Rb. A model is proposed in which fluid or melt from a sub-lithospheric source region precipitates phlogopite en route to metasomatizing the overlying subcontinental mantle lithosphere, imprinting its geochemical signature on a source region previously depleted in HREE relative to primitive mantle. Subsequent 1% melting of the metasomatized source produces a kimberlite with compatible element characteristics strongly influenced by depleted lithospheric peridotite (high Mg number, high Ni, low HREE), but with incompatible elements (and their isotope ratios) characteristic of the deeper source. The similarity of incompatible element ratios (Nb/U, Nb/Th, Ce/Pb) in the kimberlite magmas to those of ocean island basalts from the South Atlantic suggests an ultimate origin in an upwelling mantle plume.

Does depleted mantle form an intrinsic part of the Iceland plume

Abstract: [1] Icelandic basalt ranges in composition from voluminous tholeiite, erupted in the rift zones, to small-volume, mildly alkaline basalt erupted off-axis. In addition, small-volume flows of primitive basalt, highly depleted in incompatible elements, are sometimes found in the actively spreading rift axes. Relative incompatible-element depletion or enrichment in Icelandic basalt is correlated with variation in radiogenic isotope ratios, implying that the mantle beneath Iceland is heterogeneous and that the relative contribution of the various mantle components relates to eruption environment (on- or off-axis) and hence to degree of melting. Thus small-degree off-axis melting preferentially samples an enriched and more fusible mantle component, whereas more extensive melting beneath the rift axes produces magma that more closely represents the bulk Iceland plume mantle composition. The small-volume flows of depleted basalt represent melts that have preferentially sampled a depleted and more refractory mantle component. A debate has arisen over the nature of the depleted component in the Iceland plume. Some authors [e.g., Hanan and Schilling, 1997] argue that the depleted component is ambient upper mantle, the source of normal mid-ocean ridge basalt (NMORB) in this region. Others [e.g., Thirlwall, 1995; Kerr et al., 1995; Fitton et al., 1997], however, have used various lines of evidence to suggest that the plume contains an intrinsic depleted component that is distinct from the NMORB source. Hanan et al. [2000] attempt to refute the existence of a depleted Iceland plume (DIP) component through a critical evaluation of the Nb-Zr-Y arguments advanced by Fitton et al. [1997] and the Hf-Nd-isotopic evidence presented by Kempton et al. [1998]. In this paper we examine the case presented by Hanan et al. [2000] and conclude that their arguments are flawed. Firstly, their trace-element data set excludes data from depleted Icelandic basalt samples and so it is not surprising that they find no evidence for a DIP component. Secondly, they present two new Hf-isotope analyses of a single depleted Icelandic basalt sample and show that the data plot in their NMORB field on an eHf versus eNd diagram. However, new data allow the resolution of distinct NMORB and depleted Icelandic basalt fields on this diagram. We conclude that trace-element and radiogenic isotope data from Iceland require the existence of a DIP component.

Incompatible element ratios in oceanic basalts and komatiites: Tracking deep mantle sources and continental growth rates with time

Abstract: [1] Ratios of elements with similar incompatibilities in the mantle can be used to characterize magma sources through time. Nb/Y and Zr/Y distributions in oceanic basalts support the existence of a long-lived, deep depleted source in mantle. Zr/Y, Nb/Y, Zr/Nb, and Nb/Th ratios in oceanic basalts and komatiites suggest that depleted and recycled components, together probably with an enriched component, were present in the deep mantle by 3.5 Ga. Low Zr/Nb and Hf/Sm ratios and high La/Yb and Nb/Y ratios in some plume basalts and Al-depleted komatiites may reflect majorite fractionation. High Zr/Nb ratios and low Nb/Y ratios in Archean Al-undepleted komatiites may record partial melting of a Mg-perovskite source in deep mantle plumes in which Mg-perovskite crystallizes and accumulates in komatiite melts during ascent. Oceanic greenstone basalts show a gradual increase in the Nb/Th ratio with time with a relatively sudden increase at about 2 Ga. This trend is consistent with gradual continental growth and with a major episode of continental growth at 2.7 Ga. Nb/Th ratios in some Early Archean basalts may record extraction of up to 25% of the present volume of continental crust from the early upper mantle. An alternative explanation for the rapid increase in Nb/Th in oceanic basalts at 2 Ga is that a catastrophic 2.7-Ga event in the mantle changed the composition or/and location of the primary volume of mantle from which continental crust was extracted.

