Showing papers on "Incompatible element published in 1995"

Calc‐alkaline magmatism, lithospheric thinning and extension in the Basin and Range

Abstract: Most of the volcanic rocks in the Colorado River Trough (CRT) and the Mogollon-Datil Volcanic Field (MDVF) in the Basin and Range exhibit calc-alkaline major element trends and relatively low high field strength element abundances, similar to those erupted from the volcanoes of Aso and Towada in Japan. Such features are widely regarded as characteristic of subduction-related magmatism, and yet the rocks in the Basin and Range were generated in response to lithospheric extension. The preextensional to synextensional rocks of the CRT and the MDVF have higher Na2O, K2O, and TiO2, in the range 47–55% SiO2, and relatively low Al2O3, and overall, they tend to have higher Sr contents and Zr/Y and La/Nb ratios than those from Aso and Towada. In addition, the basalts in the Basin and Range tend to be more aphyric than those in Japan, consistent with more rapid movement of magma through the crust during extension in the Basin and Range, and the rate of melt generation appears to have been significantly less in the Basin and Range than along recent destructive plate margins. The geochemical differences are attributed to smaller degrees of partial melting in the Basin and Range and to source regions that had been enriched in incompatible elements since the Proterozoic, resulting in parental magmas with higher alkali contents than those commonly observed in subduction-related calc-alkaline suites. Within the CRT the subsequent calc-alkaline trend was due at least in part to mixing with crustal derived melts, whereas in the MDVF such trends reflect both crustal contamination and fractional crystallization involving magnetite and amphibole. The small volumes of magma with minor and trace element features similar to oceanic basalts indicate that relatively little melt was generated in underlying asthenosphere. Thus it is inferred that magmatism in the Basin and Range was not associated with a significant increase in temperature, such as might be attributed to a mantle plume, but rather it was in response to lithospheric extension. Calculations are presented which demonstrate that the magma volumes and inferred source regions, extension, present-day heat flow, and topography are consistent with a model of convective lithospheric thinning after thickening in the Laramide and Sevier orogenies.

Petrology, Mineral and Isotope Geochemistry of the External Liguride Peridotites (Northern Apennines, Italy)

Abstract: Mantle peridotites of the External Liguride (EL) units (Northern Apennines) represent slices of subcontinental lithospheric mantle emplaced at the surface during early stages of rifting of the Jurassic Ligurian Piemontese basin. Petrological, ion probe and isotopic investigations have been used to unravel the nature of their mantle protolith and to constrain the timing and mechanisms of their evolution. EL peridotites are dominantly fertile spinel Iherzolites partly recrystallized in the plagioclase Iherzolite stability field. Clinopyroxenes stable in the spinelfacies assemblage have nearly flat REE patterns (Ce-s/ SmN = 0-6-0-8) at (10-16) xCl and high Na, Sr, Ti and %r contents. Kaersutitic—Ti-pargasitic amphiboles also occur in the spinelfacies assemblage. Their LREE-depleted REE spectra and very low Sr, %r and Ba contents indicate that they crystallized from hydrous fluids with low concentrations of incompatible elements. Thermometric estimates on the spinelfacies parageneses yield lithospheric equilibrium temperatures in the range 1000-1100°C, in agreement with the stability of amphibole, which implies T<1100°C. Sr and Nd isotopic compositions, determined on carefully handpicked clinopyroxene separates, plot within the depleted end of the MORB field CSr/Sr = 0 • 70222-0 • 70263; Nd/Nd = 0 -5130470 -513205) similar to many subcontinental orogenic spinel Iherzolites from the western Mediterranean area (e.g. Ivrea Z and Lanzo N). The interpretation of the EL Iherzolites as subcontinental lithospheric mantle is reinforced by the occurrence of one extremeh depleted isotopic composition (Sr/Sr-0-701736; Nd/Nd = 0-513543). Sr and Nd model ages, calculated assuming both CHUR and DM mantle sources, range between 2-4 Ga and 780 Ma. In particular, the 1-2-Ga Sr age and the 780-Ma Nd age can be regarded as minimum ages of differentiation. The transition from spinelto plagioclase-fades assemblage, accompanied by progressive deformation (from granular to tectonite—mylonite textures), indicate that the EL Iherzolites experienced a later, subsolidus decompressional evolution, starting from subcontinental lithospheric levels. Sm/Nd isochrons on plagioclase—clinopyroxene pairs furnish ages of ~165 Ma. This early Jurassic subsolidus decompressional history is consistent with uplift by means of denudation in response to passive and asymmetric lithospheric extension. This is considered to be the most suitable geodynamic mechanism to account for the exposure of huge bodies of subcontinental lithospheric mantle during early stages of opening of an oceanic basin.

