Alkali Basaltic Melts Research Articles

Mineral inclusions in placer zircon from the Ohře (Eger) Graben: new data on “strontiopyrochlore”

After qualitative examination of the inclusion inventory of 80 zircon megacrysts from alluvial placers of the Cenozoic alkali basalt area of northern Bohemia and south-eastern Saxony, three representative megacrysts from the Podsedice deposit were selected for an in-depth study of inclusion minerals. Primary and secondary inclusions were distinguished and used as monitor for zircon-related syn- and epigenetic processes. The trace-element characteristics of the zircon hosts imply an alkali silicatic parental rock, probably nepheline syenites or fenites, which occur as enclaves in placer-near breccia fillings. The genetically most interesting inclusion species is “strontiopyrochlore”, for which its presence within areas of oscillatory zonation suggests a magmatic origin. This mode of origin contrasts to previous observations indicating formation of “strontiopyrochlore” as secondary mineral in altered carbonatite. The Sr concentration of “strontiopyrochlore” from Podsedice varies between 7.4 and 12 wt.% SrO, corresponding to a proportion of Sr of 29–48 at.% of the total A-site cations. The infiltration of late- or post-magmatic Fe-rich hydrothermal fluids along fractures crosscutting the zircon host caused a partial substitution of Nb5+ by Fe3+ in the B-site of the pyrochlore structure and the breakdown of primary fergusonite-like inclusions into undefined Nb–Y–REE–Fe oxide phases usually associated with goethite-like minerals. Thorite and related Y–REE–Th silicate phases are the most frequent secondary inclusions originated by dissolution−re-precipitation. Baddeleyite probably formed in response to desilicification that the zircon host has experienced during the contact with the SiO2-undersaturated alkali basalt melt.

Ore geochemistry, zircon mineralogy, and genesis of the Sakharjok Y-Zr deposit, Kola Peninsula, Russia

The Sakharjok Y-Zr deposit in Kola Peninsula is related to the fissure alkaline intrusion of the same name. The intrusion ∼7 km in extent and 4–5 km2 in area of its exposed part is composed of Neoarchean (2.68–2.61 Ma) alkali and nepheline syenites, which cut through the Archean alkali granite and gneissic granodiorite. Mineralization is localized in the nepheline syenite body as linear zones 200–1350 m in extent and 3–30 m in thickness, which strike conformably to primary magmatic banding and trachytoid texture of nepheline syenite. The ore is similar to the host rocks in petrography and chemistry and only differs from them in enrichment in zircon, britholite-(Y), and pyrochlore. Judging from geochemical attributes (high HSFE and some incompatible element contents (1000–5000 ppm Zr, 200–600 ppm Nb, 100–500 ppm Y, 0.1–0.3 wt % REE, 400–900 ppm Rb), REE pattern, Th/U, Y/Nb, and Yb/Ta ratios), nepheline syenite was derived from an enriched mantle source similar to that of contemporary OIB and was formed as an evolved product of long-term fractional crystallization of primary alkali basaltic melt. The ore concentrations are caused by unique composition of nepheline syenite magma (high Zr, Y, REE, Nb contents), which underwent subsequent intrachamber fractionation. Mineralogical features of zircon-the main ore mineral—demonstrate its long multistage crystallization. The inner zones of prismatic crystals with high ZrO2/HfO2 ratio (90, on average) grew during early magmatic stage at a temperature of 900–850°C. The inner zones of dipyramidal crystals with average ZrO2/HfO2 = 63 formed during late magmatic stage at a temperature of ∼500°C. The zircon pertaining to the postmagmatic hydrothermal stage is distinguished by the lowest ZrO2/HfO2 ratio (29, on average), porous fabric, abundant inclusions, and crystallization temperature below 500°C. The progressive decrease in ZrO2/HfO2 ratio was caused by evolution of melt and postmagmatic solution. The metamorphic zircon rims relics of earlier crystals and occurs as individual rhythmically zoned grains with an averaged ZrO2/HfO2 ratio (45, on average) similar to that of the bulk ore composition. The metamorphic zircon is depleted in uranium in comparison with magmatic zircon, owing to selective removal of U by aqueous metamorphic solutions. Zircon from the Sakharjok deposit is characterized by low concentrations of detrimental impurities, in particular, contains only 10–90 ppm U and 10–80 ppm Th, and thus can be used in various fields of application.

