10 research outputs found
Petrological relationships among lavas, dikes, and gabbros from Integrated Ocean Drilling Program Hole 1256D: Insight into the magma plumbing system beneath the East Pacific Rise
No full textA continuous section from extrusive lavas, through sheeted dikes, and uppermost gabbros recovered from Integrated Ocean Drilling Program Hole 1256D provides important information regarding magma plumbing systems beneath superfast spreading ridges. Petrological examination demonstrates that a model of fractional crystallization from a magma of composition similar to one of the more primitive gabbros in a shallow (?50–100 MPa) melt lens reasonably explains mineral and whole rock compositions of many lavas and dikes. Elevated concentrations of trace elements in some rocks appear to have resulted from mixing between primitive magma and highly evolved magma. About half of the dike samples have more evolved Fe-rich compositions than the extrusive lava samples. Magma densities of the Fe-rich dikes are a little higher (?30 kg/m3) than those of lavas, suggesting that these dike magmas would not reach the surface. Mineralogical investigations reveal that both lavas and dikes contain oscillatory zoned plagioclase xenocrysts, implying magma mixing caused by successive episodes of fractionation and magma replenishment in the melt lens. The plagioclase xenocrysts contain high-Anorthite sections [An: 100 × Ca/(Ca+Na) in mole percent] whose compositions are not in equilibrium with host liquids. The high-An sections were likely crystallized when primitive magmas with high CaO/Na2O were injected in the melt lens. Since the oscillatory zoned plagioclase generally forms crystal clots, they were probably accumulated in a mush zone. The petrographical examination favors a model suggesting that injection of primitive magma into the melt lens broke the mush zone and pushed out the oscillatory zoned plagioclase
Eruption of the Grande Ronde Basalt lavas, Columbia River Basalt Group: Results of numerical modeling
No full textOrigin of the Grande Ronde Formation flows, Columbia River flood basalt group
No full textLavas belonging to the Grande Ronde Formation (GRB) constitute about 63% of the Columbia River Basalt Group (CRBG), a flood basalt province in the NW United States. A puzzling feature is the lack of phenocrysts (\u3c 5%) in these chemically evolved lavas. Based mainly on this observation it has been hypothesized that GRB lavas were nearly primary melts generated by large-scale melting of eclogite. Another recent hypothesis holds that GRB magmas were extremely hydrous and rose rapidly from the mantle such that the dissolved water kept the magmas close to their liquidi. I present new textural and chemical evidence to show that GRB lavas were neither primary nor hydrous melts but were derived from other melts via efficient fractional crystallization and mixing in shallow intrusive systems. Texture and chemical features further suggest that the melt mixing process may have been exothermic, which forced variable melting of some of the existing phenocrysts. Finally, reported here are the results of efforts to simulate the higher pressure histories of GRB using COMAGMAT and MELTS softwares. The intent was to evaluate (1) whether such melts could be derived from primary melts formed by partial melting of a peridotite source as an alternative to the eclogite model, or if bulk melting of eclogite is required; and (2) at what pressure such primary melts could have been in equilibrium with the mantle. I carried out both forward and inverse modeling. The best fit forward model indicates that most primitive parent melts related to GRB could have been multiply saturated at ∼1.5--2.0 GPa. I interpret this result to indicate that the parental melts last equilibrated with a peridotitic mantle at 1.5--2.0 GPa and such partial melts rose to ∼0.2 GPa where they underwent efficient mixing and fractionation before erupting. These models suggest that the source rock was not eclogitic but a fertile spinel lherzolite, and that the melts had ∼0.5% water
Cycles of ∼32.5 My and ∼26.2 My in correlated episodes of continental flood basalts (CFBs), hyper-thermal climate pulses, anoxic oceans, and mass extinctions over the last 260 My: Connections between geological and astronomical cycles
No full textPotential temporal and causal connections among various geologic events have long been discussed in the geological literature. More recently, signs of common periodicities in these episodes have been reported. In this study of correlation and cyclicity of geologic occurrences, we review and synthesize previous work, and utilize the newest data for various major events over the the last 260 My. These include, 1) high-quality radio-isotopic age determinations (U-Pb zircon and 40Ar/39Ar) for continental flood-basalt (CFB) eruptions; 2) the dates of widespread intervals of ocean anoxia; 3) the latest published dates of marine and non-marine extinction events, 4) hyper-thermal climate intervals and 5) the occurrences of stratigraphic Hg anomalies, and non-radiogenic Os-isotope anomalies as potential proxies for large-scale basaltic volcanism.
