Degree Of Mantle Melting Research Articles

Trace elements in olivine fingerprint the source of 2018 magmas and shed light on explosive-effusive eruption cycles at Kīlauea Volcano

Understanding magma genesis and the evolution of intensive parameters (temperature, pressure, composition, degree of melting) in the mantle source of highly active volcanic systems is crucial for interpreting magma supply changes over time and recognizing cyclic behavior to anticipate future volcanic behavior. Major and trace elements in olivine are commonly used to study variations in mantle lithologies and melting conditions (e.g., temperature, pressure, oxygen fugacity) affecting the mantle over time. Here, we track the temporal evolution of primary melts through the most recent cycle of explosive and effusive eruptions at Kīlauea (Hawai‘i), which spans the last ∼500 years. We report major and trace elements in olivine from the last explosive period (∼1500 – early 1820’s Keanakāko‘i Tephra) and the most recent decade of the current effusive period (2018 LERZ, 2015–2018 Pu‘u‘ō‘ō, 2008–2018 lava lake and 2020 eruption in Halema‘uma‘u). Scandium concentrations in olivine allow characterizing changes in mantle source between 1500 and 2018, and suggest that the recent (2015–2018) magma feeding the Pu‘u‘ō‘ō cone did not significantly interact with the magma that erupted in the LERZ in 2018. The evolution of olivine and melt compositions over the past 500 years is not easily reconcilable with variations in mantle potential temperature, pressure of mantle melt pooling and storage, or oxygen fugacity. Instead, Sc, Mn, and Co concentrations and Ni/Mg ratio in high forsterite (Fo >87) olivine advocate for an increase in the proportion of clinopyroxene in the mantle source associated with a slightly higher degree of partial melting from 1500 to 2018. Changes in primitive melt compositions and degrees of mantle melting may well modulate magma supply to the crust and formation-replenishment of steady or ephemeral summit reservoirs, and thereby control transitions between explosive and effusive periods at Kīlauea. Analyzing trace elements in olivine at Kīlauea and elsewhere could therefore provide important clues on subtle changes occurring at the mantle level that might herald changes in volcanic behavior.

Highly refractory dunite formation at Gibbs Island and Bruce Bank, and its role in the evolution of the circum-Antarctic continent

ABSTRACT The continental margin is of profound importance as it records continental growth by accretion of orogenic magmas and following continental rifting. A high degree of mantle melting due to hydrous fluid input is expected to simultaneously stimulate continental growth and lower the intrinsic density of the mantle than more fertile mantle, which in turn isolates the continental lithosphere from the convective mantle. The mantle peridotites from Gibbs Island (South Shetland Islands) and Bruce Bank in the Drake Passage provide us an insight into the tectonic history in the circum-Antarctic region. To elucidate the continental growth of Antarctica, we present geochemical data of eight dunites from Gibbs Island and one dunite from Bruce Bank, including Re–Os isotope and highly siderophile element compositions. The dunites are severely affected by serpentinization as evidenced by antigorite + brucite or lizardite (loss on ignition = LOI ranging from 3 to 34 wt.%) but contain primary euhedral to subhedral chromites with or without spherical inclusions. The chromites rarely form lens-shaped aggregates. A dunite from Gibbs Island contains fresh olivine grains filling a fracture in the chromite with low LOI (3 wt.%), indicating a deserpentinization origin from a precursor serpentinized dunite. The dunites show highly depleted bulk-rock major element compositions (Mg/Si = 1.4–1.6 and Al/Si = 0.004–0.01 for Gibbs Island dunites, Mg/Si = 0.66 and Al/Si = 0.008 for Bruce Bank dunite), overlapping a compositional field defined by forearc peridotites. The positive correlation in Re/Ir–LOI space corroborates Re input during the later serpentinization process. The 187Os/188Os ratios of the dunites range from 0.11907 to 0.14493. Phanerozoic Re-depletion (melt depletion) ages of ca. 535–129 Ma are recorded in the Gibbs Island dunites, except for one with a Mesoproterozoic Re-depletion age of ca. 1.2 Ga. Since there exists serpentinization-related perturbation of Re, the ages provide minimum time estimates for melt depletion events. The early Paleozoic melt depletion is inferred to have occurred at a very early stage of Antarctic Peninsula formation in response to plate convergence along the margin of Gondwana, whereas the Mesoproterozoic Re-depletion age reflects convecting mantle heterogeneity unrelated to any nearby crust-forming events. The petrographic characteristics of the chromites and highly depleted nature of the dunites are attributed to melt–peridotite reaction in a subduction zone setting. A feasible interpretation for the dunite formation is that the mantle had experienced two stages of melting with the final stage occurring along the Gondwana continental margin in the subduction zone setting. Resultant highly refractory lithospheric mantle was later displaced and dispersed during the Gondwana breakup. Widespread existence of the dunite may be attributed to multi-stage melt depletion along the continental margin.

