Origin of Basalt Magmas: An Experimental Study of Natural and Synthetic Rock Systems

Natural basalts and eclogites were investigated experimentallyat a series of temperatures in the pressure range 1 atm to 40kb and with water pressures 1 to 10 kb. Some runs were alsomade on related synthetic systems at 10 and 33 kb. The two principal magma types recognized by field investigators—tholeiiteand alkali basalt types-appear to be separated by equilibriumthermal divides at 1 atm. The principal divides were found byexperiment at elevated pressures to give way to a new set ofequilibrium thermal divides resulting from a new mineralogy.The change of the equilibrium thermal divides with pressureleads to the derivation of the two principal magma trends fromthe same bulk composition. The melting behavior of basalts and eclogites indicates thatboth are the partial melting products of a more primitive rock(e.g. garnet peridotite). In the region of magma generation(below 60 km) the parental material, presumed to be garnet peridotite,yields an eclogitic magma and its fractionation depends on thegarnet and omphacite of the eclogite, not on plagioclase andclinopyroxene of a basaltic magma. Increase of the garnet constituentsin the magma at high pressure by effective removal of omphaciteor shift of the garnet-omphacite boundary ‘surface’will give rise to a tholeiite-type magma at low pressure. Similarly,increase of the omphacite constituents in the magma at highpressure by physical or physicochemical means will give riseto an alkali basalt-type magma at low pressure. In general,alkali basalt-type magmas are to be expected to be generatedat greater depths than tholeiite-type magmas from the same primarysource rock. Establishment of the two major basalt series takes place inthe region of generation; additional minor diversification ofeach series may come about after emplacement in or on the crustby crystal settling, oxidation or reduction, gas fluxing, contamination,and other processes. The derivative magmas are greatly restrictedby the course of liquid thermal descent imposed at generation. Pressure-temperature limits established experimentally suggestthat the basalt-eclogite transformation may be responsible forthe Mohorovii discontinuity under the continents, but not underthe oceans. The field of stability of basalt is drastically reduced in thepresence of water, and amphibolite is produced. The meltingof amphibolite takes place over a much greater range of temperaturethan basalt. At 10 kb water pressure the beginning of meltingof amphibolite closely approaches that of granite. Partial meltingof amphibolite may yield anorthositic liquids having a relativelylow anorthite content at exceptionally low temperatures. Eclogiteitself is not stable in the presence of water and gives placeto amphibolite or pyroxene hornblendite. Magmas which crystallizeto basalt, gabbro, or eclogite must have had a low water-contentat the time of crystallization. Fifteen rock and twenty-three mineral analyses as well as numerouspartial chemical analyses of experimental products were madeby J. H. Scoon in the course of the investigation. These chemicalanalyses bear on many mineralogical and petrological problems.     

Ultramafic Xenoliths from the Bearpaw Mountains, Montana, USA: Evidence for Multiple Metasomatic Events in the Lithospheric Mantle beneath the Wyoming Craton

