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1.
New experiments have been performed in the system CaO+MgO+Al2O3+SiO2 (CMAS)+FeO at atmospheric pressure. Most of the experiments were conducted on Fe-rich compositions, in the low-temperature field of the assemblage liq(liquid)+an(anorthite) +aug(augite)+ol(olivine), and mostly along five isotherms. Others were located on, or nearby the assemblage boundaries. These experiments, together with the previously reported high temperature experiments (Shi and Libourel 1991; Libourel et al. 1989), permit contouring the complete liq+an+aug+ol divariant field, and tracing out some of its boundaries. The boundary of the assemplage liq+an+aug+ol consists of six segments, with the appearance of one of the following phases, orthopyroxene, pigeonite, tridymite, bustamite, kirschsteinite, and spinel, as an additional phase. Within the stability field of the assemblage liq+an+aug+ol, the compositions of all the coexisting phases have been described as functions of temperature and silica content in the melt by applying a multiple linear regression method. This allows a quantitative characterization of the divariant assemblage liq+an+aug+ol in the system CMAS+FeO, from 1273°C to 1055°C, with olivine compositions ranging from Mg*[Mg/(Mg+Fe)]=1 to 0.08. Knowing the composition-temperature relationships, the basic T-X configuration of the assemblage liq+an +aug+ol has been analysed, and mass-balance calculations have been performed to examine the FeO effect on different crystallization processes. Addition of FeO to the system CMAS transforms the thermal divide in the assemblage liq+an+di(diopside)+fo(forsterite) into a thermal ridge. With decreasing temperature, the spine of the thermal ridge moves towards Si-poor compositions at Mg-rich end but towards Si-rich compositions at the Fe-rich end. This indicates that late-stage tholeiitic liquids can follow a trend of silica enrichment without the crystallization of an oxide phase. Crystallization paths of the assemblage liq+an+aug+ol are determined by the detailed T-X relations of the thermal ridge with the melt evolving away from the spine. The boundary reactions with decreasing temperature have also been characterized numerically.  相似文献   

2.
The 150 m thick late Miocene Graveyard Point sill (GPS) is situated at the Idaho-Oregon border near the southwestern edge of the western Snake River Plain. It records from bottom to top continuous fractional crystallization of a tholeiitic parent magma (lower chilled border, FeO/(FeO+MgO) = 0.59, Ni = 90 ppm) towards granophyres (late pods and dikes, FeO/(FeO+MgO) = 0.98, 78 wt% SiO2 3.5 wt% K2O, <4 ppm Ni) showing a typical trend of Fe and P enrichment. Fractionating minerals are olivine (Fo79-Fo2), augite (X Fe = 0.18−0.95), feldspars (An80Or1-An1Or62), Fe-Ti oxides (Ti-rich magnetite and ilmenite), apatite and in two samples super-calcic pigeonite (Wo18–28 Fs41–54). The granophyres may bear some quartz. Compositionally zoned minerals record a large interval of the fractionation process in every single sample, but this interval changes with stratigraphic height. In super-calcic pigeonite-bearing samples, olivine is scarce or lacking and because super-calcic pigeonite occurs as characteristic overgrowths on augite, its formation is interpreted to be related to the schematic reaction: augite + olivine (component in melt) + SiO2 (in melt) = pigeonite, that defines the cotectic between augite and pigeonite in olivine-saturated basaltic systems. Line measurements with the electron microprobe reveal that the transition from augite to super-calcic pigeonite is continuous. However, some crystals show an abrupt “reversal” towards augite after super-calcic pigeonite growth. Two processes compete with each other in the GPS: fractional crystallization of the bulk liquid (the bulk melt separates from solids and interstitial liquids in the solidification front) and fractional crystallization of interstitial melt in the solidification front itself. Interplay between those two processes is proposed to account for the observed variations in mineral chemistry and mineral textures. Received: 25 November 1998 / Accepted: 14 June 1999  相似文献   

