Extent Of Partial Melting Research Articles

Early Eocene Arctic volcanism from carbonate-metasomatized mantle

Melilitite, nephelinite, basanite, and alkali basalt, along with phonolite differentiates, form the Freemans Cove Complex (FCC) in the south-eastern extremity of Bathurst Island (Nunavut, Canada). New 40Ar/39Ar chronology indicates their emplacement between ~ 56 and ~ 54 million years ago within a localized extensional structure. Melilitites and nephelinites, along with phonolite differentiates, likely relate to the beginning and end phases of extension, whereas alkali basalts were emplaced during a main extensional episode at ~ 55 Ma. The melilitites, nephelinites, and alkali basalts show no strong evidence for significant assimilation of crust, in contrast to some phonolites. Partial melting occurred within both the garnet- and spinel-facies mantle and sampled sources with He, O, Nd, Hf, and Os isotope characteristics indicative of peridotite with two distinct components. The first, expressed in higher degree partial melts, represents a relatively depleted component (“A”; 3He/4He ~ 8 RA, εNdi ~ + 3 εHfi ~ + 7, γOsi ~ 0). The second was an enriched component (“B” 3He/4He < 3 RA, εNdi < – 1 εHfi < + 3, γOsi > + 70) sampled by the lowest degree partial melts and represents carbonate-metasomatized peridotite. Magmatism in the FCC shows that rifting extended from the Labrador Sea to Bathurst Island and reached a zenith at ~ 55 Ma, during the Eurekan orogeny. The incompatible trace-element abundances and isotopic signatures of FCC rocks indicate melt generation occurred at the base of relatively thin lithosphere at the margin of a thick craton, with no mantle plume influence. FCC melt compositions are distinct from other continental rift magmatic provinces worldwide, and their metasomatized mantle source was plausibly formed synchronously with emplacement of Cretaceous kimberlites. The FCC illustrates that the range of isotopic compositions preserved in continental rift magmas are likely to be dominated by temporal changes in the extent of partial melting, as well as by the timing and degree of metasomatism recorded in the underlying continental lithosphere.

Insights into the Tethyan mantle heterogeneity: Trace element evidence from the Karakaya Complex, Central Anatolia

The Nilüfer Unit of the Karakaya Complex (northern Turkey) preserves the remnants of the voluminous Triassic Tethyan magmatism. In the Imrahor area (Ankara), the Nilüfer Unit is characterized by a megablock consisting of hydrothermally altered basalts, gabbros, and subordinate wehrlitic cumulates. These lithologies display marked positive Nb anomalies with depleted Th/La ratios and varying degrees of LREE enrichment. They can be subdivided into two groups based on Nb/Nb*, which are not related to each other via fractional crystallization or extent of partial melting. With their strong Nb-kick and broad La/Sm range, the Imrahor lithologies are more akin to FOZO (Focus Zone)- or C (common component)-type melts than EM (Enriched Mantle) and HIMU (high μ). The Zr-Nb systematics reveals that melt mixing was an essential process in the petrogenesis of these mafic rocks. Based on geochemical modeling, the trace element systematics of the İmrahor lithologies can be explained by melting metasomatized oceanic lithospheric mantle (OLM) infiltrated by very low-degree melt fractions. The metasomatized OLM may represent a recycled, plume-related material that has generated Nb-enriched melts during the Triassic Tethyan magmatism.

Petrogenesis and tectonic implications of the late Neoproterozoic mafic dykes in the South Qinling Belt, China

The mafic dykes exposed in the Suizhou area are crucial for understanding the late Neoproterozoic tectonic evolution of the South Qinling Belt. However, it remains unclear whether these rocks were formed in a continental rift or a back-arc tectonic regime. In this contribution, we present a series of new results of zircon U–Pb geochronology, whole-rock geochemistry, and Sr–Nd–Pb isotopes on mafic dykes in the Suizhou area. Three mafic dyke samples give zircon U–Pb ages of 635 ± 2 Ma, 631 ± 3 Ma and 630 ± 2 Ma, respectively, indicating that they were emplaced during the late Neoproterozoic. The mafic dykes are tholeiitic in composition and have low concentrations of SiO2 (44.26–49.08 wt%), varying TiO2 (0.50–1.71 wt%) and MgO (6.43–14.24 wt%) with Mg# values of 53 to 74. They are enriched in LREEs with (La/Yb)N ratios of 2.10–5.20 and display E-MORB-like trace element patterns. Isotopically, the mafic dykes show negative εNd(t) values of −5.09 to −7.15 and low ratios of 206Pb/204Pb (16.77 to 16.92), 207Pb/204Pb (15.39 to 15.41), and 208Pb/204Pb (37.23 to 37.51). These characteristics illustrate that the mafic dykes were mainly derived from the enriched lithospheric mantle, since the negligible crustal contamination has been assessed by the energy-constrained assimilation and fractional crystallization simulation of Sr-Nd isotopes. In addition, the mafic dykes have ΔNb values of −0.46 to + 0.15 with zircon εHf(t) values of −1.5 to + 10.0 previously reported, indicating that the depleted mantle source had participated in their formation. Furthermore, the non-modal batch melting calculation infer that, a mixing of melts from the asthenospheric mantle (18% extent of partial melting) at garnet-stable depth with peridotites from the lithospheric mantle could fit well with geochemical composition of the mafic dykes. The correlation of incompatible elements reveals that these rocks are akin to the intra-continental mafic rocks. Combined with the coeval magmatism in the study area, we conclude that the Suizhou mafic dykes were formed in a rifting continental margin during the late Neoproterozoic.