Two late Carboniferous belts of Nb-enriched mafic magmatism in the Eastern Tianshan: Heterogeneous mantle sources and geodynamic implications

AbstractIdentification of subduction to post-collisional tectonic transitions is critical to the study of orogenic belts. To characterize such a transition in the Tianshan Orogenic Belt, a systematic study was conducted on the late Carboniferous (305–301 Ma) Hongshankou dolerite and Dikan’er basalt of Eastern Tianshan. The Hongshankou dolerites have relatively high Ti and Nb contents, akin to Nb-enriched arc basalts. Based on the Nb/La ratios, these dolerites can be divided into low-Nb/La (0.35–0.40) and high-Nb/La (0.67–1.4) groups, which were likely derived respectively from slab melt-metasomatized mantle wedge and a mixed mantle source involving depleted super-slab and enriched sub-slab asthenospheric components. Like the low-Nb/La dolerites, the Dikan’er basalts possess low Nb/La (0.42–0.46) ratios, suggesting a mantle source previously modified by slab components. In addition, the Dikan’er basalts have variable Nb contents and can be grouped into normal arc basalts and Nb-rich basalts that can be attributed to a common mantle source with different degrees of mantle melting, as demonstrated by the positive correlations of La/Sm with La and Nb. By integrating available data, two late Carboniferous belts of Nb-enriched mafic magmatism are recognized in the Eastern Tianshan, with one in the Yamansu arc (336–301 Ma) and the other in the Bogda Mountains (305–301 Ma). The former is characterized by low Nb/La (<0.6) ratios, reflecting derivation from mantle metasomatized by slab-derived melt during a subduction process; the latter exhibits an abrupt Nb/La increase from 0.6 to 1.4, indicating significant input of sub-slab asthenospheric mantle that was probably induced by slab break off. Accordingly, we propose that the tectonic transition from subduction to post-collision in the Eastern Tianshan occurred in the latest Carboniferous (305–301 Ma) and was marked by the abrupt input of deep and enriched asthenospheric mantle.

Cr-spinel records metasomatism not petrogenesis of mantle rocks

Mantle melts provide a window on processes related to global plate tectonics. The composition of chromian spinel (Cr-spinel) from mafic-ultramafic rocks has been widely used for tracing the geotectonic environments, the degree of mantle melting and the rate of mid-ocean ridge spreading. The assumption is that Cr-spinel’s core composition (Cr# = Cr/(Cr + Al)) is homogenous, insensitive to post-formation modification and therefore a robust petrogenetic indicator. However, we demonstrate that the composition of Cr-spinel can be modified by fluid/melt-rock interactions in both sub-arc and sub-mid oceanic mantle. Metasomatism can produce Al-Cr heterogeneity in Cr-spinel that lowers the Cr/Al ratio, and therefore modifies the Cr#, making Cr# ineffective as a geotectonic and mantle melting indicator. Our analysis also demonstrates that Cr-spinel is a potential sink for fluid-mobile elements, especially in subduction zone environments. The heterogeneity of Cr# in Cr-spinel can, therefore, be used as an excellent tracer for metasomatic processes.

The Role of Inherited Lithospheric Heterogeneities in Defining the Crustal Architecture of Rifted Margins and the Magmatic Budget During Continental Breakup

AbstractDuring the final stage of continental rifting, stretching localizes in the future distal domain where lithospheric necking occurs resulting in continental breakup. In magma‐poor margins, the lithospheric necking is accompanied by crustal hyperextension, serpentinization, and exhumation of mantle lithosphere in the continent‐ocean transition domain. In magma‐rich margins, the necking is accomplished by the emplacement of large amounts of volcanics in the continental‐ocean transition, in the form of seaward dipping wedges of flood basalts (seaward dipping reflections). This study examines the factors controlling the final crustal architecture observed in rifted margins and the magmatic budget during continental breakup, using observations from the Labrador Sea. The latter shows magma‐rich breakup with seaward dipping reflections documented in the north and magma‐poor breakup with a wide domain of exhumed serpentinized mantle recorded in the south. The pre‐rift strength of the lithosphere, defined by the inherited thermal structure, composition, and thickness of the lithospheric layers, controls the structural evolution during rifting. While variations in the magmatic budget associated with breakup are controlled primarily by the interaction between the pre‐rift inheritance, the timing and the degree of mantle melting, in relation to lithospheric thinning and mantle hydration.

