Determining the inputs to the Mariana Subduction Factory: Using core‐log integration to reconstruct basement lithology at ODP Hole 801C
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Abstract:
Calculating elemental mass balance across subduction zones enhances our understanding of global geochemical budgets and large‐scale Earth processes. However, to accurately constrain the input flux, it is critical to know the lithological diversity and chemical characteristics of the downgoing oceanic plate. The west Pacific altered ocean crust that was drilled during Ocean Drilling Program (ODP) Leg 185 represents a significant component of the input to the Mariana Subduction Factory. The lithological sequence in Hole 801C consists of aphyric basalt, occurring as thick massive units, pillow units, and breccia units. The shallowest basalts are intercalated with sediments and two hydrothermal deposits. Core recovery was good for a basement hole (average 47%); however, over half the lithological section was unaccounted for. Downhole logging data provide a continuous record of physical, chemical, and structural properties of the rocks at the borehole wall, thus, when calibrated using available cored material, they can be used to reconstruct lithology in unrecovered intervals. Core‐log integration results reveal a significant bias in core recovery, with infrequent retrieval of delicate breccia units and preferential recovery of more massive, competent, and less altered flow units. This is important because the breccia units are host to many of the key tracer elements used in mass balance calculations. The massive basalts exhibit high density, resistivity and velocity values and low porosity and gamma ray values. Formation MicroScanner (FMS) images of massive basalts are bright (reflecting their resistive nature) with a homogenous texture and regular fracture pattern. Breccia or pillow basalts are characterised by low resistivity, density and velocity, and high porosity and gamma ray values and unrecovered intervals displayed these same characteristics. The reconstructed log‐based lithological sequence consists of thick massive flow units (27.4%), pillow units (33%), breccia units (31%), sediments (1.4%), and hydrothermal deposits (1.3%), with 5.9% unclassified due to unreliable tool response in intervals where hole conditions were poor. These findings have a significant bearing on the Subduction Factory recycling equation. The proportion of pillow basalts doubled and the amount of breccia increased six‐fold from that reported using core description alone, demonstrating convincingly that core‐log integration is essential to provide an accurate representation of the input flux. The log‐based stratigraphy reconstructed for Hole 801C represents the first example of Jurassic fast‐spread (160 km/m.y.) ocean crust and provides constraints on the relationships between crustal structure, age, alteration, and spreading rate.Keywords:
Breccia
Lithology
Pillow lava
Basement
Pillow breccias consist of whole or disaggregated volcanic pillows in an abundant matrix of cogenetic basic tuff. They form in water and are thought to be common rocks, confused at times with other volcanic breccias. On Quadra Island, British Columbia, the breccias usually overlie ordinary pillow lava which rests in turn upon basalt flows or upon pelagic limestone. In the transition to isolated-pillow breccia the matrix increases gradationally upward to as much as 80 per cent; the pillows become isolated, diversiform, and ill-sorted. Some pillows were broken after settling but before compaction. The matrix, consisting of cogenetic globules, granules, shards, and basaltic fragments produced beneath water, is an aquagene tuff. Subaqueous shard formation is discussed. Broken-pillow breccia forming the top of the successions consists of unsorted pillow fragments set in similar aquagene tuff which is irregularly flow-laminated and somewhat resembles an ignim-brite. Laminated aquagene tuff beds with load casts in their upper surfaces occur within broken-pillow breccia. Ordinary pillow lava accumulated in relatively clear water, isolated pillows in increasingly turbid, vapor-charged water. Increasing thicknesses of incoherent, steam-laden pillows and tuff led to instability, subaqueous slumping, and fragmentation.
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Pillow lava
Slumping
Lapilli
Diamictite
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Seafloor Spreading
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Breccia
Phyllite
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Introduction and aim of study: Relatively shallow marine successions of Early Cambrian age are preserved at many places along the Caledonian Mountain chain in Sweden and Norway. In the Tornetrask area, northern Sweden, the local Lower Cambrian deposits, up to about 100 m thick, are assigned to the Tornetrask and Grammajukku Formations [1, 2]. The Tornetrask Fm rests unconformably on the basement and comprises a number of sandand siltstone dominated members signaling changes in sea level during deposition. In this part of the sequence an enigmatic occurrence of a mostly 2-4 m thick, crystalline-rich, polymict breccia, the Vakkejokk Breccia (VB), has long attracted the attention of geologists working in the area [1, 3, 4]. The breccia occupies a stratigraphic position between the ‘Lower Siltstone mbr’ (LSM) and the ‘Red and Green Siltstone mbr’ (RGSM) of the Tornetrask Fm. The lower boundary is erosive and locally the breccia even rests directly on the basement. The VB contains a mixture of clasts of Lower Cambrian sediments as well as the crystalline basement. The clast size varies from gravel to boulders and some of the larger basement blocks exceed 100 m in length although being less than 10 m thick, but even basements rafts > 200 m long have been reported [4]. The VB has been reported from a series of exposures on the northern side of Lake Tornetrask, where it is semi-continuously exposed between the Vakkejokk and Tjaurajokk rivers, i.e. for a stretch of about 7 km [1, 4]. It can be traced further eastwards in disjunct river sections for another c. 7 km [cf. 1]. Thin conglomerates in the same stratigraphic position occur also south of Lake Tornetrask, e.g., at Luopakte, possibly representing an equivalent to the VB [1, 4], but may just as well be normal sedimentary conglomerates, marking a sequence boundary [cf. 2]. The breccia has previously been interpreted as a tillite or being fault-related [see summary by 1, 2]. However, the latter authors found the previous explanations unsatisfactory due to the rather local distribution and in particular the occurrence of the very large basement boulders and suggested the breccia to be impact related. This re-interpretation sparked the new investigations reported here. Methods: The study area was visited by Ormo and Nielsen during a one week field campaign in the summer of 2012 in order to study field relations of the VB and to collect samples that potentially could provide evidence of shock diagnostic for impact. Thin sections of VB samples obtained during the 2012 field campaign were studied by Alwmark using a Leitz 5-axes universal stage [5] mounted on an optical microscope in search of planar deformation features (PDFs) in quartz grains. The crystallographic orientations of identified PDFs were determined according to techniques described in [6, 7]. Results and discussion: Shock metamorphic features, in the form of PDFs, have been found in six quartz grains from one of the breccia samples (Fig. 1). Of the six shocked grains, four display a single set of PDFs, all oriented parallel to the basal plane c (0001). Two grains contain two sets; one set oriented along (0001) and one parallel to crystallographic plane ω {10 3}.