A petrogenetic model for the origin and compositional variation of the martian basaltic meteorites

Abstract: The major element, trace element, and isotopic compositional ranges of the martian basaltic meteorite source regions have been modeled assuming that planetary differentiation resulted from crystallization of a magma ocean. The models are based on low to high pressure phase relationships estimated from experimental runs and estimates of the composition of silicate Mars from the literature. These models attempt to constrain the mechanisms by which the martian meteorites obtained their superchondritic CaO/Al2O3 ratios and their source regions obtained their parent/daughter ( 87 Rb/ 86 Sr, 147 Sm/ 144 Nd, and 176 Lu/ 177 Hf) ratios calculated from the initial Sr, Nd, and Hf isotopic compositions of the meteorites. High pressure experiments suggest that majoritic garnet is the liquidus phase for Mars relevant compositions at or above 12 GPa. Early crystallization of this phase from a martian magma ocean yields a liquid characterized by an elevated CaO/Al2O3 ratio and a high Mg#. Olivine-pyroxene-garnet-dominated cumulates that crystallize subsequently will also be characterized by superchondritic CaO/Al2O3 ratios. Melting of these cumulates yields liquids with major element compositions that are similar to calculated parental melts of the martian meteorites. Furthermore, crystallization models demonstrate that some of these cumulates have parent/daughter ratios that are similar to those calculated for the most incompatible-element-depleted source region (i.e., that of the meteorite Queen Alexandra (QUE) 94201). The incompatible-element abundances of the most depleted (QUE 94201-like) source region have also been calculated and provide an estimate of the composition of depleted martian mantle. The incompatible-element pattern of depleted martian mantle calculated here is very similar to the pattern estimated for depleted Earth's mantle. Melting the depleted martian mantle composition reproduces the abundances of many incompatible elements in the parental melt of QUE 94201 (e.g., Ba, Th, K, P, Hf, Zr, and heavy rare earth elements) fairly well but does not reproduce the abundances of Rb, U, Ta and light rare earth elements. The source regions for meteorites such as Shergotty are successfully modeled as mixtures of depleted martian mantle and a late stage liquid trapped in the magma ocean cumulate pile. Melting of this hybrid source yields liquids with major element abundances and incompatible-element patterns that are very similar to the Shergotty bulk rock.

Komatiites, kimberlites, and boninites

Abstract: When the mantle melts, it produces ultramafic magma if the site of melting is unusually deep, the degree of melting is unusually high, or the source is refractory. For such melting to happen, the source must be unusually hot or very rich in volatiles. Differing conditions produce a spectrum of ultramafic magma types. Komatiites form by high degrees of melting, at great depths, of an essentially anhydrous source. Barbertontype komatiites are moderately high degree melts from a particularly hot and deep source; Munro-type komatiites are very high degree melts of a slightly cooler source. Kimberlites result from low-degree melting, also at great depth, of sources rich in incompatible elements and CO 2 + H 2 O. They become further enriched through interaction with overlying asthenospheric or lithospheric mantle. Boninites form by hydrous melting of metasomatized mantle above a subduction zone. Just like basalts, the different types of ultramafic magma, and the conditions in which they form, are readily identified using major and trace element criteria.

Extreme chemical variability as a consequence of channelized melt transport

Abstract: [1] The spatial and temporal variability of chemical signals in lavas, residues, and melt inclusions provides important constraints on source heterogeneity, the efficiency of convection, and melt transport processes in the mantle. The past decade has seen an impressive increase in the number, precision, and spatial resolution of chemical analyses. However, as resolution has increased, the picture of variation that emerges has become increasingly difficult to understand. For example, mid-ocean ridge basalts can display large variations in trace element concentration on scales from 1000 km of ridge to melt inclusions in 500 micron crystals. These observations suggest that melt transport processes do not readily homogenize partially molten regions. While some of the observed variability is due to source variations, a large proportion could be the consequence of magma transport in channelized systems. We present results of numerical models that calculate the trace element signatures of high-porosity dissolution channels produced by reactive fluid flow. These models were originally developed to explain the organization of melt transport networks, based on observations of “replacive dunites” found in ophiolites. Channelized flow can produce orders of magnitude variation in the concentrations of highly incompatible elements, even for idealized systems with a homogeneous source, constant bulk partition coefficients, and equilibrium transport. Most importantly, the full range of variability may be found in each channel because channelization can transpose the chemical variability produced by melting throughout the melting column into horizontal variability across the width of the channel. The centers of channels contain trace element–enriched melts from depth, while the edges of the channels transport highly depleted melts extracted from the interchannel regions at shallower levels. As dunite channels may be spaced on scales of 1–100 m in the mantle, this mechanism allows highly variable melt compositions to be delivered to the Moho on small length scales. The chemical variation produced in the models is consistent with that seen in melt inclusion suites, lavas, and residual mantle peridotites dredged from the ridges and sampled in ophiolites.