H2O in basaltic glasses from the slow-spreading Carlsberg Ridge: Implications for mantle source and magmatic processes

H2O in the mantle is of great interest in the fields of geochemistry and geophysics on account of its significant influence on the melting and physical properties of mantle rocks. The abundance of water in derivative mantle melts beneath mid-ocean ridges can provide important insights into the abundance and distribution of water in the mantle. For this study, we analyzed the H2O, CO2, and major- and trace-element contents in basaltic glass samples collected from the slow-spreading Carlsberg Ridge from 59°E to 66°E in the Indian Ocean. All of the studied samples are found to be normal mid-ocean ridge basalt (N-MORB) in composition. The H2O concentrations range from 0.13 to 0.29wt% with a mean value of 0.22±0.10wt%. Along the ridge from 59°E to 66°E, H2O/La, H2O/Ce, and trace-element ratios show systematic variation whereby H2O/La and H2O/Ce ratios increase with longitude, whereas K2O/Ti2O and (La/Sm)N decrease. The Nb/U (36.4–42.3) and Ce/Pb (17.6–20.8) ratios of the samples are lower in comparison with typical values reported for global MORBs and OIBs (ocean island basalts) (Nb/U=ca. 47, Ce/Pb=ca. 25). In addition, the ratios of incompatible trace elements display wide variation (e.g.,Ba/La and Nb/Zr values of 2.04–6.02 and 0.02–0.07, respectively). The limited variation in the degree of mantle melting cannot yield the observed level of variation in the basaltic glass trace-element ratios. Therefore, we suggest that the mantle source of the studied MORBs has been mixed with subducted oceanic/continental crust and that mantle heterogeneity is the main factor responsible for the chemical variation of MORB along the Carlsberg Ridge axis from 59°E to 66°E.

Geochemistry of peridotites from the Yap Trench, Western Pacific: implications for subduction zone mantle evolution

ABSTRACTThis study examines the major and trace elements of peridotites from the Yap Trench in the western Pacific to investigate mantle evolution beneath a subduction zone. Major element results show that the peridotites are low in Al2O3 (0.31–0.65 wt.%) and CaO (0.04–0.07 wt.%) contents and high in Mg# (Mg/(Mg+Fe)) (0.91–0.92) and have spinels with Cr# (Cr/(Cr+Al)) higher than 0.6 (0.61–0.73). Trace element results show that the peridotites have extremely low heavy rare earth element (HREE) contents compared with abyssal peridotites but have U-shaped chondrite-normalized rare earth element (REE) patterns. The degree of mantle melting estimated based on the major elements, HREEs, and spinel Cr# range from 19% to 25%, indicating that the Yap Trench peridotites may be residues of melting associated with the presence of water in the mantle source. In addition to light rare earth element (LREE) enrichment, the peridotites are characterized by high contents of highly incompatible elements, positive U and Sr anomalies, negative Ti anomalies, and high Zr/Hf ratios. The correlations between these elements and both the degree of serpentinization and high field strength element (HFSE) contents suggest that fluid alteration alone cannot account for the enrichment of the peridotites and that at least the enrichment of LREEs was likely caused by melt–mantle interaction. Comparison between the peridotites and the depletion trend defined by the primitive mantle (PM) and the depleted mantle (DM) suggests that the Yap Trench mantle was modified by subduction-related melt characterized by high contents of incompatible elements, high Zr/Hf ratios, and low HFSE contents. Hydrous melting may have been enhanced by tectonic erosion of the subducting Caroline Plate with complex tectonic morphostructures at the earliest stages of subduction initiation.

Volatile (Cl, F and S) and major element constraints on subduction-related mantle metasomatism along the alkaline basaltic backarc, Payenia, Argentina

We present data on volatile (S, F and Cl) and major element contents in olivine-hosted melt inclusions (MIs) from alkaline basaltic tephras along the Quaternary Payenia backarc volcanic province (~34°S–38°S) of the Andean Southern Volcanic Zone (SVZ). The composition of Cr-spinel inclusions and host olivines in Payenia are also included to constrain any variations in oxygen fugacity. The variation of potassium, fluorine and chlorine in MIs in Payenia can be modelled by partial melting (1–10%) of a variously metasomatised mantle. The high chlorine contents in MIs (up to 3200 ppm) from Northern Payenia require addition of subduction-related fluids to a mantle wedge, whereas volatile signatures in the southern Payenia are consistent with derivation from an enriched OIB source. Cl and Cl/K ratios define positive correlations with host olivine fosterite content (Fo80-90) that cannot be explained by olivine fractionation, degassing and/or degree of mantle melting. Neither can the correlation between SiO2 and TiO2 in the MIs and host olivine Fo-content be explained by magmatic differentiation processes. Instead these correlations essentially require a south to north mantle source transition from a low Mg# pyroxenite (from recycled eclogite) to a high Mg# fluid metasomatised peridotite. The Cl/K and S/K ratios in Payenia MIs extend from enriched OIB-like signatures (south) to Andean SVZ arc like signatures (north). We show that the northward increase in S, Cl and S/K is coupled to a northward increase in melt oxidation states and thus in Fe3+/Fetot ratios in the magmas. The increase in oxidation state also correlates with an increase of Mn/Fe (olivine) ratios. We calculate that 25% of the apparent north–south pyroxenite–peridotite source variation in Payenia (based on olivine Mn/Fe ratios) can be explained by the south to north variation in melt oxidation states.