Nb-Ta-rich mantle amphiboles and micas: Implications for subduction-related metasomatic trace element fractionations

Abstract: We report the trace element compositions of amphibole and mica separated from mantle xenoliths in alkali basalts and analyzed by ICP-MS. Nb and Ta are highly (50-200-fold) enriched in vein amphibole and mica relative to primitive mantle compositions, whereas Th and U are depleted. Some disseminated amphiboles do not have such extreme Nb-Ta enrichments, but Nb-Ta partition coefficients between amphibole and clinopyroxene are remarkably high, ranging from 10 to 85. In apparent contrast with the results on natural mantle amphiboles, recently reported Nb and Ta partition coefficients between amphibole and melts are very low [1,2]. The reason for the apparent contradiction may lie in either the composition of the amphibole or the fluid phase (silica-rich aqueous fluid rather than silicate or carbonate melt). In either case, our observations show that amphibole and mica can be important hosts for Nb and Ta and cannot be ignored in identifying the underlying cause of the nearly universal relative Nb-Ta depletion of subduction-related volcanic rocks. We propose a metasomatic model for creating source regions that are depleted in Nb and Ta relative to Th, U and the LREE. Fluids generated by dehydration of the subducted slab ascend through the overlying mantle wedge and precipitate amphiboles. Highly incompatible elements including Nb and Ta are transferred with the fluid into the wedge where the ‘open-system’ precipitation of amphibole fractionates the trace elements and thus generates low (Nb,Ta)/(Th,U,LREE) ratios in the residual fluid. As this fluid travels further it either directly induces partial melting in hot regions of the wedge or is consumed through ‘closed-system’ crystallization of disseminated amphibole in host peridotite that can later undergo partial melting. In either case the resulting source regions of subduction-related magmas are enriched in highly incompatible trace elements but not in Nb and Ta. This model may be considered either as a complement or as an alternative to published models explaining the chemistry of arc magmas.

Melt inclusions in high-An plagioclase from the Gorda Ridge: an example of the local diversity of MORB parent magmas

Abstract: The use of ocean floor basalt chemistry as a tool to investigate mantle composition and processes requires that we work with basalts that have been modified little since leaving the mantle. One source of such basalts is melt inclusions trapped in primitive crystals. However, obtaining information from these melt inclusions is complicated by the fact that melt inclusions in natural basalts are essentially always altered by post-entrapment crystallization. This requires that we develop techniques for reconstructing the original trapped liquid compositions. We conducted a series of experiments to reverse the effects of post-entrapment crystallization by re-heating the host crystals to their crystallization temperature. For these experiments we used plagioclase crystals separated from a single Gorda Ridge lava. The crystallization temperature for these crystals was determined by a set of incremental re-heating experiments to be ∼1240–1260° C. The inclusions are primitive, high Ca-Al basaltic melts, saturated with plagioclase, olivine and Al-rich chromite at low pressure. The inclusion analyses can be linked to the host lava composition by low pressure fractionation. The major element composition of the re-homogenized melt inclusions within each crystal is relatively constant. However, the incompatible element analyses have extremely wide ranges. The range of La/Sm and Ti/Zr from inclusions analyzed from a single sample from the Gorda Ridge exceeds the range reported for lavas sampled from the entire ridge. The pyroxene compositions predicted to be in equilibrium with the melt inclusion trace element signature cover much of the range represented by pyroxenes from abyssal peridotites. The volumetric proportions of the magmas entering the base of the crust can be evaluated using frequency distribution of melt inclusion compositions. This distribution suggests that the array of magmas was skewed towards the more depleted compositions, with little evidence for an enriched component in this system. This pattern is more consistent with a dynamic flow model of the mantle or of a passive flow model where the melts produced in the peripheral areas of the melting regime were not focused to the ridge.