3.
 Ultramafic xenoliths in Cenozoic alkali basalts from Yitong, northeast China comprise three types in terms of their modal mineralogy: lherzolite, pyroxenite and wehrlite. The wehrlite suite always contains interstitial pale/brown glass which occupies several per cent by volume of the whole rock. The texture of the wehrlites is porphyroclastic with some large strained grains of olivine (0.5–1 mm) scattered in a very fine grained matrix (0.1 mm), implying a metamorphic origin for the protolith rather than an igneous origin. The host minerals are compositionally zoned, showing evidence of reaction with a melt. Petrological evidence for resorption of spinel (lherzolite) and orthopyroxene (wehrlite) by infiltrating melt further supports the hypothesis that the wehrlites result from interaction between a partial melting residue and a melt, which preferentially replaced primary spinel, Cr-diopside and enstatite to produce secondary clinopyroxene (cpx) + olivine (ol) ± chromite ± feldspar (fd). The composition of the mineral phases supports this inference and, further indicates that, prior to melt impregnation, the protoliths of these wehrlites must have been subjected to at least one earlier Fe-enrichment event. This explanation is consistent with the restricted occurrence of glasses in the wehrlite suite. The glass is generally associated with fine-grained (0.1 mm) minerals (cpx+ol+chromite ±fd). Electron microprobe analyses of these glasses show them to have high SiO2 content (54–60 wt%), a high content of alkalis (Na2O, 5.6–8.0%; K2O, 6.3–9.0%), high Al2O3 (20–24%), and a depletion in CaO (0.13–2.83%), FeO (0.89–4.42%) and MgO (0.29–1.18%). Ion probe analyses reveal a light rare earth element-enrichment in these glasses with chondrite normalised (La)n = 268–480. The high K2O contents in these glasses and their mode of occurrence argue against an origin by in-situ melting of pre-existent phases. Petrographic characteristics and trace element data also exclude the possibility of percolation of host-basalt related melts for the origin of these glasses. Thus the glasses must have resulted from local penetration of mantle metasomatic melts which may have been produced by partial melting of peridotites with involvement of deep-seated fluids. Such melts may have been significantly modified by subsequent fractional crystallization of ol, cpx and sp, extensive reaction with the mantle conduit and the xenolith transport process. Received: 1 August 1995 / Accepted: 19 June 1996  相似文献   

4.
The anhydrous melting behaviour of two synthetic peridotite compositions has been studied experimentally at temperatures ranging from near the solidus to about 200° C above the solidus within the pressure range 0–15 kb. The peridotite compositions studied are equivalent to Hawaiian pyrolite and a more depleted spinel lherzolite (Tinaquillo peridotite) and in both cases the experimental studies used peridotite –40% olivine compositions. Equilibrium melting results in progressive elimination of phases with increasing temperature. Four main melting fields are recognized; from the solidus these are: olivine (ol)+orthopyroxene (opx)+clinopyroxene (cpx)+Al-rich phase (plagioclase at low pressure, spinel at moderate pressure, garnet at high pressure)+liquid (L); ol+opx+cpx+Cr-spinel+L; ol+opx+Cr-spinel +L: ol±Cr-spinel+L. Microprobe analyses of the residual phases show progressive changes to more refractory compositions with increasing proportion of coexisting melt i.e. increasing Mg/(Mg+Fe) and Cr/(Cr+Al) ratios, decreasing Al2O3, CaO in pyroxene.The degree of melting, established by modal analysis, increases rapidly immediately above the solidus (up to 10% melting occurs within 25°–30° C of the solidus), and then increases in roughly linear form with increasing temperature.Equilibrium melt compositions have been calculated by mass balance using the compositions and proportions of residual phases to overcome the problems of iron loss and quench modification of the glass. Compositions from the melting of pyrolite within the spinel peridotite field (i.e. 15 kb) range from alkali olivine basalt (<15% melting) through olivine tholeiite (20–30% melting) and picrite to komatiite (40–60% melting). Melting in the plagioclase peridotite field produces magnesian quartz tholeiite and olivine-poor tholeiite and, at higher degrees of melting (30–40%), basaltic or pyroxenitic komatiite. Melts from Tinaquillo lherzolite are more silica saturated than those from pyrolite for similar degrees of partial melting, and range from olivine tholeiite through tholeiitic picrite to komatiite for melting in the spinel peridotite field.The equilibrium melts are compared with inferred primary magma compositions and integrated with previous melting studies on basalts. The data obtained here and complementary basalt melting studies do not support models of formation of oceanic crust in which the parental magmas of common mid-ocean ridge basalts (MORB) are attributed to segregation from source peridotite at shallow depths ( 25 km) to leave residual harzburgite. Liquids segregating from peridotite at these depths are more silica-rich than common MORB.  相似文献   