Crustal structure and origin of the Eggvin Bank west of Jan Mayen, NE Atlantic

AbstractThe Eggvin Bank, located between the Jan Mayen Island and Greenland, is an unusually shallow area containing several submarine volcanic peaks, confined by two transforms on the Northern Kolbeinsey Ridge (NKR). We represent P and S wave velocity models for the Eggvin Bank based on an Ocean Bottom Seismometer profile collected in 2011, showing igneous crustal thickness variations from 8 km to 13 km. A 2–5 km increase is associated with two separate 20–30 km wide segments under the main seamounts. The oceanic crust has three layers: upper crust (L2A: 2.8–4.8 km/s), middle crust (L2B: 5.5–6.5 km/s), and lower crust (L3: 6.7–7.35 km/s). Both the thick layer 2(A/B) and the high ratio of layer 2(A/B) thickness to total crustal thickness indicate that secondary, intraplate magmatism built the seamounts of the Eggvin Bank. The seamount in the north where the crust is thickest has a flat top indicating subaerial exposure but is deeper than those with rounded tops in the south and is therefore probably older. Comparing lower crustal seismic velocity with crustal thickness also indicates that the degree of mantle melting may be higher in the north than in the south. An enriched mantle source presently feeds the NKR magmatism and probably influenced the Eggvin Bank development also at earlier times. To what extent the Eggvin Bank has been influenced by the Iceland plume is uncertain, both an enriched mantle component and elevated mantle temperature may have played a role at different times and locations.

Precambrian plate tectonic setting of Africa from multidimensional discrimination diagrams

New multi-dimensional discrimination diagrams have been used to identify plate tectonic setting of Precambrian terrains. For this work, nine sets of new discriminant-function based multi-dimensional discrimination diagrams were applied for thirteen case studies of Precambrian basic, intermediate and acid magmas from Africa to highlight the application of these diagrams and probability calculations. The applications of these diagrams indicated the following results: For northern Africa: to Wadi Ghadir ophiolite, Egypt indicated an arc setting for Neoproterozoic (746 ± 19 Ma). For South Africa: Zandspruit greenstone and Bulai pluton showed a collision and a transitional continental arc to collision setting at about Mesoarchaean and Neoarchaean (3114 ± 2.3 Ma and 2610-2577 Ma); Mesoproterozoic (1109 ± 0.6 Ma and 1100 Ma) ages for Espungabera and Umkondo sills were consistent with an island arc setting. For eastern Africa, Iramba–Sekenke greenstone belt and Suguti area, Tanzania showed an arc setting for Neoarchaean (2742 ± 27 Ma and 2755 ± 1 Ma). Chila, Bulbul-Kenticha domain, and Werri area indicated a continental arc setting at about Neoproterozoic (800-789 Ma); For western Africa, Sangmelima region and Ebolowa area, southern Cameroon indicated a collision and continental arc setting, respectively for Neoarchaean (∼2800–2900 Ma and 2687-2666 Ma); Finally, Paleoproterozoic (2232-2169 Ma) for Birimian supergroup, southern Ghana a continental arc setting; and Paleoproterozoic (2123-2108 Ma) for Katiola-Marabadiassa, Côte d'Ivoire a transitional continental arc to collision setting. Although there were some inconsistencies in the inferences, most cases showed consistent results of tectonic settings. These inconsistencies may be related to mixed ages, magma mixing, crustal contamination, degree of mantle melting, and mantle versus crustal origin.

Major and trace elements of abyssal peridotites: evidence for melt refertilization beneath the ultraslow-spreading Southwest Indian Ridge (53° E segment)

This study examines the geochemistry of major and trace elements of abyssal peridotites from the Southwest Indian Ridge (SWIR) (53° E amagmatic segment), to determine the influence of mafic melts on mantle peridotites during melt extraction. The results show a great geochemical variability in the ~90 km-long ridge segment, with a degree of mantle melting ranging from 4% to 24%. An ancient melting event may explain the presence of highly depleted peridotites at the ultraslow-spreading ridge. The 53° E segment peridotites show enrichment of light rare earth elements (LREEs) (average LaN/SmN = 1.87) and significant positive anomaly of U and Pb normalized to primitive mantle (PM). The positive correlations between LREEs (La, Ce, Pr, Nd) and high field strength elements (HFSEs; e.g. Nb and Zr) suggest that the enrichment of LREEs is caused by melt refertilization, which is also supported by prevalent magmatic microstructures in the peridotites. The melt refertilization model shows that the addition of 0.02–2.7% basaltic melts to peridotites can be responsible for the LREE enrichment. We suggest that the positive anomaly of U is probably attributed to fluid alteration whereas the enrichment of Pb is probably attributed to both melt refertilization and fluid alteration. Melt refertilization in the 53° E segment peridotites may be a result of melt–rock reaction and crystallization of melts trapped in peridotites. These processes may be enhanced by increased melt permeability in the mantle owing to the refractory peridotites produced by ancient melting and the decreasing efficiency of melt extraction in the cold and thick lithosphere at the 53° E ridge segment. The presence of melt refertilization implies that melt extraction is incomplete in the ridge mantle, which may be one of the reasons for the extremely thin and irregular variation of the crustal thickness at ultraslow-spreading ridges.