Petrology and geochemistry of South Mid-Atlantic Ridge (19°S) lava flows: Implications for magmatic processes and possible plume-ridge interactions

The South Mid-Atlantic Ridge (SMAR) 19°S segment, approximately located along the line of Saint Helena volcanic chain (created by Saint Helena mantle plume), is an ideal place to investigate the issue whether the ridge-hotpot interaction process affected the whole MAR. In this study, we present major and trace elemental compositions and Sr-Nd-Pb isotopic ratios of twenty fresh lava samples concentrated in a relatively small area in the SMAR 19°S segment. Major oxides compositions show that all samples are tholeiite. Low contents of compatible trace elements (e.g., Ni ​= ​239–594 ​ppm and Cr ​= ​456–1010 ​ppm) and low Fe/Mn (54–67) and Ce/Yb (0.65–1.5) ratios of these lavas show that their parental magmas are partially melted by a spinel lherzolite mantle source. Using software PRIMELT3, this study obtained mantle potential temperatures (Tp) beneath the segment of 1321–1348 ​°C, which is lower relative to those ridges influenced by mantle plumes. The asthenospheric mantle beneath the SMAR 19°S segment starts melting at a depth of ~63 ​km and ceases melting at ~43 ​km with a final melting temperature of ~1265 ​°C. The extent of partial melting is up to 16%–17.6% with an average adiabatic decompression value of 2.6%/kbar. The correlations of major oxides (CaO/Al2O3) and trace elements (Cr, Co, V) with MgO and Zr show that the parental magma experienced olivine and plagioclase fractional crystallization during its ascent to the surface.87Sr/86Sr (0.702398–0.702996), 143Nd/144Nd (0.513017–0.513177) and 206Pb/204Pb (18.444–19.477) ratios of these lavas indicate the mantle source beneath the SMAR 19°S segment is composed of a three-component mixture of depleted MORB mantle, PREMA mantle, and HIMU mantle materials. The simple, binary mixing results among components from plume-free SMAR MORB, Saint Helena plume and Tristan plume show that asthenospheric mantle beneath the SMAR 19°S segment may be polluted by both Saint Helena and Tristan plume enriched materials. The abovementioned mantle potential temperatures, together with the low Saint Helena (<10%) and Tristan (<5%) components remaining in the asthenospheric mantle at present, show that the physically ridge-hotspot interactions at SMAR 19°S segment may have ceased. However, the trace element and Sr-Nd-Pb isotopically binary mixing calculation results imply that these lavas tapped some enriched pockets left when Saint Helena and/or Tristan plume were once on the SMAR during earlier Atlantic rifted history.

Causes of the Compositional Variability among Ocean Floor Basalts

The chemistry of ocean floor basalts (OFB) has been much studied for the insights that the compositions of their parental magmas may give into the thermal structure of the mantle, and its compositional heterogeneity. But as Mike O’Hara pointed out, OFBs are not primary magmas, and their extensive low-pressure differentiation ‘must be quantified before chemical features of lavas are ascribed to the nature of the underlying upper mantle’. O’Hara also emphasized that individual magmatic eruptions are products of complex magmatic systems. Volcanoes are not formed by isolated events, but are sites of long-term activity at which eruptions are caused by the arrival of replenishing magma that mixes with earlier magma through repeated cycles. In the intervals between replenishment events, part of the magma crystallizes, concentrating incompatible elements in the remaining magma. If the compositions of OFB are to be used to infer differences in the conditions of melting owing to mantle thermal structure, first it is necessary to disentangle the effects of this low-pressure evolution. It is a remarkable feature of OFB magmatism that these potentially complex replenish–mix–tap–crystallize (RMTX) cycles give rise to a simple pattern of chemical evolution when averaged at the global scale. Here we look at the deviations of 45 minor and trace element concentrations from these global average trends. The global trends are taken as the logarithms of the concentrations of the elements as a function of MgO concentration (log [M] vs [MgO]), and the deviations from these trends are defined as ‘variabilities’. An element’s variability is expected to result from a combination of source heterogeneity, partial melting in the mantle and melt extraction, and from the local variations in the RMTX process during crustal evolution. There is a strong correlation of an element’s variability with its slope in the global average trend, where the slope is a proxy for the element’s incompatibility (bulk partitioning) during global average RMTX. This correlation implies that the chemical controls on source heterogeneity and partial melting and melt extraction are similar to those during RMTX. Striking exceptions are the variabilities in plagioclase-hosted Na and Sr, indicating that a substantial part of the variability of Na in OFBs is due to variations in RMTX, rather than reflecting differences in extent of partial melting. If Iceland is excluded, there is no evidence from OFB chemistry to suppose that the distribution of potential temperatures in the sub-ridge mantle has a standard deviation, σ(Tp), larger than about 10°C, and if variability of the mantle source composition is allowed for, σ(Tp) could be even less. A principal component analysis of the variabilities reveals two more-or-less equally important principal components, suggesting that two distinct factors have affected incompatible trace element concentrations in global OFB. We suggest that these two factors are source heterogeneity and the RMTX process rather than the extent of partial melting or shape of the melting regime. An unexpected finding is the decoupling of the variabilities of very incompatible elements (VICE) such as Ba, Th and Nb, from those of the highly incompatible elements (HICE), such as La and the other light rare earth elements, Zr, P and Be.

