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首頁(yè)> 外文學(xué)位 >Shape-preferred orientation (SPO) of oceanic gabbros at ODP Hole 1256D: Implications for magmatic processes.
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Shape-preferred orientation (SPO) of oceanic gabbros at ODP Hole 1256D: Implications for magmatic processes.

機(jī)譯：ODP 1256D孔處的海洋輝長(zhǎng)巖的形狀優(yōu)先取向（SPO）：對(duì)巖漿作用的影響。

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The magmatic processes involved in building the plutonic section of the oceanic crust at fast-spreading centers remain debated. At this stage, no intact section of this important lithospheric layer, known as seismic layer 3, has been drilled. Yet, Ocean Drilling Program Hole 1256D, located on the eastern flank of the East Pacific Rise, provides unparalleled opportunities to investigate the mode of emplacement and fabric development of the two uppermost gabbro bodies in this crust. Although inferences made from a drill core remain limited due to the intrinsically linear nature of observations, the samples recently recovered hold potential clues on fabric-forming processes and the magma convection in such small intrusions.;Gabbro 1 forms a 52 m-thick body intruded in the texturally granoblastic altered zone of the sheeted dike complex. Gabbro 2, situated below gabbro 1, is only 24 m-thick. Both gabbro bodies lack a macroscopically visible fabric and were, until this study, considered structurally isotropic.;We use digital image analysis of petrographic thin-sections and the intercept method (Launeau and Robin, 1996) to determine the shape-preferred orientation of plagioclase phenocrysts in 3-D. Thirty-three sets of three mutually perpendicular thin-sections were prepared and analyzed in this manner. The mode of plagioclase grains ranges from 15 to 20% in the upper interval of gabbro 1, 13 to 36% in the lower interval of gabbro 1, and 9 to 28% in gabbro 2. These values refer to the mode of euhedral to subhedral grains only and do not reflect the total mode of plagioclase in the rock. The two gabbros display a weakly anisotropic shape-preferred orientation characterized by an average shape ratio > 1.050. The aspect ratio indicates plagioclase fabric strength deviation from a perfectly anisotropic distributed population. The aspect ratio in the upper interval of gabbro 1 ranges from 1.068 to 1.153. The aspect ratio in the lower interval of gabbro 1 ranges from 1.074 to 1.183. The aspect ratio in gabbro 2 ranges from 1.056 to 1.220. The fabric of these gabbros also displays broad consistency between nearby specimens. Lineation plunges in the upper interval of Gabbro 1 range from 1° to 36°. Lineations plunge in the lower interval of gabbro 1 range from 0° to 44°. Lineations in gabbro 2 plunge from 6° to 69°. In general, the symmetry of plutonic fabrics provides clues on the nature of magmatic fabric-forming processes. Prolate fabrics support magmatic flow while oblate fabrics rather suggest gravitational settling of crystals. Fabrics in gabbro 1 and gabbro 2 are both prolate and oblate. Fabric in the upper interval of gabbro 1 is more prolate than oblate whereas the lower