Melt–peridotite reactions in upwelling eclogite bodies: Constraints from EM1-type alkaline basalts in Payenia, Argentina

The processes of magma generation in upwelling eclogite bodies of recycled lithospheric material are not fully understood but are important for our understanding and modelling of major and trace element variations in many ocean island basalts (OIB). The primitive alkaline intraplate basalts from the Payenia volcanic province (34–38 °S) in Argentina, for which Sr, Nd and double-spike Pb isotope ratios are presented, and from other north Patagonian volcanic fields may provide details of the eclogite melt–peridotite reactions taking place in the melting column of an upwelling OIB-type mantle. The isotopic composition of the uncontaminated lavas is highly restricted but the Payenia basalts fall in two distinct trace element groups termed the high and low Nb/U groups, which both have EM1-type trace element patterns but with subtle differences that cannot be explained by contamination, fractionation or simple variations in degrees of mantle melting. The difference is also clear in major elements where the low Nb/U basalts have markedly higher alkali contents but lower FeO and Ni than the high Nb/U basalts. Four melt components have been identified based on olivine fractionation corrected compositions: a low Nb/U pyroxenite melt component which is interpreted as an eclogite melt that experienced low degrees of melt–peridotite interaction, a high Nb/U pyroxenite melt component which is interpreted as an eclogite melt that experienced high degrees of melt–peridotite interaction, a low pressure peridotite melt component which contributes to most high Nb/U, Payún Matrú, Matancilla and Auca Mahuida basalts and finally, a melt component with similarities to peridotite melts contributing to many low Nb/U lavas. The high Nb/U type basalts are interpreted to have been formed from mantle with higher temperature than ambient mantle while the low Nb/U basalts may have been formed from normal temperature mantle.

Panxi region (South-West China): Tectonics, magmatism and metallogenesis. A review

Abstract The Panxi region of SW China makes up most of the western margin of the Yangtze craton. Its structural pattern, defined by NS-trending deep faults, is superimposed on a zone of late Proterozoic crustal extension. The Panxi region seems to have evolved as a continental rift in the Permian, and was subsequently subjected to compression, caused mainly by the Himalayan collision. This induced the uplift of its axial and western parts, generating a horst-type structure within the former rift graben. At ca. 260 Ma, mantle plume-related magmatism in the Panxi region generated the Emeishan flood basalts together with ultramafic and silicic volcanic rocks and with numerous intrusive bodies (peridotites, pyroxenites, layered gabbros, syenites, granites). A geochemical distinction between high-Ti and low-Ti Emeishan basalts can be made, but just with the significance of compositional end-members since the published data define a continuous variation of the entire compositional range. Similar compositional variation occurs in the ultramafic lavas (picrites and komatiites) intercalated in the sequence of the Emeishan basalts, which are considered to be the products of undifferentiated primary magmas. Based on the geochemical data, the primary magmas could have been generated from distinct source materials (mantle plume, asthenosphere, lithospheric mantle), from compositionally heterogeneous mantle plume source or from a common source but with various degrees of mantle melting and crustal contamination. Considering the picritic composition of the primary magmas, a deep-seated differentiation of large volume of magma is needed to produce the extrusion of the relatively evolved Emeishan basalts. World class Fe–Ti–V oxide ore deposits are associated with the layered gabbros, while the ultramafic intrusions can host small Ni–Cu and PGE sulfide deposits. The intrusions containing Fe–Ti–V oxide deposits show remarkably continuous layering and probably derived from mafic magmas differentiated in relatively large chambers. The ultramafic intrusions hosting Ni–Cu(–PGE) sulfide ores most likely formed from sulfide saturated magma moving through conduits.