Mechanisms of differentiation in the Skaergaard Intrusion

Abstract: Thanks to its magnificent exposures and extraordinary petrologic development, the Skaergaard Intrusion provides an excellent test for three basic mechanisms of crystal-liquid fractionation: diffusive exchange, compaction, convective exchange. (1) Diffusive exchange at the solidification front does not seem to have played a major role, for no obvious difference is seen in the behavior of components of widely differing diffusivities. The effects of diffusion were probably masked by porous flow of the interstitial liquid. (2) Compaction and 9filter pressing9 of residual liquids can be tested by comparing concentrations of incompatible elements in parts of the intrusion that crystallized in different orientations with respect to gravity. The contrast between the large concentrations of these elements in the Upper Border Series and consistently smaller abundances in the Layered Series, long thought to ‘be due to contamination of the upper levels with crustal rocks, is more likely a result of compaction that took place at the floor but not under the roof. (3) Convective exchange driven by differences in the compositional densities of the main magma and interstitial liquids seems to have been important at the steep walls and, to a lesser degree, under the roof. It may also have been responsible for a large compositional anomaly near the centre of the Layered Series where an iron-rich liquid percolated down through parts of Lower and Middle Zones. At later stages of crystallization when density relations were reversed, a buoyant liquid, rich in volatiles and incompatible elements, rose from the Layered Series and permeated the upper part of the intrusion. A liquid of this latter kind could have caused the pervasive metasomatic alteration seen in much of the Layered Series. In both compaction and convective exchange, re-equilibration and reactions between migrating liquids and early-formed crystals augmented permeabilities and brought about extensive compositional and textural changes. Although inhomogeneities in the initial magma could account for some of the compositional variations, the main trend of differentiation can be ascribed to a combination of compaction and convective fractionation. The convective flow was of two kinds, one of descending iron-rich liquids and another of more evolved rising liquids (and fluids) with low densities and large concentrations of incompatible elements. In the early stages, the proportions of iron-rich liquids were much greater than those of the silica-rich variety, and until the crystallization fronts converged at the Sandwich Horizon, the former made the greatest contribution to evolution of the main body of magma. As crystallization advanced, however, light, silica-rich liquids became more important; they permeated the Upper Border Series and much of the upper part of the Layered Series. This late-stage liquid continued to differentiate after the crystallization fronts met; a secondary Sandwich Horizon was formed where incompatible elements reach maximum concentrations in the lower part of the Upper Border Series.

Constraints on the Mantle Sources of the Deccan Traps from the Petrology and Geochemistry of the Basalts of Gujarat State (Western India)