5.
Mid-ocean ridge basalts (MORBs) from East Pacific Rise (EPR) 13°N are analysed for major and trace elements, both of which show a continuous evolving trend. Positive MgO-Al2O3 and negative MgO-Sc relationships manifest the cotectic crystallization of plagioclase and olivine, which exist with the presence of plagioclase and olivine phenocrysts and the absence of clinopyroxene phenocrysts. However, the fractionation of clinopyroxene is proven by the positive correlation of MgO and CaO. Thus, MORB samples are believed to show a "clinopyroxene paradox". The highest magnesium.bearing MORB sample E13-3B (MGO=9.52%) is modelled for isobaric crystallization with COMAGMAT at different pressures. Observed CaO/Al2O3 ratios can be derived from E13-3B only by fractional crystallization at pressure >4±1 kbar, which necessitates clinopyroxene crystallization and is not consistent with cotectic crystallization of olivine plus plagioclase in the magma chamber (at pressure～1 kbar). The initial compositions of the melt inclusions, which could represent potential parental magmas, are reconstructed by correcting for post-entrapment crystallization (PEC). The simulated crystallization of initial melt inclusions also produce observed CaO/Al2O3 ratios only at >4±1 kbar, in which clinopyroxene takes part in crystallization. It is suggested that MORB magmas have experienced clinopyroxene fractionation in the lower crust, in and below the Moho transition zone. The MORB magmas have experienced transition from clinopyroxene+plagioclase+olivine crystallization at >4±1 kbar to mainly olivine+plagioclase crystallization at <1 kbar, which contributes to the explanation of the "clinopyroxene paradox".  相似文献   

6.
Aleutian tholeiitic and calc-alkaline magma series I: The mafic phenocrysts   总被引：1，自引：0，他引：1  
Diagnostic mafic silicate assemblages in a continuous spectrum of Aleutian volcanic rocks provide evidence for contrasts in magmatic processes in the Aleutian arc crust. Tectonic segmentation of the arc exerts a primary control on the variable mixing, fractional crystallization and possible assimilation undergone by the magmas. End members of the continuum are termed calc-alkaline (CA) and tholeiitic (TH). CA volcanic rocks (e.g., Buldir and Moffett volcanoes) have low FeO/MgO ratios and contain compositionally diverse phenocryst populations, indicating magma mixing. Their Ni and Cr-rich magnesian olivine and clinopyroxene come from mantle-derived mafic olivine basalts that have mixed with more fractionated magmas at mid-to lower-crustal levels immediately preceding eruption. High-Al amphibole is associated with the mafic end member. In contrast, TH lavas (e.g., Okmok and Westdahl volcanoes) have high FeO/MgO ratios and contain little evidence for mixing. Evolved lavas represent advanced stages of low pressure crystallization from a basaltic magma. These lavas contain groundmass olivine (FO 40–50) and lack Ca-poor pyroxene. Aleutian volcanic rocks with intermediate FeO/MgO ratios are termed transitional tholeiitic (TTH) and calc-alkaline (TCA). TCA magmas are common (e.g., Moffett, Adagdak, Great Sitkin, and Kasatochi volcanoes) and have resulted from mixing of high-Al basalt with more evolved magmas. They contain amphibole (high and low-Al) or orthopyroxene or both and are similar to the Japanese hypersthene-series. TTH magmas (e.g., Okmok and Westdahl) contain orthopyroxene or pigeonite or both, and show some indication of upper crustal mixing. They are mineralogically similar to the Japanese pigeonite-series. High-Al basalt lacks Mg-rich mafic phases and is a derivative magma produced by high pressure fractionation of an olivine tholeiite. The low pressure mineral assemblage of high-Al basalt results from crystallization at higher crustal levels.  相似文献   

7.
 A new method for modeling fractional crystallization processes that involve olivine (ol), plagioclase (plag) and augite (aug) is presented. This crystallization assemblage is the major control on the chemical variations in mid-ocean ridge basalts. The compositional and temperature variations in ol-plag-aug saturated basalts over a range of pressures are described using empirical expressions. A data base of 190 experiments in natural and basalt-analog chemical systems is used to describe temperature, Al, Ca and Mg molar fractions as functions of Si, Fe, Na, Ti and K molar fractions and pressure. Increases in the abundances of Na and K cause Ca and Mg abundances to decrease and Al abundance to increase in ol-plag-aug saturated melts. The equations can be used to predict pressure and temperature and thus provide a useful thermobarometer. A model is described to calculate ol-plag-aug fractional crystallization as a function of pressure and melt composition, using melt and augite models developed here, combined with existing models for olivine-melt and plagioclase-melt equilibria. We compare the fractional crystallization sequence of ALV-2004-3-1 predicted from the models presented in this paper, Langmuir et al. (1992) modified by Reynolds (1995), Ghiorso and Sack (1995) and Ariskin et al. (1993) at 0.001 and 4 kbar. As an example the model is applied to estimate pressure of crystallization of glasses from the east flank of the East Pacific Rise at 11°45′N. Received: 24 July 1995 / Accepted: 12 January 1996  相似文献   