interval of gabbro 1 and gabbro 2 have even distributions of fabric ellipsoids.;Detailed observations of petrographic thin-sections reveal several microstructures suggestive of brittle and plastic deformation in plagioclase grains. Microstructures indicative of plastic deformation include kink-banding, mechanical twinning, and undulose extinction. Microstructures indicative of brittle deformation include submagmatically-fractured laths. These subtle features appear in at least one crystal per thin-section analyzed. With the exception of mechanical twins, shipboard scientists of Expeditions 312 and 335 have not documented these microstructures.;Numerical calculations reveal that for both gabbro bodies the Rayleigh Number ranges between 1015 and 1018, depending primarily on the kinematic viscosity of the magmas and temperature change across the intrusions. These values indicate that turbulent flow (Rayleigh number > 106) occurred during emplacement. This type of flow may provide an explanation for the variability of aspect ratio and the variability in linear crystal fabric.
機(jī)譯：在快速擴(kuò)展的中心構(gòu)造洋殼的巖漿剖面所涉及的巖漿作用過(guò)程仍存在爭(zhēng)議。在此階段，尚未鉆探該重要巖石圈層的完整部分，即地震層3。然而，位于東太平洋上升東部側(cè)面的遠(yuǎn)洋鉆探計(jì)劃洞1256D提供了無(wú)與倫比的機(jī)會(huì)來(lái)研究該地殼中兩個(gè)最上部輝長(zhǎng)巖體的進(jìn)位模式和織物發(fā)育。盡管由于觀測(cè)值的內(nèi)在線性特性，鉆頭巖心的推論仍然受到限制，但最近回收的樣本在如此小的侵入體中對(duì)織物形成過(guò)程和巖漿對(duì)流具有潛在的線索.Gabbro 1形成了一個(gè)52 m厚的侵入體在薄片狀堤防復(fù)合體的質(zhì)構(gòu)粒狀改變區(qū)中。位于gabbro 1下方的Gabbro 2僅24 m厚。這兩個(gè)輝長(zhǎng)巖體都缺乏肉眼可見(jiàn)的織物，并且在此研究之前被認(rèn)為是各向同性的。我們使用巖相薄片的數(shù)字圖像分析和攔截方法（Launeau and Robin，1996）來(lái)確定斜長(zhǎng)石的形狀優(yōu)先取向3-D中的phenocrysts。以這種方式準(zhǔn)備并分析了33組三個(gè)相互垂直的薄片。斜長(zhǎng)石紋的模式在長(zhǎng)三角1的上部區(qū)間中為15％至20％，在長(zhǎng)三角1的下部區(qū)間中為13％至36％，在長(zhǎng)三角2中為9％至28％。這些值是指從直面到半面體的模式僅顆粒，不能反映巖石中斜長(zhǎng)石的總模式。兩個(gè)輝長(zhǎng)巖顯示出弱各向異性的形狀優(yōu)先取向，其特征在于平均形狀比> 1.050。長(zhǎng)寬比表示斜長(zhǎng)石織物強(qiáng)度與完全各向異性的分布種群之間的偏差。輝長(zhǎng)巖1的較高區(qū)間的長(zhǎng)寬比為1.068至1.153。輝長(zhǎng)巖1的下半部分的寬高比為1.074至1.183。 gabbro 2中的長(zhǎng)寬比為1.056至1.220。這些長(zhǎng)袍的織物在附近的標(biāo)本之間也顯示出廣泛的一致性。襯線在Gabbro 1的較高區(qū)間內(nèi)下降，范圍為1°至36°。條紋在gabbro 1的較低間隔中插入，范圍從0°到44°。輝長(zhǎng)鳥(niǎo)2中的線條從6度開(kāi)始暴跌。至69°。一般而言，深部構(gòu)造的對(duì)稱(chēng)性提供了巖漿構(gòu)造過(guò)程的性質(zhì)的線索。長(zhǎng)扁織物支持巖漿流動(dòng)，而扁扁織物則提示晶體在重力作用下沉降。 gabbro 1和gabbro 2中的織物均為長(zhǎng)扁形和扁圓形。輝長(zhǎng)巖1上部區(qū)間的織物比扁長(zhǎng)扁長(zhǎng)，而輝長(zhǎng)巖1和輝長(zhǎng)巖2的下部區(qū)間的織物橢球分布均勻。;巖相薄片的詳細(xì)觀察表明，有幾個(gè)微觀結(jié)構(gòu)表明斜長(zhǎng)石晶粒有脆性和塑性變形。指示塑性變形的微觀結(jié)構(gòu)包括扭結(jié)帶，機(jī)械孿晶和過(guò)度消光。指示脆性變形的微觀結(jié)構(gòu)包括巖漿破裂的板條。這些細(xì)微特征出現(xiàn)在每個(gè)分析的薄板中至少有一個(gè)晶體中。除機(jī)械雙胞胎外，遠(yuǎn)征312和335的船上科學(xué)家沒(méi)有記錄這些微觀結(jié)構(gòu)。數(shù)值計(jì)算表明，對(duì)于兩種輝長(zhǎng)巖體，瑞利數(shù)的范圍在1015和1018之間，主要取決于巖漿的運(yùn)動(dòng)粘度和溫度變化?？缭饺肭?。這些值表明在安置期間發(fā)生了湍流（瑞利數(shù)> 106）。這種類(lèi)型的流動(dòng)可以解釋縱橫比的變化性和線性晶體織物的變化性。

著錄項(xiàng)

  • 作者

    Trela, Jarek.;

  • 作者單位

    Southern Illinois University at Carbondale.;

  • 授予單位 Southern Illinois University at Carbondale.;
  • 學(xué)科 Geology.
  • 學(xué)位 M.S.
  • 年度 2013
  • 頁(yè)碼 91 p.
  • 總頁(yè)數(shù) 91
  • 原文格式 PDF
  • 正文語(yǔ)種 eng
  • 中圖分類(lèi)
  • 關(guān)鍵詞

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