Abstract: The late Cretaceous-early Tertiary flood basalts in the Gujarat area of the northwestern Deccan Traps (Kathiawar peninsula, Pavagadh hills and Rajpipla) exhibit a wide range of compositions, from picrite basalts to rhyolites; moreover, the basaltic rocks have clearly distinct TiO2 contents at any given degree of differentiation and strongly resemble the low-titanium and hightitanium basalts found in most of the Gondwana continental flood basalt (CFB) suites. Four magma groups are petrologically and geochemically distinguished: (1) A low-Ti group, characterized by rocks with varying SiO2 saturation, and with TiO2 1.8 wt% in picrites), Nb (>19 p.p.m.) Zr/γ (av. 6.5) and Tt/V (av. 47). (3) An intermediate-Ti group, with TiO2 contents slightly lower than the high-Ti rocks at the same degree of evolution, and with correspondingly lower incompatible trace element contents and ratios, in particular K2O, Nb, Ba and Zr/Y (av. 5.2). (4) A potassium-rich group (KT), broadly similar in geochemical character to the high-Ti group but showing more extreme K, Rb and Ba enrichment (av. K20/Na20~l; Ba/Y~20). The most primitive low-Ti and high-Ti picrites, when corrected for low-pressure olivine fractionation, show distinct major (and trace) element geochemistry, in particular for CaO/AI2O3, CaO/TiO2 and Al2O3/TiO2, and moderate but significant variations in their SiO2 and Fe2Ost contents; these characteristics strongly suggest the involvement of different mantle sources, more depleted for the low-Ti picrites, and richer in cpxfor the high-Ti picrites, but with broadly the same pressures of equilibration (27-14 kbar). This, in turn, suggests a strong lateral heterogeneity in the Gujarat Trap mantle. Low-Ti picrites and related differentiates in Kathiawar are reported systematically for the first time here, and suggest the existence of HFSE-depleted mantle in the northwestern Deccan Traps, with extension at least to the Seychelles Islands and to the area of the Bushe Formation near Bombay in the pre-drift position, before the development of the Carlsberg Ridge. The absence of correlations between LILE/HFSE ratios and SiO2 argues against crustal contamination processes acting on the low-Ti picrites, possibly owing to their probably rapid uprise to the surface. Consequently, the mantle region of this rock group was probably re-enriched by small amounts of ULE-rich materials. The substantially higher, trace element enrichment of the least differentiated high-Ti picrites, relative to the basalts of the Ambe-noli and Mahableshwar Formations of the Western Ghats, testifies also to the presence of more incompatible element rich, OIB4ike mantle sources in northern and northwestern Gujarat. These sources were geochemicaily similar to the present-day Reunion mantle sources.

Melting of metasomatized subcontinental lithosphere: undersaturated mafic lavas from Rungwe, Tanzania

Abstract: This paper uses the geochemistry of primitive mafic lavas from the Rungwe volcanic province (southwestern Tanzania) to infer the source mineralogy and melting history. Post-Miocene mafic lavas from Rungwe include alkali basalts, basanites, nephelinites and picrites with up to 18.9 wt% MgO; nephelinites (>13.5% normative nepheline) are restricted to Kiejo volcano in the southern portion of the province. Rungwe lavas differ from most Western Rift volcanics in that they are not unusually potassic (K2O/Na2O ca. 0.40). Sparsely phyric mafic lavas contain phenocrysts and xenocrysts of plagioclase (An82–90), clinopyroxene (4.5–9.5 wt% Al2O3), and olivine (Fo79–88); one basanite contains a 1 mm xenocryst of apatite included in magnesian clinopyroxene. All samples have high abundances of incompatible elements (e.g., 0.7–2.2 wt% P2O5) and are enriched in REE relative to HFSE (Hf, Zr, Ti, Y), Cs, Ba, and K. Some incompatible element ratios are constant throughout the Rungwe suite (e.g., Zr/Nb, Sr/Ce, K/Rb), but other ratios are extremely variable and exceed the range measured in global Ocean Island Basalts (OIB) (e.g., Ba/Nb, Sm/Zr, La/Nb, Pb/Ce, Nb/U). The range in degree of silica saturation, and its excellent correlation with P2O5/Al2O3, indicate that the Rungwe suite records variable degrees of melting. Variations of individual incompatible trace element abundances in nephelinite and basanite samples suggest that the source contains metasomatic amphibole, ilmenite, apatite, and zircon. The Rungwe suite is interpreted as a series of low-percentage melts of CO2-rich peridotite at pressures that span the garnet-spinel transition. A geochemical comparison of Rungwe samples to lavas from other Western Rift volcanic centers requires that the source mineralogy varies along the rift axis, although each province is underlain by metasomatized peridotite. The incompatible trace element signatures of Western Rift lavas indicate that the source area is typically homogeneous on the scale of individual volcanoes, although lavas from each volcano reflect a range in degree of melting. Significantly, volcanoes with distinct geochemistry are always separated by major rift faults, suggesting that volcanic and tectonic surface features may correspond to metasomatic provinces within the subcontinental lithospheric mantle.