REE systematics of fluorides, calcite and siderite in peralkaline plutonic rocks from the Gardar Province, South Greenland

The Gardar failed-rift Province is world-famous for its (per-)alkaline plutonic rocks. Elevated contents of F in the mantle source and F-enrichment in the parental melts have been suggested to account for the peculiarities of the Gardar rocks (e.g. their rare mineralogy, extreme enrichment of HFSE elements, Be or REE in the Ilímaussaq agpaites, and the formation of the unique Ivigtut cryolite deposit). To constrain the formation and chemical evolution of F-bearing melts and fluids, fluorides (fluorite, cryolite, villiaumite, cryolithionite), calcite and siderite from the Ilímaussaq, Motzfeldt and Ivigtut complexes were analysed for their trace element content focusing on the rare earth elements and yttrium (REE). The various generations of fluorite occurring in the granitic Ivigtut, agpaitic Ilímaussaq and miaskitic to agpaitic Motzfeldt intrusions all share a negative Eu anomaly which is attributed to (earlier) feldspar fractionation in the parental alkali basaltic melts. This interpretation is supported by the abundance of anorthositic xenoliths in many Gardar plutonic rocks. The primary magmatic fluorites from Ilímaussaq and Motzfeldt display very similar REE patterns suggesting a formation from closely related parental melts under similar conditions. Hydrothermal fluorites from these intrusions were used to constrain the multiple effects responsible for the incorporation of trace elements into fluorides: temperature dependence, fluid migration/interaction and complexation resulting in REE fractionation. Generally, the REE patterns of Gardar fluorides reflect the evolution and migration of a F/CO 2-rich fluid leading to the formation of fluorite and fluorite/calcite veins. In certain units, this fluid inherited the REE patterns of altered host rocks. In addition, there is evidence of an even younger fluid of high REE abundance which resulted in highly variable REE concentrations (up to three orders of magnitude) within one sample of hydrothermal fluorite. The REE patterns of the granitic Ivigtut intrusion show flat to slightly heavy-REE-enriched patterns characterised by a strong tetrad effect. This effect is interpreted to record extensive fluid–rock interaction in highly fractionated, Si-rich systems. Interestingly, the fluorides appear to record different source REE patterns, as the spatially close Motzfeldt and Ilímaussaq intrusions show strong similarities and contrast with the Ivigtut intrusion located 100 km NE. These variations may be attributed to differences in the tectonic position of the intrusions or mantle heterogeneities.

Amphibole crystallization in the Etnean feeding system: mineral chemistry and trace element partitioning between Mg-hastingsite and alkali basaltic melt

Amphiboles are rather rare in the volcanics of the whole Etnean succession and commonly are represented by kaersutites to titanian pargasites, mostly found in differentiated products. Titanian Mg-hastingsites have been found in lavas and tephra from the 2001 eruption at Mt. Etna. New major (EMPA) and trace element (LAM-ICP/MS) data on amphiboles from this eruption have been compared with reference data for kaersutites from prehistoric eruptions. The two amphibole groups significantly differ from each other in their AlIV, AlVI, K and Mg# values, which are higher in Mg-hastingsite than in kaersutite. Ti and Na are lower in Mg-hastingsite than in kaersutite. REE and trace element patterns for all the analysed Mg-hastingsite crystals are quite homogeneous. Kaersutite patterns generally conform to those of Mg-hastingsite but display higher concentrations for most of the trace elements. The exceptional occurrence, exclusively in tephra, of some crystals under equilibrium conditions with the coexisting residual glass has made it possible to calculate the partition coeffcients between amphibole and melt (Amph/melt D ) for trace elements. A new set of partition coeffcients is then provided, deriving from analyses on five amphibole/melt pairs at equilibrium. These data highlight the effects of amphibole crystallization in controlling some trace element ratios ( e.g. Th (or U)/Ta, Th/Nb, La/Nb) in residual melts of alkali basaltic systems, and suggest new hints for interpreting possible geochemical anomalies of these magmas. In addition, the comparison between the calculated Amph/melt D of Mg-hastingsite and those from literature relative to kaersutite from prehistoric eruptions shows that they are generally lower in the former than in the latter one for most of the trace elements. All of the available data provide constraints on the physical growth conditions for the 2001 Mg-hastingsite. Temperatures around 980 °C and volatile pressures in the range of 200–300 MPa have been estimated by integrating geophysical and petrological data. The highest pressure values are however larger than the lithostatic pressure alone acting on the magma reservoir (~ 6 km b.s.l.), as defined on the grounds of the hypocentres depth of the seismic events associated to the magma rise. This implies that Mg-hastingsite was probably crystallizing in a closed reservoir under overpressure conditions. Finally, microchemical data and trace element partitioning suggest that the differences between Mg-hastingsites and kaersutites in the Etnean products are mainly due to the less differentiated character of the magmas emitted after the 1971, and particularly after the 2001 eruption, compared to the compositions that characterize products of the prehistoric events. Furthermore, also a higher pressure of the system where Mg-hastingsite crystallized would account for its compositional differences respect to prehistoric kaersutite.