8.
Variation of major and trace elements in drilled basalts from the Mid-Atlantic Ridge (DSDP Leg 37) reflects distinct cycles of low pressure fractionation operating independently within a complex network of magma storage reservoirs beneath the crustal spreading axis. Low pressure phase relations are determined by parental magma composition, which varies from An-rich (An/Di > ca. 1.4) to Di-rich (An/Di < ca. 1.4). High An/Di magmas probably formed under slightly hydrous conditions in the mantle. They have low LIL element contents, low P/Y and high Mg/(Mg + Fe) ratios. Zr, P and Y abundance and inter-element ratios are highly diagnostic of primary magma type, and are used to quantify fractional crystallization models.Low pressure fractionation hypotheses were tested by least-squares modelling of whole-rock and phenocryst chemistry, which indicated removal or addition of phenocryst assemblages: ol; pl; ol + pl; ol + pl + cpx; pl + cpx, (± sp). Accumulation of plagioclase or olivine is an important mechanism for generating highly porphyritic rocks. A rare 3-phase (ol + pl + cpx) cumulate resulted from cotectic fractionation of a low An/Di magma type. Olivine and plagioclase cumulates appear to be related to high An/Di magmas. Olivine accumulation has been monitored by comparison of olivine/bulk rock partitioning of Fe and Mg to experimental measurements of the equilibrium KD value. A single extensive sub-axial magma chamber could not account for the observed chemical variation and would probably be dynamically unstable.  相似文献   

9.
Mid-ocean ridge basalts (MORBs) from East Pacific Rise (EPR) 13°N are analysed for major and trace elements, both of which show a continuous evolving trend. Positive MgO–Al2O3 and negative MgO–Sc relationships manifest the cotectic crystallization of plagioclase and olivine, which exist with the presence of plagioclase and olivine phenocrysts and the absence of clinopyroxene phenocrysts. However, the fractionation of clinopyroxene is proven by the positive correlation of MgO and CaO. Thus, MORB samples are believed to show a “clinopyroxene paradox”. The highest magnesium-bearing MORB sample E13-3B (MgO=9.52%) is modelled for isobaric crystallization with COMAGMAT at different pressures. Observed CaO/Al2O3 ratios can be derived from E13-3B only by fractional crystallization at pressure >4 ±1 kbar, which necessitates clinopyroxene crystallization and is not consistent with cotectic crystallization of olivine plus plagioclase in the magma chamber (at pressure ~1 kbar). The initial compositions of the melt inclusions, which could represent potential parental magmas, are reconstructed by correcting for post-entrapment crystallization (PEC). The simulated crystallization of initial melt inclusions also produce observed CaO/Al2O3 ratios only at >4±1 kbar, in which clinopyroxene takes part in crystallization. It is suggested that MORB magmas have experienced clinopyroxene fractionation in the lower crust, in and below the Moho transition zone. The MORB magmas have experienced transition from clinopyroxene+plagioclase+olivine crystallization at >4±1 kbar to mainly olivine+plagioclase crystallization at <1 kbar, which contributes to the explanation of the “clinopyroxene paradox”.  相似文献   