Ultramafic xenoliths in Eocene minettes of the Bearpaw Mountainsvolcanic field (Montana, USA), derived from the lower lithosphereof the Wyoming craton, can be divided based on textural criteriainto tectonite and cumulate groups. The tectonites consist ofstrongly depleted spinel lherzolites, harzburgites and dunites.Although their mineralogical compositions are generally similarto those of spinel peridotites in off-craton settings, somecontain pyroxenes and spinels that have unusually low Al₂O₃contents more akin to those found in cratonic spinel peridotites.Furthermore, the tectonite peridotites have whole-rock majorelement compositions that tend to be significantly more depletedthan non-cratonic mantle spinel peridotites (high MgO, low CaO,Al₂O₃ and TiO₂) and resemble those of cratonic mantle. Thesecompositions could have been generated by up to 30% partialmelting of an undepleted mantle source. Petrographic evidencesuggests that the mantle beneath the Wyoming craton was re-enrichedin three ways: (1) by silicate melts that formed mica websteriteand clinopyroxenite veins; (2) by growth of phlogopite fromK-rich hydrous fluids; (3) by interaction with aqueous fluidsto form orthopyroxene porphyroblasts and orthopyroxenite veins.In contrast to their depleted major element compositions, thetectonite peridotites are mostly light rare earth element (LREE)-enrichedand show enrichment in fluid-mobile elements such as Cs, Rb,U and Pb on mantle-normalized diagrams. Lack of enrichment inhigh field strength elements (HFSE; e.g. Nb, Ta, Zr and Hf)suggests that the tectonite peridotites have been metasomatizedby a subduction-related fluid. Clinopyroxenes from the tectoniteperidotites have distinct U-shaped REE patterns with strongLREE enrichment. They have ¹⁴³Nd/¹⁴⁴Nd values that range from0·5121 (close to the host minette values) to 0·5107,similar to those of xenoliths from the nearby Highwood Mountains.Foliated mica websterites also have low ¹⁴³Nd/¹⁴⁴Nd values (0·5113)and extremely high ⁸⁷Sr/⁸⁶Sr ratios in their constituent phlogopite,indicating an ancient (probably mid-Proterozoic) enrichment.This enriched mantle lithosphere later contributed to the formationof the high-K Eocene host magmas. The cumulate group rangesfrom clinopyroxene-rich mica peridotites (including abundantmica wehrlites) to mica clinopyroxenites. Most contain >30%phlogopite. Their mineral compositions are similar to thoseof phenocrysts in the host minettes. Their whole-rock compositionsare generally poorer in MgO but richer in incompatible traceelements than those of the tectonite peridotites. Whole-rocktrace element patterns are enriched in large ion lithophileelements (LILE; Rb, Cs, U and Pb) and depleted in HFSE (Nb,Ta Zr and Hf) as in the host minettes, and their Sr–Ndisotopic compositions are also identical to those of the minettes.Their clinopyroxenes are LREE-enriched and formed in equilibriumwith a LREE-enriched melt closely resembling the minettes. Thecumulates therefore represent a much younger magmatic event,related to crystallization at mantle depths of minette magmasin Eocene times, that caused further metasomatic enrichmentof the lithosphere. KEY WORDS: ultramafic xenoliths; Montana; Wyoming craton; metasomatism; cumulates; minette     

Size Selective Predation by the Asteroid Pisaster ochraceus on the Bivalve Mytilus calif ornianus: A Cost-Benefit Analysis

Abstract. When simultaneously presented with equal numbers of four sizes of clumped Mytilus californianus (20, 35, 55 and 85 mm shell length), Pisaster ochraceus (arm length of 70-110mm R) showed a preference for the medium sizes (35 and 55mm length). Prey-size preference was positively correlated with predator size. Mean prey-digestion times (insertion of the stomach, opening of the valves and digestive period) decreased significantly following fiiing of an artificial gape between the valves of mussels. When presented with different sizes of clumped filed mussels, starfish still chose medium mussels, indicating that stomach insertion, valve opening, and digestion rate may not be important constraints determining size selective predation. Measurements of the force required to dislodge mussels from the substratum and conspecifics showed an increasing exponential relationship with mussel size. Size selective feeding may be an energy maximizing and time minimizing strategy, where energy gained from choosing larger mussels is balanced against disproportionately increased costs in energy and time of removing large mussels from the substratum.     