Role of H2O in subduction-zone magmatism: New insights from melt inclusions in high-Mg basalts from central Mexico

Abstract: Although there is a growing body of data on H 2 O in arc magmas, there is still considerable uncertainty about the relationship between H 2 O and various incompatible elements during enrichment of the mantle wedge by subduction processes. We report data for H 2 O, other volatiles (CO 2 , S, Cl), and trace elements in olivine-hosted melt inclusions from high-Mg basalts in central Mexico that exhibit varying degrees of subduction-related enrichment. Most melt inclusions were trapped at low pressure, but rare inclusions (Mg# 65-78, olivine hosts Fo 8 5 - 9 0 ) trapped at upper to middle crustal pressures (1-6 kbar) contain high CO 2 (250-2120 ppm). The high-pressure inclusions indicate magmatic H 2 O contents from 1.3 to 5.2 wt%. Enrichment of H 2 O relative to Nb correlates positively with K/Nb, Ba/Nb, and La/Nb, indicating a clear link between H 2 O) and trace element enrichment of the mantle wedge. Our results show that fluxing of the wedge with an H 2 O-rich component from the subducted slab is important in formation of magmas that are enriched in large ion lithophile (LILE) and light rare earth (LREE) elements relative to high field strength elements (HFSE). In contrast, magmas with low LILEs and LREEs relative to HFSEs have relatively low H 2 O, and must have formed largely by decompression melting of unmodified mantle. Our data for volcanoes <50 km apart show evidence of significant variability in the composition of H 2 O-rich subduction components that are added to the mantle wedge beneath central Mexico.

Geochemical variability of the Oman ophiolite lavas: Relationship with spatial distribution and paleomagnetic directions

Abstract: [1] The multielemental composition of 31 lavas sampled in the extrusive section of the Oman ophiolite was determined by inductively coupled plasma-source mass spectrometry (ICP-MS). This study allowed us to define clear geochemical criteria to characterize the different lavas types in Oman. Most of the Oman ophiolite extrusive sequence is composed of lavas of composition similar to present-day MORB with the exception of slight Nb-Ta negative anomalies (V1 magmatism). V1 lavas display REE patterns and Zr/Hf ratios in the range of MORB (Zr/Hf = 33.15–38.7). In contrast, the overlaying V2 lavas, which outcrop only in the northern part of the ophiolite, are REE depleted relative to V1 and display low Zr/Hf ratios (23.6–30.5). V2 lavas may be further classified into two sub-groups. V2 type I lavas display a continuous decrease from HREE to LREE. The upper V2 type II lavas are more depleted in HREE and MREE but they are enriched in LREE. They are also distinguished by their low Nb/Ta ratios (10.53–11.65) relative to other Oman basalts (V1: 12.5–14; V2 Type I: 12.35–14.4). As for N-MORB, melting of an upwelling mantle beneath an oceanic spreading centre formed V1 lavas. V2 resulted from fluid-enhanced melting of previously depleted mantle residual after V1 extraction. This process probably occurred at different melt/rock ratios thus resulting in the two V2 sub-groups, and was enhanced in the northern part of the ophiolite. V3 lavas overlie the V1–V2 sequence in the Salahi area. They display incompatible element rich patterns (LREE > MREE > HREE) and a high Nb/Ta ratio typical of within-plate basalts. In the different studied areas, the transition from one lava type to the other seems to correlate with the block rotations revealed by paleomagnetic data.