Birth of an ocean in the Red Sea: Initial pangs

We obtained areal variations of crustal thickness, magnetic intensity, and degree of melting of the sub‐axial upwelling mantle at Thetis and Nereus Deeps, the two northernmost axial segments of initial oceanic crustal accretion in the Red Sea, where Arabia is separating from Africa. The initial emplacement of oceanic crust occurred at South Thetis and Central Nereus roughly ∼2.2 and ∼2 Ma, respectively, and is taking place today in the northern Thetis and southern Nereus tips. Basaltic glasses major and trace element composition suggests a rift‐to‐drift transition marked by magmatic activity with typical MORB signature, with no contamination by continental lithosphere, but with slight differences in mantle source composition and/or potential temperature between Thetis and Nereus. Eruption rate, spreading rate, magnetic intensity, crustal thickness and degree of mantle melting were highest at both Thetis and Nereus in the very initial phases of oceanic crust accretion, immediately after continental breakup, probably due to fast mantle upwelling enhanced by an initially strong horizontal thermal gradient. This is consistent with a rift model where the lower continental lithosphere has been replaced by upwelling asthenosphere before continental rupturing, implying depth‐dependent extension due to decoupling between the upper and lower lithosphere with mantle‐lithosphere‐necking breakup before crustal‐necking breakup. Independent along‐axis centers of upwelling form at the rifting stage just before oceanic crust accretion, with buoyancy‐driven convection within a hot, low viscosity asthenosphere. Each initial axial cell taps a different asthenospheric source and serves as nucleus for axial propagation of oceanic accretion, resulting in linear segments of spreading.

Hydrogen and oxygen isotope behaviors during variable degrees of upper mantle melting: Example from the basaltic glasses from Macquarie Island

We present measurements of hydrogen and oxygen isotopes in MORB glasses from Macquarie Island (SW. Pacific Ocean) coupled with determination of bulk H2O content by two independent techniques: total dehydration and FTIR. The incompatible trace elements in these glasses vary by a factor of 12 to 17, with K2O varying from 0.1 to 1.7wt.%; these ranges reflect a variable degree of closed-system mantle melting, estimated from 1 to 15%. Water concentrations determined by the two techniques match well, yielding a range from 0.25 to 1.49wt.% which correlates positively with all of the measured incompatible trace elements, suggesting that water is un-degassed, and behaves conservatively during mantle melting. Also, the agreement between the FTIR-determined and extracted water contents gives us confidence that the measured isotopic values of hydrogen reflect that of the mantle. Comparison of the range of water content with that of other incompatible trace elements allows estimation of the water partition coefficient in lherzolite, 0.0208 (ranging from 0.017 to 0.023), and the water content in the source, 386ppm (ranging from 370 to 440ppm). We observe a fairly narrow range in δD and δ18O values of −75.5±4.5‰ and 5.50±0.05‰ respectively, that can be explained by partial melting of normal lherzolitic mantle. The measured δD and δ18O values of Macquarie Island glasses that range from nepheline- to hypersthene-normative, and from MORB to EMORB in composition, are identical to those in average global MORB. The observed lack of variation of δD and δ18O with 1 to 15% degree of mantle melting is consistent with a bulk melting model of δD and δ18O fractionation, in which water is rapidly scavenged into the first partial melt. The narrow ranges of δD and δ18O in normal mantle are mostly due to the buffering effect of clino- and orthopyroxenes in the residual assemblage; additionally, fast “wet” diffusion of oxygen and hydrogen isotopes through the melting regions may further smooth isotopic differences.

Genesis of 230Th excess in basalts from mid-ocean ridges and ocean islands: Constraints from the global U-series isotope database and major and rare earth element geochemistry

Based on Th-230-U-238 disequilibrium and major element data from mid-ocean ridge basalts (MORBs) and ocean island basalts (OIBs), this study calculates mantle melting parameters, and thereby investigates the origin of Th-230 excess. (Th-230/U-238) in global MORBs shows a positive correlation with Fe-8, P (o), Na-8, and F-melt (Fe-8 and Na-8 are FeO and Na2O contents respectively after correction for crustal fractionation relative to MgO = 8 wt%, P (o)=pressure of initial melting and F (melt)=degree of melt), while Th-230 excess in OIBs has no obvious correlation with either initial mantle melting depth or the average degree of mantle melting. Furthermore, compared with the MORBs, higher (Th-230/U-238) in OIBs actually corresponds to a lower melting degree. This suggests that the Th-230 excess in MORBs is controlled by mantle melting conditions, while the Th-230 excess in OIBs is more likely related to the deep garnet control. The vast majority of calculated initial melting pressures of MORBs with excess Th-230 are between 1.0 and 2.5 GPa, which is consistent with the conclusion from experiments in recent years that D (U)> D (Th) for Al-clinopyroxene at pressures of > 1.0 GPa. The initial melting pressure of OIBs is 2.2-3.5 GPa (around the spinel-garnet transition zone), with their low excess Ra-226 compared to MORBs also suggesting a deeper mantle source. Accordingly, excess Th-230 in MORBs and OIBs may be formed respectively in the spinel and garnet stability field. In addition, there is no obvious correlation of K2O/TiO2 with (Th-230/U-238) and initial melting pressure (P (o)) of MORBs, so it is proposed that the melting depth producing excess Th-230 does not tap the spinel-garnet transition zone. OIBs and MORBs in both (Th-230/U-238) vs. K2O/TiO2 and (Th-230/U-238) vs. P (o) plots fall in two distinct areas, indicating that the mineral phases which dominate their excess Th-230 are different. Ce/Yb-Ce curves of fast and slow ridge MORBs are similar, while, in comparison, the Ce/Yb-Ce curve for OIBs shows more influence from garnet. The mechanisms generating excess Th-230 in MORBs and OIBs are significantly different, with formation of excess Th-230 in the garnet zone only being suitable for OIBs.