10.
Martian meteorites Sayh al Uhaymir (SaU) 005 and lithology A of EETA79001 (EET-A) belong to a newly emerging group of olivine-phyric shergottites. Previous models for the origin of such shergottites have focused on mixing between basaltic shergottite-like magmas and lherzolitic shergottite-like material. Results of this work, however, suggest that SaU 005 and EET-A formed from olivine-saturated magmas that may have been parental to basaltic shergottites.SaU 005 and EET-A have porphyritic textures of large (up to ∼3 mm) olivine crystals (∼25% in SaU 005; ∼13% in EET-A) in finer-grained groundmasses consisting principally of pigeonite (∼50% in SaU 005; ∼60% in EET-A), plagioclase (maskelynite) and < 7% augite. Low-Ti chromite occurs as inclusions in the more magnesian olivine, and with chromian ulvöspinel rims in the more ferroan olivine and the groundmass. Crystallization histories for both rocks were determined from petrographic features (textures, crystal shapes and size distributions, phase associations, and modal abundances), mineral compositions, and melt compositions reconstructed from magmatic inclusions in olivine and chromite. The following observations indicate that the chromite and most magnesian olivine (Fo 74-70 in SaU 005; Fo 81-77 in EET-A) and pyroxenes (low-Ca pyroxene [Wo 4-6] of mg 77-74 and augite of mg 78 in SaU 005; orthopyroxene [Wo 3-5] of mg 84-80 in EET-A) in these rocks are xenocrystic. (1) Olivine crystal size distribution (CSD) functions show excesses of the largest crystals (whose cores comprise the most magnesian compositions), indicating addition of phenocrysts or xenocrysts. (2) The most magnesian low-Ca pyroxenes show near-vertical trends of mg vs. Al2O3 and Cr2O3, which suggest reaction with a magma. (3) In SaU 005, there is a gap in augite composition between mg 78 and 73. (4) Chromite cores of composite spinel grains are riddled with cracks, indicating that they experienced some physical stress before being overgrown with ulvöspinel. (5) Magmatic inclusions are absent in the most magnesian olivine, but abundant in the more ferroan, indicating slower growth rates for the former. (6) The predicted early crystallization sequence of the melt trapped in chromite (the earliest phase) in each rock produces its most magnesian olivine-pyroxene assemblage. However, in neither case is the total crystallization sequence of this melt consistent with the overall crystallization history of the rock or its bulk modal mineralogy.Further, the following observations indicate that in both SaU 005 and EET-A the fraction of solid xenocrystic or xenolithic material is small (in contrast to previous models for EET-A), and most of the material in the rock formed by continuous crystallization of a single magma (possibly mixed). (1) CSD functions and correlations of crystal size with composition show that most of the olivine (Fo 69-62 in SaU 005; Fo 76-53 in EET-A) formed by continuous nucleation and growth. (2) Groundmass pigeonites are in equilibrium with this olivine, and show continuous compositional trends that are typical for basalts. (3) The CSD function for groundmass pigeonite in EET-A indicates continuous nucleation and growth (Lentz and McSween, 2000). (4) The melt trapped in olivine of Fo 76 to 67 in EET-A has a predicted crystallization sequence similar to that inferred for most of the rock and produces an assemblage similar to its modal mineralogy. (5) Melt trapped in late olivine (Fo ∼ 64) in SaU 005 has a composition consistent with the inferred late crystallization history of the rock.The conclusion that only a small fraction of either SaU 005 or EET-A is xenocrystic or xenolithic implies that both rocks lost fractionated liquids in the late stages of crystallization. This is supported by: (1) high pigeonite/plagioclase ratios; (2) low augite contents; and (3) olivine CSD functions, which show a drop in nucleation rate at high degrees of crystallization, consistent with loss of liquid. For EET-A, this fractionated liquid may be represented by EET-B.  相似文献   

11.
Group II xenoliths, corresponding to the lithology of dunite, wehrlite to olivine clinopyroxenite and olivine websterite to websterite, occur in Pleisto-Holocene alkali basalts from Jeju Island, South Korea. The large grain size (up to 5?mm), moderate mg# [=100?×?Mg/(Mg?+?Fetotal) atomic ratio] of olivine (79–82) and pyroxenes (77–83), and absence of metamorphic textural features indicate that they are cumulates of igneous origin. Based on textural features, mineral equilibria and major and trace element variations, it can be inferred that the studied xenoliths were crystallized from basaltic melts enriched in incompatible trace elements and belong to the Jeju Pleisto-Holocene magma system. They appear to have been emplaced near the present Moho, an estimated 5–8?kbars beneath Jeju Island. Consolidation of cumulates was followed by infiltration of silica-enriched metasomatic melt, producing secondary orthopyroxenes at the expense of olivine. The metasomatic agent appears to have been a silica-enriched residual melt evolved from an initially slightly silica-undersaturated alkali basalt to silica-saturated compositions by fractional crystallization under relatively high pressure conditions. The result of this study indicates that relatively young olivine-bearing cumulates could have been metasomatized by a silica-enriched melt within underplates, suggesting that silica enrichment can occur in intraplate Moho-related rocks as well as in the upper mantle of the subarc area.  相似文献   