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

Abstract: 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 # > 64)(Mg # = 100 × Mg/(Mg + (0.85 × FeTotal))), which carried occasional small xenoliths. As the SQVF evolved with time, differentiated magmas (Mg # < 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, eNd, 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.

Late Cenozoic basaltic magmatism in the Western Great Basin, California and Nevada

Abstract: Alkali-rich basaltic rocks have been erupted in small volumes throughout the Western Great Basin (WGB) since 17 Ma. SiO2 ranges from 46 to 53% in samples with >4% MgO, whereas Fe2O3 ranges from 8 to 10% and TiO2 from 1 to 2% in the most magnesian rocks. They have high incompatible element contents, except for relatively low high field strength elements abundances, and thus their minor and trace element patterns closely resemble subduction-related rather than intraplate magmas. Their radiogenic isotope ratios are also enriched with 87Sr/86Sr = 0.7040–0.7078, 143Nd/144Nd = 0.5129–0.5120 and variable Pb isotopes (206Pb/204Pb = 18.0–19.2), generally displaced above the Northern Hemisphere Reference Line in both the 207Pb/204Pb and 208Pb/204Pb diagrams. The WGB straddles the isotopic and tectonic boundary between continental North America, underlain by Proterozoic basement, and the younger accreted terrain that forms the basement to most of California. There are no discernible differences in the major and trace element contents of the basalts either side of this boundary except that those erupted through Proterozoic basement tend to lower silica contents and have higher 87Sr/86Sr and lower 143Nd/144Nd ratios. Nd model ages and a secondary Pb isochron from basalts east of the boundary suggest an age of 1.6–1.8 Ga, significantly younger than the age of crust generation inferred from Nd model ages of granites. Correlated trace element and isotope ratios in basaltic rocks to the west of the boundary indicate a contribution from recent subduction, recognized by a decrease in 87Sr/86Sr as Nb/Sr and Zr/Sr decrease. A second, less well defined trend to higher Nb/Sr and Zr/Sr and lower 87Sr/86Sr is interpreted to reflect a minor asthenospheric contribution to contemporary magmatism. Relative to basalts from the Basin and Range, the basaltic rocks from the WGB have higher SiO2 and lower TiO2 and Fe2O3 at similar degrees of fractionation. These differences are interpreted to reflect different pressures of segregation from their mantle source regions, the WGB basalts being derived from shallower depths than the Basin and Range basalts, within the lithospheric mantle. This conclusion is consistent with their lithospheric trace element and isotope characteristics and trace element models that allow minimal garnet in the source region. By contrast the lower silica contents of the Basin and Range alkali basalts are more consistent with derivation from depths of 90 km or more, which may place their source regions in the asthenosphere, also consistent with their trace element and isotope variations.