A new type of orthopyroxenite xenolith from Takashima, Southwest Japan: silica enrichment of the mantle by evolved alkali basalt

We found fine-grained Fe-rich orthopyrox- ene-rich xenoliths (mainly orthopyroxenite) containing partially digested dunite fragments of Group I from Takashima, Southwest Japan. Orthopyroxenite vein- lets, some of which contain plagioclase at the center, also replace olivine in dunite and wehrlite xenoliths of Group I. This shows high reactivity with respect to olivine of the melt involved in orthopyroxenite for- mation, indicating its high SiO2 activity. The secondary orthopyroxene of this type is characterized by low Mg# (= Mg/(Mg + total Fe) atomic ratio) (down to 0.73) and high Al2O3 contents (5-6 wt%). It is differ- ent in chemistry from other secondary orthopyroxenes found in peridotite xenoliths derived from the mantle wedge. Clinopyroxenes in the Fe-rich orthopyroxenite show a convex-upward REE pattern with a crest around Sm. This pattern is strikingly similar to that of clinopyroxenes of Group II pyroxenite xenoliths and of phenocrystal and xenocrystal clinopyroxenes, indicat- ing involvement of similar alkali basaltic melts. The Fe- rich orthopyroxenite xenoliths from Takashima formed by reaction between evolved alkali basalt melt and mantle olivine; alkali basalt initially slightly undersat- urated in silica might have evolved to silica-oversatu- rated compositions by fractional crystallization at high- pressure conditions. The Fe-rich orthopyroxenites oc- cur as dikes within the uppermost mantle composed of dunite and wehrlite overlying pockets of Group II pyroxenites. The orthopyroxene-rich pyroxenites of this type are possibly common in the uppermost mantle beneath continental rift zones where alkali basalt magmas have been prevalent.

Geochemistry and petrogenesis of spinel lherzolite xenoliths from Boeun, Korea

Geochemical characteristics of spinel lherzolite xenoliths, enclosed in Miocene alkali basalt from Boeun, Korea, provide important clues for understanding the lithosphere composition, equilibrium temperature and pressure conditions, and depletion and enrichment processes of subcontinental lithospheric mantle beneath Boeun. The spinel lherzolite xenoliths with protogranular to porpyroclastic textures were accidentally trapped by the ascending alkali basalt magma. The spinel lherzolite xenoliths originated at depths between 50 and 63 km with equilibrium temperatures ranging from 847 to 1030 °C. These xenoliths may have undergone small degrees (1–2%) of partial melting and cryptic metasomatism by an alkali basaltic melt. Based on Sr and Nd isotope compositions, the subcontinental lithospheric mantle beneath Boeun was heterogeneous and similar to that beneath East China and Central Mongolia rather than the Japanese Island Arc.

N-MORB crust beneath Fuerteventura in the easternmost part of the Canary Islands: evidence from gabbroic xenoliths