Highly Siderophile Element and Os Isotope Systematics of Volcanic Rocks at Divergent and Convergent Plate Boundaries and in Intraplate Settings

Terrestrial magmatism is dominated by basaltic compositions. This definition encompasses mid-ocean ridge basalts (MORB), which account for more than eighty percent of Earth’s volcanic products and which are formed at divergent oceanic plate margins, as well as intraplate volcanic rocks such as ocean island basalts (OIB), continental flood basalts (CFB) and continental rift-related basalts, and highly magnesian ultramafic volcanic rocks that dominantly occur in Archean terranes, termed komatiites. All of these broadly basaltic rocks are considered to form by partial melting of the upper mantle, followed by extraction from their source regions and emplacement at the Earth’s surface. For these reasons, basalts can be used to examine the nature and extent of partial melting in the mantle, the compositions of mantle sources, and the interactions between the crust and mantle. Because much of Earth’s mantle is inaccessible, basalts offer some of the best ‘proxies’ for examining mantle composition, mantle convection and crust–mantle interactions. By contrast, at arcs, volcanism is dominated by andesitic rock compositions. While some arcs do have basaltic and picritic magmatism, these magma types are rare in convergent plate margin settings and reflect the complex fractional crystallization and often associated concomitant assimilation processes occurring in arcs. Despite the limited occurrence of high MgO magmas in arc volcanic rocks, magmas from this tectonic setting are also important for elucidating the behavior of the HSE from creation of basaltic compositions at mid-ocean ridges to the subduction of this crust beneath arcs at convergent plate margins. The highly siderophile elements (HSE; comprising Re and Au, along with the six platinum-group elements [PGE] Os, Ir, Ru, Rh, Pt, and Pd) combined with the 187Re–188Os and 190Pt–186Os systems that are embedded within these elements, have found significant utility in the study of basaltic rocks (e.g., Shirey and …

Mantle heterogeneity controls on small-volume basaltic volcanism: REPLY

Eruptions of basaltic material in small-scale volcanic fields located in intraplate settings display a very diverse range in physical and chemical characteristics. Despite its relevance to the understanding of volcanic hazards, the relationship between physical properties of eruptions (explosivity, volume, location) and chemical composition of erupted products has, to date, not been investigated. Here we present a relationship between mantle heterogeneity and extents of partial melting, and both erupted volumes and eruptive style from the Auckland Volcanic Field (New Zealand), and we suggest that this provides a general model for small-scale “monogenetic” magmatic systems globally. Small volcanic centers consistently take the form of nephelinitic tuff rings and scoria cones, whereas larger centers are produced from effusive eruptions of less alkalic magmas. Nephelinitic melts are generated by melting of a deep, carbonated source, whereas less alkalic melts are the products of melting of a shallower, noncarbonated source. U-Th-Ra isotope data from eruptions closely paired in space and time show that mixing between magmas is extremely limited as a consequence of different ascent mechanisms due to differential segregation of melts from varying sources (early, carbonated melts ascending by higher porosity channels, and later, uncarbonated melts by a more diffusive regime). This suggests that extraction of melt is nearly instantaneous in these environments. Our results stress the importance of melting and magma dynamics in determining the size and style of eruptions in small volcanic fields, and suggest that mantle controls should be an important consideration in volcanic hazard assessment.

Mantle heterogeneity controls on small-volume basaltic volcanism

Eruptions of basaltic material in small-scale volcanic fields located in intraplate settings display a very diverse range in physical and chemical characteristics. Despite its relevance to the understanding of volcanic hazards, the relationship between physical properties of eruptions (explosivity, volume, location) and chemical composition of erupted products has, to date, not been investigated. Here we present a relationship between mantle heterogeneity and extents of partial melting, and both erupted volumes and eruptive style from the Auckland Volcanic Field (New Zealand), and we suggest that this provides a general model for small-scale “monogenetic” magmatic systems globally. Small volcanic centers consistently take the form of nephelinitic tuff rings and scoria cones, whereas larger centers are produced from effusive eruptions of less alkalic magmas. Nephelinitic melts are generated by melting of a deep, carbonated source, whereas less alkalic melts are the products of melting of a shallower, noncarbonated source. U-Th-Ra isotope data from eruptions closely paired in space and time show that mixing between magmas is extremely limited as a consequence of different ascent mechanisms due to differential segregation of melts from varying sources (early, carbonated melts ascending by higher porosity channels, and later, uncarbonated melts by a more diffusive regime). This suggests that extraction of melt is nearly instantaneous in these environments. Our results stress the importance of melting and magma dynamics in determining the size and style of eruptions in small volcanic fields, and suggest that mantle controls should be an important consideration in volcanic hazard assessment.