Geochronology of Mount Morning, Antarctica: two-phase evolution of a long-lived trachyte-basanite-phonolite eruptive center

Mount Morning is a Cenozoic, alkaline eruptive center in the south-west Ross Sea, Antarctica. New ages on 17 Mount Morning volcanic rocks (combined with 34 existing ages) allows division of Mount Morning volcanism into two phases, erupted between at least 18.7 Ma and 11.4 Ma, and 6.13 and 0.02 Ma. The position of Mount Morning on the active West Antarctic Rift System within the stationary Antarctic plate is a key factor in the eruptive center’s longevity. The earliest, mildly alkaline, Phase I volcanism comprises predominantly trachytic rocks produced by combined assimilation and fractional crystallization processes over 7.3 m.y. Strongly alkaline Phase II volcanism is dominated by a basanite – phonolite lineage, with the youngest (post last glacial maximum) activity dominated by small volume primitive basanite eruptions. The evolution from mildly to strongly alkaline chemistry between phases reflects magma residence time in the crust, the degree of mantle melting, or the degree of magma—country-rock interaction. Phase I magmatism occurred over a comparable area to the present-day, Phase II shield. The 5.2 m.y. volcanic hiatus separating Phase I and II coincides with a cycle of eruption and glacial erosion at the nearby Minna Bluff eruptive center. Mount Morning is the likely source of volcanic detritus in Cape Roberts drill-core (about 24.1 to 18.4 Ma) and in ANDRILL drill-hole 1B (about 13.6 Ma), located 170 km north and 105 km north-east respectively, of Mount Morning. Based upon the timing of eruptions and high heat-flow, Mount Morning should be considered a dormant volcano.

PGE-rich Podiform Chromitites in the Al 'Ays Ophiolite Complex, Saudi Arabia: An Example of Critical Mantle Melting to Extract and Concentrate PGE

The Al ‘Ays ophiolite complex in Saudi Arabia is an example of an ophiolite that contains anomalous concentrations of all six platinum group elements (PGE) in podiform chromitite with maximum values of 2,570 ppb Pt, 6,870 ppb Pd, 840 ppb Rh, 5,800 ppb Ru, 6,200 ppb Ir, and 3,300 ppb Os. Smooth chondrite-normalized PGE profiles indicate igneous PGE ratios. These suggest that in situ alteration of the PGM caused only minor mobility of PGE during secondary modification of the mineralogy. Thus the geochemistry of the igneous concentration processes can be examined despite the mineralogical changes caused by subsequent alteration. Three main types of PGE mineralization are observed that are defined by their relative abundances of individual PGE. Type 1 has Ru > both Pt and Pd, with negative-slope chondrite-normalized profiles. Type 2 has Ru < either Pt or Pd, (Pt+Pd)/Ir ratios of 1 to 5, and convex upward chondrite-normalized profiles. Type 3 has Ru < either Pt or Pd, (Pt+Pd)/Ir ratios of 5 to 60, positive-slope chondrite-normalized profiles and is associated with elevated Cu and Ni concentrations. The unaltered centers of chromite grains in the chromitite within this complex have an unusually large range of composition; for example, Cr2O3 varies from 39 to 69 wt percent. PGE mineralization types 1, 2, and 3 are related to the composition of the chromite. Type 1 occurs across the range of chromitite compositions from 39 to 69 wt percent Cr2O3, type 2 occurs in chromitite having a range of 53 to 61 wt percent Cr2O3 , and type 3 occurs in chromitite having a range of 39 to 51 wt percent Cr2O3. The PGE form a great variety of platinum group minerals (PGM) and they differ among the three types of PGE mineralization. Type 1 is characterized by euhedral Os, Ir, and Ru (IPGE) alloys and laurite, both commonly enclosed in chromite, as well as members of the irarsite hollingworthite solid-solution series and Pt-IPGE-bearing PGM, both commonly interstitial to the chromite grains. Type 2 PGE enrichment is characterized by IPGE-, Pt- and Rh-bearing PGM. Type 3 PGE enrichment hosts predominantly Pd- and Pt-bearing PGM associated with Ni- and Cu-bearing minerals. Where exposed to the serpentinization process, the PGM are altered to alloys, arsenides, antimonides, and oxides that form irregular shapes or may form pseudomorphs of former PGM. They are commonly associated with Ni- and Cu-bearing minerals, including ruthanian pentlandite, millerite, arsenides, and PGE-bearing awaruite. Mantle melting and subsequent crystallization were at an optimum to concentrate PGE in the Al ‘Ays ophiolite complex. Crystallization of IPGE, commonly prior to chromite crystallization and latterly with some Pt and Rh, occurred across the range of chromitite composition. Crystallization of Pd with some remaining Pt occurred during sulfur saturation in chromitite formed from a more evolved magma. We propose that this crystallization was from a magma that was enriched in PGE because the degree of mantle melting was just sufficient to extract the PGE, but not dilute them in a melt that includes further mantle melting. This feature is likely to be common to other PGE-rich ophiolite complexes such as in Shetland in the United Kingdom, Leka in Norway, Thetford in Canada, Pindos in Greece, Tropoja in Albania and in New Caledonia. If equilibrium partial melting had continued in Al ‘Ays, then the magma would have been diluted by subsequent PGE-poor melt. This would have prevented sulfur saturation until much higher in the sequence, producing Pt- and Pd-bearing base metal sulfides in the crustal wehrlite and gabbro, as has occurred in the Cyprus and Oman ophiolites.