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Isotopes of boron
Metasomatism
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A basaltic tuff formation (Upper Basaltic Tuff of the Janggi Group) occurs in close association with basalt (Yeonil Basalt) at the Tertiary Janggi basin. The purpose of this paper is to describe the occurrence of the basaltic tuff and associated basalt and to determine their mode of formation. The basaltic rocks of the study area show four distinct lithofacies, all of which are originated from the interaction of basaltic magma with external water. The four lithofacies include (l) sideromelane shard hyaloclastite, (2) pillow breccia, (3) entablature-jointed basalt, and (4) in-situ breccia. The sideromelane shard hyaloclastite constitutes most of the Upper Basaltic Tuff and has a gradual contact with the pillow breccia. The pillow breccia consists of a poorly sorted mixture of isolated and broken pillows, and small basalt globules and fragments engulfed in a volcanic matrix of sideromelane shard hyaloclastite. The entablature-jointed basalt occurs as a small body within the hyaloclastite. It is characterized by irregularly-curved joints known as entablature. The in-situ breccia occurs as a marginal facies of entablature-jointed basalt, and its width varies from 10 to 30m. The result of this study indicates that the basaltic tuff and associated basalts of the study area were produced by the volcanic activity of same period and the basaltic tuff was formed by subaqueous eruption of basaltic lava followed by nonexplosive quench fragmentation.
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Deep drill holes bored into in situ oceanic crust were reviewed. Since the beginning of the Deep Sea Drilling Project, 45 drill holes have penetrated over ca. 50 m into normal oceanic crust, however, most are concentrated in the north Atlantic and east Pacific Oceans. Basement ages of most holes are younger than 20 Ma, and are rather biased toward younger crust compared to the average age of the oceanic crust (61 Ma). Only five deep drill holes have penetrated over 500 m into the basement, three of which drilled into slow spread crust formed at < 4 cm/yr, with only one hole is fast spread crust formed at >8 cm/yr and one in intermediately spread crust.Three deep holes (332B, 395A, 418A) drilled into slow spread crust formed at Mid-Atlantic Ridge gave the first evidence of magnetic reversals through the vertical oceanic crust, and showed that the slow spread upper oceanic crust away from hot spots is dominantly composed of pillow lavas with a normal MORB-like affinity. Younger 332B and 395A holes (3.5 and 7.3 Ma) gave poor core recovery (18-21%), while the oldest Hole 418A (110 Ma) yielded a fairly high recovery of 72%.Hole 504B is the only hole to penetrate the extrusive rocks and most of the way through the sheeted dike complex (1836.5 m sub-basement). Average core recovery dropped from 29.8-25.3% in the lava and transition zone down to 14.3% in the sheeted dike complex. Unfortunately, the 504B lava is the depleted extremity of MORBs from intermediate-fast spread ridges. One of the most important findings of Hole 504B is a discrepancy between the seismic velocity structure and the downhole lithology in that the Layer 2/3 boundary resides in the middle of the sheeted dikes, as interpreted by the difference in porosity and bulk density.Hole 1256D is dedicated to coring typical oceanic crust and ultimately penetrates the entire crust into the upper mantle. The site is located on the 15-Ma Cocos plate generated at a superfast rate (22 cm/yr). 502-m-long cores of basement (48% recovery) are lavas showing moderately evolved MORB-like compositions similar to those from the present fast spread ridges. The hole has been cleaned and left ready for future drilling, possibly into Layer 3.The above examples of deep drill holes show that the major obstacles to ultradeep drilling are hole collapse and poor core recovery. Riser drilling is expected to overcome these obstacles for “the 21-century Mohole”.
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