Geochemistry and origin of the basal lherzolites from the northern Oman ophiolite (northern Fizh block)

Abstract: [1] Abundances of major and trace elements in whole rocks and minerals in lherzolites and harzburgites from the northern Oman ophiolite are used to understand the mantle processes creating compositional variation in oceanic lithospheric mantle. Detailed mapping shows that lherzolites occur near the base of a mantle section in the northern Fizh block. Geochemical analyses identify two types of basal lherzolite. The first type (Type I lherzolite) displays porphyroclastic microstructure and occurs sporadically in the basal mylonite zone. Whole rock and clinopyroxene are highly depleted in incompatible elements such as Na, Ti, Zr, and rare earth elements (REE). The chondrite-normalized patterns of Type I lherzolites show steep slopes from heavy REE (HREE) to light REE (LREE) that are ascribed to melt extraction, up to 12–18%, from a source containing a small amount of garnet. The chondrite-normalized patterns have slight enrichment in LREE relative to the patterns expected for residues of partial melting thereby indicating reaction with a LREE-enriched melt or fluid at a low melt/rock ratio. The second type (Type II lherzolite) shows mylonitic microstructure and only occurs at the contact between the mantle section and the metamorphic sole. Abundances of incompatible elements in whole rocks and clinopyroxenes are greater than those of Type I lherzolites, and clinopyroxenes in Type II lherzolites have high Na2O contents (>1 wt.%). To a first approximation, the high Na content of clinopyroxenes and whole rocks and the LREE-depleted, chondrite-normalized whole rock REE patterns are consistent with Type II lherzolite being in equilibrium with a mid-ocean ridge basalt (MORB)-type melt at relatively high pressure (>2 GPa). However, the flatness of chondrite-normalized patterns for middle and heavy REE are inconsistent with residual garnet peridotite. The characteristics of Type II lherzolites are better explained by a mixing process in which residual peridotite was refertilized by addition of a LREE-depleted melt. The large compositional gradient near the basal thrust in the northern Fizh block may have recorded a transient state in which the degree of partial melting was progressively decreased as a result of reducing mantle temperature and upwelling rate. This scenario is consistent with the inferred failing ridge associated with a transform zone in the western side of the northern Fizh block proposed by Nicolas et al. [2000]. In the detachment stage of the Oman ophiolite, a small amount of ascending melt may have crystallized near the basal part of mantle section thereby forming Type II lherzolites. Basal lherzolites and their spatial chemical variations in the northern Fizh block may provide a key for understanding the processes of ridge segmentation and detachment at fast spreading ridges.

Constraints on the Source Components of Lavas Forming the Hawaiian North Arch and Honolulu Volcanics

Abstract: Hawaiian volcanoes, dominantly shields of tholeiitic basalt, form as the Pacific Plate migrates over a hotspot in the mantle. As these shields migrate away from the hotspot, highly alkalic lavas, forming the rejuvenated stage of volcanism, may erupt after an interval of erosion lasting for 0 25±2 5Myr. Alkalic lavas with geochemical characteristics similar to rejuvenatedstage lavas erupted on the sea floor north of Oahu along the Hawaiian Arch. The variable Tb/Yb, Sr/Ce, K/Ce, Rb/La, Ba/La, Ti/Eu and Zr/Sm ratios in lavas forming the North Arch and the rejuvenated-stage Honolulu Volcanics were controlled during partial melting by residual garnet, clinopyroxene, Fe±Ti oxides and phlogopite. However, the distinctively high Ba/Th and Sr/Nd ratios of lava forming the North Arch and Honolulu Volcanics reflect source characteristics. These characteristics are also associated with shield tholeiitic basalt; hence they arise from the Hawaiian hotspot, which is interpreted to be a mantle plume. Inversion of the batch melting equation using abundances of highly incompatible elements, such as Th and La, requires enriched sources with 10±55% clinopyroxene and 5±25% garnet for North Arch lavas. The Sr/Sr and Nd/Nd ratios in lavas forming the North Arch and Honolulu Volcanics are consistent with mixing between the Hawaiian plume and a depleted component related to midocean ridge basalts. Specifically, the enrichment of incompatible elements coupled with low Sr/Sr and high Nd/Nd relative to bulk Earth ratios is best explained by derivation from depleted lithosphere recently metasomatized by incipient melt (52% melting) from the Hawaiian plume. In this metasomatized source, the incompatible element abundances, as well as Sr and Nd isotopic ratios, are controlled by incipient melts. In contrast, the large range of published Os/Os data (0 134±0 176) reflects heterogeneity caused by various proportions of pyroxenite veins residing in a depleted peridotite matrix.