Times of at least 13 of 17 intervals of anoxic oceans are marked by stratigraphic Hg-anomalies, pointing to contemporaneous LIP eruptions, and 5 anoxic intervals in the warm Cretaceous Period are correlated with marine Os-isotope ratios suggesting potential LIP hydrothermal activity. Nine of the ocean-anoxic intervals are thus far correlated with times of marine-extinction episodes, and 8 of those anoxia/extinction co-events are significantly correlated with the ages of the well-dated CFB eruptions. Seven of the marine-extinction events and associated CFB volcanism are coeval with extinctions of non-marine vertebrates, supporting global catastrophic volcano-climatic episodes devastating both marine and terrestrial environments.
New digital circular spectral analyses revealed significant underlying cycles of ∼32.5 My and ∼ 26.2 My in the ages of the anoxic events and marine extinctions. Spectral analysis of the latest high-quality ages of the CFBs resulted in similar significant periodicities of 32.8 My and 12.9 My (∼26.2/2 My harmonic). High-frequency periods at various harmonics appear at ∼6.4 My, 8.4 My and 9.7 My in each of the three spectra.
These findings support a multi-factor extinction scenario in which release of massive amounts of CO2 and perhaps CH4 mostly from CFB magmas (and in some cases sub-volcanic intrusions into carbon-rich deposits), led to very warm climate intervals with near-lethal to lethal hyper-thermal conditions on land and in the sea. Concurrent release of halogens from CFB eruptions could also have decimated the global ozone layer. In many cases, the warm oceans became acidic, and developed anoxic to euxinic conditions, even up to the ocean surface, contributing to the causes of the marine extinctions. Additionally, four extinction events (late Eocene, end-Cretaceous, end-Jurassic and mid-Norian) correlate closely with the ages of the 4 largest impacts (craters ≥100 km in diameter) over the same period, capable of producing severe climatic effects and extinctions.
The potential dominant underlying ∼33-My and 26-My cycles, reported in these and other correlated tectonic, climatic, and biotic events over the last 260 My and beyond, are likely related to the Earth's tectonic-volcanic rhythms, but the similarities with known Milankovitch Earth orbital periods and their amplitude modulations, and with known Galactic cycles, suggest that, contrary to conventional wisdom, the geological events and cycles may be paced by astronomical factors
Global nickel anomaly links Siberian Traps eruptions and the latest Permian mass extinction
No full textAbstractAnomalous peaks of nickel abundance have been reported in Permian-Triassic boundary sections in China, Israel, Eastern Europe, Spitzbergen, and the Austrian Carnic Alps. New solution ICP-MS results of enhanced nickel from P-T boundary sections in Hungary, Japan, and Spiti, India suggest that the nickel anomalies at the end of the Permian were a worldwide phenomenon. We propose that the source of the nickel anomalies at the P-T boundary were Ni-rich volatiles released by the Siberian volcanism, and by coeval Ni-rich magma intrusions. The peaks in nickel abundance correlate with negative δ13C and δ18O anomalies, suggesting that explosive reactions between magma and coal during the Siberian flood-basalt eruptions released large amounts of CO2 and CH4 into the atmosphere, causing severe global warming and subsequent mass extinction. The nickel anomalies may provide a timeline in P-T boundary sections, and the timing of the peaks supports the Siberian Traps as a contributor to the latest Permian mass extinction.</jats:p
Rock Magnetic Characterization Through an Intact Sequence of Oceanic Crust, IODP Hole 1256D
No full textCoring at Site 1256 (6.736◦N, 91.934◦W, 3635 m water depth) during Ocean Drilling
Program (ODP) Leg 206 and Integrated Ocean Drilling Program (IODP) Expeditions
309 and 312 successfully sampled a complete section of in situ oceanic crust, including
sediments of Seismic Layer 1, lavas and dikes of Layer 2, and the uppermost
gabbros of Layer 3. The crust at this site was generated by superfast seafloor spreading
(>200 mm/yr full spreading rate) along the East Pacific Rise some 15 Ma ago.
