Host Alkali Basalts Research Articles

Isotope and trace-element geochemistry of alkali basalts and associated megacrysts from the Huangyishan volcano, Kuandian, Liaoning, NE China

Trace-element abundances and Sr, Nd isotopic compositions have been measured on three megacrysts of clinopyroxene, three of garnet and one of amphibole, as well as their host alkali basalts from the Huangyishan volcano, Kuandian, NE China. The Ce isotopic compositions of three host basalt samples have been also analyzed. All megacrysts studied here are shown to be cognate with the host basalts mainly based on the isotopic compositions and major-element distributions between silicate crystal and melt. The source mantle of Huangyishan basalts is isotopically and chemically heterogeneous, and must have been enriched in Rb, La and Sm relative to Sr, Ce and Nd, respectively, for a relatively long time. Highly incompatible trace-element ratios are also suggestive of enrichment of large-ion lithophile elements in the mantle compared with the mantle source of MORB and most OIB. The fine structure, i.e. the double-concave shape of REE patterns commonly found in alkali basalts, is considered to be an important clue to the origin of alkali basalts. Based on the REE partition between the megacrysts and host basalts, evaluation of several theoretical models which can account for the double-concave shape of REE patterns shows that alkali basalts with such patterns must have been in equilibrium or quasi-equilibrium with both garnet and clinopyroxene solid phases. The small positive Eu anomaly commonly observed in REE patterns in alkali basalts, in most cases, is actually the turning point of the double-curved REE pattern. Partial melting of a garnet peridotite source is the most probable process which can generate the alkali basalts under consideration.

Evidence for pressure dependence of the peak position in the REE mineral/melt partition patterns of clinopyroxene

Major element compositions, REE abundances and strontium and neodymium isotope ratios were measured for three megacrysts of garnet, five of clinopyroxene, one of amphibole and the host alkali basalts from two volcanoes in northeastern China. Strontium and neodymium isotopic data and major elements (Fe and Mg) indicate a cognate origin of megacrysts and host lavas. Logarithmic values of REE partition coefficients for these minerals were found to vary parabolically with ionic radius and hence the REEs in these three minerals are considered to substitute primarily into only one lattice site of each mineral. The ionic radius (optimum radius) defined by the position of peak top of the parabola is variable. Based on the REE partition coefficient data for both experimental and naturally existing clinopyroxene/melt pairs, the change of the optimum radius may be interpreted to be mainly pressure dependent.

Petrology and geochemistry of spinel peridotite xenoliths from the western Pannonian Basin (Hungary): evidence for an association between enrichment and texture in the upper mantle

Twenty spinel peridotite xenoliths from Pliocene alkali basaltic tuffs and lavas of the western Pannonian Basin (Hungary) have been analysed for bulk rock major and trace elements, electron probe mineral compositions, and REE and Sr, Nd isotopes on separated and leached clinopyroxenes. The xenoliths are texturally diverse, including protogranular, porphyroclastic, equigranular and poikilitic textures which can generally be correlated with geochemical features. Protogranular xenoliths are relatively undepleted in Ca, Al, Ti and Na, whereas poikilitic xenoliths are more refractory. LREE-depleted patterns. and MORB-like eNd and eSr values are associated with protogranular peridotites. In contrast, xenoliths with complex textures are generally LREE-enriched. Much of the isotopic variation in the suite (eSr=−20.4 to +10.4, +Nd=+1.8 to +13.7) can be related to interaction between protogranular mantle and melts resembling the host alkali basalts, but a third (high eSr) component may be due to Miocene subduction beneath the region.

Organic compounds on crack surfaces in olivine from San Carlos, Arizona and Hualalai Volcano, Hawaii

Organic compounds associated with thin carbonaceous films on crack surfaces have been detected by thermal-desorption photoionization mass spectrometry in large single crystals of olivine from San Carlos, Arizona and Hualalai Volcano, Hawaii. Alkalis, silicon, aluminum and halogens are also present in the 3–4 nm thick carbonaceous films. The organics probably were not derived from the upper mantle or lower crust or from environmental biogenic contamination after eruption and cooling. It is likely that the carbonaceous films and organics were deposited or formed on crack surfaces created during eruption and cooling of the host alkali basalts. Whether the organics were produced abiotically by FischerTropsch-like reactions involving volcanic gases and fresh-fractured surfaces where reduced carbon was deposited, or whether the organics represent biogenic material that was assimilated into the magmatic system prior to or during magma ascent, cannot be ascertained at this time due to their low abundance.