Significance of large, refractory dunite bodies in the upper mantle of the Bay of Islands Ophiolite

Abstract: [1] The origin of large mantle dunites is considered critical for models of melt migration in the mantle. Their presence is not compatible with formation synchronous to a fracture-related melt transport event. In models of porous channel systems for melt transport, they represent a strongly coalesced, high-flux conduit. Dunites from the lower parts of the mantle sections in the Bay of Islands Ophiolite are investigated by detailed geochemical traverses and with single samples. Dunites tend to cluster in the sense that several smaller dunites are associated with larger dunites or several dunites occur together. The chemistry of the large bodies is very depleted (Mg# in olivine 92–94, CaO in olivine 0.05–0.08%, Cr# [100 Cr/(Cr + Al)] in spinel 65–85, TiO2 in clinopyroxene 0.01–0.04%, Sm/Yb 0.2 to 0.7 relative to N-MORB). Detailed traverses across the dunites commonly show a decrease of NiO in olivine associated with an increase in the Mg# along the harzburgite-dunite boundary. Internally, dunite bodies are nearly homogeneous. Thickness of dunite bodies correlates with chemistry, in particular Mg# in olivine and probably Cr# and ferric iron in spinel, but not NiO in olivine. Incompatible element data for the largest dunites argue for their formation by an extremely depleted, high Mg# (boninitic?) melt. We suggest that integrated refractory melt: rock ratios in the largest dunites (up to 40 m) were below 8, because of a low abundance of refractory melts in the crust, and a lack of a systematic change of NiO in olivine with dunite width or across single dunites in detailed chemical traverses. Tectonically, the formation of depleted melts in a late stage of the spreading center is indicated. Their melt feeders failed when approaching the base of the mantle lithosphere and generated large dunites as replacive bodies. The latest expression of this magmatism are orthopyroxenite dykes, in part draining the large dunites. Since the large majority of all deeper mantle dunites are of refractory chemical nature and not akin to MORB, we caution as universally taking large dunite bodies to represent deep-reaching channels with high melt flux and to take the abundance and size distribution of all dunites in an ophiolitic mantle section to infer melt migration mechanisms. In the Bay of Islands Ophiolite, the largest dunites in the mantle section appear to have little to do with the main constructional stage of the spreading center.

The ‘zero charge’ partitioning behaviour of noble gases during mantle melting

Abstract: Noble-gas geochemistry is an important tool for understanding planetary processes from accretion to mantle dynamics and atmospheric formation1,2,3,4. Central to much of the modelling of such processes is the crystal–melt partitioning of noble gases during mantle melting, magma ascent and near-surface degassing5. Geochemists have traditionally considered the ‘inert’ noble gases to be extremely incompatible elements, with almost 100 per cent extraction efficiency from the solid phase during melting processes. Previously published experimental data on partitioning between crystalline silicates and melts has, however, suggested that noble gases approach compatible behaviour, and a significant proportion should therefore remain in the mantle during melt extraction5,6,7,8. Here we present experimental data to show that noble gases are more incompatible than previously demonstrated, but not necessarily to the extent assumed or required by geochemical models. Independent atomistic computer simulations indicate that noble gases can be considered as species of ‘zero charge’ incorporated at crystal lattice sites. Together with the lattice strain model9,10, this provides a theoretical framework with which to model noble-gas geochemistry as a function of residual mantle mineralogy.

The behaviour of boron in a peraluminous granite-pegmatite system and associated hydrothermal solutions: a melt and fluid-inclusion study