Late Cenozoic magmatism of the Bolivian Altiplano

Abstract: Small basalt to dacite volcanic centers are distributed sparsely over the Bolivian Altiplano, behind the Andean volcanic front. Most are Pliocene to Recent in age, and are characterized by textural and mineralogical disequilibrium with abundant xenoliths and xenocrysts. True phenocrysts are rare in the more mafic samples. Compared with Recent volcanic rocks from Andean stratovolcanoes, the Bolivian centers overlap in major element trends. Incompatible element contents tend to be higher, particularly in the eastern Altiplano. The ranges of isotopic compositions reflect ubiquitous crustal contamination. Pb isotope compositions are dominated by Pb from isotopically heterogeneous basement, resulting in a wide scatter of data lying between inferred crustal compositions and showing little overlap with possible mantle sources in the region. Rocks sampled from the Bolivian Altiplano were probably derived from asthenospheric mantle and subjected to extensive open system differentiation during ascent through the 70 km thick crust of the region. Major element trends are largely controlled by the fractionating phase assemblage (olivine, clinopyroxene and amphibole). Trace element and isotope systematics, however, defy realistic attempts at modeling due to the geographic scatter of samples, the uniformity of compositions at a given center, and the heterogeneity of the contaminant. Nevertheless, there are first order systematic trace element variations that appear to relate to the geometry of the subduction zone. In particular, LIFE/HFSE (exemplified by Ba/Nb), and Zr/Nb ratios decrease from the arc front eastward into the Altiplano. These variations are not easily reconciled with control by crustal contamination alone. A model is proposed in which the asthenospheric source is fluxed by high Ba/Nb slab-derived fluid to induce melting. Beneath the arc, high fluid flux increases the Ba/Nb ratio of the asthenosphre and leads to high degrees of partial melting (high Zr/Nb). To the east, lower or no fluid flux leads to low Ba/Nb and low degrees of partial melting (low Zr/Nb). Melting in the back arc region of the Altiplano may be facilitated by lithospheric delamination that leads to decompression melting of counter-flowing asthenosphere. There is no unequivocal evidence that requires a significant role for the lithospheric mantle.

Constraints on the origin of alkaline and calc-alkaline magmas from the Tuxtla Volcanic Field, Veracruz, Mexico

Abstract: Lavas erupted in the Tuxtla Volcanic Field (TVF) over the last 7 Ma include primitive basanites and alkali basalts, mildly alkaline Hy-normative mugearites and benmoreites, and calc-alkaline basalts and basaltic andesites. The primitive lavas are silica-undersaturated, with high concentrations of both incompatible and compatible trace elements, variable La/Yb with constant Yb at 6 to 8 times chondritic, and low Sr and O and variable Pb and Nd isotopic ratios. The primitive magmas originated by increasing degrees of melting with pressure decreasing from greater than 30 kbar to 20 kbar, in the garnet stability field. Another group of alkali basalts and hawaiites has lower Ni and Cr concentrations and higher Fe/Mg ratios, and was derived from the primitive group by crystal fractionation at pressures of several kbar. Incompatible trace elements in these silica undersaturated lavas show depletion in high field strength elements (HFSE) relative to large ion lithophile elements, similar to subduction-related basalts. Ba/Nb ratios are nearly constant and thus the HFSE depletion cannot be the result of a residual HFSE-bearing phase in the source, but could be the result of generation from a source contaminated by fluids or melts from the subducted lithosphere. The silica-saturated mugearites and benmoreites, and the calc-alkaline basalts and basaltic andesites, were erupted only between 3.3 and 1.0 Ma. These have incompatible element concentrations generally lower than in the silica-undersaturated lavas, and thus could not have been derived by crystal fractionation from the silica-undersaturated alkaline magmas. Magmas parental to the silica-saturated magmas originated by higher degrees of melting at lower pressures than the primitive magmas. Melting may have been promoted by an influx of fluid from the subducted lithosphere. Trace element and Sr, Nd, Pb and O isotopic data suggest that three components are involved in the generation of TVF magmas: the mantle, a fluid from the subducted lithosphere, and continental crust. TVF alkaline lavas are similar to those erupted in the back-arc region of the MVB and Japan, and show characteristics similar to alkaline magmas erupted in the southern Andean volcanic arc. These low degree melts reach the surface along with calc-alkaline lavas in the TVF due to an extensional stress field that allows their passage to the surface.