Gabbro xenoliths reported in this paper were collected in northern Fuerteventura, the Canary Island located closest to the coast of Africa. The xenoliths are very fresh and consist of Ti–Al-poor clinopyroxene + plagioclase (An87–67) + olivine (Fo72–86) ± orthopyroxene. Clinopyroxene and orthopyroxene are constantly and markedly depleted in light rare earth elements (LREE) relative to heavy REE (HREE), as expected for cumulus minerals formed from highly refractory N-MORB-type melts. In contrast, whole-rock Primordial Mantle-normalized trace element patterns range from mildly S-shaped (mildly depleted in Pr–Sm relative to both the strongly incompatible elements Rb–La and the HREE) to enriched. Estimates show that the trace element compositions of the rocks and their minerals are compatible with formation as N-MORB gabbro cumulates, which have been infiltrated at various extents (≤1% to >5%) by enriched alkali basaltic melts. The enriched material is mainly concentrated along grain boundaries and cracks through mineral grains, suggesting that the infiltration is relatively recent, and is thus associated with the Canary Islands magmatism. Our data contradict the hypothesis that a mantle plume was present in this area during the opening of the Atlantic Ocean. No evidence of continental material that might reflect attenuated continental crust in the area has been found. Gabbro xenoliths with REE and trace element compositions similar to those exhibited by the Fuerteventura gabbros are also found among gabbro xenoliths from the islands of La Palma (western Canary Islands) and Lanzarote. The compositions of the most depleted samples from these islands are closely similar, implying that there was no significant change in chemistry during the early stages of formation of the Atlantic oceanic crust in this area. Strongly depleted gabbros similar to those collected in Fuerteventura have also been retrieved in the MARK area along the central Mid-Atlantic Ridge. The presence of N-MORB oceanic crust beneath Fuerteventura implies that the continent–ocean transition in the Canary Islands area must be relatively sharp, in contrast to the situation both further north along the coast of Morocco, and along the Iberian peninsula.

Origin of megacrysts and pyroxenite xenoliths from the Pliocene alkali basalts of the Pannonian Basin (Hungary)

Clinopyroxene and amphibole megacrysts and type II pyroxenite xenoliths occur in young (mainly Pliocene) alkali basalts of the Pannonian Basin. Major and trace element contents (the latter were determined by LAM-ICP-MS) of the clinopyroxenes and amphiboles suggest that megacrysts and type II xenoliths share a common origin. Both suites are fragments of igneous cumulates and pegmatitic veins that crystallised from alkali basaltic melts in the upper mantle at 11-14 kbar pressures. Trace element contents of the calculated hypothetical melts in equilibrium with the megacrysts and type II xenoliths are very similar to those of the host alkali basalts, but 8 7 Sr/ 8 6 Sr ratios are significantly lower and 1 4 3 Nd/ 1 4 4 Nd ratios are higher in the megacrysts and xenoliths than in the Pliocene alkali basalts that host them. The source rocks of the megacrysts and type II xenoliths had probably crystallised from earlier intrusions of alkali basaltic melts in the lithospheric mantle and were disrupted later by more voluminous intrusions of compositionally similar but isotopically more enriched alkali basaltic magmas.

Experimental investigation of large-ion-lithophile-element-, high-field-strength-element- and rare-earth-element-partitioning between calcic amphibole and basaltic melt: the effects of pressure and oxygen fugacity

The effects of pressure and oxygen fugacity (fO2) on trace element partitioning between pargasitic amphibole and alkali-basaltic melts have been determined at pressures from 1.5 to 2.5 GPa and oxygen fugacities at 2 log units above and below the nickel–nickel oxide buffer. Amphibole crystallization experiments were performed in a piston cylinder apparatus and partition coefficients between amphibole and quenched melt of large-ion-lithophile elements (LILE: Rb, Sr, Ba), high-field-strength elements (HFSE: Zr, Nb, Ta, Hf, U, Th) and rare-earth elements (REE: La to Lu; +Y) were measured with a LASER ablation inductively coupled plasma – mass spectrometer. Increasing pressure from 1.5 to 2.5 GPa at similar temperatures and approximately constant fO2 increases DRb but decreases DZr and DHf and DREE (DLa, DCe, DPr). An empirical relationship was observed between DZr and (Ti/Al)M2 in the amphibole, which can be described by:

Pyroxenite Xenoliths from Al Ashaer Volcano, Syria: Constraints on the Thermal State of the Subcontinental Arabian Lithosphere