The Mineral Chemistry of Ultramafic Xenoliths of Eastern China: Implications for Upper Mantle Composition and the Paleogeotherms

Ultramafic xenoliths of garnet lherzolite (±rare spinel), spinel lherzolites, spinel harzburgites, clinopyroxenites, and clinopyroxene megacrysts were collected from Cenozoic basalts in all parts of eastern China. From their modal composition and mineral chemistry all the xenoliths may be placed into three types representing: a fertile or more primitive mantle (garnet lherzolite and spinel lherzolite), a refractory or more depleted mantle (spinel harzburgite and dunite), and inclusions cognate with the host alkali basalts at mantle pressures (pyroxenite and megacrysts). There are systematic differences between the mineral compositions of each type. Spinel shows a wide compositional range and the spinel cr-number [ 100Cr/(Cr + Al)] is a significant indicator of the xenolith type. Spinel cr-number and Al2O3 of coexisting minerals (spinel, clinopyroxene, and orthopyroxene) are useful as refractory indicators for spinel peridotite in that the cr-number increases and the percentage of Al2O3 decreases with increasing degrees of melting. In garnet peridotite, however, the same functions vary with pressure, not degree of melting. According to P–T estimates, the various xenoliths were derived from a large range of depths in the upper mantle: spinel peridotite from approximately 11 to 22 kb (37–66 km), spinel/garnet lherzolite from 19 to 24 kb (62–80 km), and garnet lherzolite from 24 to 25 kb (79–83 km). We conclude that the uppermost mantle beneath eastern China is heterogeneous, with a north-northeast zone of more depleted mantle lying beneath the continental margin and a more primitive mantle occurring towards the continental interior.

The southeast Australian lithospheric mantle: isotopic and geochemical constraints on its growth and evolution

Trace elements and isotopic compositions of whole rocks and mineral separates are reported for 15 spinel-bearing harzburgite and lherzolite xenoliths from southeastern Australia. These samples have an exceedingly large range in isotopic compositions, with 87Sr/ 86Sr ranging from 0.70248 to 0.70834 and ε Nd values ranging from +12.7 to −6.3. This range in isotopic compositions can be found in xenoliths from a single locality. The isotopic compositions of clinopyroxene separates and their whole rocks were found to be different in some xenoliths. Samples containing small glass pockets, which replace pre-existing hydrous minerals, generally show only small differences in isotopic composition between clinopyroxene and whole rock. In a modally metasomatized peridotite, significant differences in the Sr and Nd isotopic compositions of a coexisting phlogopite-clinopyroxene pair are present. Coexisting clinopyroxenes and orthopyroxenes from an anhydrous lherzolite have Sr isotopic compositions that are significantly different (0.70248 versus 0.70314), and yield an apparent age of 625 Ma, similar to that found previously by Dasch and Green [1]. However, the Nd isotopic compositions of the clinopyroxene and orthopyroxene are identical indicating recent (within 40 Ma) re-equilibration of Nd. Sr and Nd concentrations in the whole rocks and clinopyroxenes show an excellent positive correlation, and have an average Sr/Nd ratio of 15. This ratio is similar to the primitive mantle value, as well as that found in primitive MORBs and OIBs, but is much lower than that measured in island arc basalts and what might be predicted for a subduction zone-derived fluid. This indicates that a significant proportion of the Sr and Nd in these peridotites is introduced as a basaltic melt with intraplate chemical characteristics. The isotopic compositions of the peridotites reflect long-term, small-scale heterogeneities in the continental lithospheric mantle, and are in marked contrast to the near uniform isotopic compositions of the host alkali basalts ( 87Sr/ 86Sr= 0.7038–0.7041andε Nd = +3.6 to +2.9). A minimum of three evolutionary stages are identified in the growth of the continental lithospheric mantle: an early basalt depletion event, recording the initial development and stabilization of the lithospheric mantle, followed by at least two enrichment episodes. These observations are consistent with continental lithospheric mantle growth involving the underplating of refractory peridotite diapirs.