Feldspar-bearing lherzolite xenoliths in alkali basalts from Hamar-Daban, southern Baikal region, Russia

Abstract: Lherzolite xenoliths in Miocene to Pleistocene basalts from five sites in the Hamar-Daban range in southern Siberia provide sampling of the mantle close to the axis of the Baikal rift. These anhydrous spinel lherzolites commonly have foliated fabrics and spongy rims around clinopyroxene, and many contain accessory feldspar. The feldspar occurs in reaction zones adjacent to spinel and orthopyroxene (where it appears to have been formed by the reaction: spl+opx+cpx+fluid →fs+ol) and less commonly as thin, irregular veins. The feldspars have variable compositions but are generally alkali-rich; their K2O content ranges from 0.3 to 11.2% and is much higher than in plagioclase from orogenic lherzolites (usually <0.1% K2O). The temperature range for the Hamar-Daban xenolith suite (950–1010° C) is more restricted than for spinel peridotite xenoliths from other occurrences in the Baikal area. The feldspar-bearing lherzolites yield equilibration temperatures similar to or slightly lower than feldspar-free ones. The majority of the Hamar-Daban lherzolites are fertile and clinopyroxene-rich, as for most other occurrences in the Baikal region. Trace element compositions of selected xenoliths and their clinopyroxenes were determined by ICP-MS, INAA and proton microprobe. Feldspar-bearing xenoliths are enriched in alkalies indicating that feldspar formation is associated with addition of material and is not simply due to isochemical phase changes. Most xenoliths and their clinopyroxenes studied are depleted in light REE and have contents of Sr, Zr and Y common for fertile or moderately depleted mantle peridotites. Few are moderately enriched in LREE, Sr, Th and U. Sr-Nd isotope compositions of clinopyroxenes indicate long-term depletion in incompatible elements similar to unmetasomatised xenoliths from other occurrences south and east of Lake Baikal. The formation of feldspar and of spongy aggregates after clinopyroxene, and the enrichment in alkalies appear to be recent phenomena related to infiltration of an alkali-rich, H2O-poor fluid into spinel peridotites.

Magmatism in the Garrett transform fault (East Pacific Rise near 13°27′S)

Abstract: The Garrett transform is characterized by recent (zero age) volcanic activity located within the active tectonic domain of the transform valley at depths greater than the 3500 m. This intratransform volcanic activity contributed to the formation of constructional edifices forming ridges (>300 m in height) and small mounds (<20 m in height) built near slivers of serpentinized peridotites. The erupted lavas are depleted mid ocean ridge basalts (MORBs) with low ratios of K/Ti (0.02–0.11), Zr (30–100 ppm), Y (18–50 ppm), and (La/Sm)N (0.25–0.60). Their more depleted nature and smaller range of variability for the compatible elements (Ni = 70–180 ppm, Mg# = 0.58–0.71, where Mg# is the magnesium number (= Mg+2/Mg+2 + Fe+2)) are the main points of difference between the Garrett intratransform volcanics (GITV) and those from the ultrafast south East Pacific Rise (SEPR). However, ferrobasalts (Mg# = 0.41–0.55) were collected from the intratransform walls as well as at the EPR-transform intersection. The GITV are even more depleted in incompatible elements than lava from the north East Pacific Rise (21°N-11°26′N). The Garrett recent lava is believed to have erupted after the successive, incremental partial melting and discontinuous melt extraction of a composite lherzolitic mantle similar to that of the SEPR. The limited range of incompatible element ratios (Zr/Y = 1–2.5, (Ce/Yb)N = 0.4–1) and K/Ti ratios (<0.14) and the samples more porphyritic nature with respect to other SEPR rocks suggest that the intratransform volcanics from the Garrett are extracted from their source and channeled directly toward the surface without extensive mixing in magma chambers. In order to explain the restricted range of compositional variabilities and the absence of the enriched basalts produced by prior melting, we postulate that even though they were produced, these most enriched end-member lavas did not reach the surface; instead, we propose these melts contribute to the formation of impregnated mantle material in the lithosphere. We suggest that this same petrogenetic style of intratransform volcanism might also characterize other oceanic provinces associated with low magmatic to quasi-amagmatic regimes.