Abstract: Detailed analyses of melt and fluid inclusions combined with an electron-microprobe survey of boron-bearing minerals reveal the evolution of boron in a highly evolved peraluminous granite-pegmatite complex and the associated high- and medium-temperature ore-forming hydrothermal fluids (Ehrenfriedersdorf, Erzgebirge, Germany). Melt inclusions in granite represent embryonic pegmatite-forming melts containing about 10 wt% H2O and 1.8 wt% B2O3. These melts are also enriched in F, P, and other incompatible elements such as Be, Sn, Rb, and Cs. Ongoing differentiation and volatile enrichment drove the system into a solvus, where two pegmatite-forming melts coexisted. The critical point is at about 712 °C, 100 MPa, 20 wt% H2O and 4.1 wt% B2O3. Cooling and concomitant fractional crystallisation from 700 to 500 °C induced development of two conjugate melts, an H2O-poor (A-melt) and an H2O-rich melt (B-melt) along the opening solvus. Boron is a major element in both melts and is preferentially partitioned into the H2O-rich melt. Temperature-dependent distribution coefficients $$ D_{{\rm{boron}}}^{{\rm{B - melt/A - melt}}} $$ are 1.3 at 650 °C, 1.5 at 600 °C, and 1.8 at 500 °C. In both melts, boron concentrations decreased during cooling because of exsolution of a boron-rich hypersaline brine throughout the pegmatitic stage. Boromuscovite containing up to 8.5 wt% was another sink for boron at this stage. The end of the melt-dominated pegmatitic stage was attained at a solidus temperature of around 490 °C. Fluid inclusions of the hydrothermal stage reveal trapping temperatures of 480 to 370 °C, along with varying densities and highly variable B2O3 contents ranging from 0.20 to 2.94 wt%. A boiling system evolved, indicating a complex interplay between closed- and open-system behaviour. Pressure switched from lithostatic to hydrostatic and back, generating hydrothermal convection cells where meteoric waters were introduced and mixed with magmatic fluids. Boron-rich solutions originated from magmatic fluids, whereas boron-depleted fluids were mainly of meteoric origin. This highlights the potential of boron for discriminating fluids of different origin. Tin is continuously enriched during the evolution because tin and boron are cross-linked by formation of boron-, fluorine- and tin-fluorine-bearing complexes and is finally deposited within quartz-cassiterite veins during the transition from closed- to open-system behaviour. Boron does not only trace the complex evolution of the Ehrenfriedersdorf complex but exerts, together with H2O, F and P, an important control on the physical and chemical properties of pegmatite-forming melts, and particularly on the formation of a two-melt solvus at low pressure. We discuss this with respect to experimental results on H2O solubility and the critical behaviour of the haplogranite-water system which contained variable concentrations of volatiles.

Geochemical variability in a single flow from northern Iceland

Abstract: [1] Compositional variability is present for almost every element analyzed in 70 whole rock samples and 40 olivine-hosted melt inclusions from the Borgarhraun lava flow in NE Iceland. The variation of incompatible element concentrations can be produced by incomplete mixing of fractional melts, while the compatible element variations in the whole rock samples can be explained by addition/removal of crystals found in the flow. The melt inclusion incompatible element compositions are more variable than the whole rock compositions, and the magnitude of variability of the whole rock samples can be matched if each hand specimen is made of magma that was formed by mixing of 20–30 batches of melt with the composition of the inclusions. Clinopyroxene barometry and the major element composition of the whole rock samples suggest that mixing took place at pressures of ∼0.9 GPa in sub-Moho magma chambers. The spatial distribution of the concentrations of incompatible elements is not random in Borgarhraun and samples with a separation of <4 km have concentrations that are more similar than expected from a random distribution of geochemistry. This separation distance corresponds to a lava volume of 0.014–0.14 km3. This volume may be controlled by magma mixing and episodic eruption of melts from a sub-Moho chamber. The concentration variations of compatible elements with position in the flow are little different to those expected for random variation. It is likely that the crystal removal and addition processes that control major element compositions generate variability on short length scales.

Nature and distribution of dykes and related melt migration structures in the mantle section of the Oman ophiolite