Amphibole-rich xenoliths and host alkali basalts: petrogenetic constraints and implications on the recent evolution of the upper mantle beneath Ahaggar (Central Sahara, Southern Algeria)

Quaternary basanitic to nephelinitic volcanoes from Tahalra (western Ahaggar, southern Algeria) contain numerous Mg-ilmenite and amphibole-rich inclusions (±olivine, ±salite) and spinel lherzolite (±pargasite) inclusions associated with kaersutite megacrysts. On the basis of petrological, geochemical and Sr isotopic study of representative xenoliths (including a composite nodule defined as a vein cross-cutting peridotite) and lavas, we attribute the series of amphibole-rich xenoliths and megacrysts to segregation under upper mantle conditions from a hydrous high Ti and LREE melt geochemically similar to the Quaternary basanite but isotopically different. Amphibole-rich rocks and megacrysts are the results of magmatic events (less than 40 Ma) probably contemporaneous with the various pre-Quaternary volcanic phases recognized in Ahaggar. The amphibole-rich veins and the Quaternary lavas have a garnet lherzolitic source enriched in REE (7 to 9 times chondritic in LREE, 2 times in HREE). This enrichment probably results from former metasomatic events unrelated to the recent magmatic history. Melts from which these veins precipitated within upper mantle peridotite also account for mantle enrichment processes; they induced a local partial melting and contact metasomatism (pargasitization). The upper mantle beneath the volcanic areas of Ahaggar is veined and hydrous, and consequently lightened: thus, the uplift of basement may be the isostatic response to magmatism and related metasomatism and therefore the result of the Cenozoic igneous activity.

Continental alkali basalts as mixtures of kimberlite and depleted mantle: Evidence from Kilbourne Hole Maar, New Mexico

A depleted‐mantle nodule from Kilbourne Hole, New Mexico, has been analyzed in detail to test a recent model for the formation of alkali basalts by two‐component mixing of depleted mantle and enriched (kimberlite) mantle. The bulkrock chemistry of the nodule can be reconstructed from its constituent phases, except for calcium, sodium and several incompatible elements. It is found that acid‐leachates derived from the surfaces of the minerals contain the "missing" complement of these elements. Furthermore, major element compositions indicate that each leachate may be a mixture of nodule mineral plus kimberlite. It is suggested that the host alkali basalt lava could have formed by the same mixing process, as the lava and enclosed nodule are in approximate Nd isotopic equilibrium, and calculated mineral/melt partition coefficients for the rare earth elements (REE) between nodule minerals and host lava match those determined experimentally.

Geochemistry of ultramafic xenoliths and their host alkali basalts from Tallante, southern Spain

AbstractUltramafic xenoliths enclosed in Plio-Quaternary alkali basalts from Tallante near Cartagne (southern Spain) are composed mainly of spinel lherzolites which are probably upper mantle residues. In many xenoliths, the spinel lherzolite is cut by pyroxenite or gabbroic anorthosite veinlets generally 0.2–3 cm thick. The clinopyroxenite veinlets were formed by high-pressure crystal-liquid segregation from alkali basalt magmas formed earlier than the host basalts, whereas mantle metasomatism played a role in the genesis of gabbroic anorthosites. Close to the contact with the veinlets, the spinel lherzolites are enriched in Ca, Fe, and some incompatible elements including light REE due to the migration of a fluid from the veinlets into the surrounding lherzolites. The host alkali basalts were derived from a heterogeneous, incompatible element-enriched upper-mantle source probably similar in composition and nature to the composite xenoliths, but were formed in a garnet stability field.