The source of Mesozoic granitoids in South China: Integrated geochemical constraints from the Taoshan batholith in the Nanling Range

A combined study of zircon U–Pb ages and Lu–Hf isotopes, whole-rock major-trace elements and Sr–Nd isotopes as well as whole-rock and mineral O isotopes was carried out for Mesozoic granitic intrusions from the Taoshan batholith in the Nanling Range, South China. The results not only place constraints on the origin of granitoids but also provide insights into the effect of melting temperature on granitoid compositions. LA-ICPMS zircon U–Pb dating yields weighted 206Pb/238U ages of 230±2Ma for the Caijiang intrusion, 167±2Ma for the Huangpi intrusion, 152±3Ma for the Daguzhai intrusion, and 146±3Ma for the Luobuli intrusion. Relict zircon cores with older Mesozoic to Paleozoic U–Pb ages are present in some samples. These Mesozoic granitoids are weakly to strongly peraluminous with A/CNK ratios of 1.05 to 1.23. They exhibit arc-like trace element distribution patterns, with enrichment of LREE and LILE (e.g., Rb, K, Pb) but depletion of HFSE (e.g., Nb, Ta, Ti). They show high whole-rock initial 87Sr/86Sr ratios of 0.7101 to 0.7156 and low εNd(t) values of −11.2 to −8.8 as well as negative zircon εHf(t) values of −14.3 to −4.8 and Paleoproterozoic Nd-Hf model ages. These results suggest their derivation from partial melting of ancient continental crust. Furthermore, they have high zircon δ18O values of 8.6 to 10.6‰ and calculated whole-rock δ18O values of 10.8 to 12.5‰. Along with their peraluminous features, high K2O/Na2O ratios, arc-like trace element distribution patterns and enriched Sr–Nd–Hf isotope compositions, these Mesozoic granitoids are unambiguously derived from partial melting of metasedimentary rocks and thus are of S-type affinity. There are significant differences in both major and trace elements between the Caijiang intrusion of Triassic age and the Huangpi, Daguzhai and Luobuli intrusions of Jurassic age. While the Triassic granites exhibit high (Fe2O3)T+MgO, P2O5, Th, LREE and Zr+Nb+Ce+Y contents but low Al2O3/TiO2, Rb/Ba and Rb/Sr ratios in association with high zircon saturation temperatures, the Jurassic granites have low (Fe2O3)T+MgO, P2O5, Th, LREE and Zr+Nb+Ce+Y contents but high Al2O3/TiO2, Rb/Ba and Rb/Sr ratios in association with low zircon saturation temperatures. These correlations indicate that melting temperature and thus extent of partial melting have played an important role in dictating the compositions of granitoids. Therefore, the geochemical compositions of S-type granitoids are dictated not only by the composition of source rocks but also by the temperature of partial melting.

Partial melting control of water contents in the Cenozoic lithospheric mantle of the Cathaysia block of South China

Major and trace element and the H2O contents of minerals in peridotite xenoliths hosted by the Cenozoic basalts of Jiande in the Cathaysia block were evaluated using electron microprobe, laser-ablation ICP-MS and Fourier transform infrared spectroscopy, respectively. The correlations among the major elements of the minerals define a melting trend, and modeling of the Y and Yb contents in the clinopyroxenes indicates that the degree of partial melting ranges from 1% to 15%. Most samples (22 out of 29) show depleted chondrite-normalized rare earth element patterns and a degree of partial melting <4%. The H2O contents (weight in ppm) of the clinopyroxenes, orthopyroxenes and olivines are 390–590ppm, 160–330ppm and ∼0ppm, respectively. Although potential H-loss during xenolith ascent cannot be excluded for olivines, pyroxenes largely preserve the H2O content they have in the mantle prior to sampling by the host basalts, as inferred from (1) the homogenous H2O content within single pyroxene grains, and (2) the equilibrium H2O partitioning between the clinopyroxene and orthopyroxene. Based on the mineral modes and assuming a partition coefficient of 10 for H2O between the clinopyroxene and olivine, the calculated whole-rock H2O contents range from 90 to 220ppm, similar to that of the MORB source. When combined with previously reported data for peridotites hosted by Cenozoic basalts at other localities of the Cathaysia block, the correlation of H2O content with melting index (such as Yb content in cpx, Cr# in spinel) suggests that extent of partial melting is the main factor controlling H2O abundance in these rocks. Nevertheless, the variations in whole-rock H2O contents cannot be perfectly modeled as a simple modal melting process using available experimental partition coefficients of H2O between peridotite and melt. The lack of coherent variations between the H2O contents of the whole rocks and the metasomatic index (La/Yb ratio in clinopyroxene) indicates that mantle metasomatism did not modify the initial H2O contents after the melting event(s). Based on the similarities in major and trace elements, and of H2O contents in the Cathaysia block peridotites and those inferred for the MORB source, we propose that the Cenozoic lithospheric mantle of the Cathaysia block is accreted from the upwelled and cooled asthenospheric mantle. In addition, the H2O contents of the Cenozoic lithospheric mantle of the Cathaysia block are much higher than those of the North China Craton of similar fertility (100ppm vs. 20ppm for whole rock H2O contents), where lithospheric thinning occurred during the Mesozoic. This implies that either the Cathaysia block did not undergo similar lithospheric thinning or that the mechanism of the lithospheric thinning was different.