Abstract: [1] We conducted a comprehensive field, petrographic, and microprobe study of the dykes and porous flow channels cropping out in the Oman harzburgites. The 36 rock types we recognized among of about 1000 samples can be grouped in two main magma suites contrasted in terms of structural and textural characteristics, modal composition, order of crystallization, and phase chemistry. One suite (troctolites, olivine gabbros, opx-poor gabbronorites, and rare oxyde gabbros) derives from MORB-like melts. The other suite (pyroxenites, opx-rich gabbronorites, diorites, and tonalite-trondhjemites) derives from melts richer in silica and water than MORBs and ultradepleted in incompatible elements. Dykes and porous flow channels from the MORB suite are restricted to a few areas, covering only 25% of the mantle section. This is an unexpected result as the deep Oman crust is made essentially of cumulates from MORB-like melts. Their composition, texture, and relations with the host harzburgites point to high mantle temperatures at the time of crystallization (likely above 1100°C, up to 1200°C for part of them), i.e., conditions close to the “asthenosphere/lithosphere” boundary. The largest outcrop of mantle harzburgites enclosing MORB like dykes is a 80 km long and 10 km wide corridor, parallel to the strike of the sheeted dyke complex and centered on an area where a former mantle upwelling has been unambiguously defined (the Maqsad “diapir”). A few other occurrences of mantle cumulates from the MORB suite are smaller than the Maqsad area and have a lesser abundance of troctolites (i.e., of high-temperature cumulates). We interpret the troctolite zones of Oman as the witnesses of former diapirs frozen at various stages of their development. Dykes belonging to the depleted suite are the most common in Oman harzburgites. Their structural and textural characteristics show that they crystallized in a mantle colder than the melt (likely in the range 600°C to 1100°C). A possible origin for the parent melts of this suite is in situ partial melting of the shallow and partly hydrated lithosphere residual after MORB extraction. Our data support the view that feeding magma chambers with MORBs is a focused (and likely episodic) process involving the rise of hot mantle to the base of the crust through a lithospheric lid accreted during a previous diapiric event. They suggest also that the shallow mantle beneath spreading centers is a place of important petrologic processes, some of them predicted on the basis of MORB composition (e.g., fractionation inside melt conduits) and other ones unexpected (e.g., remelting of the depleted lithosphere).

Trace-element partitioning between alkali feldspar and peralkalic quartz trachyte to rhyolite magma. Part I: Systematics of trace-element partitioning

Abstract: New alkali feldspar/felsic magma trace-element partition coefficients ( D -values) for Rb, Sr, Ba, Eu, Y, Zr, Nb, Ga, Zn, trivalent REE, Be, Cs, Hf, Pb, Th, and U for 30 samples of peralkalic quartz trachyte and rhyolite are presented. D -values of incompatible elements vary systematically with melt polymerization parameters, increasing with whole-rock silica, but decreasing in rocks with higher agpaitic indices [A.I. = mol (Na + K)/Al]. D -values for Sr and Ba (evaluated to be accurate) vary systematically with crystal chemistry, probably substituting for Na in the Ca-poor alkali feldspar phases. Apparent D -values for Sr and Ba from pre-Quaternary systems are fraught with contamination and analytical errors, respectively, and should be used with caution. D Eu decreases exponentially with A.I., ranging from compatible in weakly peralkalic (A.I. 1.2) rocks. These regular variations strongly suggest that partition coefficients for these elements may be predicted accurately if whole-rock and crystal-chemical parameters are known.

Crustal origin for coupled 'ultra-depleted' and 'plagioclase' signatures in MORB olivine-hosted melt inclusions: evidence from the Siqueiros Transform Fault, East Pacific Rise