Geochemistry of spinel peridotite inclusions in basalts from Hoggar, Algeria

Ultramafic xenoliths enclosed in Cenozoic alkali basaltic rocks from Hoggar (central Sahara, Algeria) are composed mainly of spinel peridotite. Some spinel lherzolites and harzburgites contain amphibole or its breakdown products or were cut by amphibolite veinlets. The spinel peridotites are upper mantle residue left after the removal of basaltic melt and were later affected by metasomatic processes which led to the formation of amphibole and the enrichment of several lithophile elements particularly in amphibole-bearing varieties. The model calculations using the total inverse method of Tarantola and Valette ( Rev. Phys. Space Phys. 20, 219–232, 1982) indicate that the original upper mantle beneath the Hoggar had a composition similar to the estimated mantle of Jagoutz et al. ( Proc. 10th Lunar Planet. Sci. Conf., pp. 2031–2050, 1979) and Taylor ( Proc. 11th Lunar Sci. Conf., pp. 333–348, 1980) and that the host alkali basalts are not genetically related to the spinel peridotite xenoliths. The basalts were derived from a deeper, garnet-bearing source and scavenged the spinel peridotite xenoliths during their ascent.

Amphibole in a spinel lherzolite xenolith: evidence for volatiles and partial melting in the upper mantle beneath southern British Columbia

Pargasitic amphibole has been observed for the first time in an ultramafic xenolith from British Columbia. The xenolith is a chrome diopside-bearing spinel lherzolite trapped within an alkali basaltic lava flow at Lightning Peak, near Vernon, British Columbia. Amphibole (<5%) occurs within the xenolith as small grains, interstitial between other xenolith mineral phases, and always shows evidence of melting. Microprobe analyses of the amphibole reveal that it is a pargasite rich in MgO (MgO = 17.1–17.7 wt.%; Mg/(Mg + Fe2+) = 0.89) and CaO (10.4–10.7 wt.%). Textural and chemical evidence suggests that the pargasite is in equilibrium with the other phases in spinel lherzolite. The pargasite probably crystallized within the spinel stability field of the upper mantle from a volatile-rich metasomatic fluid that was produced by dehydration of subducted material. Melting in the amphibole may have been caused by one of three processes: superheating by the host alkali basalt, decompression as the magma ascended, or by in situ partial melting within the upper mantle. The partial melting of amphibole-bearing spinel lherzolite provides a possible mechanism for the generation of late Cenozoic alkalic magmas of the Intermontane Belt of British Columbia.

Nd and Sr isotope geochemistry of hydrous mantle nodules and their host alkali basalts: implications for local heterogeneities in metasomatically veined mantle

Nd and Sr isotopic data on pargasite Iherzolite inclusions, kaersutite megacrysts and their host alkali basalts are presented here to clarify some questions regarding isotopic equilibration during mantle metasomatism and the role of metasomatism in basalt genesis. Five alkali basalts from Nunivak Island within the Aleutian back-arc basin, have 87Sr/ 86Sr ratios of 0.70251–0.70330 and 143Nd/ 144Nd ratios of 0.51289–0.51304. On a Nd versus Sr isotope composition diagram the basalts overlap the fields of MORB and ocean island basalts. Pargasites and mica separated from hydrous nodules found in these basalts have a range in 87Sr/ 86Sr of 0.70256–0.70337 but identical 143Nd/ 144Nd ratios of 0.51302. The metasomatic fluid represented by the pargasite is in isotopic equilibrium, both for Sr and Nd, with the dry mantle as represented by diopside. Eight alkali basalts from the Ataq diatreme, South Yemen, have 87Sr/ 86Sr range of 0.70335–0.70426 and 143Nd/ 144Nd range of 0.51252–0.51305. On a Nd versus Sr isotope composition diagram the basalts from Ataq plot in two distinct fields, (1) within the field of ocean island basalts, and (2) within the range of continental rift basalts but to the left of the Nd-Sr correlation line, somewhat similar to the Skye and Oslo rift basalts. Diopside and pargasite separated from three nodules at Ataq have a more complex history than those at Nunivak. Two nodules contain pargasite and diopside with identical 87Sr/ 86Sr ratios but different 143Nd/ 144Nd ratios. A third nodule contains diopside with a 143Nd/ 144Nd ratio similar to that of other diopsides. The Nunivak basalts are derived from a source with a time-integrated light-REE depletion, in contrast to the light-REE-enriched nature of the basanites. This is best explained by a recent metasomatic event in the source region which increased the LIL element content of the peridotite thus accommodating higher degrees of melting. The Ataq volcanic rocks seem to tap different sources characterized by both light-REE enrichment and depletion, in contrast to the uniform source of the Nunivak basanites. Production of the Ataq basanites is believed to involve anataxis of metasomatically veined continental mantle where local mantle heterogeneities survived the melting event.