12.
Near-liquidus crystallization experiments have been carried out on two basalts (12.5 and 7.8 wt% MgO) from Soufriere, St Vincent (Lesser Antilles arc) to document the early stages of differentiation in calc-alkaline magmas. The water-undersaturated experiments were performed mostly at 4 kbar, with 1.6 to 7.7 wt% H2O in the melt, and under oxidizing conditions (ΔNNO = −0.8 to +2.4). A few 10 kbar experiments were also performed. Early differentiation of primitive, hydrous, high-magnesia basalts (HMB) is controlled by ol + cpx + sp fractionation. Residual melts of typical high-alumina basalt (HAB) composition are obtained after 30–40% crystallization. The role of H2O in depressing plagioclase crystallization leads to a direct relation between the Al2O3 content of the residual melt and its H2O concentration, calibrated as a geohygrometer. The most primitive phenocryst assemblage in the Soufriere suite (Fo89.6 olivine, Mg-, Al- and Ti-rich clinopyroxene, Cr–Al spinel) crystallized from near-primary (Mg# = 73.5), hydrous (∼5 wt% H2O) and very oxidized (ΔNNO = +1.5–2.0) HMB liquids at middle crustal pressures and temperatures from ∼1,160 to ∼1,060°C. Hornblende played no role in the early petrogenetic evolution. Derivative HAB melts may contain up to 7–8 wt% dissolved H2O. Primitive basaltic liquids at Soufriere, St Vincent, have a wide range of H2O concentrations (2–5 wt%).  相似文献   

13.
BEST  M. G. 《Journal of Petrology》1975,16(1):212-236
Rare inclusions in Holocene basanite within the western GrandCanyon are comprised of poikilitic titaniferous amphibole togetherwith variable proportions of relatively Fe-rich clinopyroxene,orthopyroxene, olivine, Cr-poor spinel, and pyropic garnet,magnesian ilmenite, and titaniferous phlogopite. No feldsparhas been found in the 219 inclusions investigated. Availableexperimental data suggest crystallization at approximately 20kb (65 km depth) in a region where the crust is 30–40km thick. On the basis of their fabric, the inclusions appear to representcumulates, but other modes of origin cannot be completely ruledout. Anhydrous grains, including some considered to be postcumulusprecipitates, experienced extensive resorption into the interstitialhydrous melt before it ultimately crystallized, perhaps 100?C below liquidus temperatures, as the poikilitic amphibole.In spite of these crystal-melt reactions, and some probablesubsolidus recrystallization as well, systematic variationsin cumulus phase compositions exist and indicate one main precipitationsequence was ol+sp, ol+sp+cpx, cpx+cpx+sp, cpx+sp, cpx+sp+ilm.The local pyropic garnet appears postcumulus in the last threecumulus assemblages. The igneous bodies represented by the inclusions comprised arelatively small portion of the upper mantle sampled by theascending basanitic magma. But in contrast to the thin amphibole-bearingveins in the mantle-derived massif at Etang de Lherz, the igneousbodies beneath the Grand Canyon are considered to be substantiallylarger in dimension, on the order of at least meters ratherthan a few centimeters. Primary nephelinite-basanite melts produced by variable butsmall degrees of partial melting of hydrous upper mantle arenot represented by the poikilitic amphiboles because of complexprocesses at the site of emplacement, including reactions betweenmelt and chromian-spinel peridotite wall rocks.  相似文献   

14.
Electron probe analyses of clinopyroxenes from several areas of the Deccan and Rajmahal Traps consisting mostly of subalkalic and alkalic basalts, picritic basalts and a few dolerite dykes have been obtained. Evaluation of the data indicate the absence of pigeonite from subalkalic basalts that occur in close spatial association with mild or strongly alkalic basalts in areas such as Rajpipla, Navagam and central Kachchh. Co-existence of augite and pigeonite, however, has been noticed in subalkalic basalts/dykes and picritic basalts from a number of Deccan localities such as Sagar, Igatpuri, Kalsubai, Triambak, Pavagarh and Girnar besides the one sample from Rajmahal. Diopside, salite, and wollastonite-rich compositions dominate the basanites and foidites of Kachchh whereas chrome-diopside and salite are the main types in the picrite basalt samples from Anila, Botad and Paliyad in Saurashtra akin to those found in contiguous areas in the east from borehole flows at Dhandhuka and Wadhwan studied in detail previously. Compositional variations in zoned clinopyroxenes indicate differentiation of the parental magma and also mixing of different magma types (subalkalic and alkalic) from areas such as Igatpuri, Rajpipla and Kachchh. Based on host-rock chemistry, total alkalis-silica plot, CIPW norms, estimated temperatures of eruption and augite – pigeonite thermometry, it has been inferred that clinopyroxene compositions, especially the incidence of pigeonite, appear to be very sensitive to bulk chemistry of host rocks, especially their Na2O, K2O, SiO2, total iron and TiO2 contents. Non-quadrilateral cationic components in the clinopyroxenes, such as Al in tetrahedral and octahedral positions together with Si, Na, Ti and Cr abundances have been found to be useful to discriminate clinopyroxenes from alkalic and subalkalic basalt types besides inferences on the ferric iron component in them. Evaluation of host-rock compositions in the ternary olivine–clinopyroxene-quartz plot indicate polybaric conditions of crystallization and evolution especially in samples that are picritic (e.g. Pavagarh, Anila and Kachchh) and which could also breach the olivine–clinopyroxene-plagioclase thermal divide that exists in part between alkalic and subalkalic basalts under atmospheric conditions.  相似文献   