Abstract: Geochemical data from melt inclusions in olivine phenocrysts in a picritic basalt from the Siqueiros Transform Fault on the northern East Pacific Rise provide insights into the petrogenesis of mid-ocean ridge basalts (MORB). The fresh lava contains approximately 10% of olivine phenocrysts (Fo89.3 - 91.2) and rare, small (<1 mm) plagioclase phenocrysts with subhedral to irregular shapes with a range of compositions (An80-90, An57-63). Melt inclusions in olivine phenocrysts are glassy, generally rounded in shape and vary in size from a few to approximately 200 lm. Although most of the inclusions have compositions that are generally consistent with being representative of parental melts for the pillow-rim glasses, several inclusions are clearly different. One inclusion, which contains a euhedral grain of high-Al, low-Ti spinel, has a composition unlike any melt inclusions previously described from primitive phenocrysts in MORB. It has a very high Al2O3 (approximately 20 wt%), very low TiO2 (approximately 0.04 wt%) and Na2O (approximately 1 wt%) contents, and a very high CaO/Na2O value (approximately 14). The glass inclusion is strongly depleted in all incompatible elements (La =0.052 ppm; Yb =0.34; La/Sm(n) approximately 0.27), but it has large positive Sr and Eu anomalies (Sr/Sr* approximately 30; Eu/Eu* approximately 3) and a negative Zr anomaly. It also has low S (0.015 wt%) and relatively high Cl (180 ppm). We suggest that this unusual composition is a consequence of olivine trapping plagioclase in a hot, strongly plagioclase-undersaturated magma and subsequent reaction between plagioclase and the host olivine producing melt and residual spinel. Two other melt inclusions in a different olivine phenocryst have compositions that are generally intermediate between 'normal' inclusions and the aluminous inclusion, but have even higher CaO and Sr contents. They are also depleted in incompatible elements, but to a lesser degree than the aluminous inclusion, and have smaller Sr and Eu anomalies. Similar inclusions have also been described in high-Fo olivine phenocrysts from Iceland and northern Mid-Atlantic Ridge. We suggest that the compositions of these inclusions represent assimilation of gabbroic material into the hot primitive magma. The localised nature of this assimilation is consistent with it occurring within a crystal mush zone where the porosity is high as primitive magmas pass through earlier formed gabbroic 'cumulates'. In such an environment the contaminants are expected to have quite diverse compositions. Although the interaction of primitive melts with gabbroic material may not affect the compositions of erupted MORB melts on a large scale, this process may be important in some MORB suites and should be accounted for in petrogenetic models. Another important implication is that the observed variability in melt inclusion compositions in primitive MORB phenocrysts need not always to reflect processes occurring in the mantle. In particular, inferences on fractional melting processes based on geochemistry of ultra-depleted melt inclusions may not always be valid.

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

Abstract: [1] 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 <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.

A Petrologic Perspective of Kīlauea Volcano's Summit Magma Reservoir

Abstract: Two hundred years of magmatic history are documented by the lavas and tephra sampled from K olauea's historical summit eruptions. This paper presents detailed petrographic and geochemical data for a comprehensive suite of samples erupted within or near K olauea's summit caldera since the 17th century. Our results elucidate the range of magmatic processes that operate within the volcano's summit magma reservoir and document two compositional trends that span nearly the entire known range for the volcano. Prior to the 1924 summit crater collapse, a trend of increasing incompatible element and CaO and decreasing SiO2 abundances (at a constant MgO) prevailed. Thereafter, the trend reversed direction and has persisted for the rest of the 20th century, including during the current PuAu A OA o eruption. The rapid and systematic nature of these temporal geochemical variations indicates that the summit reservoir is a single, relatively small body rather than a plexus of dikes and sills. Olivine fractionation is the dominant petrologic process within this reservoir. Petrographic observations and olivine and whole-rock geochemical data suggest that the summit reservoir has a crown of aphyric, more evolved, low-density magma. Differentiation within this crown involving clinopyroxene and plagioclase is more extensive than previously recognized in K olauea summit lavas. The effects of crystal fractionation are superimposed upon an evolving hybrid magma composition produced by mixing new, mantle-derived magmas with more fractionated reservoir magma. Frequent eruptions of these hybrid reservoir magmas document the rapid variation in parental magma composition. These compositional variations correlate with magma supply rate; both are thought to be influenced by the degree of melting of small-scale source heterogeneities within the Hawaiian plume. However, K olauea's source compositions and partial-melting processes have varied only within a narrow range over the past 350 kyr.

Sr–Nd isotopic characteristics of the Mesozoic magmatism in the Taihang–Yanshan orogen, North China craton, and implications for Archaean lithosphere thinning

Abstract: Voluminous felsic rocks (mainly monzonitic) and coeval mafic rocks (mainly monzogabbro–diorites) were emplaced in the Taihang–Yanshan orogen of eastern North China craton in Mesozoic time. The monzogabbro–diorites have high Sr (mostly >1300 ppm) and low e Nd (t) values (−9.5 to −15), indicating a long-term incompatible element enriched subcontinental lithospheric mantle source for their genesis. The monzonitic rocks show elemental geochemistry (e.g. high Sr, and REE patterns) and isotopic compositions similar to the monzogabbro–diorites, which leaves little doubt that the two rock suites share a similar origin. These mafic and felsic rocks thus represent a significant addition of juvenile continental crust from an enriched lithospheric mantle source in the Mesozoic, and their generation via melting of enriched portions of the subcontinental lithospheric mantle is probably an important mechanism responsible for the lithosphere thinning beneath eastern North China craton.