Nd and Sr isotope geochemistry of hydrous mantle nodules and their host alkali basalts: implications for local heterogeneities in metasomatically veined mantle : Menzies, Martin and V. Rama Murthy, 1980. Earth planet. Sci. Letts, 46(3): 323–334

Nd and Sr isotope geochemistry of hydrous mantle nodules and their host alkali basalts: implications for local heterogeneities in metasomatically veined mantle : Menzies, Martin and V. Rama Murthy, 1980. Earth planet. Sci. Letts, 46(3): 323–334

Textures, microstructures and deformation of ultramafic xenoliths from San Quintin, Baja California

The spinel lherzolite xenoliths from San Quintin, Baja California show evidence of plastic deformation in the upper-mantle conditions prior to their inclusion by the host alkali basalt. This is reflected by stretching and flattening of all the four constituent minerals — olivine, orthopyroxene, clinopyroxene and spinel. Deformed clinopyroxene porphyroclasts are a conspicuous feature of the more lherzolitic xenolith population. Spinel and orthopyroxene define strong lineation. The microstructures of the mylonitic xenoliths such as kink bands, polygonization, subgrain structures, serrated grain boundaries and grain aggregates suggest that such structures are caused by plastic flow, recovery and recrystallization processes. The textures of the xenoliths are divided into four types. The coarse granular texture, considered to be the most primitive type, is relatively undeformed; it grades into the porphyroclastic type with increasing degree of deformation. The porphyroclastic texture further grades into the tabular and equigranular mosaic types which are considered to be the most deformed in the xenolith suite. This progressive variation of the textural types is similar to those found in the high-temperature peridotite massif of Mt. Albert where the relatively undeformed core grades into the more deformed varieties towards the margin. The orthopyroxenes show many features of deformation and recrystallization that are rarely seen, or some of which have not yet been reported from xenolilhs in basaltic or kimberlitic suites. A detailed description of these features is given. Lamellar orthopyroxenes show a variation of 1–12 in their length: width ratios measured on the plane of foliation. The increase in this ratio corresponds with decreasing pressure—temperature conditions of equilibration. The textures and microstructures of the constituent minerals of the xenoliths suggest the following deformational history: plastic deformation → recrystallization → rupture and ductile fracture → brittle fracture (during unloading). Although the four textural types show overlapping relationship in pressure—temperature space, there is a strong correspondence between texture, bulk composition (indicated by spinel chemistry and modal amount of clinopyroxene), degree of deformation and the pressure—temperature condition of equilibration. The more deformed rocks show lower pressure—temperature conditions than the less deformed ones. The most deformed rocks are also most refractory in terms of the (Cr/Cr + Al) ratios of the spinels and the modal amounts of clinopyroxenes. It is suggested that the growth of thermal diapirs by plastic flow in the lithosphere possibly provides the mechanism for the origin of the tectonite textures and fabrics of the xenoliths.

Ancient lithospheric lherzolite xenolith in alkali basalt from Baja California

K, Rb, Sr contents and Sr isotopic composition are reported for (1) the coexisting silicate minerals of a spinel lherzolite xenolith, (2) the whole rock xenolith, and (3) the host alkali basalt from the Pleistocene-Recent San Quintin volcanic field in Baja California. The data also include major element chemistry of the four mineral phases of the xenolith. The olivine-spinel and pyroxene thermometers indicate that the last temperature of equilibration of the xenolith in the upper mantle was about 1100°C. K, Rb, and Sr abundances are extremely low in the minerals of the xenolith, in contrast with the general enrichment of Ca, Al, Fe and Na in the whole xenolith. Furthermore, the abundance levels and the 87Sr/ 86Sr ratios of the minerals are greatly lowered by surface washing of the minerals in 2N cold HCl for three minutes. It is suggested that this is due to grain boundary and surface contamination of the minerals which took place in the upper mantle by a vapor phase deposition, prior to the inclusion of the xenolith in the basalt. The source of the vapor phase must have a 87Sr/ 86Sr ratio greater than 0.7070, the highest measured ratio in the unwashed orthopyroxene. Sr in the host alkali basalt has a 87Sr/ 86Sr ratio of 0.7031, unrelated to the grain boundary material. The acid-washed minerals, the unwashed minerals and the acid-washed whole rock xenolith show a scatter on a 87Rb/ 86Sr versus 87Sr/ 86Sr diagram. However, the surface-washed minerals and the whole rock alone define a straight-line relationship with a positive slope, which corresponds to an age 3.4 ± 0.3AE(2σ) for the rock and an initial 87Sr/ 86Sr of 0.70057 ± 0.0004 (2σ). The age of 3.4 AE for the lherzolite is interpreted as its last involvement in a small degree of partial melting and the consequent extreme depletion of the large ion lithophile elements from the constituent minerals.