Pyroxenite xenoliths from the Pliocene Al Ashaer volcano, Syria, have been investigated to expand the database on pyroxenite xenoliths in the western part of the Arabian plate, and to complement the existing data for associated spinel peridotite xenoliths. Such data are essential for establishing the chemical composition and thermal state of the Arabian plate lithosphere, and for evaluating possible geographic and secular variations in the mantle. Sixteen samples of Al Ashaer pyroxenite xenoliths were analyzed by X-ray fluorescence for major and minor elements, and mineral compositions in six samples were determined by electron microprobe. Al Ashaer pyroxenites belong to the Al-augite group of mantle xenoliths, include spinel- and garnet-bearing varieties, and are characterized by a mosaic texture. Rock and mineral compositions indicate that the pyroxenites formed by high-pressure crystal accumulation from alkali basaltic melts in the upper mantle. Equilibration conditions for Al Ashaer pyroxenites are 970 to 1140°C and 11.5 to 13.8 kbar; these values are comparable to those for other pyroxenite xenoliths in the Arabian plate, and are consistent with T-P conditions for accompanying spinel peridotites. T-P conditions for xenolith suites from different locations in the western Arabian plate are unrelated to geographic position and tectonic setting. Pyroxenite and peridotite equilibration conditions are well above those predicted by the present-day surface heat flow of ∼45 mW/m2, and reflect a geologically young thermal pulse whose effects have yet to reach the surface. This thermal perturbation is caused most likely by advective heating of the subcontinental lithosphere by extensive and voluminous Neogene magmas, although high mantle temperatures immediately east of the Red Sea may be caused by convective thinning of the lithosphere associated with rifting.

Reaction of orthopyroxene in peridotite xenoliths with alkali-basalt melt and its implication for genesis of alpine-type chromitite

The Kawashimo alkali basalt of the southwest Japan arc has peridotite xenoliths with a wide lithological range, from lherzolite {Fo of olivine, 89; Cr' [=Cr/(Cr + AI) atomic ratio] of spinel O.IO}to harzburgite (Fo of olivine, 91; Cr' of spinel, 0.54). Reaction zones between orthopyroxene and alkali-basalt melt are low-pressure analogues of mantle-melt interaction products and consist of two subzones: a fme-grained inner subzone (adjacent to orthopyroxene) and a relatively coarse-grained outer subzone. In both xenolith types the reaction products are olivine + diopsidic clinopyroxene :t spinel :t glass, but the spinel concentrations are remarkably different around lherzolite from those around harzburgite. Cr-bearing spinel is concentrated only in the outer subzone on harzburgite orthopyroxene (Cr' > 0.14); the inner subzone on harzburgite orthopyroxene and both the inner and outer subzones on lherzolite orthopyroxene (Cr' = 0.05) are almost free of spinel. The remarkable enrichment of spinel in the outer subzone on the harzburgite orthopyroxene suggests a mechanism of spinel concentration, Le., the origin of podiform chromitite is related to interaction between Cr-rich orthopyroxene and basaltic melt. This observation for the Kawashimo xenoliths is concordant with the near absence ofpodiform chromitite in Iherzolitic mantle. Chromian spinel could be concentrated if a relatively silica-rich secondary melt, produced by interaction between pyroxene-undersaturated magma and harzburgite orthopyroxene, is mixed with a primitive magma in the upper mantle.

The Role of Partial Melting in the 15-Ma Geochemical Evolution of Gran Canaria: A Blob Model for the Canary Hotspot