15.
 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. Received: 5 January 1995 / Accepted: 13 June 1995  相似文献   

16.
Peridotite mantle xenoliths collected north of Gobernador Gregores, Patagonia, affected by cryptic and modal metasomatism bear melt pockets of unusually large size. Melt pockets consist of second generation olivine (ol2), clinopyroxene (cpx2) and spinel (sp2) ± relict amphibole (amph) immersed in a yellowish vesicular glass matrix. Amphibole breakdown was responsible for melt pocket generation as suggested by textural evidence and proved by consistent mass-balance calculations: amph → cpx2 + ol2 + sp2 + melt. Composition of calculated amphibole in amphibole-free melt pockets is very similar to that measured in amphibole-bearing melt pockets from the same xenolith, i.e. amphibole was consumed in the melt pocket generation process. In melt pockets devoid of relict amphibole, mass-balance calculations show remarkable differences between the calculated amphibole and the measured amphibole compositions in melt pockets from the same xenolith. The participation of minor proportions of a consumed reactant phase could be a reasonable explanation. In some samples the calculated phase proportion of glass is in excess compared to modal estimations based on backscattered electron images, probably because a portion of the generated melt was able to migrate out of the melt pockets. Compositional inhomogeneity of cpx2 and variable Ti Kd in cpx2 vs. glass in the same melt pocket reflect fast nucleation and growth and disequilibrium crystallisation, respectively. This and the difference between forsterite content in calculated equilibrium olivine and second generation olivine, suggest that mineral equilibrium was inhibited by rapid quenching of melt pockets.  相似文献   

17.
This paper reports experiments carried out at 1-atm under conditions of controlled oxygen fugacity, using natural andesites and andesite mixed with augite+synthetic pigeonite or augite+orthopyroxene. The experimental results are used (1) to investigate the controls of Mg# (Mg/[Mg+Fe2+]) and temperature on low-Ca pyroxene stability (pigeonite vs orthopyroxene), (2) to quantify the effects of variations in bulk composition on the position of multiple saturation boundaries in mineral component projection schemes and (3) to develop a thermodynamic model for silica activity for melts saturated with olivine and pyroxene. Over the Mg# range of 0.80–0.30 the minimum temperature of pigeonite stability in natural compositions is equivalent to the Lindsley (1983) boundary determined for pure Ca-Mg-Fe pigeonites. For the low variance, 5-phase assemblage oliv-aug-low-Ca pyroxene-plag-liquid, expressions involving liquid (Na2O+K2O)/(Na2O+K2O+CaO),Mg# and TiO2 content predict temperature and the movement of multiple saturation boundaries in pseudoternary projections in response to changing melt composition. The equilibrium for the low pressure melting of low-Ca pyroxene to olivine+liquid is formulated as a geothermometer and monitor of silica activity. Equilibrium constants estimated from thermochemical data and activities calculated for experimentally produced olivine and pyroxenes are used to develop a model for silica activity in liquid.  相似文献   

18.
Olivine phenocrysts in microporphyritic xenoliths in the St. Mesmin chondrite (LL-breccia) show parallel rimward variations of FeO (10 to > -30wt.%), CaO (0·1–0·4%), MnO (0·2–0·8%) and Cr2O3 (0·2–0·7%). Aluminum is near background levels and does not vary systematically with iron. Titanium, sodium and nickel are below the limit of detection. Covariation of Fe and Cr in this olivine distinguishes it from the olivines in lunar mare basalts, in which Cr varies inversely with Fe.Transmission electron microscopy of the St. Mesmin olivine suggests that it is free of submicroscopic inclusions and exsolution lamellae and that the chromium present occurs in solid solution in the olivine. Charge balance and ionic radius considerations suggest that it occurs as Cr2+, whose effective ionic radius is nearly identical to that of Mn2+.The different Fe-Cr relationships observed in the lunar basaltic and St. Mesmin olivines reflect different crystallization sequences. Chromian spinel coprecipitated with olivine in the lunar basalts, reducing the activity of chromium in the melt and leading to the observed anticorrelation of Fe and Cr in olivine (butler, 1972). By contrast, olivine precipitated first in the St. Mesmin microporphyry and was the only solid phase present until more than half of the rock had crystallized. Parallel variation of Fe and Cr reflects crystallization from a melt in which the activity of chromium was increasing.  相似文献   