Lead concentration and isotopic composition in five peridotite inclusions of probable mantle origin

The lead content of five whole-rock peridotite inclusions (four lherzolites and one harzburgite) in alkali basalt ranges from 82 to 570 ppb (parts per billion). Approximately 30–60 ppb of this amount can be accounted for by analyzed major silicate minerals (olivine ≤ 10 ppb; enstatite 5–28 ppb; chrome diopside ∼400 ppb). Through a series of acid leaching experiments, the remainder of the lead is shown to be quite labile and to reside in either glassy or microcrystalline veinlets or accessory mineral phases, such as apatite and mica. The lead isotopic composition of the peridotites ( 206Pb/ 204Pb= 18.01–18.90; 207Pb/ 204Pb= 15.52–15.61; 208Pb/ 204Pb= 37.80–38.86) lies within the range of values defined by many modern volcanic rocks and, in particular, is essentially coextensive with the abyssal tholeiite field. In all but one instance, isotopic differences were found between the peridotite and its host alkali basalt. Two of the peridotites clearly demonstrated internal isotopic heterogeneity between leachable and residual fractions that could not simply be due to contamination by the host basalt. However, there is no evidence that these ultramafic rocks form some layer in the mantle with isotopic characteristics fundamentally different from those of the magma sources of volcanic rocks.

The spacing of windrows of Sargassum in the ocean : Faller A.J. and A.H. Woodcook, 1964. J. Mar. Res., 22 (1): 22–29

A small isolated Neogene, possibly Quaternary, monogenetic alkali olivine basalt cone in northeastern Iran contains both mantle peridotite and crustal gabbroic xenoliths, as well as plagioclase megacrysts. The basaltic magma rose to the surface along pathways associated with local extension at the junction between the N–S right-lateral and E–W left-lateral strike slip faults that form the northeastern boundary of the Lut microcontinental block. This basalt is enriched in LREE relative to HREE, and has trace-element ratios similar to that of oceanic island basalts (OIB). Its 87Sr/86Sr (0.705013 to 0.705252), 143Nd/144Nd (0.512735 to 0.512738), and Pb isotopic compositions all fall in the field of OIB derived from enriched (EM-2) mantle. It formed by mixing of small melt fractions from both garnet-bearing asthenospheric and spinel-facies lithospheric mantle. Plagioclase (An26-32) megacrysts, up to 4 cm in length, have euhedral crystal faces and show no evidence of reaction with the host basalt. Their trace-element concentrations suggest that these megacrysts are co-genetic with the basalt host, although their 87Sr/86Sr (0.704796) and 143Nd/144Nd (0.512687) ratios are different than this basalt. Round to angular, medium-grained granoblastic meta-igneous gabbroic xenoliths, ranging from ~ 1 to 6 cm in dimension, are derived from the lower continental crust. Spinel–peridotite xenoliths equilibrated in the subcontinental lithosphere at depths of 30 to 60 km and temperatures of 965 °C to 1065 °C. These xenoliths do not preserve evidence of extensive metasomatic enrichment as has been inferred for the mantle below the Damavand volcano further to the west in north-central Iran, and clinopyroxenes separated from two different mantle xenoliths have 87Sr/86Sr (0.704309 and 0.704593) and 143Nd/144Nd (0.512798) ratios which are less radiogenic than either their host alkali basalt or Damavand basalts, implying significant regional variations in the composition and extent of metasomatism in the sub-Iranian mantle.