The subaerial portion of Gran Canada, Canary Islands, was built by three cycles of volcanism: a Miocene Cycle (8-5—15 Ma), a Pliocene Cycle (1-8-60 Ma), and a Quaternary Cycle (1-8-0 Ma). Only the Pliocene Cycle is completely exposed on Gran Canaria; the early stages of the Miocene Cycle are submarine and the Quaternary Cycle is still in its initial stages. During the Miocene, SiO2 saturation of the mafic volcanics decreased systematically from tholeiite to nephelinite. For the Pliocene Cycle, SiO2 saturation increased and then decreased with decreasing age from nephelinite to tholeiite to nephelinite. SiO2 saturation increased from nephelinite to basanite and alkali basalt during the Quaternary. In each of these cycles, increasing melt production rates, SiO2 saturation, and concentrations of compatible elements, and decreasing concentrations of some incompatible elements are consistent with increasing degrees of partial melting in the sequence melilite nephelinite to tholeiite. The mafic volcanics from all three cycles were derived from CO2-rich garnet lherzolite sources. Phlogopite, ilmenite, sulfide, and a phase with high partition coefficients for the light rare earth elements (LREE), U, Th, Pb, Nb, and Zr, possibly zircon, were residual during melting to form the Miocene nephelinites through tholeiites; phlogopite, ilmenite, and sulfide were residual in the source of the Pliocene-Quaternary nephelinites through alkali basalts. Highly incompatible element ratios (e.g., Nb/U, Pb/Ce, K/U, Nb/Pb, Ba/Rb, Zr/Hf, La/Nb, Ba/Th, Rb/Nb, K/Nb, Zr/Nb, Th/Nb, Th/La, and Ba/La) exhibit extreme variations (in many cases larger than those reported for all other ocean island basalts), but these ratios correlate well with degree of melting. Survival of residual phases at higher degrees of melting during the Miocene Cycle and differences between major and trace element concentrations and melt production rates between the Miocene and Pliocene tholeiites suggest that the Miocene source was more fertile than the Pliocene-Quaternary source(s). We propose a blob model to explain the multi-cycle evolution of Canary volcanoes and the temporal variations in chemistry and melt production within cycles. Each cycle of volcanism represents decompression melting of a discrete blob of plume material. Small-degree nephelinitic and basanitic melts are derived from the cooler margins of the blobs, whereas the larger-degree tholeiitic and alkali basaltic melts are derived from the hotter centers of the blobs. The symmetrical sequence of mafic volcanism for a cycle, from highly undersaturated to saturated to highly undersaturated compositions, reflects melting of the blob during its ascent beneath an island in the sequence upper margin-corelower margin. Volcanic hiatuses between cycles and within cycles represent periods when residual blob or cooler entrained shallow mantle material fill the melting zone beneath an island.

Alkali basalts from Shuqra, Yemen: magmas generated in the crust-mantle transition zone?

Abstract The alkali basalts from Shuqra, Yemen, contain a megacryst suite consisting of olivine, aluminous diopside, kaersutite, pleonaste, plagioclase, and apatite. Deformational kink bands in the olivines, and the existence of multi-phase inclusions (e.g. olivine + pleonaste, aluminous diopside + pleonaste, kaersutite + pleonaste, aluminous diposide + plagioclase) suggest that the megacrysts are derived from disaggregated plutonic rocks. Whole-rock compositional variation cannot have been generated by the fractionation of the phenocrysts present (olivine, augite, labradorite), but is readily explicable as a mixing line between alkali basalt melt and restite, represented by the megacryst suite. It is suggested that the source rock was a deep-crustal, or uppermost mantle, hydrous, and very alkalic, picritic, gabbroic, or syenogabbroic intrusion, which was remelted, perhaps by the injection of further picritic magma. Although the Shuqra rocks themselves do not appear to contain mantle xenoliths, the proposed mechanism of magma generation, if operating on an interleaved crust/mantle transition zone, could provide a general explanation for the occurrence of occasional alkali basalt flows which are extremely rich in spinel lherzolite inclusions.

Abundance and distribution of gold in the orogenic-type spinel peridotites from Ariège (Northeastern Pyrenees, France)

Fifty-one spinel peridotite samples from the Eastern Pyrenean massifs (Ariège, France) have been analyzed for gold by instrumental neutron activation (INA) and radiochemical neutron activation (RNA). Two groups of peridotites are distinguished. The first one includes all the samples except the amphibole-rich peridotites from Caussou in the northeastern Pyrenees of France. In this group, the Au content varies widely between 0.1 and 4.0 ppb, irrespective of the refractory character of peridotites and Ce Yb ratios. LREE-rich peridotites which have been percolated by alkali basaltic melts (harzburgites and peridotite wall-rocks of amphibole-pyroxenite veins) are lacking in Au-enrichment. The second group of analyzed peridotites (amphibole-rich samples from Caussou) displays significant Au-enrichments in the concentration range of 5 to 18 ppb. We infer from these data that gold may have a significantly different behaviour during mantle metasomatism: gold behaves as a relatively inert element during mantle metasomatism dominated by silicate melts. In contrast, H 2O-CO 2 fluids leading to amphibole crystallization could have transported gold.