Mantle heterogeneity beneath the Antarctic–Phoenix Ridge off Antarctic Peninsula

Abstract We determined the Sr, Nd and Pb isotopic compositions of basalts recovered from the Antarctic–Phoenix Ridge (APR), a fossil spreading center in the Drake Passage, Antarctic Ocean, in order to understand the nature of the subridge mantle source. There are no known hotspots in close proximity to the site. We observe that small‐scale isotopic heterogeneity exists at a shallow level in the subaxial mantle of the APR. Enriched (E‐type) mid‐ocean ridge basalts (MORB) coexist with normal (N‐type) MORB in this region. The E‐type basalts are: (i) relatively young compared to the N‐type samples; (ii) were erupted after the extinction of the APR; and (iii) have been generated by low‐degree partial melting of an enriched mantle source. Extinction of the APR likely caused the extent of partial melting in this region to decrease. We interpret that the geochemically enriched materials dispersed in the ambient depleted mantle were the first fraction to melt to form the E‐type MORB.

Partitioning of U and Th during garnet pyroxenite partial melting: Constraints on the source of alkaline ocean island basalts

Uranium series disequilibria in ocean island basalts (OIB) provide evidence for the presence of garnet in their source region. It has been suggested that enriched OIB signatures derive from mantle lithologies other than peridotite, such as eclogite or pyroxenite, and, in particular, that silica-poor garnet pyroxenite is the source lithology for alkali basalts. To test the ability of such a source to produce the U–Th disequilibria observed in alkali OIB, we determined experimentally clinopyroxene-melt and garnet-melt partition coefficients for a suite of trace elements, including U and Th, at 2.5 GPa and 1420–1450 °C. The starting composition for the experiments was a 21% partial melt of a silica-poor garnet pyroxenite. Experimentally determined clinopyroxene-melt partition coefficients range from 0.0083 ± 0.0006 to 0.020 ± 0.002 for Th and from 0.0094 ± 0.0006 to 0.024 ±0.002 for U, and garnet-melt partition coefficients are 0.0032 ± 0.0004 for Th and 0.013 ± 0.002 for U. Comparison of our experimental results with partition coefficients from previous experimental studies shows that the relative compatibilities of U and Th in both garnet and clinopyroxene are different for different mineral compositions, leading to varying degrees of U/Th fractionation with changing lithology. For a given melting rate and extent of partial melting, mafic lithologies tend to produce larger 230Th excesses than peridotite. However, this effect is minimized by the greater overall extents of melting experienced by eclogites and pyroxenites relative to peridotite. Results from chromatographic, batch, and fractional melting calculations with binary mixing between partial melts of pyroxenite and peridotite, carried out using our new partitioning data for the pyroxenite component and taking into account variable productivities and different solidus depths for the two lithologies, suggest that OIB are not the product of progressive melting of a source containing a fixed quantity of garnet pyroxenite. Melting a peridotite with enriched signatures, and mixing those melts with melts of a depleted, “normal” peridotite, is an alternative explanation for the trends seen in Hawaiian, Azores and Samoan lavas.

Geochemistry of Cenozoic basalts and mantle xenoliths in Northeast China

Volcanic activity in Northeast China and adjacent regions is widespread, with eruption ages ranging from Late Cretaceous (80 Ma) to about 300 years ago. Volcanic rock types are principally basanite, alkali olivine basalt, and tholeiite, with minor evolved trachyte and rhyolite. Ultramafic xenoliths, mainly spinel lherzolite and harzburgite, are common in alkali olivine basalt and basanite. We present new major and trace element data from 11 volcanic fields, and helium isotope data from ultramafic xenoliths of three of those fields. 3He/ 4He ratio ranges from 5 to 7 times the atmospheric ratio, indicating that previously reported high 3He/ 4He ratios are likely due to the contamination by cosmogenic 3He. There is currently no evidence for a high- 3He/ 4He mantle plume component beneath NE China. Sr–Nd–Pb isotopic data from the literature are reinterpreted to result from mixing between a FOZO end-member and a LoMu end-member. The LoMu end-member appears to reside within continental lithosphere. Major and trace elements do not indicate significant contributions from a subducted slab. Differences in depth and the extent of partial melting in the asthenosphere, both locally within individual fields and regionally between the different volcanic fields, explain most of the correlations between major and trace elements observed in this study, with deeper melting usually associated with a smaller degree of melting. Highly potassic basalts are associated with the LoMu end-member and are interpreted to be the products of lithosphere melting.