19.
The peridotitic and gabbroic rocks described occur a) as a tectonically emplaced layered body in Piton des Neiges volcano, b) as blocks in basaltic agglomerate, Piton des Neiges, and c) as nodular inclusions in lavas of both Piton des Neiges and Piton de la Fournaise volcanoes. All are associated with the olivine basalts of the early shield-forming growth stages and not later alkaline lavas, thereby contrasting with the Hawaiian situation. Rock-types include dunite, clinopyroxenite, wehrlite, feldspathic wehrlite, olivine eucrite, allivalite, (bytownite) anorthosite and gabbro. The peridotites and most of the gabbroic rocks are inferred to be cumulates formed in floored magma chambers occurring at depths from 30 km upwards. The inclusion suite is probably derived from repetitive layered units consisting predominantly of ol + sp cumulates with sporadic development of ol + cpx±sp and ol + cpx + plag cumulate horizons.  相似文献   

20.
Pantelleria, Italy, is a continental rift volcano consisting of alkalic basalt, trachyte, and pantellerite. At 1 atm along the FMQ buffer, the least-evolved basalt (Mg #= 58.5% norm ne) yields olivine on the liquidus at 1,180° C, followed by plagioclase, then by clinopyroxene, and by titanomagnetite and ilmenite at 1,075°. After 70% crystallization, the residual liquid at 1,025° is still basaltic and also contains apatite and possibly kaersutite. A less alkalic basalt shows the same order of phase appearance. Glass compositions define an Fe-enrichment trend and a density maximum for anhydrous liquids that coincides with a minimum in Mg#.During the initial stages of crystallization at 1 atm, liquids remain near the critical plane of silica-undersaturation until, at lower temperatures, Fe-Ti oxide precipitation drives the composition toward silica saturation. Thus the qtz-normative trachytes and pantellerites typically associated with mildly ne-normative basalts in continental rifts could be produced by low-pressure fractional crystallization or by shallow-level partial melting of alkali gabbro. At 8 kbar, clinopyroxene is the liquidus phase at 1,170° C, followed by both olivine and plagioclase at 1,135°. Because clinopyroxene dominates the crystallizing assemblage and plagioclase is more albitic than at 1 atm, liquids at 8 kbar are driven toward increasingly ne-normative compositions, suggesting that higher-pressure fractionation favors production of phonolitic derivatives.Natural basaltic samples at Pantelleria are aphyric or contain 1–10% phenocrysts of plag olcpx or ol>cpx, with groundmass Fe-Ti oxides and apatite. The lack of phenocrystic plagioclase in two of the lavas suggests that crystallization at slightly higher PH2O may have destabilized plagioclase relative to the 1-atm results, but there is no preserved evidence for significant fractionation at mantle depths as clinopyroxene is the least abundant phenocryst phase in all samples and contains only small amounts of octahedral Al. The liquid line and phenocryst compositions match more closely the 1-atm experimental results than those at 8 kbar.Although major-element trends in natural liquids and crytals reflect low-pressure fractionation, minor- and trace-element concentrations preserve evidence of multiple parental liquids. Scatter in variation diagrams exceeds that attributable to crystal accumulation in these phenocryst-poor rocks, and the large range in concentrations of P and Ti at high MgO contents cannot be produced by polybaric fractionation nor by mixing with coexisting felsic magmas. Sr and O isotope ratios rule out significant interaction with crystalline upper crust, Mesozoic shelf sediments, or Tertiary evaporites. Positive correlations of compatible and incompatible elements suggest that the basalts are not simply related to one another by closed-system fractional crystallization of a single parental magma. Increasing Ce/Yb with Ce suggests that these relations are not a product of mixing within a replenished magma chamber, nor of mixing with more felsic members of the suite, which have smaller Ce/Yb ratios. Low-pressure fractional crystallization of ol+cpx+ plag±oxides from slightly different parental magmas produced by varying degrees of melting of garnet-bearing peridotite is a possible scenario.Small and infrequently replenished magma reservoirs in this continental rift environment may account for the strongly differentiated nature of the Pantellerian basalts. There is no correlation between Mg# and eruptive frequency, in part because concentration of volatiles in residual liquids offsets the effect of Fe-enrichment on melt density, such that strong Fe-enrichment is no hindrance to eruption.  相似文献   

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