Dissolution Rates of Upper Mantle Minerals in an Alkali Basalt Melt at High Pressure: An Experimental Study and Implications for Ultramafic Xenolith Survival

Dissolution Rates of Upper Mantle Minerals in an Alkali Basalt Melt at High Pressure: An Experimental Study and Implications for Ultramafic Xenolith Survival

Petrology and geochemistry of differentiated teschenite intrusions from the Hunter Valley, New South Wales, Australia

In the upper Hunter Valley of New South Wales a high level teschenitic sill complex emplaced into Permian coal measures derives from parent magmas which were themselves crystal fractionation products of alkali basaltic melts. The sills crystallised in situ and produced a spectrum of rock types ranging from olivine teschenite to teschenite, syenoteschenite and, ultimately, syenite. The olivine teschenites are also enriched in biotite and are crudely interlayered with teschenite and syenoteschenite. The lineage from olivine teschenite to syenoteschenite is characterised by a progressive decrease in olivine and a build-up of alkaline mesostasis which is accompanied by strong chemical zonation in abutting silicate minerals. The alkaline mesostasis and syenites are identical mineralogically. Primary crystallisation of olivine in the olivine teschenite-teschenite-syenoteschenite continuum and data from coexisting iron-titanium oxide pairs suggest that oxygen fugacity was constrained to a path parallel to the QFM buffer curve. Absence of olivine from the alkaline mesostasis and syenite veins, together with the appearance of sphene, indicates buffering of oxygen fugacity by other assemblages (probably annite-alkali feldspar-magnetite) and generally higher fO2 in the residual liquids. Here, a build-up of CO2, F and Cl, in addition to H2O, influenced the relative stabilities of the pyroxene and amphibole minerals.

Lithium tracer-diffusion in an alkali-basaltic melt — An ion-microprobe determination

An ion-microprobe-based technique has been used to measure lithium tracer-diffusion coefficients ( D Li ) in an alkali-basaltic melt at 1300, 1350 and 1400°C. The results can be expressed in the form: D Li=7.5 ×10 −2exp(−27,600/RT)cm 2S −1 The results show significantly faster diffusion rates than those previously recorded for other monovalent, divalent and trivalent cations in a tholeiitic melt. Consequently, diffusive transport of ions acting over a given time in a basaltic melt can produce a wider range of transport distance values than hitherto supposed. Hence, it is concluded that great care should be exercised when applying diffusion data to petrological problems.

The mantle of Mars: Some possible geological implications of its high density

The density of the Martian mantle is estimated to be about 3.55 g/cm 3 ( Reasenberg, 1977). Model mineral assemblages for the Martian mantle (at 30 kbar) were calculated using a modified CIPW norm scheme by adding FeO to model terrestrial mantle compositions. The density of the resulting mineral assemblages vary with increasing FeO content. With pyrolite starting compositions for the terrestrial mantle, the resulting model Martian mantle with density of 3.55 g/cm 3 is not garnet-lherzolite like the Earth; rather it is an assemblage properly called oxide-garnet wehrlite: oxide (periclase-wüstite) 2%; garnet 11%; olivine 73%; clinopyroxene 12%; with no orthopyroxene. Partial melting of such an assemblage wouldyield iron-rich, ultrabasic lavas, with extremely low viscosities. Specifically, model partial melts, assuming production from the quaternary eutectic (inferred to be near: op 7 g 42 cpx 43 ox 8) yields an ultrabasic (SiO 2, 41 to 44%) picritic alkali-basaltic melt (norm composition ne 2.5, plag 32, or 2.4, di 20, ol 37, mt 4.4 and ilm, tr), with a computed viscosity of about 12 P at 1200°C. This model for the composition of the Martian surface lavas (derived from geophysical data and petrologic arguments) is in remarkable agreement with a recently published model by Maderazzo and Huguenin (1977) (derived from reflection spectroscopy, experimental and theoretical models for weathering in the Martian environment). The result also appears to be consistent with recent interpretations (Rasool and Le Sergeant, 1977) of Viking atmospheric chemistry results, namely that the Martian crust is potassium poor. There are a number of geological implications which follow, including (1) superfluid lavas may account for some flood and erosional features observed on Mars; (2) the XRF inorganic chemistry experiment on Vikings 1 and 2 (Baird, 1976) indeed may be measuring compositions approaching primary lavas, contrary to current interpretations which favor a rather mature (weathered) soil; (3) ultrabasic (ferrokimberlitic) ash might be a major constituent of the Martian soil, especially if cosmological models concerning the incorporation of a much volatile material within the early accreting Mars are correct—a matter of current debate; (4) a number of mineral assemblages not previously considered are possible in the Martian mantle depending principally on the activity of volatile substances, (S, O, C, H); it is possible that some very unusual magmas are produced on partial melting; and (5) some ferro-granite melts might be produced by liquid immiscibility.