Petrology and Geochemistry of Volcan Cerro Azul: Petrologic Diversity among the Western Galapagos Volcanoes

INTRODUCTION Cerro Azul, one of the large shield volcanoes in the western Galapagos archipelago, has erupted a wide range of tholeiitic to alkalic basalts. The Galapagos archipelago is a hotspot-generated collection of island volcanoes that differs in many respects These diverse compositions include some of the most primitive yet from more familiar examples, such as Hawaii. The arreported from the western archipelago and are unlike those of the chipelago is located adjacent to the Galapagos Spreading other, well-studied, neighboring volcanoes of Sierra Negra and Center and constructed atop a shallow volcanic platform Alcedo, which have erupted basalt of fairly uniform composition. on young, thin ocean lithosphere (Fig. 1). Although it Majorand trace-element modeling shows that Cerro Azul, Alcedo has been long recognized that the Galapagos islands and Sierra Negra share a similar depth of melting and source reflect intraplate volcanism resulting from a mantle plume composition. Modeling also reveals that there are small, systematic (Morgan, 1971), a number of tectonic and petrologic differences in the extent of partial melting between the volcanoes features conflict with a simple mantle plume model. that can be related to their distance from the proposed plume center These include: (1) a non-linear arrangement of volcanoes; below the westernmost island of Fernandina. However, even though (2) a wide distribution of simultaneously active volcanoes; melts segregating from the plume in the western Galapagos reflect a (3) a distinct geographic pattern to the petrologic and narrow range of temperatures and source compositions, there are geochemical diversity of the volcanoes; (4) a lack of wide variations in the enrichments of major and trace elements any consistent petrological and geochemical evolutionary between Cerro Azul, Alcedo and Sierra Negra that cannot be patterns like those documented for Hawaii (Macdonald attributed to mantle processes. We believe the observed intershield & Katsura, 1964; Frey & Clague, 1983) Chen & Frey, geochemical differences result from magma supply and cooling rates 1985; and Reunion (Albarede et al., 1997). Much of that are unique to each volcano, and reflect the variations in the geochemical variability between volcanoes in the lithospheric transport and storage processes across the western Galapagos has been ascribed to mantle processes conarchipelago. nected with a well-defined, archipelago-wide pattern of depleted and enriched mantle source compositions (Geist et al., 1988; Geist, 1992; Graham et al., 1993; White et al., 1993; Harpp, 1995; Kurz & Geist, 1999; Harpp & White, 2001). These geochemical patterns are proposed to result from both intrinsic differences in plume com

Large [formula omitted]-excesses in basalts produced by partial melting of spinel lherzolite

Excesses of 230 Th over 238 U in mid-ocean ridge basalts (MORB) require that the mantle source region preferentially retains U over Th during partial melting. Based on existing mineral-melt partitioning data, 230 Th excesses are widely cited as evidence that partial melting beneath ridges begins within the garnet stability field, at pressures over 2.8 GPa. However, recent experimental and theoretical studies of U–Th partitioning show that melting in the presence of aluminous mantle clinopyroxene may also generate 230 Th -excess. In order to try to distinguish between these models we sought basalts with independent constraints on their depth and extent of partial melting. We report data from alkali basalts from the Antarctic Peninsula whose tectonic setting indicates that they formed by <6% partial melting at pressures of <2 GPa, well within the spinel stability field. Their major and trace element chemistry is best modelled by ∼4% partial melting at pressures of 1–2 GPa, in excellent agreement with that inferred from the plate structure. However, these rocks preserve large (6–26%) 230 Th -excesses, which would conventionally be ascribed to the involvement of garnet. Instead we show that the trace element signature and isotopic data can be reconciled with partial melting involving residual aluminous–clinopyroxene within the spinel stability field. These Antarctic Peninsula basalts provide the first observational evidence that significant 230 Th -excesses can be produced by partial melting of spinel lherzolite and challenge the perceived importance of garnet in MORB petrogenesis.