Wet and Dry Basalt Magma Evolution at Torishima Volcano, Izu–Bonin Arc, Japan: the Possible Role of Phengite in the Downgoing Slab

The arc-front volcanoes of Sumisu (31 58N, 1408E) andTorishima (30 58N, 140 38E) in the central Izu^Bonin arc are similar in size and rise as relatively isolated edifices from the seafloor.Together they provide valuable along-arc information about magma generation processes. The volcanoes have erupted low-K basalts originating from both wet and dry parental basaltic magmas (low-Zr basalts and high-Zr basalts, respectively). Based on models involving fluid-immobile incompatible element ratios (La/Sm), the parental basalts appear to result from different degrees of partial melting of the same source mantle ( 20% and 10% for wet and dry basalt magmas, respectively). Assuming that the wet basalts contain greater abundances of slab-derived components than their dry counterparts, geochemical comparison of these two basalt types permits the identification of the specific elements involved in fluid transport from the subducting slab. Using an extensive set of new geochemical data fromTorishima, where the top of the downgoing slab is about 100 km deep, we find that Cs, Pb, and Sr are variably enriched in the low-Zr basalts, which cannot be accounted for by fractional crystallization or by differences in the degree of mantle melting. These elements are interpreted to be selectively concentrated in slab-derived metasomatic fluids. Variations in K, high field strength element and rare earth element concentrations are readily explained by variations in the degree of melting between the lowand high-Zr basalts; these elements are not contained in the slab-derived fluids. Rb and Ba exhibit variable behaviour in the low-Zr basalts, ranging from immobile, similar to K, to mildly enriched in some low-Zr basalts.We suggest that the K-rich mica, phengite, plays an important role in determining the composition of fluids released from the downgoing slab. In arcfront settings, where slab depth is 100 km, phengite is stable, and the fluids released from the slab contain little K. In back-arc settings, however, where the slab is at 100^140 km depth, phengite is unstable, and K-rich fluids are released.We conclude that cross-arc variations in the K content of arc basalts are probably related to differing compositions of released fluids or melts rather than the widely held view that such variations are controlled by the degree of partial melting.

Variations in Melt Productivity and Melting Conditions along SWIR (70°E–49°E): Evidence from Olivine-hosted and Plagioclase-hosted Melt Inclusions

Melt inclusion and host glass compositions from the eastern end of the Southwest Indian Ridge show a progressive depletion in light rare earth elements (LREE), Na8 and (La/Sm)n, but an increase in Fe8, from the NE (64°E) towards the SW (49°E). These changes indicate an increase in the degree of mantle melting towards the SW and correlate with a shallowing of the ridge axial depth and increase in crustal thickness. In addition, LREE enrichment in both melt inclusions and host glasses from the NE end of the ridge are compatible with re-fertilization of a depleted mantle source. The large compositional variations (e.g. P2O5 and K2O) of the melt inclusions from the NE end of the ridge (64°E), coupled with low Fe8 values, suggest that melts from the NE correspond to a variety of different batches of melts generated at shallow levels in the mantle melting column. In contrast, the progressively more depleted compositions and higher Fe8 values of the olivine- and plagioclase-hosted melt inclusions at the SW end of the studied region (49°E), suggest that these melt inclusions represent batches of melt generated by higher degrees of melting at greater mean depths in the mantle melting column. Systematic differences in Fe8 values between the plagioclase- and the olivine-hosted melt inclusions in the SW end (49°E) of the studied ridge area, suggest that the plagioclase-hosted melt inclusions represent final batches of melt generated at the top of the mantle melting column, whereas the olivine-hosted melt inclusions correspond to melts generated from less depleted, more fertile mantle at greater depths.