Origin of some magmas in oceanic and circum-oceanic regions

Partial melting experiments on spinel-lherzolite, a rock which probably occurs in relatively shallow parts of the oceanic upper mantle, demonstrate that alkali basaltic melt is formed at depths of at least 20 kbar whereas tholeiitic melt is formed at lower pressures (< 15 kbar) under anhydrous conditions. The specimen studied was a relatively iron-rich natural spinel-lherzolite (Fe/Mg+Fe=0.15) and the melts produced have ratios comparable to those obtained in basalts. Slight increase of degree of partial melting produces picritic melt over a wide pressure range. Under hydrous (water-excess) conditions, andesitic melt is produced by partial melting of the same natural spinel-lherzolite and a synthetic lherzolite. The melting experiments on two different abyssal tholeiites from the Mid-Atlantic Ridge suggest that the derivation of olivine tholeiite from a more mafic magma or a mantle peridotite (lherzolite) is possible, but is limited to depths shallower than 25 km under essentially anhydrous conditions, whereas plagioclase tholeiite may have been formed by fractional crystallization at depths of about 20 km in the presence of a small amount (∼ 2 wt.%) of water. It is suggested that under mid-ocean ridges, partial melting of spinel-lherzolite at depths shallower than 60 km would produce olivine-tholeiitic magma, which differentiates at shallower levels (20–25 km) under either essentially anhydrous or hydrous (but vapor-absent) conditions to produce abyssal tholeiites of olivine-tholeiite type or plagioclase-tholeiite type. It may be also possible that the former olivine-tholeiite is generated by direct partial melting of plagioclase-lherzolite. Alkali basalts in the oceanic region may be generated at depths greater than 50 km by relatively small degree of partial melting. Along island arcs and continental margins, where the subduction zones probably exist, partial melting of lherzolite would take place in the presence of water that may be supplied by breakdown of hydrous minerals in the subducted oceanic crust, thereby producing andesitic magmas. High-alumina basalt magma could be produced by partial melting of the dehydrated oceanic crust in the subduction zone at depths between 40 and 60 km, where garnet is unstable above the solidus.

Quantitative Untersuchungen mit der Elektronen-Mikrosonde an Pyroxenen aus Basalten und Peridotit-Einschl�ssen

The chemical composition of the pyroxenes and olivines of 12 basaltic rocks and 5 lherzolite nodules was determined quantitatively by electron micro-probe analysis. The composition of the pyroxenes depends on the type of basalt in which they occur. Tholeiitic basalts with normative quartz contain three pyroxenes: orthorombic pyroxenes, pigeonites and augites. All pyroxene phases are zoned and do not show any exsolution. Their Ti and Al contents (Ca-Tschermaks and Ti-augite molecules) are small. All pyroxene phases were formed under disequilibrium with each other and with the melt because of rapid quenching. The sequence of crystallization: orthopyroxene—pigeonite—augite could be established by their Cr content. The alkali olivine basalts undersatured in SiO2 and the olivine nephelinites are characterized by Ti and Al-rich clinopyroxenes. The distribution of Ti and Al in the pyroxenes of the alkali olivine basalts shows a differentiation trend from the cores of the phenocrysts to their outer zones and to the crystals of the ground mass. Thereby the Ca-Tschermaks molecule is being replaced more and more by the Ti-augite molecule. The Ti content of the pyroxenes of the olivine nephelinites decreases in the last stage of differentiation because simultaneously increasing amounts of titaniferous magnetite crystallize. The pyroxenes of lherzolite peridotite nodules are characterized by high Al and low Ti contents which differ according to the type of basalt (alkali olivine basalt or olivine nephelinite) in which the nodules occur. The homogeneous distribution of the elements within the single grains indicates crystallization under equlibrium conditions. The conditions of their formation are comparable to those of Al-pyroxene peridotites in the upper mantle. The composition of pyroxenes of early accumulates of alkali basaltic melts differ from those of peridotite nodules. Therefore lherzolite nodules can be taken as residues of deeper peridotite masses.