Structural and petrologic evolution of the Lihue basin and eastern Kauai, Hawaii

The topography of the eastern part of the Hawaiian island of Kauai is dominated by the Lihue basin, a large (∼110 km 2 ) semicircular depression bounded by steep cliffs and partly filled by late rejuvenated-stage (or posterosional stage) volcanic material. As with other large, semicircular basins on ocean-island volcanoes, the subsurface geology and origin (e.g., structural collapse vs. fluvial erosion) of the Lihue basin are poorly understood. New analyses of samples collected from eastern Kauai and drill holes within the basin document several important features of the late-stage geologic evolution of Kauai. First, thick (>300 m) sequences of rejuvenated-stage Koloa Volcanics in the Lihue basin show systematic, basin-wide geochemical trends of increasingly incompatible elements with time, indicating a gradual decrease in the extent of partial melting of mantle sources with time. Second, beneath the rejuvenated-stage volcanics in the basin, a thin layer of postshield alkalic stage lavas (e.g., hawaiites and mugearites) overlies older shield-stage tholeiitic lavas of the Napali Member, indicating that the Lihue basin formed by structural collapse, not fluvial erosion. Third, a large (∼2–5 km 3 ) matrix-supported breccia, interpreted as deposits of one or more debris flows, is within the rejuvenated-stage volcanics throughout the basin, and correlates with surficial exposures of the Palikea Breccia west of the basin. Isotopic compositions of the bulk breccia are similar to those of tholeiites from the east side of Kauai, and distinct from those of west Kauai tholeiites. Clasts within the breccia are dominantly hawaiite and alkali gabbro. The source region of the breccia in the steep cliffs and highlands of the central massif to the west of the basin must contain magmatic products of an extensive postshield alkalic stage, including hawaiite flows and one or more large intrusive bodies or ponded sequences of alkali gabbro.

Postshield volcanism and catastrophic mass wasting of the Waianae Volcano, Oahu, Hawaii

The 3.9- to 2.9-Ma Waianae Volcano is the older of two volcanoes making up the island of Oahu, Hawaii. Exposed on the volcanic edifice are tholeiitic shield lavas overlain by transitional and alkalic postshield lavas. The postshield "alkalic cap" consists of aphyric hawaiite of the Palehua Member of the Waianae Volcanics, overlain unconformably by a small volume of alkalic basalt of the Kolekole Volcanics. Kolekole Volcanics mantle erosional topography, including the uppermost slopes of the great Lualualei Valley on the lee side of the Waianae Range. Twenty new K–Ar dates, combined with magnetic polarity data and geologic relationships, constrain the ages of lavas of the Palehua member to 3.06–2.98 Ma and lavas of the Kolekole Volcanics to 2.97–2.90 Ma. The geochemical data and the nearly contemporaneous ages suggest that the Kolekole Volcanics do not represent a completely independent or separate volcanic event from earlier postshield activity; thus, the Kolekole Volcanics are reduced in rank, becoming the Kolekole Member of the Waianae Volcanics. Magmas of the Palehua and Kolekole Members have similar incompatible element ratios, and both suites show evidence for early crystallization of clinopyroxene consistent with evolution at high pressures below the edifice. However, lavas of the Kolekole Member are less fractionated and appear to have evolved at greater depths than the earlier Palehua hawaiites. Postshield primary magma compositions of the Palehua and Kolekole Members are consistent with formation by partial melting of mantle material of less than 5–10% relative to Waianae shield lavas. Within the section of Palehua Member lavas, an increase with respect to time of highly incompatible to moderately incompatible element ratios is consistent with a further decrease in partial melting by approximately 1–2%. This trend is reversed with the onset of eruption of Kolekole Member lavas, where an increase in extent of partial melting is indicated. The relatively short time interval between the eruption of Palehua and Kolekole Member lavas appears to date the initial formation of Lualualei Valley, which was accompanied by a marked change in magmatic conditions. We speculate that the mass-wasting event separating lavas of the Palehua and Kolekole Members may be related to the formation of a large submarine landslide west and southwest of Waianae Volcano. Enhanced decompression melting associated with removal of the equivalent volume of this landslide deposit from the edifice is more than sufficient to produce the modeled increase of 1–2% in extent of melting between the youngest Palehua magmas and the posterosional magmas of the Kolekole Member. The association between magmatic change and a giant landsliding event suggests that there may be a general relationship between large mass-wasting events and subsequent magmatism in Hawaiian volcano evolution.

San Quintín Volcanic Field, Baja California Norte, México: Geology, petrology, and geochemistry

The San Quintin Volcanic Field (SQVF) is unique for the Baja California peninsula as the only known location of intraplate‐type mafic alkalic volcanism and the only known source of peridotitic and granulitic xenoliths. It consists of 10 distinct Quaternary volcanic complexes. The oldest cones mainly erupted primitive magmas (Mg # &gt; 64)(Mg # = 100 × Mg/(Mg + (0.85 × FeTotal))), which carried occasional small xenoliths. As the SQVF evolved with time, differentiated magmas (Mg # &lt; 64) became increasingly common, but primitive magmas, virtually devoid of xenoliths and unusually rich in olivine phenocrysts, dominanted at the youngest cones. Abundances of incompatible elements declined during evolution of the SQVF, implying a temporal increase in the extent of partial melting in the mantle, or progressive exhaustion of these elements in the source. Samples from two cones, Mazo and Ceniza, show relatively low Ce/Pb, εNd, and 206Pb/204Pb and high 87Sr/86Sr, which we interpret as evidence for crustal contamination of these magmas. Small isotopic variations for the other cones are collectively interpreted to reflect involvement of at least three mantle components beneath the SQVF. Ranges in isotopic composition overlap for primitive and differentiated rocks, supporting fractional crystallization as the mechanism for deriving the latter from the former. Most differentiated rocks can be successfully modeled by fractional crystallization of olivine, plagioclase, clinopyroxene, and spinel from primitive parents. The largest and most abundant xenoliths were carried by differentiated magmas, indicating that fractional crystallization took place within the mantle, below the level of peridotite entrainment, and reflecting the importance of fractionation‐elevated volatile contents for driving these differentiated magmas rapidly to the surface. Primitive rocks of the SQVF are unusual compared to other reported intraplate‐type mafic alkalic suites from around the world in having relatively high Al2O3 and Yb, as well as low La/Yb and CaO/Al2O3. These characteristics and trends of rising Al2O3 and falling CaO with decreasing incompatible element abundances are all consistent with origins for the SQVF primitive magmas by progressive partial melting of spinel lherzolite at unusually shallow levels in the mantle.