Flow properties of turbidity currents in Bute Inlet, British Columbia

Bute Inlet, a fiord along the southwestern coast of British Columbia, Canada, includes a sea-floor sedimentation system 70 km in length which resembles those developed on some large submarine fans. Turbidity currents originate at the head of the flord on the submerged delta fronts of the Homathko and Southgate rivers. They move downslope for about 30 km within a single large incised channel, spill onto a depositional area termed the channel lobe complex, and finally spread out over a low-relief distal splay area that passes 55 km downslope into a flat basin floor. During the present study, turbidity currents in Bute Inlet were studied using sea-floor morphology, bottom sediment distribution, and in-situ instrument packages. The mean velocities of the most recent flows, estimated from surface sediment grain size, has varied between 100–120 cm s^–1 in the incised channel, 20–50 cms^–1 in the channel lobe complex, and < 5 cm s^–1 on the basin floor. Velocities based on channel morphology are poorly constrained but are in the range of 160-425 cm s^–1 in the upper part of the incised channel and 66 cm s^–1 in the lower channel. Calculated flow densities range from 1.049 to 1.028g cm^–3. Turbidity flows monitored in 1986 using submerged instrument packages exceeded 32 m in thickness in the upper part of the incised channel, where the maximum measured velocity was 330 cm s^–1. At the head of the channel lobe complex the maximum velocity had declined to 75 cm s^–1. The density of the monitored flows is estimated at 1.025-1.03g cm^–3. The cored sediments and channel morphology yield estimates of mean flow velocities that are generally greater than those measured by the in-situ instrument packages and estimated from modern surface sediments. The former suggest past flow velocities up to 500 cm s^–1 in the incised channel, about 20 cm s^–1 in spillover deposits along the lower part of the incised channel, and 100-140 cm s^–1 in the distal splay. The contrast between the velocities of modern and past flows suggests that past flows may have been considerably larger and more energetic than those presently occurring in Bute Inlet. The size properties of sediments in the monitored turbidity flows suggest a strong vertical size gradient in the suspended load during transport. The surface and cored sediments fine downslope from the channel lobe complex to distal splay area. Distinctive sedimentary sequences are recognized in cores from the spillover lobes, channel lobe complex, distal splay, and basin floor depositional areas. Many individual turbidites grade downslope from massive T_a divisions in the channel lobe complex and probably in the incised channel to T_a divisions overlain by slurried divisions on the distal splay and largely slurried beds on the basin floor. These facies suggest that individual currents commonly evolve from largely cohesionless suspensions in the incised channel and channel lobe complex to dilute cohesive slurries downslope on the distal splay and basin floor. Many flows in Bute Inlet fail to develop a traction state of sedimentation and the resulting turbidites lack well-developed T_b. T_c, and T_d divisions.     

Oxygen and Carbon Isotope Studies of Contact Metamorphism of Carbonate Rocks

O¹⁸/O¹⁶ and C¹²/C¹³ ratios have been determined for carbonaterocks and coexisting minerals from two contact metamorphic aureolesat Birch Creek, California and Marble Canyon, Texas. The peliticmetasediments and granitic intrusions of the Birth Creek localitywere also analyzed for their O¹⁸/¹⁶ and D/H ratios. Oxygen andcarbon isotope fractionations in coexisting dolomite and calciteare interrelated but show no obvious correlation with sampledistance from the intrusive contact. Small-scale oxygen isotopicexchange effects between rock units are observed within a fewfeet of the intrusive-country rock contacts and the marble-schistcontacts at Birch Creek. Oxygen isotopic temperatures obtainedfrom quartz—biotite fractionations in the biotite schistsof Birch Creek show a systematic decrease with increasing distanceaway from the intrusive contact; the isotopic contact temperatureis calculated to be 535–45°C. Low O¹⁸/¹⁶ and C¹³/¹²ratiosof the contact metamorphic marbles generally correlate wellwith presence of calc-silicate minerals, indicating that theCO₂ liberated during metamorphic decarbonation reactions isenriched in both O¹⁸and C¹² relative to the carbonates. Materialbalance calculations indicate that the liberated CO₂ is about5 per mille richer in O¹⁸ and about 6 per mille richer in C¹³than coexisting calcite.     