One goal of drilling a complete oceanic crust section is to determine the source of
marine magnetic anomalies. For crust generated by fast seafloor spreading, is the
signal dominated by the upper extrusive layer, do the sheeted dikes play any role,
how significant is the magnetic signal from gabbros relative to that at slow spreading
centers and what is the timing of acquisition of the magnetization? To address
these questions, we have made a comprehensive set of rock magnetic and paleomagnetic
measurements that extend through the igneous interval. Continuous downhole
variations in magnetic grain size, coercivity, mass-normalized susceptibility, Curie
temperatures, and composition have been mapped. Compositionally, we have found
that the iron oxides vary from being titanium-rich titanomagnetite (TM60), which
are commonly partially oxidized to titanomaghemites, to titanium-poor magnetite
as determined semi-quantitatively from Curie temperature analyses and microscopy
studies. Skeletal titanomagnetite with varying degrees of alteration is the most common
magnetic mineral throughout the section and is often bordered by large iron
sulfide grains. The low-Ti magnetite or stoichiometric magnetite is present mainly
in the dikes and gabbros and is associated with higher Curie temperatures (550◦C
to near 580◦C) and higher coercivities than in the extrusive section. Magnetic grain
sizes predominantly fall in the pseudo single domain (PSD) grain size region on Day
diagrams, with only a small numbers of samples falling within the single domain (SD) or multi-domain (MD) regions. Overall the magnetic properties of this
hole are strongly influenced by post-emplacement alteration, particularly the lower
part of the section from the gabbros up into the transition zone. Some of the more
prominent features of the rock magnetic data are the gradual increase in Curie
temperatures with depth from about 200–350◦C at the top of the extrusives to about
425◦C just above the transition zone, the more variable Curie temperatures and less
variable susceptibility and coercivity of remanence in the upper half of the extrusives
relative to the lower half the near constant composition (x = 0.6) and oxidation
(z = 0.6) of the iron oxide grains (>5νm) in the extrusives (Chapter 12 this volume),
the highly irreversible nature of thermomagnetic curves in the extrusives, in which
the cooling curve has Curie temperatures higher (generally > 500◦C) than indicated
by the heating curve, the abrupt change in rock magnetic properties across the
transition zone, particularly the Curie temperature., a somewhat finer grain size and
increased intensity in the sheeted dike zone relative to the extrusives and gabbros,
and the nearly constant Curie temperatures (530 and 585◦C) for the dikes and
gabbros
Proceedings of the Integrated Ocean Drilling Program 309/312: Superfast Spreading Rate Crust 2 and 3
No full text<p>The Superfast Spreading Rate Crust mission is a multicruise program to drill, for the first time, a complete section of the upper oceanic crust from the extrusive lavas, through the dikes, and into the underlying gabbros. Hole 1256D was initiated during Ocean Drilling Program Leg 206 in the eastern equatorial Pacific and is drilled into 15 Ma crust that formed at the East Pacific Rise during a period of superfast spreading (>200 mm/y). This site is chosen to exploit the inverse relationship between spreading rate and the depth to axial low-velocity zones, thought to be magma chambers now frozen as gabbros, observed from seismic experiments. During Integrated Ocean Drilling Program (IODP) Expedition 309, Hole 1256D was deepened to a total depth of 1255 m below seafloor (mbsf) (1005 m subbasement), penetrating >800 m of extrusive normal mid-ocean-ridge basalt, and entering the sheeted dike complex. Expedition 312 deepened Hole 1256D to 1507.1 mbsf. The hole now extends through the 346 m sheeted dike complex and 100.5 m into the upper portions of the plutonic section. The uppermost crust at Site 1256 comprises a >74 m thick ponded lava overlying massive, sheet, and minor pillow flows, some of which exhibit inflation structures requiring eruption onto a subhorizontal surface, suggesting 284 m thickness of off-axis lavas. Sheet and massive lava flows make up the remaining extrusive section above subvertical cataclastic zones, intrusive contacts, and mineralized breccias denoting a lithologic transition zone. Below 1061 mbsf, massive basalts, some with doleritic textures, dominate the sheeted dikes, which exhibit increased thermal conductivity and P-wave velocity. Numerous subvertical dikes, commonly with brecciated and mineralized chilled margins, crosscut the sheeted dikes. The upper dikes (<1255 mbsf) contain greenschist-facies minerals, actinolite becomes abundant below ~1300 mbsf, and hornblende and secondary plagioclase are present below ~1350 mbsf, reflecting a steep thermal gradient in the dikes.</p>
Drilling to gabbro in intact ocean crust
No full textSampling an intact sequence of oceanic crust through lavas, dikes, and gabbros is necessary to advance the understanding of the formation and evolution of crust formed at mid-ocean ridges, but has been an elusive goal of scientific ocean drilling for decades. Recent drilling in the eastern Pacific Ocean in Hole 1256D reached gabbro within seismic layer 2, 1157 meters into crust formed at superfast spreading rate. The gabbros are the crystallized melt lenses that formed beneath a mid-ocean ridge. The depth at which gabbro was reached confirms predictions extrapolated from seismic experiments at modern ridges that melt lenses occur at shallower depths at faster spreading rates. The gabbros intrude metamorphosed sheeted dikes and have compositions similar to the overlying lavas, precluding formation of the cumulate lower oceanic crust from melt lenses so far penetrated by Hole 1256D