Development and Evolution of the Lithospheric Mantle: A Case Study from the Horoman Peridotite, Japan

The Horoman peridotite, Hokkaido, Japan is a layered upper mantle peridotite that is a piece of heterogeneous mantle created by melt formation, segregation, migration and waUrock/melt interaction. (Obata and Nagahara, 1987; Takahashi, 1991, 1992; Takazawa et al., 1992b). In order to understand the scale and timing of these processes we studied 60 samples from a 40 m stratigraphie section (the Bozu section). This section is well exposed and several rock types as layers, 10's of meters thick. Abundances of REE, Sr, Zr, Ti, Y, Cr and V in clinopyroxenes (cpxs) from 60 peridotites were analyzed by a secondary ion mass spectrometer at WHOI. Abundances of REE, Sr, Zr, Nb, Hf, Th and REE in 34 whole rocks were measured by ICP-MS at Universit6 de Montpellier II, France. Plagioclase lherzolites are generally depleted in incompatible elements (Fig. 1). It is noted that Zr fractionates from Hf as the bulk rocks becomes more depleted in incompatible elements. Fig. 2 shows variation of chondrite-normalized La/Nd ratio [(La/Nd)N] in cpxs in terms of stratigraphic height. Plagioelase lherzolites have the lowest values (< 1), except in a subsection of LREE enriched plagioclase lherzolite (~2) . In the lherzolite layer the ratio increases from near unity to 10 towards the boundary with the harzburgite, then decreases gradually within 3 meters from the boundary because abundances of Nd in the cpxs increase. Lherzolites near the phlogopite-bearing zone have (La/Nd)N ratio higher than unity. Harzburgite has constant (La/Nd)N ratio of 1 ~ 2 while some harzburgite near a mafic layer has (La/Nd)N ratio lower than unity. Generally the lithological boundary is associated with steep compositional gradient. Fig. 3 shows variation of chondrite normalized patterns of REE, Sr, Zr and Ti along a traverse in the porphyroclastic cpx in a ptagiodase lherzolite. The rim is enriched in REE, especially in HREE, and depleted in Sr relative to the core. It is noted that the core is relatively depleted in Zr but this depletion decreases toward the rim. The geochemical data provide evidence for five sequential events: Event 1. Formation as asthenospheric mantle. The plagioclase lherzolites are fertile peridotites, 34 % CaO and A1203, which have been variably depleted in incompatible elements by segregation of small and variable extents of partial melting (Fig. I). Because the plagioclase lhcrzolite is the most

MORBCAL: a program for calculating the compositions of primary basaltic melts produced by decompression-induced melting below mid-ocean ridges

A widely accepted model proposes that mid-ocean ridge basalts (MORB) are produced by decompression-induced partial melting of oceanic upper mantle which adiabatically upwells in response to plate separation (McKenzie, 1984; Klein and Langmuir, 1987; McKenzie and Bickle, 1988). In this model, rising mantle begins to melt as it intersects the solidus at a depth determined by its temperature and melting characteristics. Continued upwelling involves continuous melting, matrix compaction, and melt segregation. To quantitatively evaluate this type of melting, Niu and Batiza (1991) developed an empirical scheme based on the peridotite melting experiments of Jaques and Green (1980), Falloon and Green (1987, 1988), and Falloon and others (1988). We used the experimental data to calculate the bulk partition coefficients (Kd) for the major elements. In addition, we determined pressure and temperature (extent of partial melting) dependance of these Kds and used these to calculate the compositions of pooled polybaric melts created by incremental melting. Finally, we developed a method for calculating the melting parameters (initial and final melting pressures and temperatures, plus total extent of melting) for actual MORB melt compositions. This method assumes that primary polybaric pooled melts fractionate only at low pressure and employs the chemical back-tracking method of Klein and Langmuir (1987). The purpose of this Short Note is to present the MORBCAL program. This program is an easy-to-use tool for readers interested in mid-ocean ridge processes and igneous petrogenesis. For details of the melting model, readers are referred to Niu and Batiza (1991). In the rest of the note, we briefly discuss MORBCAL and its availability.