新疆博格达造山带晚古生代构造-岩浆演化过程:火山岩组合及地球化学证据

最近,笔者在博格达山东段北部的西地-伊齐-小红柳峡一带的地质调查中发现,该区发育有大量晚石炭世柳树沟组双峰式火山岩及早二叠世卡拉岗组酸性火山岩建造,但双峰式火山岩性质及成因有别于其南侧七角井早石炭世双峰式火山岩。玄武岩富钠贫钾(K_2O=0.18%～0.45%,Na_2O=2.24%～3.63%),属拉斑系列;TiO_2=1.6%～1.7%,略高于MORB,较高的Al(Al_2O_3=16.2%～16.7%)、高Mg(MgO=8.12%～9.54%,Mg~#=61～64),以及低K_2O/TiO_2和K_2O/P_2O_5比值(分别为0.1～0.27、0.63～1.68),反映了在岩浆演化过程中分离结晶作用不明显;Rb/Sr比值0.01～0.02,Zr/Nb=21.6～39.7,Zr/Y=5.38～7.47,以及不相容元素Ba、Zr、Hf相对略富集、Nb-Ta和Th相对亏损,显示岩石具有板内玄武岩的特点;稀土元素球粒陨石标准化配分图上整体接近于平坦型,(La/Yb)_N=1.8～1.9,Eu无异常至轻微正异常(δEu=1.07～1.12),正ε_(Nd)(t)值(+5.63～+5.89),(~(143)Nd/~(144)Nd)_I=0.512927～0.512944,Th/Yb0.2,Ta/Yb=0.1,表明玄武岩浆源于亏损软流圈地幔,且在演化过程中不曾发生过斜长石的分离结晶作用,并暗示当时的大陆地壳可能由于拉张而变得较薄,玄武岩浆形成后快速上升至地表喷发。双峰式火山岩中的流纹岩Rb-Sr等时线年龄为296±2Ma(1σ),具高Si(SiO_2=76%～80%),富钾贫钠(K_2O=5.1%～5.7%,Na_2O=0.94%～2.03%);低Al(Al_2O_3=7.9%～10.4%);低Ti、Ca和P含量,属高钾钙碱性系列;微量元素Rb、Th、Zr、Hf、K相对富集,Ba、Sr、P、Ti、Nb、Ta为显著亏损;轻稀土元素适度富集且轻、重稀土分馏程度低,(La/Yb)_N=5.1～7.1,(La/Sm)_N=2、(Gd/Yb)_N=1.6～2.2,以及强烈的负Eu异常(δEu=0.17～0.2),(~(87)Sr/~(86)Sr)_I=0.7051～0.7052,δ~(18)O=11.6‰,指示岩石源于地壳物质的部分熔融,源区存在有斜长石残留,形成于大陆裂谷环境。早二叠世末流纹岩(Rb-sr等时线年龄为278±2Ma)具高Si(SiO_2=74%),富钾贫钠(K_2O/Na_2O2),低Al(Al_2O_3=11.0%)以及较低的Ti和P含量的特征,岩石为高钾钙碱性系列;微量元素PM标准化图解上表现为Rb、Ba、Th、U、K、La、Ce不相容元素相对富集,高场强元素Nb、Ta、P、Ti以及Sr为明显的负异常;轻稀土轻度富集,(La/Yb)_N=5～6,(La/Sm)_N=3,(Gd/Yb)_N=1.3～1.4,以及强烈的负Eu异常(δEu=0.31～0.39),(~(87)Sr/~(86)Sr)_I为0.7069,δ~(18)O=11.97‰,指示源于地壳物质部分熔融的产物,形成于伸展垮塌的构造环境。综合研究结果表明,博格达山前身裂谷岩浆作用始于早石炭世,结束于晚石炭世末期,早二叠世末进入后造山伸展的演化阶段。     

The Cansiwang Melange of Southeast Bohol (Central Philippines): Origin and tectonic implications

Abstract The Cansiwang Melange underlies the Southeast Bohol Ophiolite Complex (SEBOC) and is composed mainly of sheared ophiolite-derived blocks such as harzburgites, microgabbros, basalts and cherts in a pervasive serpentinite matrix. Available field, as well as geophysical evidence show that this melange unit is not diapiric, nor does it have a sedimentary origin considering that it lacks slump and flow structures. A tectonic origin for the Cansiwang Melange is favored in view of the numerous thrust faults, which cut across the exposures, as well as the tectonic contacts that the melange has with the overlying and underlying formations. The presence of the Cansiwang Melange in between the SEBOC and the Alicia Schist provides evidence that the amphibolite of the Alicia Schist do not correspond to the metamorphic sole of SEBOC. Similar to what is recognized in the Josephine Ophiolite, this suggests a 'cold' emplacement of the ophiolite over the Alicia Schist. The Cansiwang Melange represents an accretionary prism product which marks the location of an ancient subduction zone in what is now Central Philippines.     

Two new eucrite breccias from Northwest Africa

Abstract– This work describes two newly discovered eucrite breccias: three presumably paired meteorites, all named Northwest Africa (NWA) 6105, and NWA 6106. For each meteorite, major‐ and minor‐element compositions of minerals were determined using the electron microprobe. Pyroxene Fe‐Mn co‐variations and bulk‐rock oxygen isotope compositions confirm their classification as eucrites. Variations in mineral compositions and textures are attributed to differences in clast types present (i.e., basaltic or cumulate eucrite). The pyroxene compositions support the hypothesis that samples NWA 6105,1; 6105,2; and 6105,3 are paired polymict eucritic breccias, whereas sample NWA 6106 is a monomict basaltic eucritic breccia. Two‐pyroxene geothermometry yields temperatures too low for igneous crystallization. The variation in temperatures among samples suggests that metamorphism occurred prior to brecciation.     

GEOMORPHOLOGY OF THE LAKE SHEWA LANDSLIDE DAM,BADAKHSHAN, AFGHANISTAN,USING REMOTE SENSING DATA

Lake Shewa in northeastern Badakhshan, Afghanistan, was dammed sometime in antiquity when a large rock avalanche (sturzstrom) from the fault‐shattered and strongly weathered Archean gneisses of the Zirnokh peaks to the north moved into the Arakht River valley. This rock avalanche dammed up the river and its tributaries to a dam thickness of c. 400 m, producing a 12‐km‐long lake that is as much as 270 m deep, leaving c. 80 m of freeboard to the top of the dam. At least four separate instances of slope failure have been mapped at the site of the landslide dam, as well as a rock glacier, using remotely sensed data, historical maps, and Google Earth?. Spring seepage through the dam face has caused several recent subsidiary debris slides, which if continued at a large enough scale for long enough, or with additional seismicity from the active strike‐slip faults that cross beneath the landslide dam, could threaten its integrity. Otherwise the clean water that emerges from the dam face could be the source of an unvarying mini‐hydroelectric power source, in addition to the agricultural irrigation that it provides at the present time.     

Anorthite--Forsterite and Anorthite--Enstatite Reactions and their bearing on the Basalt--Eclogite Transformation

The reactions betweeon anorthite and forsterite and betweenanorthite and enstatite have been studied at high pressuresand high temperatures with solid-media and gas-media apparatus.For the 1: 1 (molecular ratio) composition of anorthite andforsterite the following three assemblages have been obtainedwith increasing pressure at a constant temperature: anorthiteforsterite,clinopyroxeneorthopyroxenespinelanorthite, and garnet (pyropegrossulariteseries). Two univariant curves between these three assemblageshave been determined. The amounts of pyroxenes relative to spineland anorthite in the second assemblage increase and the contentsof the Tschermak's components in the pyroxenes increase withincreasing pressure. The 1:2 (molecular ratio) composition of anorthite and forsteritewas also studied, and the univariant curve for the reactionclinopyroxeneorthopyroxenespinel garnet (pyropegrossulariteseries)forsterite has been determined. For the 1: 2 (molecular ratio) composition of anorthite andenstatite the following assemblages have been obtained withincreasing pressure at a constant temperature: anorthiteenstatite,anorthiteorthopyroxeneclinopyroxenequartz, and garnet (pyrope-grossulariteseries)quartz. The univariant curve between the last two assemblageshas been determined. Both the pyroxenes in the second assemblagecontain Tschermak's components, which increase in content withincreasing pressure at the expense of anorthite. These experimental results suggest that basalts, either saturatedor undersaturated with silica, transform into aluminous pyroxene-richassemblages at lower pressures or higher temperatures than thestability field of eclogites, and that pyroxene-spinel symplectitesfound in some metamorphosed gabbro, troctolite, and norite mayhave formed in the stability field of the pyroxene-rich assemblages.The stability field of the pyroxene-rich assemblages is probablyincluded within the pressure-temperature conditions of the granulitefacies. It is also suggested that the garnet-free peridotites,such as the peridotite inclusions in basaltic rocks and theintrusive peridotites in the orogenic belts, are stable at lowerpressures or higher temperatures than garnet peridotites, suchas the inclusions in kimberlite and intrusive garnet peridotitein the orogenic belts.